DE102024106903B3 - Illumination device and device and method for generating multispectral images - Google Patents

Abstract

An illumination device (13) for generating multispectral images is provided. The illumination device (13) comprises a light source arrangement (21, 23A-D, 33A-F) which is designed to illuminate an object (5) with illumination light having a broad spectral range and a hue corresponding to a specific point in a color space, and which makes it possible to independently vary the radiant powers arriving at the object (5) in at least two narrow spectral ranges (I, II). Furthermore, the illumination device (13) comprises a control device (25) for controlling the light source arrangement such that the radiant powers in the at least two narrow spectral ranges (I, II) are varied sequentially over time, wherein the sequential changes each include a change in the radiant power in at least one of the at least two narrow spectral ranges (I, II). The control device (25) is designed to control the light source arrangement in such a way that the change in the radiant power in at least one of the at least two narrow spectral ranges (I, II) in the radiant power arriving at the object (5) only causes color tone changes which do not exceed the predetermined maximum permissible deviation from the specific point of the color space.
Furthermore, a device for generating multispectral images with such an illumination device (13) and a method for generating multispectral images are provided.

Description

The present invention relates to an illumination device for use in generating multispectral images. The invention also relates to a device and a method for generating multispectral images.

In many areas of medicine, tissue should be differentiated as minimally as possible for the patient. Examples include distinguishing dental fillings from healthy teeth, distinguishing tumor tissue from non-tumor tissue during resection surgeries, distinguishing active cortical tissue from inactive cortical tissue, etc.

There are various approaches to distinguishing tissue. One approach, for example, is to use fluorescence to distinguish tissue. This involves using filters during the acquisition of fluorescence images that essentially only allow fluorescent light to pass through to the image sensor. This changes the color impression of the object and significantly reduces the intensity of the light emitted by the object. Therefore, a surgeon usually has to switch back and forth between the fluorescence mode and the white light mode of a surgical microscope and interrupt their work while acquiring fluorescence images. Furthermore, acquiring fluorescence images often requires the injection of a fluorescent dye.

An alternative to distinguishing tissue using fluorescence is the acquisition of so-called multispectral or hyperspectral images. An imaging system that can capture white-light and hyperspectral images is available, for example, in WO 2018/049215 A1 described. A conventional color image detects light reflected and/or scattered by the object in three color channels, namely a red, a green, and a blue color channel. The recording of the individual color channels can be done sequentially with a monochromatic image sensor by taking an image for the red color channel, an image for the green color channel, and an image for the blue color channel, which are then combined to form a color image. Alternatively, all three color channels can be recorded simultaneously. For this, the image sensor must be designed as a color sensor, which is sometimes also referred to as a Bayer sensor. In a color sensor, a matrix is connected upstream of the image sensor, which arranges either a red, a green, or a blue spectral filter in front of each pixel of the image sensor. This matrix is called a Bayer matrix.

In multispectral and hyperspectral images, the number of color channels is increased and can also include color channels in the infrared or ultraviolet spectral range. To capture multispectral or hyperspectral images, the time-sequential recording method is often used. The object is illuminated with white light and spectral filters are successively inserted into the beam path between the object being observed and the camera to record the individual color channels in a time-sequential manner. Such a procedure is used, for example, in CA 2 942 069 A1 described. In this way, a monochrome image is obtained for each color channel, which represents the intensity distribution of the light reflected and/or scattered by the object under observation in the respective color channel. For each pixel, the spectral distribution of the reflected and/or scattered light can then be determined from the images recorded in the respective color channels. The spectral distribution can be determined with greater resolution the more color channels are used. As an alternative to inserting spectral filters into the beam path between the object under observation and the camera, it is also possible to illuminate the object under observation only with the corresponding narrow wavelength ranges to record the individual color channels of a hyperspectral image or a multispectral image. Such a procedure is also described in CA 2 942 069 A1 described.

In CA 2 942 069 A1 A method is also described that is intended to enable a surgeon to continue working while hyperspectral images are being acquired. For this purpose, hyperspectral images are acquired between two white light images. However, this requires a high acquisition frequency if a surgeon is to be presented with a flicker-free white light image. A flicker-free image requires a frame rate of approximately 30 frames per second. If, for example, the hyperspectral image is to contain n color channels, this requires the acquisition of n images in different wavelength ranges in addition to the acquisition of a white light image within a thirtieth of a second. This is why the images in the individual color channels must be acquired with short exposure times, especially if a large number of color channels are to be used.

Out of DE 10 2011 053 250 A1 A method for taking images of an object under observation is known in which a time-sequenced cial illumination with N different wavelengths.

Out of DE 10 2017 108 036 A1 A method for controlling a lighting device having at least two electric light sources is described. The light sources have at least a first and a second spectral light distribution, which are superimposed on one another to generate a superimposed light distribution. The superimposed light distribution is adjusted based on the incoming light measured at the illuminated object such that a target value of the incoming light at the illuminated object is achieved.

In view of this prior art, a first object of the present invention is to provide an illumination device which makes it possible to record multispectral images without a surgeon having to interrupt his work during the recording of the multispectral images, whereby the problem of short exposure times described above can be avoided.

Furthermore, it is a second object of the present invention to provide an apparatus and a method for generating multispectral images without a surgeon having to interrupt his work during the acquisition of the multispectral images, whereby the above-described problem of short exposure times can be avoided.

The first object is achieved by an illumination device according to claim 1, the second object by an apparatus for generating multispectral images representing spectral properties of an object according to claim 9 and by a method for generating multispectral images representing spectral properties of an object according to claim 12. The dependent claims contain advantageous embodiments of the invention.

An illumination device according to the invention comprises a light source arrangement and a control device. Wherever reference is made to a control device or controller within the scope of this description, this also includes a closed-loop control device or controller that adjusts a controlled variable to a reference variable using a feedback loop. The light source arrangement is designed to illuminate an object with illuminating light having a broad hue corresponding to a specific point in a color space. The term "light" is intended to encompass not only wavelengths in the visible spectral range, but also wavelengths in the ultraviolet (UV) and infrared (IR) spectral ranges. The color space can be any color space, e.g., an RGB color space such as the sRGB color space, a CIE color space such as the CIElab color space, or the CIExy color space, which was introduced by the International Commission on Illumination to characterize the perception of color contrasts between hues. The color tone can, in particular, be a white tone, for example, a color tone that can be characterized by a color temperature. Instead of a white tone, however, the color tone can also be a colorful tone.

The light source arrangement is also designed such that it allows the radiation power arriving at the object to be changed independently of one another in at least two narrow spectral ranges. The control device is designed to control the light source arrangement such that the radiation power in the at least two narrow spectral ranges can be changed sequentially over time, wherein the sequential changes each include a change in the radiation power in at least one of the at least two narrow spectral ranges. The narrow wavelength ranges can partially overlap.

Typically, the broad spectral range has a spectral width of at least 200 nm, in particular at least 300 nm, and more particularly a width of at least 400 nm. In special cases, it can even have a width of at least 700 nm. If the broad spectral range is intended to encompass visible light, it has a width of at least 300 nm (e.g., at least from 400 to 700 nm) and in particular at least 370 nm (e.g., at least from 380 to 750 nm). If, for example, it is also intended to encompass the infrared spectral range and parts of the ultraviolet spectral range, the broad spectral range can also be more than 400 nm or even more than 700 nm wide (e.g., from 360 to 800 nm or even from 360 nm to 1100 nm). A narrow spectral range, on the other hand, is considered to be a spectral range whose spectral width is a maximum of 35% of the wide spectral range. In particular, the spectral width of the narrow spectral range can also be a maximum of 20% of the wide spectral range, a maximum of 15% of the wide spectral range, or even a maximum of 10% of the wide spectral range. A narrow spectral range has a centroid wavelength that characterizes the spectrum of the narrow spectral range and represents the weighted arithmetic mean of the wavelengths of the narrow spectral range. The width of the narrow spectral range is considered to be the half-width, which is given by the amount of spectral separation between those wavelengths at which the intensity value has dropped from a maximum intensity of the narrow spectral range to half the maximum intensity value. A central wavelength and a half-width can also be present in the wide spectral range. However, a wide spectral range can also be made up of a number of narrow spectral ranges, which can then also have several local maxima in intensity or transmission. A spectral range does not have to be symmetrical around a central wavelength. Typically, the wide spectral range in particular is asymmetrical when it is produced by an incandescent lamp whose glow is based on the temperature of a filament.

According to the invention, a maximum permissible deviation from the specific point is specified in the control device for the color space. Furthermore, the control device is configured to control the light source arrangement such that the change in the radiant power in at least one of the at least two narrow spectral ranges causes only hue changes in the radiant power arriving at the object that do not exceed the specified maximum permissible deviation from the specific point of the color space.

The maximum permissible deviation from the specific point can, for example, be specified by a fixed maximum value. However, the permissible deviation from the specific point can also depend on the direction in the color space, for example to take into account different sensitivities of the eye to different colors. In addition, the maximum permissible deviation can also depend on the location of the specific point in the color space. Due to the lower sensitivity of the human eye in the red and violet spectral ranges compared to the green spectral range, wavelengths in the transition region between violet and ultraviolet and in the transition region between red and infrared can be changed more significantly than wavelengths in the green spectral range without causing a disruptive change in color perception.

A maximum permissible deviation, which depends on both the direction in the color space and the specific point in the color space, can be specified, for example, using the so-called MacAdam ellipse. 11 shows a schematic of the CIExy color space 94 with MacAdam ellipses around specific hues (centers of the ellipses). Furthermore, the figure shows the so-called Planck curve 99, which represents the colors of a blackbody thermal radiator as a function of its temperature, with the temperature of the blackbody thermal radiator increasing from right to left along the Planck curve 99 in the figure. 11 The points 96, 98 and 100 represent white light with different color temperatures. In the figure, the lower corner of the color space 94 represents blue light with a wavelength of 380 nm, the right corner of the color space 94 represents red light with a wavelength of 700 nm and the rounded tip of the color space 94 represents green light with a wavelength of 520 nm. 11 As can be seen, the size and ellipticity of a MacAdam ellipse depend on its location in the color space. However, it should be noted that the MacAdam ellipses shown are enlarged by about ten times for better visibility. The actual MacAdam ellipses are therefore considerably smaller than those shown in the 11 drawn ellipses.

Color tones that lie within a MacAdam ellipse cannot be distinguished by the human eye, so that when the radiation power arriving at the object is changed sequentially in time, the color tone changes of the light arriving at the object cannot be perceived by the human eye. A viewer of the object illuminated with the illumination device according to the invention thus does not perceive any color tone change in the illumination light, so that their color impression when viewing the object is maintained during the time-sequential change of the radiation power arriving at the object in the at least two narrow spectral ranges. However, if the images are recorded by an image sensor, an evaluation device can evaluate the color tone changes lying within a MacAdam ellipse and thus extract the signal attributable to the respective narrow wavelength range. This enables the recording of multispectral images during a surgeon's work without impairing the surgeon's perception of the object being observed. This eliminates the need to switch between a mode for capturing multispectral images and a mode for capturing white light images. Since color changes are not noticeable during multispectral image capture, there is no need to limit the exposure time for capturing multispectral images.

Hue changes are particularly noticeable when compared to each other. For example, if the hue changes rapidly sequentially, i.e. at a high frequency, the rapid succession of hue changes can be perceived as irritating color flickering. perceived when the successive color tones exceed a small, specified maximum permissible deviation, i.e., for example, do not lie within a MacAdam ellipse. If, on the other hand, the time-sequential change in the color tones occurs slowly, a change in color tone is perceived as less disturbing. In such cases, a larger maximum permissible deviation can be specified. For example, it may be sufficient in such cases if the change in the radiant power in at least one of the at least two narrow spectral ranges in the radiant power arriving at the object only causes changes in color tone that lie within an ellipse whose semi-axes each correspond to a multiple of a MacAdam ellipse in the CIExy color space, e.g., twice a MacAdam ellipse, four times a MacAdam ellipse, or even six times a MacAdam ellipse. Furthermore, the acceptable color tone changes may depend on the application in which the light source is used (intraoperative generation of multispectral images, preoperative generation of multispectral images, postoperative generation of multispectral images, etc.). The control device can therefore, for example, be configured to control the light source arrangement such that the change in the radiant power in at least one of the at least two narrow spectral ranges in the radiant power arriving at the object only causes color tone changes that lie within an ellipse whose semi-axes each correspond at most to a predetermined multiple of a MacAdam ellipse in the CIExy color space, e.g., twice a MacAdam ellipse, four times a MacAdam ellipse, or even six times a MacAdam ellipse. In particular, the control device can also be configured to control the light source arrangement such that changing the radiant power in at least one of the at least two narrow spectral ranges in the radiant power arriving at the object only causes hue changes that lie within a MacAdam ellipse. Furthermore, the control device can comprise an adjustment device with which the maximum permissible deviation from the specific point in the color space can be set. This adjustment device can be part of a control panel, integrated into a graphical user interface (GUI), part of a voice input module for entering voice commands, etc.

Instead of specifying a maximum permissible deviation based on a MacAdam ellipse, other specifications that are not based on the MacAdam ellipse can also be applied. Other color spaces can also be used in this case. For example, in the sRGB color space, a sphere with a specified radius can be placed around a point that represents a hue. If the radius is small enough, the sphere will only contain hues that are indistinguishable to the human eye. The corresponding radius can then represent the maximum permissible deviation in the sRGB color space. In principle, such a definition of the maximum permissible deviation is also possible in any other color space. Since color spaces can be transformed into one another, it is also possible to transfer an ellipse or a sphere given in one color space to another color space. For example, the ellipse can be transferred from the CIExy color space to another color space, such as the sRGB color space, although the simple geometric shape of an ellipse is not necessarily retained. Another way to specify a maximum permissible deviation, especially for white colors, is to specify a maximum permissible Duv value. The Duv value indicates the position of the color tone in relation to a white point 96, 98, 100 on the Planck curve 99 (see 11 ). A positive Duv value represents a shift in hue towards a yellow-greenish hue, a negative Duv value a shift towards a purple hue. The smaller the Duv value, the closer the hue is to the corresponding white point 96, 98, 100 and the more difficult it is to perceive the difference between the hue and the corresponding white point 96, 98, 100. If a small Duv value is specified as the maximum permissible deviation, the difference between the hue and the white point is difficult to perceive, and if a Duv value is specified that is small enough, the difference may not be perceptible at all.

In a first embodiment of the light source arrangement, it can comprise a broadband light source emitting across the entire broad spectral range and, for at least one of the narrow spectral ranges, at least one spectral filter that can be introduced into the beam path by the control device. In addition or alternatively to the at least one spectral filter that can be introduced into the beam path by the control device, the light source arrangement can comprise at least one light source whose radiant power can be adjusted by the control device and which emits in one of the at least two narrow spectral ranges. The light source arrangement can comprise at least two light sources whose radiant power can be adjusted by the control device, for example, two, three, four or more light sources, wherein the light sources each emit in a different narrow spectral range. In some variants, the light source arrangement can comprise at least six and in particular comprise at least twelve light sources whose radiant power can be adjusted and which emit in different narrow spectral ranges.

Broadband light sources can be at least one incandescent lamp such as a halogen lamp or at least one luminescent lamp such as a gas discharge lamp, light-emitting diodes (LEDs), in particular white light LEDs, organic light-emitting diodes (OLEDs), etc. The broadband light source can in particular also contain a plurality of light sources which only form the broadband light source when they interact. The spectral filters can each have a band with a bandwidth of no more than 50 nm, in particular no more than 30 nm, more particularly no more than 20 nm, and the centers of the bands can be at least 35 nm, in particular at least 45 nm, and more particularly at least 60 nm apart in the spectrum. The band of the respective spectral filter can lead to either increased or reduced transmission of the filter compared to the broad spectral range.

The light sources, which can be adjusted in their radiant power, can be present in addition to a broadband light source, as described.

Alternatively, the narrow spectral ranges of the light sources, the radiant power of which can be adjusted, can also jointly form at least a portion of the broad spectral range or even the entire broad spectral range. In the latter case, no additional broadband light source is necessary. The narrowband light sources can be LEDs, OLEDs, lasers, etc., which emit in different narrow spectral ranges. The width of the narrow spectral ranges for LEDs and OLEDs is typically no greater than 75 nm, in particular no greater than 50 nm, and more particularly no greater than 25 nm. For lasers, it can be even smaller, e.g., no greater than 10 nm, in particular no greater than 5 nm, more particularly no greater than 3 nm, and even more particularly no greater than 1 nm or even no greater than 0.5 nm.

If two or more narrowband light sources are to be combined to create a broader spectral range, there are basically several ways to implement this combination. For example, the narrowband light sources can be arranged so close to one another that the individual spectral components cannot be perceived with spatial resolution, as is the case with RGB displays, for example. The narrowband light sources can be arranged in a linear arrangement, particularly if the broader spectral range is only intended to cover a few narrowband spectral ranges and therefore only a few narrowband light sources are required. Alternatively, the narrowband light sources can be arranged in a matrix-like manner in a two-dimensional arrangement. A matrix-like arrangement can, for example, repeat the linear arrangement of narrowband light sources perpendicular to the direction of extension of the linear arrangement, whereby the arrangement of the light sources in the individual linear arrangements can also be permuted. In addition, a matrix-like arrangement offers the possibility of repeating an already basic two-dimensional arrangement of narrowband light sources in one or two dimensions. For each narrow spectral range, the matrix-like arrangement thus contains as many narrowband light sources as there are repeats. This can be advantageous in terms of homogeneous illumination of the object under observation.

The combination of the narrowband spectral ranges through spatial proximity of the narrowband light sources is advantageous in that no additional optical elements are required for combination, provided the narrowband light sources are small enough, as is the case with LEDs, OLEDs, semiconductor lasers, etc. For larger narrowband light sources, it is possible to arrange the narrowband light sources as primary light sources with a greater distance from each other and to couple their light into optical fibers, whose outputs can then be close to each other and form secondary light sources.

Furthermore, it is possible to combine narrowband light sources emitting in different narrow spectral ranges using spectral filters. For example, interference filters can be equipped with one or more passbands in which they allow certain wavelengths to pass while reflecting the remaining wavelengths. Incorporating such a filter into the beam path makes it possible to combine light that passes through the filter due to its transmission properties with light that is reflected by the filter. This makes it possible to superimpose different wavelengths along the same optical path.

It is also possible to provide light in different spectral ranges with mutually orthogonal polarizations and then, for example, to use a polarization beam splitter that allows light with one polarization to pass through and reflects light with the orthogonal polarization. This makes it possible, in particular, to combine narrow spectral bands that are very close to one another. ranges, which would place very high demands on the spectral filter if combined using a spectral filter, to form a somewhat broader spectral range. Such a somewhat broader spectral range can then, if necessary, also be combined with other spectral ranges using spectral filters, as described above. As an alternative to polarization beam splitters, which allow light with one polarization to pass through and reflect light with the orthogonal polarization, there is also a crystalline form of polarization beam splitter that utilizes birefringence. In such polarization beam splitters, orthogonal polarizations have different refractive indices. This results in different exit angles from such a polarization beam splitter and thus in different light paths after exiting the polarization beam splitter.

If the light source arrangement comprises more than two spectral filters that can be introduced into the beam path or more than two light sources whose radiant power is adjustable, the control device can be configured in a variant of the invention to control the light source arrangement such that a change in the radiant power in at least one of the at least two narrow spectral ranges is compensated for by a change in the radiant power in at least one other of the at least two narrow spectral ranges such that the change in the radiant power in the narrow spectral ranges only causes hue changes in the radiant power arriving at the object that do not exceed the predetermined maximum permissible deviation from the specific point of the color space. In a simple example, the radiant power can be increased or decreased simultaneously at least in a first narrow spectral range and in a second narrow spectral range. With a suitable distance between the first narrow spectral range and the second narrow spectral range, the resulting color tone change can be smaller than if only one of the two narrow spectral ranges were increased or decreased, so that spectral changes that are easy to evaluate for an evaluation unit can be brought about, which in total do not exceed the specified maximum permissible deviation from the specific point of the color space. If, for example, in the case of lighting with a white hue, there are two narrow spectral ranges in the red spectral range, two narrow spectral ranges in the green spectral range and two narrow spectral ranges in the blue spectral range and one of the narrow spectral ranges in the red spectral range, one of the narrow spectral ranges in the green spectral range and one of the narrow spectral ranges in the blue spectral range is increased or reduced in its radiant power, it can be achieved that the specified maximum permissible deviation from the white hue will not be exceeded even with a significant increase or reduction in the respective radiant powers and thus the original white hue is retained within the limits of perception of the human eye.

In another example, for illumination with a white hue, the light source arrangement can be controlled such that the radiant power is increased in at least one narrow spectral range and, at the same time, the radiant power is reduced in at least a second narrow spectral range, wherein the centroid wavelengths of the two spectral sub-ranges whose radiant power is increased and reduced are close to one another. This can compensate for the effect of increasing the radiant power in one narrow spectral range on the hue by reducing it in the other narrow spectral range to such an extent that the associated change in hue lies below the perception threshold of the human eye, i.e., the predetermined maximum permissible deviation from the white hue is not exceeded. For example, the radiant power can be increased in a first blue spectral range and decreased in a second blue spectral range without exceeding the predetermined maximum permissible deviation from the white hue. The same applies to changes in the radiant power in the green or red spectral range.

In the light source arrangement according to the invention, the control device can also be designed to control the light source arrangement in such a way that when the radiation power is changed in at least one of the at least two narrow spectral ranges, only brightness changes in the radiation power arriving at the object occur which amount to a maximum of 10%, in particular a maximum of 5%, further in particular a maximum of 3% and even further in particular a maximum of 1% or even a maximum of 0.1%. The accepted maximum brightness change, as already with the accepted maximum color tone change, can depend on the frequency with which the changes occur and/or on the application in which the light source is used (intraoperative generation of multispectral images, preoperative generation of multispectral images, postoperative generation of multispectral images, etc.). Any brightness change caused by changing the radiation power in at least one of the at least two narrow spectral ranges can be limited by limiting the change in the Radiant power can be adjusted in at least one spectral range or by compensating for a brightness change caused by changing the radiant power in at least one of the at least two narrow spectral ranges by changing the radiant power of the light source arrangement outside the at least one spectral range. In particular, if a plurality of spectral ranges outside the at least one spectral range are used for compensation, the brightness fluctuations can be kept low. The previously described increase in the radiant power in at least one narrow spectral range while simultaneously reducing the radiant power in at least a second narrow spectral range can also be used to keep brightness fluctuations low.

According to the invention, a device for generating multispectral images representing spectral properties of an object is also provided. This device includes at least one image sensor for recording images of an object and an illumination device according to the invention. The device also includes an evaluation unit that generates the multispectral images of the object based on the changes in the radiation power reflected and/or scattered by the object and arriving at the image sensor resulting from the time-sequential change in the radiation power arriving at the object. The use of the light source arrangement according to the invention enables the recording of multispectral images during a surgeon's work without impairing the surgeon's perception of the object being observed. It is therefore not necessary to switch between a mode for recording multispectral images and a mode for recording white light images. Since the color tone changes during the recording of the multispectral images are not perceived as disturbing, no limitation of the exposure time for recording the multispectral images is necessary. The evaluation unit can, in particular, be designed to perform the evaluation in real time, i.e., with a delay of only a few frames, e.g., with a delay of a maximum of five frames, preferably a maximum of two, and ideally less than one. This can be used, for example, to overlay the multispectral images or parts thereof on the white light images in real time.

The image sensor used in the device can be a monochromatic image sensor. However, this is not a requirement. If radiant power is supplied to different areas of the image sensor via one of at least two different spectral channels, the evaluation unit is designed to evaluate the differences in the radiant power supplied to the respective areas when determining the spectral properties of the object. This means that image sensors in which different spectral channels are assigned to different areas of the image sensor, as is the case with color sensors, for example, can also be used to capture multispectral images. A color sensor would have three different spectral channels: a red, a green, and a blue color channel. Each of these channels would accordingly only allow the respective spectral component of the light reflected and/or scattered by the object to reach the image sensor. Depending on the spectral distribution of the illuminating light, the proportions of light reaching the image sensor via the respective spectral channel change, so that the ratios of the proportions in the respective spectral channels can be used to determine the spectral distribution of the light reflected and/or scattered by the object. The spectral channels of the image sensor generally do not correspond to the spectral bands of the light source arrangement.

The evaluation unit of the device according to the invention can also be designed to generate at least one digital image with the specified color tone, for example with a specified white tone, during the time-sequential change of the radiation power arriving at the object. The evaluation unit is designed to further reduce, in the at least one digital image with the specified color tone, a color tone deviation lying within the specified maximum permissible deviation from the specific point of the color space by means of digital color tone adjustment. In this way, a digital image with the specified color tone, for example a white light image with a specified white tone, can be generated without flickering in the color tone even if a relatively large maximum permissible deviation is specified.

• - Beleuchten eines Objekts mit Licht, welches einen breiten Spektralbereich und einen einem bestimmten Punkt eines Farbraums entsprechenden Farbton aufweist.
• - Zeitsequentielles Verändern der am Objekt ankommenden Strahlungsleistung, wobei das zeitsequentielle Verändern der am Objekt ankommenden Strahlungsleistung jeweils eine Veränderung der Strahlungsleistung in wenigstens einem von wenigstens zwei schmalen Spektralbereichen beinhaltet. Die schmalen Wellenlängenbereiche können dabei teilweise überlappen.
• - Aufnehmen wenigstens eines Bildes mit der jeweiligen am Objekt ankommenden Strahlungsleistung nach jeder zeitsequentiellen Veränderung der am Objekt ankommenden Strahlungsleistung in den wenigstens zwei schmalen Spektralbereichen.
• - Generieren eines Multispektralbildes aus den aufgenommenen Bildern, wobei die spektralen Eigenschaften des Objekts aus zeitsequentiellen Änderungen der vom Objekt reflektierten und/oder gestreuten Strahlungsleistung ermittelt werden, wobei die zeitsequentiellen Änderungen der vom Objekt reflektierten und/oder gestreuten Strahlungsleistung durch das zeitsequentielle Verändern der am Objekt ankommenden Strahlungsleistung verursacht werden.

Furthermore, the invention provides a method for generating multispectral images representing spectral properties of an object. The method comprises the steps:

• - Illuminating an object with light that has a wide spectral range and a hue corresponding to a specific point in a color space.
• - Time-sequential change of the radiation power arriving at the object, whereby the time-sequential change of the radiation power arriving at the object each Involves a change in the radiant power in at least one of at least two narrow spectral ranges. The narrow wavelength ranges may partially overlap.
• - Recording at least one image with the respective radiant power arriving at the object after each time-sequential change in the radiant power arriving at the object in the at least two narrow spectral ranges.
• - generating a multispectral image from the recorded images, wherein the spectral properties of the object are determined from time-sequential changes in the radiant power reflected and/or scattered by the object, wherein the time-sequential changes in the radiant power reflected and/or scattered by the object are caused by the time-sequential change in the radiant power arriving at the object.

The radiant power is changed in at least one of the at least two narrow spectral ranges in such a way that the change in the radiant power in at least one of the at least two narrow spectral ranges only causes hue changes in the radiant power arriving at the object that do not exceed a predetermined maximum permissible deviation from the specific point of the color space. For example, the maximum permissible deviation can be defined on the basis of the MacAdam ellipse, as described with reference to the lighting device according to the invention, so that, for example, only hue changes are caused that lie within a MacAdam ellipse, or that only hue changes are caused that lie within an ellipse whose semi-axes correspond at most to a predetermined multiple of a MacAdam ellipse in the CIExy color space, e.g., twice a MacAdam ellipse, four times a MacAdam ellipse, or even six times a MacAdam ellipse. However, other ways of specifying a maximum permissible deviation than the specification based on a MacAdam ellipse can also be used, as has been described with reference to the light source arrangement according to the invention.

The method according to the invention enables the acquisition of multispectral images during a surgeon's work without disrupting the surgeon's perception of the object being observed. Thus, it is not necessary to switch between a mode for capturing multispectral images and a mode for capturing white light images. Since the color changes during the acquisition of multispectral images are not or hardly noticeable, no limitation of the exposure time for capturing the multispectral images is necessary.

In one embodiment of the method, a change in the radiant power in at least one of the at least two narrow spectral ranges can be compensated by changing the radiant power in at least one other of the at least two narrow spectral ranges such that the change in the radiant power in the narrow spectral ranges only causes hue changes in the radiant power arriving at the object that do not exceed the predetermined maximum permissible deviation from the specific point of the color space. This was explained with reference to the lighting device according to the invention and will therefore not be repeated here.

In the method according to the invention, the recording of at least one image can comprise the recording of image signals in at least two different spectral channels. When generating the multispectral images, differences between the image signals in the at least two different spectral channels are then evaluated. This embodiment of the method makes it possible to use image sensors in which different regions of the image sensor are assigned different spectral channels, as is the case with color sensors, for recording the multispectral images. A color sensor, for example, would have three different spectral channels: a red, a green, and a blue channel. Each of these channels would accordingly only allow the respective spectral component of the light reflected and/or scattered by the object to reach the image sensor. Depending on the spectral distribution of the illuminating light, the proportions of light reaching the image sensor via the respective spectral channel change, so that the ratios of the proportions in the respective spectral channels can be used to determine the spectral distribution of the light reflected and/or scattered by the object. The spectral channels of the image sensor generally do not correspond to the spectral bands of the light source arrangement.

Within the scope of the method according to the invention, the object can be illuminated in such a way that when the radiation power is changed in at least one of the at least two narrow spectral ranges, only brightness changes in the radiation power arriving at the object occur which amount to a maximum of 10%, in particular a maximum of 5%, further in particular a maximum of 3% and even further in particular a maximum of 1% or even a maximum of 0.1%, as has already been described with reference to the light source arrangement according to the invention.

If images in a given color tone, such as a given shade of white, are generated for an observer not only purely optically but also digitally, at least one digital image with the given color tone can be generated during the time-sequential variation of the radiant power arriving at the object. In this digital image with the given color tone, a color tone deviation that lies within the specified maximum permissible deviation from a specific point in the color space can be further reduced by digital color tone adjustment. In this way, a digital image with the specified color tone, such as a white light image with a given shade of white, can be generated without flickering in the color tone, even if a relatively large maximum permissible deviation is specified.

• 1 zeigt eine Operationsmikroskop mit einer Beleuchtungsvorrichtung, die das Erzeugen von spektrale Eigenschaften eines Objekts repräsentierenden Multispektralbildern mit dem Operationsmikroskop ermöglicht.
• 2 zeigt ein Schaltschema zum Zuschalten von LEDs mit schmalen Spektralbereichen zu einer Weißlichtbeleuchtung beim Operationsmikroskop aus 1.
• 3 zeigt eine alternative Ausgestaltung der Beleuchtungsvorrichtung.
• 4 zeigt eine weitere Alternative Ausgestaltung der Beleuchtungsvorrichtung.
• 5 zeigt ein Filterrad, wie es in der in 4 gezeigten Beleuchtungsvorrichtung zum Einsatz kommen kann.
• 6 zeigt ein Schaltschema zum Verändern der Strahlungsleistung in schmalen Spektralbereichen des Beleuchtungslichtes.
• 7 zeigt ein weiteres Schaltschema zum Verändern der Strahlungsleistung in schmalen Spektralbereichen des Beleuchtungslichtes.
• 8 zeigt noch ein weiteres Schaltschema zum Verändern der Strahlungsleistung in schmalen Spektralbereichen des Beleuchtungslichtes.
• 9 zeigt die spektralen Kanäle eines Farbsensors.
• 10 zeigt typische Transmissionskurven von Rotfiltern, Grünfiltern und Blaufiltern, wie sie in einem Farbsensor üblicherweise Verwendung finden.
• 11 zeigt schematisch den CIExy-Farbraum mit eingezeichneten MacAdam-Ellipsen und dem Planck'schen Kurvenzug.

Further features, properties and advantages will become apparent from the following description of embodiments of the invention with reference to the accompanying figures.

• 1 shows a surgical microscope with an illumination device that enables the generation of multispectral images representing spectral properties of an object with the surgical microscope.
• 2 shows a circuit diagram for connecting LEDs with narrow spectral ranges to a white light illumination in the surgical microscope from 1 .
• 3 shows an alternative design of the lighting device.
• 4 shows another alternative design of the lighting device.
• 5 shows a filter wheel as it is in the 4 shown lighting device can be used.
• 6 shows a circuit diagram for changing the radiant power in narrow spectral ranges of the illuminating light.
• 7 shows another circuit diagram for changing the radiant power in narrow spectral ranges of the illuminating light.
• 8 shows another circuit diagram for changing the radiant power in narrow spectral ranges of the illuminating light.
• 9 shows the spectral channels of a color sensor.
• 10 shows typical transmission curves of red filters, green filters and blue filters as they are commonly used in a color sensor.
• 11 shows schematically the CIExy color space with drawn MacAdam ellipses and the Planck curve.

The following is based on 1 An exemplary embodiment of a device for generating multispectral images representing spectral properties of an object is described. In the exemplary embodiment, the device is designed as a surgical microscope or integrated into a surgical microscope. The surgical microscope can be a digital surgical microscope, in which intermediate images generated by an optical system are recorded by image sensors and displayed on a display, or an analog surgical microscope, in which the intermediate images are viewed optically by means of eyepieces and to which at least one camera is also connected, to which a part of the optical path of the optical system is coupled out for recording an image by means of an image sensor, as shown schematically in the 1 shown exemplary embodiment.

In the device for generating multispectral images of the exemplary embodiment, a monochrome image sensor is used to capture the multispectral images. However, it is also possible to capture the multispectral images with a color sensor, as will be explained later. Color sensors differ from monochrome image sensors in that they are preceded by a so-called Bayer matrix, in which a spectral filter is arranged in front of each pixel of the image sensor, which allows either red, green, or blue light to pass through. Each pixel is thus only supplied with information in the red color channel, the green color channel, or the blue color channel. The color information of the other pixels is then interpolated from the existing color information. For example, the color information of the green color channel for those pixels that are supplied with only red or blue light due to the Bayer matrix is interpolated based on the surrounding pixels that are supplied with green light.

The surgical microscope 1 of the exemplary embodiment comprises two stereoscopic partial beam paths, a main objective 3, which images a divergent beam 7 emanating from a focal plane F in the object 5 into two parallel partial beams, one for each stereoscopic partial beam path of the surgical microscope 1. In the surgical microscope 1, a magnification changer (not shown) is usually present. which can be used to change the magnification factor of the image generated by the surgical microscope 1. This magnification changer can, for example, be designed as a Galilean changer, in which different lens combinations can be alternately inserted into the beam path, with each lens combination providing a different magnification factor. Alternatively, the magnification changer can be designed as a zoom system, with which a continuous change of the magnification factor can be realized. The magnification changer can be a "large" magnification changer, which is passed through by both stereoscopic partial beam paths. Alternatively, it is possible for the magnification changer to have its own Galilean changer or its own zoom system for each stereoscopic partial beam path.

In the present exemplary embodiment, beam splitters 9, 11 are also arranged in the parallel partial beam paths. Each of these beam splitters 9, 11 can be designed as a partially transparent mirror or as a prism. The first beam splitter 9 is arranged in the beam path such that the illumination beam path B emanating from an illumination device 13 is deflected in the direction of the observation object 5. This beam splitter can be a "large beam splitter" through which both stereoscopic partial beam paths of the observation beam path B pass, as shown in the 1 illustrated exemplary embodiment. However, there can also be a small beam splitter in each stereoscopic partial beam path, which is then only passed through by one of the two stereoscopic partial beam paths. This is particularly the case if the illumination is to be parallel to the stereoscopic partial beam paths of the observation beam path A (so-called coaxial illumination). Alternatively, the illumination beam path B can also be directed between the two stereoscopic partial beam paths of the observation beam path A in the direction of the observation object 5. In this case, a reflective element, e.g. a mirror or a reflection prism, can be used instead of a beam splitter. It is also possible to guide the illumination through an edge region of the main objective 3 at an angle to the observation beam path A to the object (5) (so-called oblique illumination). In this case, too, a mirror or another reflective element without transmission properties can be used instead of a beam splitter. The angle of oblique lighting can, for example, be between 2° and 10° and, in particular, around 6°.

The second beam splitter 11 couples light from the observation beam path toward an image sensor 15, which in the present exemplary embodiment is symbolized by a camera. Typically, the light coupled toward the image sensor 15 is coupled from one of the stereoscopic partial beam paths. If a stereoscopic image is to be recorded, two image sensors are present, each of which receives light from a different one of the two stereoscopic partial beam paths, so that each image sensor can record a stereoscopic partial image. In the present exemplary embodiment, the image sensor 15 is a monochrome image sensor.

Furthermore, the surgical microscope 1 in the exemplary embodiment comprises a binocular tube. This comprises a tube objective for each stereoscopic partial beam path, with which the parallel stereoscopic partial beam paths are each focused onto an intermediate image plane. The aerial images thus created in the intermediate image plane can be viewed using eyepieces 19A, 19B. Alternatively, image sensors can also be arranged in the respective intermediate image plane, onto which the parallel stereoscopic partial beam paths are focused and which record the stereoscopic partial images for display on one or more monitors. These image sensors are typically color sensors. In this case, the image sensors can also be used to record the multispectral images, so that the second beam splitter 11 and the 1 the image sensor symbolized by camera 15 can be dispensed with.

If the recorded stereoscopic partial images are displayed on a single monitor, this can be done in particular in such a way that the left and right stereoscopic partial images are displayed alternately in time, which can be viewed by a user, for example, using shutter glasses or polarized glasses. Another option for displaying the stereoscopic partial images is a head-mounted display (HMD), in which a stereoscopic partial image is presented to each eye via a display assigned to the respective eye. It is also possible to split the stereoscopic partial beam paths into two beam paths using additional beam splitters, one of which is fed to an image sensor and the other to a tube lens, in order to enable purely visual observation and simultaneous display on a monitor or in an HMD.

The lighting device 13 comprises in the present exemplary embodiment a halogen lamp 21 as a white light source, which emits in a broad spectral range and has a specific white tone in a color space, e.g., the CIExy color space. Instead of a halogen lamp 21, other broadband light sources can also be used, such as white-light LEDs or gas discharge lamps such as xenon lamps. Furthermore, the broadband light source does not necessarily have to emit white light. The broadband light source can also emit colored light, i.e., light that has a specific colorful hue in the color space, for example, light with a green, blue, or red hue. Furthermore, the broadband light source can also emit light in the infrared and/or ultraviolet spectral range.

In addition to the halogen lamp 21, the illumination device 13 includes a number of LEDs 23A to 23D that emit in different narrow spectral ranges. The width of the narrow spectral ranges is typically no greater than 75 nm, in particular no greater than 50 nm, and more particularly no greater than 25 nm. The halogen lamp 21, together with the LEDs 23A-23D, forms a light source arrangement that is designed to illuminate the observation object 5 with light in a wide spectral range, in the present exemplary embodiment with white light, and is also capable of illuminating the observation object 5 in different narrow spectral ranges. By appropriately controlling the radiation outputs emitted by the respective LEDs, it is possible to independently vary the radiation output arriving at the observation object 5 in the narrow spectral ranges.

The surgical microscope 1 is also assigned a control device 25, which 1 is depicted as a computer, but can also be integrated as a dedicated control device, for example, into the surgical microscope 1 or into the illumination device 13. The control device 25 acts on the illumination device 13 to control the radiant power emitted by the respective LEDs 23A-23D of the light source arrangement. The radiant power emitted by an LED can be controlled, for example, via pulse width modulation of the current supplied to the respective LED. The radiant power emitted by the respective LEDs 23A-23D of the light source arrangement is controlled in such a way that the radiant powers emitted by the LEDs 23A-23D in the narrow spectral ranges are changed sequentially over time. In addition to the light source arrangement, the control device 25 also acts on the image sensor 15 to synchronize the recording of images with the sequential change in the radiant power emitted by the LEDs 23A-23D. The aim is to enable a temporal assignment of the images captured by the image sensor 15 to the LED 23A-23D active at the respective capture time. This goal can be achieved, for example, by assigning each captured image an identifier that is characteristic of the respective active LED. Alternatively, it is possible to store the times at which the respective LEDs were active and the times at which the images were captured, so that the images can subsequently be assigned to the LEDs based on the stored times. This requires that the switching of the LEDs 23A-23D, on the one hand, and the image capture, on the other, occur with the same time signal, or that the time signals are synchronized if different time signals are used for switching the LEDs and image capture.

A time-based circuit diagram for switching on the LEDs 23A-23D for white light illumination is shown in 2 shown. The 2 shows the switching on of the individual LEDs 23A-23D to the white light of the halogen lamp 21 for various times t0 to t3. The white light of the halogen lamp 21 is shown as a straight line, which represents a constant radiant power of the halogen lamp 21. In addition to the halogen lamp 21, the LEDs 23A-23D are switched on at the different times t0 to t3. The respectively switched on LED 23A-23D leads to an increased radiant power at the observation object 5 in the corresponding narrow spectral range, which in turn leads to an increased luminance in the recorded monochrome image. The luminance represents a measure of the brightness registered by the pixels of the image sensor 15. The luminance contains no color information and is determined by the luminance that results at the location of the image sensor from the radiant power reflected and/or scattered by the observation object 5. For each pixel of a monochrome image recorded at a time t0, t1, t2, t3, the luminance value of the pixel is a superposition of a luminance component resulting from the broadband spectral range of the halogen lamp 21 and the luminance component resulting from the narrowband spectral range of the LED active at the time of recording. By subtracting the luminance component attributable to the halogen lamp 21 from the luminance value of the respective pixel, an evaluation unit 27 can therefore extract for each pixel the luminance component of the luminance value attributable to the narrowband spectral range of the active LED. The result is monochrome images in which the luminance values of the individual pixels are only attributable to the narrowband spectral range of the LED active at the time of recording. These images can be combined by the evaluation unit 27, for example, using a suitable false color display to form a multispectral image. To determine the luminance component attributable to the halogen lamp 21, an image can be taken at the beginning of the recording or at regular intervals without the LEDs 23A-23D switched on.

In the present exemplary embodiment, the evaluation unit 27, like the control unit 25, is implemented by a computer, in particular by the same computer as the control unit 25. However, it need not be implemented by the same computer as the control unit 25 and, in particular, does not need to be implemented as a computer at all. Rather, it can be a computer separate from the control unit 25 or a dedicated unit that can be integrated, for example, into the surgical microscope.

If the difference between the luminance component of the currently active LED and the luminance component of the halogen lamp 21 does not exceed a predetermined maximum permissible deviation, it is possible to ensure that the color impression does not change perceptibly for the user of the surgical microscope, i.e., that the color tone changes, for example, only within the MacAdam ellipse around the white color tone of the halogen lamp 21. In other words, in the present exemplary embodiment, the MacAdam ellipse specifies a limit up to which the radiation power can be increased in the narrow spectral range without changing the color tone. This limit also depends on the width of the narrow spectral range. If this is very narrow, the limit can be higher than for a not quite as narrow spectral range. For example, increasing the radiant power by a certain amount in a narrow spectral range corresponding to 10% of the blue spectral range has less effect on the color tone than increasing the radiant power by the same amount if the narrow spectral range corresponds to 50% of the blue spectral range.

Although in 2 a circuit diagram is shown in which only one LED is switched on at a time, it is also possible to switch on more than one LED 23A-23F at a time. The luminance component extracted in each case is then representative of more than one narrow spectral range. By cyclically varying the combinations of the switched on LEDs and calculating the difference between the respectively extracted luminance components, the luminance components of the individual narrow spectral ranges can then be determined. By switching on more than one LED at the same time, it is possible to switch them on in such a way that the color tone changes caused by the switched on LEDs compensate for each other. If, for example, an LED in a narrow red spectral range, an LED in a narrow green spectral range and an LED in a narrow blue spectral range are switched on at the same time, the sum of the switched on LEDs can result in light with a white color tone. Compared to switching on only a single LED, this makes it possible to increase the radiant power of the switched LEDs without leaving the MacAdam ellipse around the hue of the halogen lamp 21. Increasing the radiant power of the switched LEDs leads to a larger difference between the luminance component caused by the respective LED in the monochrome image and the luminance component caused by the halogen lamp 21 in the monochrome image, thereby reducing the noise in the monochrome image after subtracting the luminance component caused by the halogen lamp 21.

If digital images are generated with the surgical microscope 1 using color sensors and presented on a display, it is possible to average the images using the connected LEDs 23A-23D, provided that the switching frequency with which the LEDs are switched is high enough to achieve at least 24 averaged images per second, preferably at least 30 averaged images per second. If such averaging occurs, the radiation conduction of the LEDs can be increased, so that a larger difference can be generated in the image of the monochrome image sensor 15 between the luminance component caused by the respective LED and the luminance component caused by the halogen lamp 21. The prerequisite is that the color tone when adding the narrow spectral ranges of the individual LEDs 23A-23D is sufficiently close to the color tone of the halogen lamp 21. The closer the color tone of the addition is to the color tone of the halogen lamp 21, the greater the difference can be between the radiant power of the individual LEDs 23A-23D and the radiant power of the halogen lamp 21, without the color tone in the averaged image of the color sensors being outside the MacAdam ellipse.

Although in the 2 While four LEDs 23A-33d are shown in the exemplary embodiment shown, the number of LEDs may be greater or less than four. For example, 12 or even more LEDs emitting in narrow spectral ranges are often used to capture multispectral images.

In the exemplary embodiment so far, the illumination device 13 has both a light emitting in a wide spectral range White light source 21 as well as LEDs 23A to 23D. In alternative exemplary embodiments, however, it may also comprise exclusively LEDs or exclusively a white light source in conjunction with a number of spectral filters. A lighting device 13 comprising exclusively LEDs is described below with reference to 3 described, a lighting device 13 which exclusively comprises a white light source in conjunction with a number of spectral filters, with reference to the 4 and 5 .

3 shows an exemplary embodiment of an illumination device 13, in which the illumination device 13 has only LEDs 33A-33F and a control device 25 for controlling the radiation outputs of the LEDs 33A-33F. The control device 25 can, for example, be the computer from the previously described exemplary embodiment or can be integrated as a dedicated control device into the surgical microscope 1 or the illumination device 13. The narrow spectral ranges of the LEDs 33A-33F and the radiation outputs of the LEDs 33A-33F relative to one another are coordinated such that the narrow spectral ranges add up to produce light with a predetermined hue when operated with the coordinated radiation outputs. The hue can be a white or a colored hue. In other words, if all LEDs 33A-33F are operated with the coordinated radiation powers, the predetermined color tone and thus a predetermined color impression is achieved in the image viewed through the eyepieces of the surgical microscope 1 and/or in the image recorded with the color sensors of the surgical microscope 1.

To capture the multispectral images, the radiant power of the LEDs 33A-33F can be individually adjusted by the control device 25. The radiant power of an LED 23A-23F can be increased or decreased compared to the coordinated radiant power. For each pixel of the monochrome image captured during the increase or decrease, the luminance value of the pixel is increased or decreased compared to the luminance value when the image was captured with the coordinated radiant power. The luminance component attributable to the narrow spectral range of the LED whose radiant power was increased or decreased can then be extracted from the difference between the luminance value achieved with the increased or decreased radiant power and the luminance value achieved with the coordinated radiant powers. If the radiant power of an LED 23A-23F is reduced compared to the coordinated radiant power, the amount of the difference is determined as the luminance component attributable to the corresponding narrow spectral range. Increasing or decreasing the radiant power of the respective LEDs 33A-33F can be kept so small that the change in the color impression only occurs within the MacAdam ellipse around the point of the specified hue. The limit for increasing or decreasing the radiant power of the respective LEDs 33A-33F is determined by the MacAdam ellipse and, as already mentioned, also depends on the width of the narrow spectral range.

However, there is also the possibility, already described with reference to the previous exemplary embodiment, of summing the images recorded with the color sensors of the surgical microscope 1 in order to average the effect of increasing or decreasing the radiant power of the individual LEDs 33A-33F in the color images. As already described with reference to the first exemplary embodiment, this allows the increase or decrease in the radiant power of the individual LEDs to be greater, thereby reducing the noise of the extracted luminance component.

Although in the 3 While six LEDs 33A-33F are shown in the exemplary embodiment shown, the number of LEDs may be greater or less than six. For example, 12 or even more LEDs emitting in narrow spectral ranges are often used to capture multispectral images. Furthermore, the LEDs do not need to be arranged in a linear array, as shown in 3 is shown merely to simplify the illustration. Rather, they can be arranged in a matrix-like arrangement. Furthermore, it is possible to repeat these matrix-like arrangements so that for each narrow spectral range there are as many LEDs as there are repetitions of the matrix-like arrangement. This can have advantages with regard to homogeneous illumination of the object being observed. Furthermore, the options mentioned above for combining the wavelength ranges of the LEDs using spectral filters or polarization beam splitters can also be used. The latter can be particularly useful if the LEDs are laser diodes with very narrow and very closely spaced spectral ranges.

In the 4 and 5 In the exemplary embodiment shown for a lighting device 13, only a halogen incandescent lamp 41 or another broadband light source such as a gas discharge lamp or a white light LED is present. The halogen incandescent lamp 41 is In the exemplary embodiment, a filter wheel 42 is arranged downstream of the illumination beam path, which can be rotated by means of a controllable or adjustable motor 44. This filter wheel 42 has a number of spectral filters 43A-43F, each of which has a filter characteristic which, except for a narrow spectral range, exhibits high transmission across the entire wide spectral range. The transmission is only reduced in the narrow spectral range. The maximum reduction in transmission in the narrow spectral range is limited by the fact that the reduction in transmission in the narrow spectral range should only change the hue of the illumination light within a MacAdam ellipse, so that the color impression for the user of the surgical microscope 1 does not change. The centroid wavelengths of the narrow spectral ranges in which the transmission is reduced differ between the individual spectral filters 43A-43F, so that by rotating the filter wheel, illumination with different, reduced narrow spectral ranges occurs one after the other. As in the previous exemplary embodiments, the reduced radiation power at the observation object 5 in the recorded monochrome image reduces the luminance values of the pixels. The luminance component attributable to the narrow spectral range can therefore be extracted by forming the pixel-by-pixel difference between the luminance values with the radiation power reduced in the narrow spectral range and the luminance values without reducing the narrow spectral range. In order to obtain the luminance values without reducing the radiation power in a narrow spectral range, the filter wheel 42 can also contain a position in which no filter is present. In this way, with each rotation of the filter wheel 42, a reference image is also recorded, representing the luminance values without reducing the radiant power in a narrow spectral range. The limit for reducing the radiant power using the spectral filters 43A-43F is determined by the MacAdam ellipse and also depends on the width of the narrow spectral range in which the reduction in radiant power occurs.

Although with reference to the 4 and 5 While filters 43A-43F have been described, in which the transmission is reduced in a narrow spectral range, it is also possible to use filters 43A-43F in which the transmission is increased in a narrow spectral range. For example, filters are conceivable which have a transmission rate of less than 100% across the entire broad spectrum, for example 70% or 50%, and only in the narrow spectral range an increased transmission compared to the other ranges, for example a transmission increased by 10%, 20% or 30%. In this case, the introduction of a filter into the illumination beam path has the same effect as that described with reference to the 1 and 2 described switching on an LED.

Furthermore, it is possible to design filters 43A-43F as narrowband filters, which only allow light in a narrow spectral range to pass through and block all other light in the wider spectral range. However, this requires that the narrow spectral ranges defined by the individual filters 43A-43F add up to the desired color tone, and that filters 43A-43F are inserted into the illumination beam path so quickly that a user of surgical microscope 1 only perceives the color tone resulting from the sum.

6 shows a time-dependent circuit diagram, as used to reduce the radiant power of the LEDs 33A-33F from 3 or to reduce the radiation output of the halogen lamp 1 by means of the spectral filters 43A-43F from the 4 and 5 can be used. The figure shows nine narrow spectral ranges (three in the blue spectral range B1-B3, three in the green spectral range G1-G3 and three in the red spectral range R1-R3), which are delimited from one another by vertical lines. At time t0, the radiant power is reduced in a first narrow spectral range of the illuminating light, at time t1 in a second narrow spectral range and so on until finally at time t8 the radiant power is reduced in the ninth narrow spectral range. At each time t0 to t8, a monochrome image is recorded, from which the luminance component resulting from the respective narrow spectral range is extracted. A multispectral image is then generated from the individual monochrome images, with the spectral information from the respective narrow spectral range at the time the respective monochrome image was recorded being derived from the extracted luminance component.

7 shows an alternative circuit diagram for reducing the radiant power of the LEDs 33A-33F from 3 or reducing the radiation output of the halogen lamp 1 by means of the spectral filters 43A-43F from the 4 and 5 . In the 7 In the circuit diagram shown, at a time t0, the radiant powers in a blue narrow spectral range B1, a green narrow spectral range G1, and a red narrow spectral range R1 are simultaneously reduced. At a time t1, the radiant powers in a blue narrow spectral range B2, a green narrow spectral range G2 and a narrow red spectral range R2, whereby, however, the respective narrow spectral ranges differ from the narrow spectral ranges reduced at time t0. Accordingly, at a time t2 the radiant powers in a narrow blue spectral range B3, a narrow green spectral range G3 and a narrow red spectral range R3 are reduced, whereby the spectral ranges differ from both the spectral ranges reduced at time t0 and those reduced at time t1. The cycle is then repeated. In the next cycle the reduced radiant powers in the narrow spectral bands can be permuted so that, for example, at time t0 of the second cycle the radiant power in the blue spectral range is reduced as at time t0 of the first cycle and the radiant power in the green spectral range is reduced as at time t1 of the first cycle. After completing a cycle, or after completing cycles with all permutations, an image can also be acquired in a narrow spectral range without reducing the radiant power to determine the luminance values for the image points in the monochrome image caused by the broad spectral range. At each point in a cycle, a monochrome image is acquired, from which the luminance component attributable to the respective narrow spectral range is extracted. Based on the permutations, the individual luminance components for each narrow spectral range can then be determined.

8 shows another circuit diagram for changing the radiation power of the LEDs 33A-33F from Figure 1. This circuit diagram is similar to the one in 6 shown circuit diagram in that the radiated power is reduced at a time t0 in a first narrow blue spectral range B1. In contrast to the circuit shown in 6 However, in the circuit diagram shown, the radiant power is increased in at least one further narrow blue spectral range. In the present exemplary embodiment, the increase even occurs in two further blue spectral ranges B2, B3, as a result of which the increase can be smaller. By increasing the radiant power in the two further narrow blue spectral ranges, the color tone change caused by the reduction in the radiant power in the first narrow blue spectral range can be reduced. At time t1, the radiant power is then reduced in the next narrow blue spectral range B2 and simultaneously increased in the two remaining blue spectral ranges B1, B3, or in at least one further blue spectral range. This scheme continues until the radiation intensity is reduced in the last narrow red spectral range R3 and increased in at least one other red spectral range R1, R2.

With the 7 and 8 The circuit diagrams shown make it possible to compensate for the radiant power in at least one of the narrow spectral ranges by changing the radiant power in at least one other of the narrow spectral ranges, so that even with relatively large changes in the radiant power in the narrow spectral ranges, the radiant power arriving at the object only causes color changes that do not exceed the specified maximum permissible deviation, i.e., lie within the required ellipse. In addition to the 7 and 8 In addition to the circuit diagrams shown, a specialist can implement further circuit diagrams that only cause color changes in the radiation power arriving at the object that do not exceed the maximum permissible deviation. For example, the 6 to 8 The circuit diagrams shown can be combined with one another. For example, a reduction in the radiant power in a narrow blue spectral range and an increase in the radiant power in at least one further narrow blue spectral range can be simultaneously achieved with a reduction in the radiant power in a narrow green spectral range and an increase in the radiant power in at least one further green spectral range, as well as a reduction in the radiant power in a narrow red spectral range and an increase in the radiant power in at least one further red spectral range.

In the previous exemplary embodiments, monochromatic image sensors were used to record the data for the multispectral images. As already mentioned, it is also possible to record the data for the multispectral images using color sensors. A color sensor differs from a monochromatic image sensor in that it has a Bayer matrix that assigns a color filter to each pixel of the sensor, so that the respective pixel is only supplied with light in a color channel defined by the assigned spectral filter. An example of a typical Bayer matrix is shown in 9 The Bayer matrix has three types of spectral filters 45, the first allowing primarily red light, the second allowing primarily green light, and the third allowing primarily blue light to pass to the respective pixels. In the following, the three types of spectral filters are therefore referred to as red filter 45R, green filter 45G, and blue filter 45B. Typical transmission curves R, G, and B of the red filter 45R, the green filter 45G, and the blue filter 45B are shown in 10 shown which is a slide diagram in which the transmission T of the red filter, the green filter and the blue filter is plotted as a function of the wavelength λ. The diagram from 10 shows that the red filters 45R, the green filters 45G and the blue filters 45B essentially have a high transmission T for red, green and blue light respectively and that their transmission T is significantly lower in the other wavelength ranges.

Typically, the number of 45G green filters is twice as high as the number of 45R red filters and 45B blue filters. This is because the human eye is most sensitive to the green spectral range. The increased number of 45G green filters therefore makes it easier to achieve a white balance that appears natural to the human eye.

The signals detected by the individual pixels of the color sensor each represent the luminance component of the color channel assigned to the respective pixel by means of a spectral filter. This means that the pixels assigned to the red filters 45R essentially detect the luminance component caused by the red spectral range, the pixels assigned to the green filters 45G essentially detect the luminance component caused by the green spectral range, and the pixels assigned to the blue filters 45B essentially detect the luminance component caused by the blue spectral range. A monochrome image sensor, on the other hand, would register the luminance caused by the entire spectral range with each pixel, i.e., the sum of all luminance components. To achieve a corresponding luminance with the color sensor, the luminance components missing from the respective pixels are supplemented. This supplementation is based on an interpolation of the luminance components of neighboring pixels. For example, for a pixel assigned to a blue filter 45B, the luminance component caused by the green spectral range is interpolated from the luminance components detected by adjacent pixels assigned to green filters 45G. Similarly, the luminance component caused by the red spectral range is interpolated from the luminance components detected by adjacent pixels assigned to red filters 45R. The interpolated luminance components are then added to the luminance component detected by the pixel with the blue filter 45B. Similarly, for pixels assigned to a red filter 45R or a green filter 45G, the undetected luminance components from neighboring pixels are interpolated, and the luminance components detected by the pixels are added. In this way, a luminance is obtained for each pixel of the color sensor that corresponds to the luminance of a monochrome image sensor. The described addition can be carried out either by the evaluation unit 27 or by an electronic unit assigned to the color sensor.

The 10 The transmission curves of the red filter 45R, the green filter 45G and the blue filter 45B shown can be used to record data for multispectral images. To illustrate this, 10 Two narrow spectral ranges I and II are shown as examples, whose center wavelengths lie in the blue-green spectral range (narrow spectral range I) and in the yellow-orange spectral range (narrow spectral range II). 10 As can be seen, the blue filters 45B and the green filters 45G have approximately the same average transmission T in the narrow spectral range I. The transmission T of the red filters 45R, on the other hand, is extremely low in the narrow spectral range I. For the narrow spectral range II, on the other hand, the red filters 45R have a very high transmission T, the green filters 45G have a low transmission T, and the blue filters 45B have an extremely low transmission T. Light in the narrow spectral range I therefore leads to a medium signal at those pixels to which the blue filters 45B and the green filters 45G are assigned, but to an extremely low signal at those pixels to which the red filters 45R are assigned. Light in the narrow spectral range II, on the other hand, results in an extremely low signal at those pixels assigned to the blue filters 45B, a low signal at those pixels assigned to the green filters 45G, and a very high signal at those pixels assigned to the red filters 45R. Using the example of the narrow spectral ranges I and II, it becomes clear that a narrow spectral range can be characterized by the signal ratios it creates between the pixels assigned to the respective color filters 45R, 45G, 45B. Therefore, from the signal ratios between the pixels assigned to the respective color filters 45R, 45G, 45B, the evaluation unit 27 can, conversely, determine the position of the narrow spectral range in the spectrum.

Since, when recording multispectral images, there is only an increase or decrease in the radiant power in the respective narrow spectral range compared to the wide spectral range, the luminance component attributable to the corresponding narrow spectral range is determined before the position of the narrow spectral range is determined as described. The luminance component attributable to the corresponding narrow spectral range is determined. As with a monochromatic image sensor, the resulting luminance component can be determined from the difference between the luminance resulting from only illumination in the broad spectral range and the luminance resulting from increasing or decreasing the illumination in the narrow spectral range. Using the image containing only the determined luminance component, the evaluation unit 27 then determines the position of the narrow spectral range, as described above, from the ratios of the signals generated by the luminance component at the pixels to which the respective color filters 45R, 45G, 45B are assigned, and assigns the position of the narrow spectral range to the image representing the determined luminance component.

After an image has been generated for each narrow spectral range that represents the luminance component attributable to it and the position of the respective narrow spectral range has been determined, the evaluation unit 27 can combine these images into a multispectral image. Since, with this procedure, the position of the narrow spectral range in the spectrum is determined from the image data recorded with the color sensor, the synchronization does not need to guarantee assignment of the images recorded by the color sensor to the narrow spectral range active at the respective recording time. Rather, it only needs to ensure that during the recording of an image with the color sensor, there is no switching from one narrow spectral range to another. The evaluation unit 27 then determines which narrow spectral range was used during the recording from the recorded image.

Although in the case of 9 and 10 While a Bayer matrix with red, green, and blue spectral filters is used in the exemplary embodiment described, the matrix can also contain other types of spectral filters, such as spectral filters for the colors cyan, magenta, and yellow. The number of different types of spectral filters is neither limited to three, nor do the wavelength ranges that can be passed through by the spectral filters have to lie exclusively in the visible spectral range. For example, the matrix can also have just two different types of spectral filters or four or five different types of spectral filters. An upper limit for the number of different types of spectral filters is given by the resolution with which the data in the respective color spectral range is actually to be recorded. Furthermore, it is possible for the matrix to comprise, instead of spectral filters that allow light in the visible spectral range to pass, or in addition to at least one type of spectral filter that allows light in the visible spectral range to pass, at least one type of spectral filter that allows light in the ultraviolet spectral range to pass, and/or at least one type of spectral filter that allows light in the infrared spectral range to pass. The described method of determining the position of the narrow spectral range can also be used when using such a modified matrix.

In the exemplary embodiments described, only hue changes were described that lie within a MacAdam ellipse in the CIExy color space. If the perception of a small color change can be accepted, a color change within an ellipse whose semi-axes correspond to a multiple of a MacAdam ellipse in the CIExy color space can also be accepted in the exemplary embodiments described. For example, a color change within an ellipse whose semi-axes correspond to a multiple of a MacAdam ellipse in the CIExy color space, e.g., twice a MacAdam ellipse, four times a MacAdam ellipse, or even six times a MacAdam ellipse, can be accepted. Since color spaces can be transformed into one another, it is also possible to transform an ellipse from the CIExy color space into another color space, such as the sRGB color space, although the simple geometric shape of an ellipse is not necessarily retained. Furthermore, the specified maximum permissible deviation does not have to be based on a MacAdam ellipse. For example, in a color space, a sphere with a specified radius can be placed around a point representing a hue. If the radius is small enough, the sphere will contain only hues that are indistinguishable to the human eye. The corresponding radius can then represent the maximum permissible deviation in the color space. Another alternative is to specify a permissible Duv value that must not be exceeded.

The present invention has been described in detail using exemplary embodiments for illustrative purposes. However, one skilled in the art will recognize from this description that variations may occur within the scope of the invention as defined in the appended claims. Therefore, the invention is to be limited only by the appended claims and not by the exemplary embodiments.

Claims (16)

Illumination device (13) for generating multispectral images, comprising - a light source arrangement (21, 23A-D, 33A-F) which is designed to illuminate an object (5) with to illuminate with illuminating light which has a broad spectral range and a hue corresponding to a specific point of a color space, and which makes it possible to change the radiant powers arriving at the object (5) independently of one another in at least two narrow spectral ranges (I, II); and - a control device (25) for controlling the light source arrangement (21, 23A-D, 33A-F) in such a way that the radiant powers in the at least two narrow spectral ranges (I, II) are changed sequentially in time, wherein the time-sequential changes each include a change in the radiant power in at least one of the at least two narrow spectral ranges (I, II), characterized in that a maximum permissible deviation from the specific point is predetermined in the control device (25) for the color space and the control device (25) is designed to control the light source arrangement in such a way that the change in the radiant power in at least one of the at least two narrow spectral ranges (I, II) only causes color tone changes in the radiant power arriving at the object (5) which do not exceed the predetermined maximum permissible deviation from the specific point of the color space. Lighting device (13) according to Claim 1 , characterized in that the light source arrangement (21, 23A-D, 33A-F) comprises a broadband light source (21) emitting in the entire broad spectral range. Lighting device (13) according to Claim 2 , characterized in that the light source arrangement (21, 23A-D, 33A-F) for at least one of the narrow spectral ranges (I, II) comprises at least one spectral filter (43A-F) which can be introduced into the illumination beam path (B) by the control device (25). Lighting device (13) according to one of the Claims 1 until 3 , characterized in that the light source arrangement (21, 23A-D, 33A-F) comprises at least one light source (23A-D, 23A-D, 33A-F) whose radiation power can be adjusted by the control device (25) and which emits in one of the at least two narrow spectral ranges (I, II). Lighting device (13) according to Claim 4 , characterized in that the light source arrangement (21, 23A-D, 33A-F) comprises at least two light sources (23A-D, 23A-D, 33A-F) whose radiation power can be adjusted by the control device (25), each emitting in a different narrow spectral range (I, II). Lighting device (13) according to Claim 5 , characterized in that the narrow spectral ranges (I, II) of the at least two light sources (33A-F) whose radiation power is adjustable jointly form at least a partial range of the broad spectral range. Lighting device (13) according to Claim 5 or Claim 6 , characterized in that a change in the radiation power in at least one of the at least two narrow spectral ranges (I, II) is compensated by a change in the radiation power in at least one other of the at least two narrow spectral ranges (I, II) in such a way that the change in the radiation powers in the narrow spectral ranges (I, II) only causes color tone changes in the radiation power arriving at the object (5) which do not exceed the predetermined maximum permissible deviation from the specific point of the color space. Lighting device (13) according to one of the Claims 1 until 7 , characterized in that the control device (25) is also designed to control the light source arrangement (21, 23A-D, 33A-F) in such a way that when the radiant power is changed in at least one of the at least two narrow spectral ranges (I, II), only brightness changes in the radiant power arriving at the object (5) occur which amount to a maximum of 10%. A device for generating multispectral images representing spectral properties of an object, comprising: - at least one image sensor (15) for recording images of an object (5), - an illumination device (13) according to one of the preceding claims, and - an evaluation unit (27) which generates the multispectral images of the object (5) based on the change in the radiation power reflected and/or scattered by the object (5) and arriving at the image sensor (15), resulting from the change in the radiation power arriving at the object (5). Device according to Claim 9 , characterized in that different areas of the image sensor (15) are each supplied with radiation power via one of at least two different spectral channels (45R, 45G, 45B) and the evaluation unit (27) is designed to evaluate the differences in the radiation power supplied to the respective areas when determining the multispectral images of the object. Device according to Claim 9 or Claim 10 , characterized in that the evaluation unit (27) is further designed to generate at least one digital image with the predetermined color tone during the time-sequential change of the radiation power arriving at the object (5) and to further reduce a color tone deviation lying within the predetermined maximum permissible deviation from the specific point of the color space in the at least one digital image with the predetermined color tone by means of a digital color tone adjustment. A method for generating multispectral images representing spectral properties of an object (5), comprising the steps of: - illuminating an object (5) with light having a broad spectral range and a hue corresponding to a specific point in a color space; - time-sequentially changing the radiant power arriving at the object (5), wherein the time-sequential changing of the radiant power arriving at the object (5) includes a change in the radiant power in at least one of at least two narrow spectral ranges (I, II); and - recording at least one image with the respective radiant power arriving at the object (5) after each time-sequential change in the radiant power arriving at the object (5); - generating a multispectral image from the recorded images, wherein the spectral properties of the object (5) are determined from time-sequential changes in the radiant power reflected and/or scattered by the object (5), wherein the time-sequential changes in the radiant power reflected and/or scattered by the object (5) are caused by the time-sequential change in the radiant power arriving at the object (5); characterized in that the change in the radiant power in at least one of the at least two narrow spectral ranges (I, II) takes place in such a way that the change in the radiant power in at least one of the at least two narrow spectral ranges (I, II) only causes hue changes in the radiant power arriving at the object (5) that do not exceed a predetermined maximum permissible deviation from the specific point of the color space. Procedure according to Claim 12 , characterized in that a change in the radiation power in at least one of the at least two narrow spectral ranges (I, II) is compensated by a change in the radiation power in at least one other of the at least two narrow spectral ranges (I, II) in such a way that the change in the radiation power in the narrow spectral ranges (I, II) only causes color tone changes in the radiation power arriving at the object (5) which do not exceed a predetermined maximum permissible deviation from the specific point of the color space. Procedure according to Claim 12 or Claim 13 , characterized in that the recording of the at least one image comprises the recording of image signals in at least two different spectral channels (45R, 45G, 45B) and, when generating the multispectral images, differences between the image signals in the at least two different spectral channels (45R, 45G, 45B) are evaluated. Method according to one of the Claims 12 until 14 , characterized in that the object (5) is illuminated in such a way that when the radiation power is changed in at least one of the at least two narrow spectral ranges (I, II), only brightness changes in the radiation power arriving at the object (5) occur which amount to a maximum of 10%. Method according to one of the Claims 12 until 15 , characterized in that , in addition, during the time-sequential change of the radiation power arriving at the object (5), at least one digital image with the predetermined color tone is generated and that in the digital image with the predetermined color tone, a color tone deviation lying within the predetermined maximum permissible deviation from the specific point of the color space is further reduced by digital color tone adjustment.

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