DE102022109577B4 - Method and measuring camera for two-dimensional measurement of objects - Google Patents

Abstract

Method for two-dimensional measurement of an inclined surface (78) of an object (26) with a measuring camera (40) which has a lens (42) with an optical axis (62), a variably adjustable pupil filter (50) arranged in a pupil plane (48) of the lens (42), and an image sensor (56), comprising the following steps: a) determining the orientation of the inclined surface (78) relative to the optical axis (62) of the lens (42); b) specifying a viewing direction from which the measuring camera (40) is to view the inclined surface (78); c) specifying a main ray direction (80) on the basis of the orientation determined in step a) and the viewing direction specified in step b); d) specifying a partial area (82) released by the pupil filter (50) in the pupil plane (48) in such a way that main rays (84) which emanate from the inclined surface (78) and impinge on the center of the partial region (82), run parallel to the main beam direction (80), wherein the partial region (82) is arranged decentered with respect to the optical axis (62);e) adjusting the pupil filter (50) such that the partial region (82) defined in step d) is exposed; andf) recording an image of the inclined surface (78) with the image sensor (56) using the pupil filter (50) adjusted in step e).

Description

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates to a method and a measuring camera for two-dimensional measurement of workpieces and other objects. Such measuring cameras are used as contactless sensors in coordinate measuring technology and can, for example, be attached to a coordinate measuring machine or be permanently installed on a coordinate measuring machine.

2. Description of the state of the art

Coordinate measuring machines are currently used to measure the geometry of workpieces and other objects. Such measurements are carried out, for example, as part of quality assurance or reverse engineering. The sometimes complex measuring tasks are usually reduced to measuring the spatial coordinates of a number of individual points.

The coordinate measuring machines contain a sensor whose position relative to the workpiece to be measured can be changed using drives. Especially with smaller coordinate measuring machines, the workpiece is located on a cross table that can be moved along two horizontal coordinate axes x, y with high precision. The sensor is usually attached to a spindle that can be moved vertically (i.e. in the z direction) with a similarly high level of precision. If particularly large or heavy workpieces are to be measured, coordinate measuring machines with a gantry design are used, in which the workpiece is stationary and only the sensor is moved.

When it comes to sensors for coordinate measuring machines, a distinction is made between optical and tactile sensors. While with tactile sensors the information about the position of a measuring point is generated by touching the measuring point with a probing element, with optical sensors the information about the position of the measuring point is recorded by light. With optical sensors, the coordinates of the workpiece can often only be measured in the focal plane of the sensors and thus in two dimensions. If measurements are taken successively in several focal planes whose distance from one another is known with a high degree of accuracy, the geometry of the workpiece can be measured in three dimensions - apart from undercuts and similar structures that are not visible to the optical sensor.

Coordinate measuring machines with optical sensors are sold by the applicant, for example, under the brand ZEISS O-INSPECT and are available in the EN 10 2016 202 928 A1 (corresponds US 2017 / 0 248 768 A1 ) described.

When measuring coordinates with optical sensors, it is generally desired that the workpieces are viewed from a specific direction by the measuring camera. For workpieces with flat surfaces, this viewing direction is usually perpendicular to the respective surfaces. However, the preferred viewing direction can also be determined by the longitudinal axis of a bore, a blind hole or a raised structure whose longitudinal axis is inclined to a surrounding surface.

In order to set the desired viewing direction, the relative spatial arrangement between the measuring camera and the workpiece must be changed. Whether the measuring camera or the workpiece is moved depends primarily on the size and weight ratios. For smaller workpieces, it is easiest to use a tilting table or a similar adjustable support for the workpiece. For heavier workpieces such as cast components, for which gantry-type coordinate measuring machines are usually used, only the measuring camera usually moves to change the viewing direction, while the heavy workpiece remains stationary. Since the coordinate measuring machine can only perform linear movements, a locking rotary swivel joint (RDS) is often used to adjust the measuring camera, which connects the measuring camera to the quill of the coordinate measuring machine.

Regardless of whether the workpiece or the measuring camera is moved, changing the relative spatial arrangement between the two parts takes time. The higher the accuracy requirements, the longer a single adjustment process takes, as the desired target position must then be approached particularly slowly. When using locking rotary swivel joints, highly precise alignment is sometimes not possible at all.

The US 2006/0007531 A1 discloses a method for measuring a surface extending obliquely to the viewing direction. An afocal optical system and a pupil filter are used, which is selected depending on a diffraction pattern on the surface of a wafer to be tested. The pupil filter can be selected, for example, according to the US 2003/0132405 A1 be trained.

The WO 2018/172027 A1 and WO 2003/093903 A2 reveal adjustable pupil filters in lighting systems to determine the angular spectrum of the illuminating radiation.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method and a measuring camera for the two-dimensional measurement of workpieces and other objects, with which the viewing direction can be changed quickly and with high accuracy.

1. a) Ermitteln der Orientierung der geneigten Oberfläche relativ zu der optischen Achse des Objektivs;
2. b) Vorgeben einer Betrachtungsrichtung, aus der die Messkamera die geneigte Oberfläche betrachten soll;
3. c) Festlegen einer Hauptstrahlrichtung auf der Grundlage der in Schritt a) ermittelten Orientierung und der in Schritt b) vorgegebenen Betrachtungsrichtung;
4. d) Festlegen eines von dem Pupillenfilter in der Pupillenebene freigegebenen Teilbereichs derart, dass Hauptstrahlen, die von der geneigten Oberfläche ausgehen und auf die Mitte des Teilbereichs auftreffen, parallel zur Hauptstrahlrichtung verlaufen, wobei der Teilbereich bezüglich der optischen Achse dezentriert angeordnet ist;
5. e) Einstellen des Pupillenfilters derart, dass der in Schritt d) festgelegte Teilbereich freigegeben wird; und
6. f) Aufnehmen eines Bildes der geneigten Oberfläche mit dem Bildsensor unter Verwendung des in Schritt e) eingestellten Pupillenfilters.

This task is solved by a method for two-dimensional measurement of an inclined surface of an object with a measuring camera that has a lens with an optical axis, a variably adjustable pupil filter arranged in a pupil plane of the lens, and an image sensor. The method comprises the following steps:

1. a) determining the orientation of the inclined surface relative to the optical axis of the objective;
2. b) Specifying a viewing direction from which the measuring camera should view the inclined surface;
3. c) determining a main ray direction on the basis of the orientation determined in step a) and the viewing direction specified in step b);
4. d) defining a partial area released by the pupil filter in the pupil plane such that main rays emanating from the inclined surface and impinging on the centre of the partial area run parallel to the main ray direction, the partial area being arranged decentered with respect to the optical axis;
5. e) adjusting the pupil filter such that the partial area defined in step d) is exposed; and
6. f) capturing an image of the inclined surface with the image sensor using the pupil filter set in step e).

The invention is based on the realization that the direction of the main rays can be determined within certain limits by the position of the partial area that is exposed in the pupil plane by the pupil filter. The partial area represents a section of the area that can be maximally illuminated in the pupil plane and defines the numerical aperture of the lens. The diameter of the maximum illuminable area specifies the maximum angle within which the main ray directions can be varied.

If, for example, a circularly outlined, released partial area moves across the pupil plane, this means that light beams with different main ray directions can enter the lens and reach the image sensor. Since the relationship between the main ray direction and the position of the partial area is known, a partial area can be defined for each desired main ray direction in step d), which leads to the viewing direction specified in step b).

A significant advantage of the invention is that inclined surfaces can be optically measured from any viewing direction within certain limits without the need for major mechanical movements such as tilting a tilting table or rotating a rotary swivel joint. Instead, different viewing directions are created by adjusting the variable pupil filter.

Depending on the pupil filter used, the adjustment times are in the order of a few milliseconds. With correspondingly fast image sensors, more than a hundred images with different viewing directions can be recorded in one second. With conventional methods, in which the object and/or the measuring camera have to be moved, the recording of a single image often takes several seconds. Using the method according to the invention, the measuring time can be shortened by up to three orders of magnitude.

However, the invention does not exclude the possibility of additionally changing the relative arrangement between the measuring camera and the object in the conventional manner. In the case of very large changes in the viewing direction, for example, a rough alignment can be carried out as before by mechanical movements, whereas the fine alignment is effected by the pupil filter. Even then, a significant reduction in the measuring time can be achieved, since with conventional mechanical movements, it is mainly the fine alignment with the low travel speeds required for this that takes up a particularly long measuring time.

The viewing direction in step b) is determined relative to the inclined surface. In most cases, the desired viewing direction is perpendicular to the inclined surface. The viewing calculation then coincides with the main ray direction determined in step c). Viewing angles other than 90° are particularly considered if the inclined surface has holes or projections. that do not extend perpendicular to the inclined surface.

The surface being observed does not have to be flat, but can also have a curvature in one or two directions. In general, a compensation plane can be defined for the image section observed by the measuring camera, in which the (possibly quadratic) deviations from the plane are minimal. The viewing direction is then set relative to this compensation plane.

In one embodiment, the partial area defined in step d) has a contour that is at least substantially circular or has a multi-cell rotational symmetry. The use of correspondingly symmetrical beam bundles generally leads to better imaging properties and thus to more precise measurement results.

Often, objects that have several differently oriented surfaces need to be measured. In this case, steps a) to f) must be repeated with a different pupil filter setting. Since adjusting the pupil filter takes little time, even complex objects can be measured in a very short time.

The lens can be telecentric on the object side. The advantage of such lenses, namely being able to image structures at different object distances with less sharpness but at the same image scale, is then retained for structures that have different distances along the set viewing direction.

In order to be able to set the pupil filter correctly, the orientation of the inclined surface to be measured relative to the optical axis of the lens must be known. In some cases, it is sufficient to derive this knowledge from the often known geometry data of the object to be measured. For higher measurement accuracy or if no geometry data is available, the orientation of the inclined surface must be measured. Additional optical sensors can be used for this purpose, which can be used to measure the inclination of surfaces. It is generally easier to use autofocus methods, as these do not require any additional sensors.

1. a) Ermitteln der Orientierung der geneigten Oberfläche relativ zu der optischen Achse des Objektivs;
2. b) Vorgeben einer Betrachtungsrichtung, aus der die Messkamera die geneigte Oberfläche betrachten soll;
3. c) Festlegen einer Hauptstrahlrichtung auf der Grundlage der in Schritt a) ermittelten Orientierung und der in Schritt b) vorgegebenen Betrachtungsrichtung;
4. d) Festlegen eines von dem Pupillenfilter in der Pupillenebene freigegebenen Teilbereichs derart, dass Hauptstrahlen, die von der geneigten Oberfläche ausgehen und auf die Mitte des Teilbereichs auftreffen, parallel zur Hauptstrahlrichtung verlaufen, wobei der Teilbereich bezüglich der optischen Achse dezentriert angeordnet ist;
5. e) Einstellen des Pupillenfilters derart, dass der in Schritt d) festgelegte Teilbereich freigegeben wird; und
6. f) Aufnehmen eines Bildes der geneigten Oberfläche mit dem Bildsensor unter Verwendung des in Schritt e) eingestellten Pupillenfilters.

With regard to the measuring camera, the object mentioned at the outset is achieved by a measuring camera for two-dimensional measurement of an inclined surface of an object, which has a lens that has an optical axis and a pupil plane, a variable pupil filter arranged in the pupil plane and a control unit. According to the invention, the control unit is set up to carry out or initiate the following steps:

1. a) determining the orientation of the inclined surface relative to the optical axis of the objective;
2. b) Specifying a viewing direction from which the measuring camera should view the inclined surface;
3. c) determining a main ray direction on the basis of the orientation determined in step a) and the viewing direction specified in step b);
4. d) defining a partial area released by the pupil filter in the pupil plane such that main rays emanating from the inclined surface and impinging on the centre of the partial area run parallel to the main ray direction, the partial area being arranged decentered with respect to the optical axis;
5. e) adjusting the pupil filter so that the partial area defined in step d) is exposed; and
6. f) capturing an image of the inclined surface with the image sensor using the pupil filter set in step e).

The variable pupil filter can be designed as an LCD panel, for example. LCD panels require very little installation space and have a switching speed that is usually less than 5 µs. Since LCD panels polarize the incident light, there may be limitations for very specific measuring tasks. In addition, an LCD panel causes intensity losses, which can lead to a lower signal-to-noise ratio on the image sensor and thus to less accurate measurement results.

These disadvantages can be avoided if a micromirror array is used as the pupil filter, which is arranged in the pupil plane and has a plurality of micromirrors, each of which can be moved into a first and a second stable tilt position. A micromirror in the first tilt position reflects light that has been reflected by the object so that it can reach the image sensor. A micromirror in the second tilt position reflects light that has been reflected by the object so that it cannot reach the image sensor.

Micromirror arrays whose micromirrors have two stable tilt positions are known in the state of the art and are often referred to as DMD mirror, where DMD stands for Digital Micromirror Device. The use of a micromirror array as a variable pupil filter is known in the state of the art. The use of such a dynamic pupil filter in a measuring camera was first described in the as yet unpublished application filed on July 15, 2021 EN 10 2021 118 327.8 described.

In order to enable a compact design, it is often useful if the measuring camera has a deflection mirror arranged in the light path between the micromirror array and the image sensor. This is preferably flat, but in principle can also have an imaging function. With a view to favorable beam folding without shadowing effects, it can be useful if the micromirrors are arranged in a mounting plane that runs at an angle to a sensor plane in which the image sensor extends.

In one embodiment, the measuring camera has an illumination system for illuminating the object. An optical axis of the illumination system is arranged such that a micromirror, which is in the second tilt position, directs the measurement light generated by the illumination system onto the object. The micromirror array thus serves not only to filter the pupil of the light reflected from the object, but also to variably adjust the illumination angle distribution of the light striking the object within certain limits. The beam splitters that are usually used when illuminating the object using the measuring camera and cause high intensity losses can be avoided in this way.

The lighting system preferably has a light source and an illumination optics that projects the light source onto the micromirror array. If the light source is a point light source, only a small area in the pupil plane is illuminated, which leads to highly directional illumination. It is therefore preferred if the light source is a surface emitter with Lambertian radiation characteristics so that the object to be measured can be illuminated evenly from different directions.

Since the micromirror array is located both in the beam path of the light used for illumination and in the beam path of the light reflected from the object, the illumination angle distribution for the illumination light and the pupil function for the reflected light cannot be adjusted independently of one another. If, for example, the object is to be illuminated with axially parallel light, which requires illumination of the center of the pupil, the micromirrors arranged there must be in the second tilt position. It is then not possible to filter the light reflected from the object in the pupil plane so that only axially parallel light falls on the image sensor. For many measuring tasks, however, it is sufficient if only one of the two properties illumination angle distribution and pupil function needs to be adjustable.

• das Mikrospiegel-Array zur Messung der Neigung einer Oberfläche des Gegenstands so zu steuern, dass während der Aufnahme mehrerer Bilder des Gegenstands die Pupillenebene in unterschiedlichen Bereichen abgeblendet wird, und
• aus einem Vergleich der Helligkeit der aufgenommenen Bilder ein Maß für die Neigung der Oberfläche zu berechnen.

With the micromirror array according to the invention in the pupil plane, the spatial orientation of the surface can also be easily measured in step a). The above-mentioned coupling of illumination angle distribution and pupil function can be used advantageously here. The control unit is then set up to

• to control the micromirror array for measuring the inclination of a surface of the object in such a way that the pupil plane is dimmed in different areas during the acquisition of several images of the object, and
• to calculate a measure of the inclination of the surface by comparing the brightness of the recorded images.

This use of the measuring camera according to the invention is based on the idea that the direction of the light used for illumination can be easily determined using the variable pupil filter and the lighting system. If the light falls perpendicularly onto the inclined surface, the intensity of the light on the image sensor is minimal. The micromirror array allows the different light directions to be tuned within a very short time, so that the inclination of the surface can be determined practically instantly. This procedure is particularly advantageous for specularly reflecting surfaces, where it is not possible to focus using the autofocus method due to a lack of recognizable structures.

In principle, when measuring the angle of inclination, the micromirror array can be controlled so that at a given time the light only falls on the surface of the object from a single direction. In this case, at a given time only the micromirrors in an approximately point-shaped area are in their second tilt position. The point-shaped area must then move over the entire pupil plane during the measurement process.

The inclination of the surface can be measured more quickly and no less accurately if the dimmed areas are selected in such a way that when taking an image for each object point, the light only comes from directions outside the measuring camera, which lie in one plane. The planes assigned to the images are different and intersect in a first straight line. If light spreads in a plane that is arranged perpendicular to the inclined surface, after reflection on the surface it can only hit micromirrors that are in the second tilt position and thus prevent it from being passed on to the image sensor.

Such a measurement can be carried out by taking an image with only a strip-shaped area in the pupil plane not stopped down, with the areas assigned to the images arranged parallel to each other. Each strip-shaped area then corresponds to one of the said planes. The planes are selected one after the other by moving the areas parallel from image to image.

If the inclination of the surface in two spatial directions is to be determined, a second measurement run can be carried out in which the blocked-off areas are selected so that when an image is taken for each object point, the light only exits the measuring chamber from directions that lie in one plane. The planes associated with the images are different and intersect in a second straight line that runs at an angle (preferably right) to the first straight line.

This measurement process can also be carried out more quickly in two spatial directions than if an approximately point-shaped area moves over the entire pupil plane.

It should be emphasized that the tilt angles are measured pixel by pixel as described above and not averaged over the entire image.

If the measuring camera receives additional distance information by setting different focal planes, the objects can be geometrically measured not only in two but in three dimensions.

BRIEF DESCRIPTION OF THE DRAWINGS

• 1 eine schematisch perspektivische Darstellung eines Koordinatenmessgeräts mit einem optischen Sensor, der eine erfindungsgemäße Messkamera enthält;
• 2 einen schematischen Meridionalschnitt durch die Messkamera;
• 3 eine perspektivische Darstellung eines Mikrospiegel-Arrays, das als variables Pupillenfilter verwendet wird;
• 4 eine Variante der in der 1 gezeigten Messkamera, bei der die Messkamera ein Beleuchtungssystem umfasst;
• 5 einen schematischen meridionalen Schnitt durch einen Abschnitt des Werkstücks mit einer geneigten Oberfläche sowie Teilen der Messkamera;
• 6 eine der 5 entsprechende Darstellung, jedoch mit einer anders angeordneten Oberfläche;
• 7: eine der 5 entsprechende Darstellung, jedoch mit einer schwächer geneigten Oberfläche, wobei ein axiales Lichtbündel eingezeichnet ist;
• 8 eine der 7 entsprechende Darstellung, in der ein außeraxiales Lichtbündel eingezeichnet ist;
• 9a, 9b und 9c jeweils eine Lichtverteilung (oben) in der Pupillenebene der Messkamera, die zur Messung der Neigung einer Planfläche eines Werkstücks verwendet wird, und einen schematischen Strahlengang (unten) zwischen dem Mikrospiegel-Array und dem Werkstück zu unterschiedlichen Zeitpunkten des Messvorgangs;
• 10a, 10b und 10c an die 9a, 9b bzw. 9c angelehnte Darstellungen für einen zweiten Messvorgang zur Messung eines zweiten Neigungswinkels.

In the following, embodiments of the invention are explained in more detail with reference to the drawings. In these:

• 1 a schematic perspective view of a coordinate measuring machine with an optical sensor containing a measuring camera according to the invention;
• 2 a schematic meridional section through the measuring camera;
• 3 a perspective view of a micromirror array used as a variable pupil filter;
• 4 a variant of the 1 measuring camera shown, in which the measuring camera includes an illumination system;
• 5 a schematic meridional section through a section of the workpiece with an inclined surface and parts of the measuring camera;
• 6 one of the 5 corresponding representation, but with a differently arranged surface;
• 7 : one of the 5 corresponding representation, but with a less inclined surface, with an axial light beam drawn in;
• 8th one of the 7 corresponding representation in which an off-axis light beam is drawn;
• 9a , 9b and 9c a light distribution (top) in the pupil plane of the measuring camera used to measure the inclination of a flat surface of a workpiece, and a schematic beam path (bottom) between the micromirror array and the workpiece at different times of the measuring process;
• 10a , 10b and 10c to the 9a , 9b or 9c Similar representations for a second measuring process to measure a second angle of inclination.

DESCRIPTION OF PREFERRED EMBODIMENTS

1. Structure of the coordinate measuring machine

The 1 shows a schematic perspective view of a coordinate measuring machine, designated 10 overall.

The coordinate measuring machine 10 comprises a base 12 which supports a table 14 to which a control panel 16 is attached. A stand 18 which supports a spindle 20 extends upwards from the table 14. As indicated by an arrow 22, the spindle 20 can be moved precisely in the vertical direction (z-direction) with the aid of a drive (not shown).

An optical sensor 24 is attached to the underside of the sleeve 20, with which an image of a workpiece 26 can be recorded. The workpiece 26 is attached to a cross table 28, with which the workpiece 26 can be precisely moved in the horizontal plane (x-direction and y-direction). can be used as in the 1 indicated by arrows 30 and 32, respectively. In this way, it is possible to measure even larger workpieces 26 successively with the aid of the sensor 24 by gradually introducing the workpiece 26 into the measuring field of the sensor 24 with the aid of the cross table 28.

If even larger or particularly heavy workpieces 26 are to be measured, the coordinate measuring machine 10 can also have a different mechanical structure and, for example, instead of the cross table 28, have a movable portal to which the sleeve 20 is attached. In this way, the sleeve 20 can be moved precisely not only along the z-direction, but also along the x-direction and y-direction, as is known in the prior art. The workpiece 26 then does not have to be moved during the measurement.

2. Structure of the sensor

The sensor 24 contains a measuring camera 40, the optical structure of which is 2 shown in a schematic meridional section.

The measuring camera 40 contains a lens 42 which comprises a first partial lens 44 and a second partial lens 46. In the illustrated embodiment, the partial lenses 44, 46 are each represented by only a single lens. However, they can also consist of more than just one lens. For reasons that will be explained in more detail below, the lens 42 should have a larger numerical aperture than conventional measuring lenses.

The focal lengths of the two lenses 44, 46 are in the 2 designated by f ₁ or f _2. The rear focal plane of the first partial objective 44 coincides with the front focal plane of the second partial objective 46. This creates an optical structure that is referred to as 4f optics. A 4f optic is afocal, so that parallel light that enters the objective 42 leaves the objective 42 again as parallel light. In the 2 The edge rays of the beams emanating from two object points are pairwise parallel.

4f optics are telecentric on both sides. This means that both the entrance pupil and the exit pupil are at infinity and the main rays run parallel to the optical axis in both the object space and the image space. The object-side telecentricity leads to the property, which is important for measuring cameras, that the image scale does not change when the object is axially shifted. The image of the object therefore always appears the same size regardless of the object distance, but of course becomes blurred if the object (workpiece 26) is located outside the object plane 47. In a 4f optic, this is the front focal plane of the first partial lens 44.

In the illustrated 4f optics, the pupil plane 48 of the objective 42 is located at the interface between the two partial objectives 44, 46, i.e. in the rear focal plane of the first partial objective 44 and thus in the front focal plane of the second partial objective 46. Light rays entering parallel into the objective 42 pass through the pupil plane 48 at a point which, in the case of axis-parallel rays, lies on the optical axis.

Conversely, different locations in the object plane 47 translate into different angles in the pupil plane 48.

A digital micromirror array 50 is arranged in the pupil plane 48 of the lens 42. The 3 shows the micromirror array 50 in a perspective view. In the enlarged section C, a single tilted micromirror can be seen and is designated 51. Such micromirror arrays are known in the prior art, so a more detailed explanation is omitted at this point. Micromirror arrays from the DLP9000 product group from Texas Instruments® are suitable, for example.

The micromirrors 51 of the micromirror array 50 each have two stable tilt positions in which the micromirrors 51 are deflected by either an angle +α or -α, where α is typically in the order of about 12°. If a micromirror 51 is stress-free, the tilt angle α is approximately 0°. The surface of the micromirror 51 then runs approximately parallel to a mounting plane of the micromirror array. However, this neutral tilt position is not defined, which is why it is usually not used in applications of the micromirror array - and thus also in the measuring camera 40.

For reasons of space and to avoid shadows, a flat deflection mirror 52 is located in the beam path between the pupil plane 48 and the second partial objective 46. Depending on the dimensions in the individual case, it may be expedient if the mounting plane of the micromirror array 50 is not perpendicular to the paper plane of the 2 , but is arranged at an angle other than 90°. The object plane 47 is then also tilted according to the Scheimpflug condition; the second partial objective 46 is accordingly located outside the paper plane of the 2 In this way, beam shadowing can be effectively avoided.

A two-dimensional image sensor 56 is arranged in an image plane 54 of the lens 42, which can be, for example, a CCD or CMOS image sensor.

The micromirror array 50 and the image sensor 56 are controlled by a control device 58. An evaluation device 60 is also connected to the image sensor 56, which evaluates the images of the workpiece 26 recorded by the image sensor 56, taking into account the filter function generated by the micromirror array 50 in the pupil plane 48. The control device 58 and the evaluation device 60 are referred to jointly as the control unit below.

The filter function that can be generated with the micromirror array 50 is digital. If a micromirror 51 is in a first tilt position, light that was reflected by the workpiece 26 is reflected by the micromirror 51 in such a way that it can reach the image sensor 56. If, however, the micromirror 51 is in the second stable tilt position, light that was reflected by the workpiece 26 is reflected in such a way that it cannot reach the image sensor 56.

If, for example, a filter function is to be implemented with the micromirror array 50, in which only the axially parallel portion of the light emanating from the workpiece 26 is to be fed to the image sensor 56, only a few micromirrors 51, which are located near the optical axis 62 of the lens 42, are moved to the first tilt position. All other micromirrors 51 are in the second tilt position, so that the light striking them cannot reach the image sensor 56.

If, for another image, only light that leaves the image sensor 56 at a certain angle to the optical axis 62 of the workpiece 26 is to reach the image sensor 56, the control device 58 controls the micromirror array 50 so that micromirrors 51 that lie in an annular region of the pupil plane 48 centered on the optical axis 62 are in the first tilt position and all other micromirrors 51 are in the second tilt position. The radius of the annular region depends on the desired angle.

The variable pupil filter in the form of the micromirror array 50 not only requires little installation space, but is also extremely fast with around 10,000 switching operations per second. This means that extensive image sequences recorded with different pupil filters can be generated almost instantly. Due to the high resolution of around 4 million pixels, the filter function of the pupil filter can be set very precisely.

Since the reflectivity of the micromirrors 51 is almost 90% and the fill factor is over 90%, the optical losses are considerably lower than is the case with LCD panels with their polarization filters.

3. Lighting system

The 4 shows in a 2 The illustration based on this variant shows a variant in which a lighting system is integrated into the measuring camera 40. For reasons of clarity, the control device 58 and the evaluation device 60 are not shown.

In contrast to the 2 The measuring camera shown contains the 4 The measuring camera 40 shown additionally has an illumination system 70 with which the workpiece 26 can be illuminated. The optical axis 72 of the illumination system 70 is arranged such that a micromirror 51, which is in the second tilt position, directs the light generated by the illumination system 70 onto the workpiece 26.

In the illustrated embodiment, the illumination system comprises two subsystems 73, 75 indicated by lenses, which have identical or almost identical focal lengths f ₃ and f ₄ , respectively. In this way, Köhler illumination can be realized, in which a light source 74 is imaged via the subsystems 73, 75 into the pupil plane 48 and thus onto the micromirror array 50.

If the light source 74 is designed as a point light source, as shown in the 4 As indicated, only a small axial area of the pupil plane 48 is illuminated, which leads to a highly directed illumination of the workpiece 26. If the light used for illumination is to have a wider angular distribution, a surface radiator with Lambertian radiation characteristics can be used instead of a point light source. Its dimensions, the focal lengths f ₃ and f ₄ , the field size and the numerical aperture of the subsystems 73, 75 are preferably coordinated so that the image of the radiation surface approximately corresponds to the dimensions of the micromirror array 50.

With the 4 With the measuring camera shown, it is possible not only to take pictures of the workpiece 26, but also to illuminate it with a coaxial incident light. The illumination angle distribution is determined by the filter function, which is implemented by the micromirror array 50. Since the illumination beam path from the light source 74 to the workpiece 26 and the imaging beam path from the workpiece 26 to the image sensor 56 use the same pupil plane 48 zen, the pupil function for the imaging beam path cannot be determined independently of the illumination angle distribution.

If, as explained in the next section, only a few micromirrors 51 are in the first tilt position at a given time, this necessarily means that all other micromirrors 51 are in the second tilt position and thus the workpiece 26 is illuminated with a wide illumination angle range with only a few illumination directions missing.

4. Adjusting the viewing angle

Like in the 1 As can be seen, the workpiece 26 has a section with a surface 78 that is not perpendicular but is oriented at an angle of less than 90° to the optical axis 62 of the measuring camera 40. Surfaces in which the surface normal does not run parallel to the optical axis 62 are referred to here as "inclined".

On the inclined surface 78 there are structures 79, the locations of which can only be recorded by the measuring camera 40 free of distortions and shadows if the structures are viewed from above by the measuring camera 40. This means that for such measurements the viewing direction must be perpendicular to the surface 78.

Such a vertical viewing direction on the inclined surface 78 is realized according to the invention by adjusting the pupil filter (ie the micromirror array 50) in a suitable manner. How this is to be done is explained below with reference to the 5 to 8 which each show in a schematic meridional section the section of the workpiece 26 with the inclined surface 78 and above it the first partial objective 44 and the micromirror array 50. A plan view of the micromirror array 50 is shown at the top.

Before an image of the inclined surface 78 can be recorded, its orientation relative to the optical axis 62 of the lens 42 must be determined. For this purpose, known autofocus methods, additional inclination sensors or the method explained in the next section 5 can be used. In the 5 the angle of inclination of the surface 78 in the illustrated cutting plane is denoted by α.

Furthermore, the viewing direction from which the measuring camera 40 is to view the surface 78 must be determined. In the present example, it is assumed that the surface 78 is to be viewed perpendicularly from the measuring camera 40. The viewing direction is not an absolute direction in space, but is determined relative to the surface 78. In the special case of the perpendicular viewing direction assumed here, this forms an angle β = 90° with the surface 78, regardless of the cutting plane.

Based on the orientation of the surface 78 and the specified viewing direction, the main ray direction to be observed can be determined, which is set relative to the optical axis 62 of the measuring camera 40. In the special case of the vertical viewing direction assumed here (β = 90°), the main ray angle γ = α applies. In the general case, the following applies to the 5 shown cutting plane the relationship γ = 90° - β + α. In the 5 The main beam direction is indicated by a double arrow and labeled 80.

In a next step, a subsection 82 of the above-mentioned 5 indicated micromirror array 50, within which all micromirrors 51 are in the first tilt position. This releases this partial area 82 so that light that enters the measuring camera 40 and reaches the released partial area 82 is reflected in the direction of the image sensor 56 and contributes to the image formation there. The released partial area 82 is defined with the proviso that main rays 84, which emanate from the inclined surface 78 and impinge on the center of the partial area 82, run parallel to the previously defined main beam direction 80. With an inclined main beam direction 80, this implies that the partial area 82 is arranged decentered with respect to the optical axis 62.

After the evaluation device 60 has mathematically determined the partial area 82, the control device 58 controls the micromirrors 51 of the micromirror array 50 such that only the micromirrors in the defined partial area 82 are in the first tilt position.

If an image of the surface 78 is now taken, the main rays 84 all impinge perpendicularly on the surface 78, which corresponds to the desired vertical viewing direction. The measuring camera thus observes the inclined surface 78 vertically from above without the workpiece 26 or the measuring camera 40 having to be moved.

The 6 illustrates that structures that are arranged one behind the other along the main beam direction 80 may be blurred, but are imaged with the same image scale, since the main rays 84 always run parallel. The measuring camera 40 is therefore telecentric along this direction.

The 7 The arrangement shown differs from that of the 5 only by the smaller angle of inclination α' of the surface 78. The area 82 to be released is therefore closer to the optical axis 62. From the comparison of the 5 and 7 It becomes clear that by selecting the partial area 82, different main beam directions 80 can be set with the help of the pupil filter.

The 8th equals to 7 with the difference that in the 8th an off-axis beam is shown which is collimated but impinges on the released portion 82 at an angle.

In the measuring camera 40 according to the invention, the main beam direction 80 can be changed within fractions of a second, since only micromirrors 51 have to change their tilt position. As already mentioned above, the switching frequencies of such micromirror arrays 50 are in the order of 10 KHz.

In the embodiment shown, the partial area 82 has a circular contour, the diameter of which determines the numerical aperture NA available for imaging. If this numerical aperture is, for example, NA = 0.05 and the numerical aperture of the objective 42 is NA _obj = 0.2, which corresponds to the diameter of the maximum area 86 that can be illuminated on the pupil filter 50, the main beam direction 80 can be varied within an angular range of approximately ±8.6° by appropriately controlling the micromirror array 50. The larger the available numerical aperture NA _obj of the objective 42, the larger the available angular range.

How precisely the main beam direction can be set depends primarily on the resolution of the pupil filter, i.e. in this case the number of micromirrors (51) per unit area. With the pupil filters currently available, for example, an angle grid of around 0.02° can be achieved.

5. Inclination measurement

With the 4 The inclination of specularly reflecting surfaces can also be measured quickly and easily using the measuring camera shown. To do this, the pupil plane 48 is dimmed in different areas while several images of the workpiece 26 are taken. A measurement of the inclination of the surface of the workpiece 26 can be calculated by comparing the brightness of the images taken.

This procedure is explained in more detail below using the 9a , 9b and 9c which illustrate the measurement process at three different points in time.

At a first point in time, which was 9a As illustrated, the micromirrors 51 of the micromirror array 50 are controlled by the control device 58 in such a way that only the micromirrors 51 within a strip-shaped area 761 are in the second tilt position. Only the portion of the light generated by the light source 74 that falls into the optically usable area 761' within the strip 761 is therefore reflected in the direction of the workpiece 26. In the lower illustration of the 9a the light path of this portion between the micromirror array 50 and the inclined surface 78 of the workpiece 26 is indicated by dark hatched arrows P1. It should be noted that - contrary to what the lower illustration seems to suggest - the surface 78 is essentially completely illuminated. However, the illumination light only has a narrow and constant angular spectrum across the surface 78, which is determined by the location and size of the area 761'.

After specular reflection on the surface 78, the light re-enters the lens 42 and hits a narrow area of the pupil plane 48, which is arranged outside the strip-shaped area 761. There, all micromirrors 51 are in the first tilt position, so that the light is directed in the direction of the image sensor 56 and largely completely illuminates it. The intensity is recorded for each pixel of the image sensor 56.

During the measuring process, the strip-shaped region 761, in which the micromirrors 51 are in the first tilt position, gradually moves across the micromirror array 50. After each adjustment of the micromirrors 51, an image is recorded by the image sensor 56 and the intensity is recorded again for each pixel of the image sensor 56.

The 9b shows the strip-shaped area, designated only with 762, at a later point in time. As can be seen in the illustration below, the light used for illumination now hits the surface 78 of the workpiece 26 perpendicularly. As a result of the specular reflection, the light is reflected in itself and returns in the same way to the micromirrors 51 of the micromirror array 50, which are located in the strip-shaped area 762 and thus in the second tilt position. Thus, the light reflected from the surface 78 is not reflected in the direction of the image sensor 56, but in the direction of the light source 74 of the illumination system 70. The image sensor 56 is in this Constellation is not illuminated, so that an intensity minimum is recorded for this constellation.

In the 9c The constellation shown again shows conditions similar to those in the 9a The light reflected from the surface 78 is deflected by the micromirror array 50 in the direction of the image sensor 56 and illuminates it, so that high intensities are again measured.

Since each location of the strip 762 in the pupil plane 48 is assigned a specific angle spectrum, by linking the angle spectrum with the total intensity detected by the image sensor 56, it is possible to determine at which angles the measuring light fell perpendicularly onto the surface 78.

With the 9a to 9c The measuring process explained above only allows the angle of inclination of the surface 78 to be measured in the plane of the paper. In fact, it is not the case that the angles 9a to 9c shown constellations, the light hits the surface 78 from only one direction. This would only be the case if the illuminated area 761 to 763 were not strip-shaped, but point-shaped. In the strip shape shown, light hits each object point from directions from the measuring camera 40 that lie in one plane. In the 9a to 9c These planes run perpendicular to the paper plane. At different times, these planes are different from each other and intersect in a straight line that lies in the object plane and runs through the respective field point.

However, for the function it does not matter whether the light comes from exactly one direction or from several directions within one of the said planes. 9b In the configuration shown, for example, light that hits the surface 78 at an angle to the plane of the paper is reflected within the plane in question and therefore also hits micromirrors 51 that are in the second tilt position. These light rays that run outside the plane of the paper therefore do not reach the image sensor 56.

In the 9a and 9c In the constellations shown, light rays which run outside the plane of the paper in the relevant planes of incidence also reach the image sensor 56 and contribute to the intensity there.

In order to be able to determine the angle of inclination in a plane perpendicular to the paper plane, the measuring process can be repeated. The strip-shaped areas 761 to 763 are then rotated by 90°, as shown in the 10a to 10c Otherwise, the measurement process is the same as described above with reference to the 9a to 9c The planes in which the directions of the light emerging from the measuring camera lie then intersect in a second straight line, which also lies in the object plane and is arranged at a right angle to the other straight line determined by the first measuring process.

If an actual spatial coordinate is known for at least one surface point, the measured surface angles can be converted into a closed surface shape by appropriate integration in a further evaluation step. Suitable methods are known in the state of the art, which is why this will not be discussed further here.

The measurement of the inclination angles explained above is carried out with spatial resolution, since each pixel of the image sensor 56 is assigned to exactly one location on the surface 78 of the workpiece 26 and the intensities at the different locations are independent of one another. This means that even complicated wave-shaped surface profiles of specularly reflecting surfaces can be measured very easily with high accuracy.

Claims (12)

Method for two-dimensional measurement of an inclined surface (78) of an object (26) with a measuring camera (40) which has an objective (42) with an optical axis (62), a variably adjustable pupil filter (50) arranged in a pupil plane (48) of the objective (42), and an image sensor (56), comprising the following steps: a) determining the orientation of the inclined surface (78) relative to the optical axis (62) of the objective (42); b) specifying a viewing direction from which the measuring camera (40) should view the inclined surface (78); c) determining a main ray direction (80) on the basis of the orientation determined in step a) and the viewing direction specified in step b); d) defining a partial area (82) released by the pupil filter (50) in the pupil plane (48) such that main rays (84) which emanate from the inclined surface (78) and impinge on the center of the partial area (82) run parallel to the main ray direction (80), wherein the partial area (82) is arranged decentered with respect to the optical axis (62); e) adjusting the pupil filter (50) such that the partial area (82) defined in step d) is released; and f) taking an image of the inclined surface (78) with the image sensor (56) using the pupil filter (50) set in step e). Procedure according to Claim 1 , characterized in that the partial region (82) has a contour which is at least substantially circular or has a multiple rotational symmetry. Procedure according to Claim 1 or 2 , characterized in that for measuring several differently oriented surfaces (78) steps a) to f) are repeated with differently adjusted pupil filters (50). Method according to one of the preceding claims, characterized in that the lens (42) is telecentric on the object side. Computer program, the program code means for controlling a method according to one of the Claims 1 until 4 when the computer program is executed on a computer contained in or connected to the measuring camera (40). Measuring camera (40) for two-dimensionally measuring an inclined surface (78) of an object (26), with a lens (42) that has an optical axis (62) and a pupil plane (48), an image sensor (56), a variable pupil filter (50) arranged in the pupil plane (48) and with a control unit (58, 60), characterized in that the control unit (58, 60) is set up to carry out or cause the following steps: a) determining the orientation of the inclined surface (78) relative to the optical axis (62) of the lens (42); b) specifying a viewing direction from which the measuring camera (40) should view the inclined surface (78); c) setting a main ray direction (80) on the basis of the orientation determined in step a) and the viewing direction specified in step b); d) defining a partial region (82) released by the pupil filter (50) in the pupil plane (48) such that main rays (84) which emanate from the inclined surface (78) and impinge on the center of the partial region (82) run parallel to the main ray direction (80), wherein the partial region (82) is arranged decentered with respect to the optical axis (62); e) adjusting the pupil filter (50) such that the partial region (82) defined in step d) is released; and f) recording an image of the inclined surface (78) with the image sensor (56) using the pupil filter (50) adjusted in step e). Measuring camera according to Claim 6 , characterized in that the pupil filter has a micromirror array (50) which is arranged in the pupil plane (48) and has a plurality of micromirrors (51), each of which can be transferred to a first and a second stable tilt position, wherein a micromirror (51) which is in the first tilt position reflects light which has been reflected by the object (26) in such a way that it can reach the image sensor (56), and wherein a micromirror (51) which is in the second tilt position reflects light which has been reflected by the object (26) in such a way that it cannot reach the image sensor (56). Measuring camera according to Claim 7 , characterized by an illumination system (70) for illuminating the object (26), wherein an optical axis (62) of the illumination system (70) is arranged such that a micromirror (51) which is in the second tilt position directs the light generated by the illumination system (70) onto the object (26). Measuring camera according to Claim 8 , characterized in that the illumination system (70) has a light source (74) and an illumination optics (73, 75) which images the light source (74) onto the micromirror array (50). Measuring camera according to one of the Claims 8 or 9 , characterized in that the control unit (58, 60) is designed to control the micromirror array (50) for measuring the inclination of a surface (78) of the object (26) in such a way that the pupil plane (48) is dimmed in different areas during the recording of several images of the object (26); and to calculate a measure of the inclination of the surface (78) from a comparison of the brightness of the recorded images. Measuring camera according to Claim 10 , characterized in that the stopped-down areas are selected such that when recording an image for each object point, the light only exits the measuring camera (40) from directions which lie in one plane, and wherein the planes associated with the images are different and intersect in a first straight line. Measuring camera according to Claim 10 or 11 , characterized in that when taking an image only a strip-shaped area (761, 762, 763) in the pupil plane (48) is not blocked out. is det, with the areas assigned to the images arranged parallel to each other.

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