MX2008006866A - Probe apparatus for measuring a color property of a liquid - Google Patents

Abstract

A probe for measuring a property of a liquid under test using interrogating radiation at a predetermined wavelength includes a housing member having a window transparent to interrogating radiation mounted at a first end thereof. A partition transparent to interrogating radiation is mounted in spaced relationship to the window. The partition being and the window cooperate to define an air cavity therebetween. The spacing between the partition and the window is such that radiation reflected from a liquid disposed in contact with the second surface of the partition is prevented from evanescently coupling into the window such that the reflected radiation undergoes total internal reflection in the partition rather than in the window.

Description

PROBE DEVICE FOR MEASURING A PROPERTY OF COLOR OF A LIQUID Field of the Invention This invention relates to a probe apparatus for measuring the color properties of a liquid, such as paint, having a transparent cover of a window at the tip of the probe.

BACKGROUND OF THE INVENTION Pigment dispersions and inks are widely used in the formulation of high performance liquid coating compositions. These compositions are used, for example, as exterior finish paints for automobiles and trucks. The dry color measurement of those liquid compositions is believed to be the most accurate indication of the color properties of the composition. This measurement is usually done manually by taking an aliquot of the composition that is prepared. The composition is sprayed as a coating on a panel and the panel is baked and dried. One or more color properties of the dry coating can be measured against a reference by the use of a colorimeter or spectrophotometer. Based on the measurement, the batch under preparation is adjusted in an effort to obtain Ref. 192771 an equalization closer to the reference. Manual color measurements are time consuming, mainly due to long preparation and drying times. Also, the procedure may have to be repeated many times before the desired color property is achieved. It is believed that manufacturing efficiencies can be achieved through the ability to measure the color properties of a liquid composition while in a wet state. However, to be effective, any wet color measurement must accurately predict the color of the composition as it dries. This goal has proven to be evasive. Instruments using a reflectance spectrophotometer have been used to obtain a free surface reflectance measurement of a wet liquid dispersion. Representative of these instruments are the devices described in the patent of E.U.A. 6,583,878 (Hustert), patent of E.U.A. 6,292,264 (Voye et al.) And German patent DE 25 25 701 (Langer). These instruments all use a free surface reflectance measurement of a wet coating film with the use of a spectrophotometer. The measurements taken from these instruments therefore modalize the best representation of the color of the coating film that could be correlated with the measurements of the same film in its dry state. However, surface non-uniformities of wet coatings, as well as variations in viscosity, sedimentation and flocculation could nonetheless lead to erroneous results and unacceptable measurement variability. It is believed that additional efficiencies could be achieved by coupling that device to a manufacturing process. However, the coupling of such devices as just described to a continuous process has its own aggravating difficulties, including but not limited to the operation of the device in the presence of volatile flammable solvents emitted from the sample surface as well as cleaning. To attach a color measuring device to a manufacturing process, in light of the possible presence of volatile flammable solvents, as well as the consideration that many processes operate at superabundant pressures, it is normal practice to contain the flow of sample of fluid through the device in a closed system, separated from the source of illumination and spectral detector by a window of sufficient strength, and therefore thickness, to withstand the pressure. The required thickness T of a window is given by the equation: s where z is a form factor for the window; P is the pressure that is contained; D'is the unsupported diameter, and s is the maximum design stress (pressure) for the window material. Instruments that measure the absorbance and / or dispersion properties of a liquid contained in a closed system have been proposed for standard spectrophotometric measurements, which include both laboratory and processing applications, either in transmission or reflectance mode. Some of these instruments also involve measuring the color of the liquid in reflectance mode through a viewing glass in the process stream or inside a sample cell that uses a window between the sample and the detector. The patent of E.U.A. 4,511,251 (Falcoff et al.) And US patent. 6,288,783 (Auad et al.) Are representative of this kind of instrument. The instrument described in the last referenced patent uses a variable path length measuring cell to measure the properties of liquids, including color. The instrument uses a closed path for the flow of liquid to be measured, which allows it to be placed in hazardous classification areas within a manufacturing plant environment. Without However, this particular instrument has multiple moving parts that are part of the liquid's path, which can cause difficulty in cleaning, and are difficult to maintain. Another disadvantage is that the instrument requires high volumes of liquid sample to take appropriate readings. Moreover, although the instrument can measure both in reflectance and transmission mode, it uses 0/0 geometry for each one. As a result, in the transmission mode, information about scattered light of the fluid being analyzed is not provided. In the reflection mode, the non-mitigated backscattered light from the source washes the color sensitivity. Finally, the most significant individual problem to be overcome in the measurement of the color of a liquid in close contact with the window of the flow cell is the disturbance of the light on its path back to the detector that occurs due to the presence of the window itself. The causes of that disturbance of light include, but are not limited to, reflection, refraction, total internal reflection, and loss or escape of light with reference to the various surfaces of the window. As a result of that disturbance, the light will eventually never reach the detector or is modified by the surfaces of the window with which it interacts, in such a way that the spectral information presented to the detector is no longer truly representative of the sample that is measured. A liquid in close contact with a viewing window looks different to the naked eye when viewed through that window than the color of the same liquid when viewed in a free-surface manner, ie with nothing between the eye and the eye. free surface of the wet liquid. Figure 1 is a stylized diagrammatic representation of the optical phenomena occurring at the interface between a liquid L and a window W. The window W may be part of a flow cell or a probe. The liquid L flows beyond the window in a flow direction G at a predetermined fluid pressure. The liquid L is in contact with the window W. The light scattering pigments of the liquid composition are usually dispersed in a solvent vehicle having a refractive index near the refractive index of the window material. To better understand the optical effects that occur when a liquid is viewed through a window, consider the situation illustrated in Figure 1. As a ray of light R propagates through a medium M (e.g., air), impinges on the outer surface E of the window W. The material of the window W refracts the ray R. The refracted ray R 'propagates through the window W towards the window / liquid interface. If the refractive indices of the window and the solvent are substantially equal (i.e. within approximately 0.2 refractive index units of each other) there is no optical interface between the liquid and the window and the beam continues along substantially the same trajectory. The light ray R 'that enters the liquid and strikes a suspended pigment particle is both specularly reflected and diffusely scattered in a solid hemisphere of 2p radians emanating from a scattering site X. (Note that although dispersion occurs within the liquid , the scattering site X is illustrated in figure 1 in the window / liquid interface). The scattered specular rays, e.g., the S-ray, strike against the surface of the window E at an angle? S (measured with respect to a normal to that surface) that is smaller than the critical angle? C of the interface window / medium. That scattered specular beam S leaves the window (at point Q) towards the display field F presented to a detector. However, some diffusely scattered rays, e.g., the U-ray, emanating from the scattering site X, impinge against the surface of the window E at an angle u that is greater than the critical angle c. That diffusely scattered U ray is fully reflected internally inside the window (at point V). The ray U diffusely dispersed propagates back to the window / liquid interface where it can suffer a secondary scattering impact at site X ', at which point its scattering angle can change direction. The secondary scattering impact on the X 'site itself produces specular and diffuse scattering. That scenario is repeated several times within the material of the window. At each scattering impact some of the light is reflected at angles that would make its direction on the surface of the window E greater than the critical angle for the window / air interface while some of the light is reflected at angles that would make its direction at the surface of the window E smaller than the critical angle for the window / air interface. The distance d between the initial impact site X and a secondary impact site X 'depends on the thickness T of the window W in accordance with the relationship: d = 2 • T tan TU / where? U is the angle that the beam U diffusely dispersed does with normal to surface E. Due to the fact that, as was originally described, the window must be thick enough to withstand the pressure of the sample stream it may be the case that there is a distance insufficient lateral available for a diffusely dispersed U ray to pass through a statistically significant number of impacts secondary before being dispersed at an angle with respect to the normal to the surface E which is less than the critical angle for the window / air interface. In that case, the U ray is more likely to come out through the peripheral surface P of the window, as indicated at point Z. This energy is outside the display field F and the detector is lost. The effect caused by the total internal reflection of the diffusely scattered rays is doubled. First, the intensity of the scattered light that finally reaches the detector is diminished. This makes the liquid darker in color. Second, the total internal reflection causes the body of the window to have a "brightness" effect. This increases the background against which the detected radiation is measured. The decrease in intensity received coupled with an increase in background intensity produces a flattening of the waveform of the intensity / wavelength curve or reflectance spectrum detected. When standard colorimetric calculations are carried out to calculate L *, a * and b * according to the ClELab76 formalism, the net effect of this is to produce a chroma loss (C * ab = [a * 2 + b * 2] 1 2), and skew the determination of the perceived color properties. Moreover, since the intensity goes through different range distortions in different domains of With localized wavelengths, the problem can not be expeditiously cured by simply scaling the resulting intensity waveform. Also, if the light is disturbed on its way back to the detector in a way that erroneously represents the measurement of the true color of the sample, adjustments to that color, as may be required in a manufacturing procedure, may also be a mistake . Accordingly, in view of the foregoing, it is believed advantageous to provide an apparatus and a method that mitigates the disturbance of light, and therefore the loss of chroma, during the measurement of color of a liquid material with the use of reflectance spectroscopy. . It is also believed to be advantageous that the liquid measurements correlate well with the measurements made on the material in its dry state. It is believed that it is of additional advantage that the apparatus and method are capable of operating in the environment of a pressurized liquid without alteration of the color measurement. It is believed that it is of further advantage to provide an apparatus wherein the pressurized liquid is introduced to a measurement region without undergoing any flow discontinuity so that a laminar flow of pressurized liquid flow is maintained beyond the window. It is believed that it is of additional advantage to provide a apparatus that is capable of being cleaned quickly (e.g., within one or two minutes) so that the cycle time of the measurement is extremely small compared to process changes; which gives the easy supply (including automatic) of a sample to the analysis cell so that color measurements can be made quickly; and that they can be placed in a potentially dangerous environment, such as the floor of a plant.

SUMMARY OF THE INVENTION In a first aspect, the present invention is directed toward a method for measuring a color property of a liquid under pressurized flow under test in a form that mitigates the disturbance of light. A liquid under test is contacted against a transparent partition that is separated at a predetermined distance from a transparent window. The partition has a predetermined refractive index and has a thickness dimension that is smaller than that of the window. A questionable radiation beam having a wavelength within a predetermined wavelength range is directed through the transparent window and the partition to the liquid. At least some of the reflected radiation of the liquid undergoes total internal reflection within the partition while, simultaneously, the evanescent coupling of that radiation reflected in the window material is prevented. The prevention of evanescent coupling in the material of the window is achieved by: i) arranging a medium that has a refractive index lower than that of the partition between the window and the partition, and ii) maintaining the separation between the window and the window. partition at a distance not less than three (3) times the wavelength of the interrogating radiation. Due to the thickness dimension of the partition, the radiation within the partition is allowed the lateral distance needed to pass through a statistically significant number of internal reflections sufficient to allow radiation to leave the partition. As a result, more reflected radiation is able to enter the display field of a detector and be collected by it than would be the case if the allowed reflected radiation entered directly into the relatively thicker window. Therefore, light disturbance and concomitant loss of chroma could be mitigated. In other aspects, the present invention is directed to a color measuring apparatus in the form of a flow cell and to a system that incorporates the same to measure the color properties of a flowing liquid. through the flow cell with the use of interrogating radiation at a wavelength within a predetermined wavelength range. The flow cell comprises a base and a cover. The cover has a window transparent to questioning radiation. A thin partition that is also transparent to the questioning radiation is mounted within the flow cell in a separate relationship between the window and the base. The partition is preferably formed of a flexible polymer membrane having a first surface and a second surface thereon. The partition has a predetermined refractive index and has a thickness dimension that is smaller than that of the window. The first surface of the partition and the window cooperate to define an air cavity therebetween, reflected from a liquid in a liquid sample chamber. A liquid sample chamber is defined between the second surface of the partition and the base. The separation between the partition and the window is such that the evanescent coupling of the radiation reflected from the liquid to the window material is prevented. Therefore, at least some of the reflected radiation of the liquid undergoes total internal reflection within the partition. Typically, this separation is a distance not less than three (3) times the maximum predetermined wavelength in the interrogating radiation wavelength range. The partition allows sufficient lateral distance for the reflected radiation to pass through a statistically significant number of reflections before dispersing at an angle less than the critical angle for the partition / air cavity interface. Thus, substantially all the reflected radiation of the liquid would pass through the air cavity, enter the window, cross the window and then exit the window on the side towards the detector, with little disturbance to the light and loss of chroma. A plurality of spacer elements may be disposed in the air cavity to maintain the separate relationship between the partition and the window. In accordance with one embodiment of the flow cell of the present invention, the spacers take the form of either cylindrical post-shaped features or irregularly shaped nodular features formed on the surface of the window. Each characteristic thus defining a separating element extends from the window to the partition. The average dimension of each measured characteristic is approximately 0.00254 cm or twenty-five (25) microns. Each characteristic is separated from an adjacent characteristic by an average distance not less than ten (10) times the average characteristic dimension. Alternatively, the separators can be formed on the first surface of the partition (the surface facing the window). If the first surface of the partition is a rough surface, then the rough irregular features on the partition can serve as the separating elements. As another alternative, the spacers may take the form of confined members within the air cavity that are not attached either to the window or to the partition. In accordance with yet another aspect of the present invention, the flow cell has a liquid supply passage and a liquid removal passage formed therein. The liquid supply passage, the sample chamber and the liquid removal passage cooperate to define a liquid flow path through the flow cell. The liquid supply passage, the sample chamber and the liquid removal passage are configured such that any cross section taken in a plane substantially perpendicular to the liquid flow path anywhere along it substantially presents the same cross-sectional area.

A system using the flow cell of the present invention includes a reflectance mode spectrophotometer located with respect to the flow cell and a pump for pumping a sample of liquid therethrough. The spectrophotometer directs the questioning radiation to a liquid that flows through the sample chamber and responds to questionable radiation reflected from the liquid to produce an electrical signal representative of a color property of the same. In accordance with another alternative embodiment of the invention, the flow cell cover has a pressurized fluid inlet flow channel and a pressurized fluid outflow channel formed therein. Each of the incoming and outgoing flow channels communicates with the air cavity. The inflow and outflow channels are sized to pass a pressurized fluid, such as pressurized air, through the air cavity such that, during use, the separated relationship between the partition and the window is maintained by fluid Pressurized in the air cavity. The pressure of the pressurized fluid in the air cavity is determined in accordance with the pressure of the liquid flowing through the cell. In accordance with yet another aspect, the present invention can be implemented in the form of a probe for measuring a property of a liquid under test by use of questionable radiation at a predetermined wavelength. The probe comprises a housing member having a window transparent to the questioning radiation mounted on a first end thereof. A partition transparent to the questioning radiation is mounted in separate relation to the window. The partition has a first surface and a second surface on it, with the first surface of the partition in confrontation with the window. The partition is arranged in such a way that the first surface of the partition and the window cooperate to define an air cavity therebetween. The separation between the partition and the window is such that the reflected radiation of a liquid arranged in contact with the second surface of the partition is prevented from evanescent coupling to the window, such that the reflected radiation undergoes total internal reflection in the partition more than in the window.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood from the following detailed description taken in connection with the appended figures, which form part of this application and in which: Figure 1 is a stylized representation of the optical effects in the interface - between a cell window of flow and a liquid in contact with the window of a flow cell of the prior art; Figure 2 is an exploded side elevational view, completely in section, of a preferred embodiment of a flow cell for measuring a color property of a liquid; Figure 3 is a plan view of the base of the flow cell of Figure 2 taken along the display lines 3-3 therein; Figure 4 is an enlarged side elevational view, completely in section, showing details of the flow cell of the present invention, and in particular, the partition assembly in separate relation between the base and the cell cover; Fig. 5 is a plan view of the inner surface of the window of the assembled cover of the flow cell of Fig. 2, taken along the lines of visualization 5-5 in Fig. 2, illustrating the arrangement of post-shaped features arranged in the window; Fig. 6 is a plan view similar to Fig. 5 showing the inner surface of the cover window of the flow cell of Fig. 2 and illustrating an arrangement of nodular features disposed in the window; Figure 7 is a side elevational view, completely in section, taken along section lines 7-7 in Figure 6; Figures 8, 9, 10 and 11 are sectional views, taken along section lines correspondingly numbered in Figures 3 and 4, illustrating the configuration of the flow path of a fluid through the cell flow; Figure 12 is a schematic representation of a measurement system incorporating a flow cell in accordance with the present invention; Figures 13A and 13B are stylized representations, similar to Figure 1, showing the optical interactions occurring within a flow cell of the present invention; Fig. 14 is an enlarged side elevational view illustrating an alternative embodiment of the flow cell of the present invention in which the flow cell cover has an inflow channel of pressurized fluid and an outflow channel of pressurized fluid formed therein; Figure 15A is a side elevation view of a probe implementation of the present invention, while Figure 15B is an enlarged view of the end of the probe of Figure 15A; Y Figure 16 is a plot of the reflectance versus wavelength for the example sample 1 as measured with each instrument described in the example.

Detailed Description of the Invention Throughout the following detailed description similar reference numerals refer to similar elements in all figures of the drawings. It is to be understood that various details of the structure and operation of the present invention shown in various figures have been stylized in form, with some enlarged or exaggerated portions, all for convenience of illustration and ease of understanding. Figure 2 is an exploded side elevational view, completely in section, of a preferred embodiment of a flow cell generally indicated by the reference character 10 for measuring a color property of a wet liquid, such as paint, as it flows under pressure through the cell. The measurement is carried out by a spectrophotometer 118 (Figure 12, which operates, e.g., in the reflectance mode) with the use of interrogating radiation in a predetermined wavelength range. A suitable interrogative wavelength range is four hundred to seven hundred (700) nanometers. A reference axis 10A extends through cell 10. It should be understood that although the description herein is coined or in terms of the measurement of one or more liquid paint color properties, the flow cell 10 can be advantageously used to measure other properties of any liquid or gaseous fluid material flowing through the cell. The flow cell 10 includes an enclosed housing formed of first and second housing members 14, 16 connectable with each other. In the illustrated arrangement, the first housing member 14 defines the base of the flow cell 10 while the second housing member 16 defines a removable cover. One of the housing members, typically the cover 16 in the preferred case, has a window 20 mounted thereon. The window 20 is optically transparent to the questioning radiation. The liquid under analysis is introduced into cell 10 through base 14. However, it can be understood that, if desired, the described arrangement of the parts can be reversed, in which case the window would be arranged in the base and the liquid would be introduced through the cover. The base 14 includes a body portion 14B machined from stainless steel or any suitable alternative stable material compatible with the liquid whose color properties are measured. A liquid supply passage 18 and a liquid removal passage 19 extend through the body portion 14B of the base 14. Each passage 18, 19 has a respective axis 18A, 19A that extends through it. The respective shafts 18A, 19A of the liquid supply passage 18 and the respective liquid removal passage 19 define respective angles 18L, 19L (FIG. 1) with respect to the reference axis 10A. The angles 18L, 19L are laid within a range of thirty to forty-five degrees (30 ° to 45 °). As seen from Figures 2 and 3 the body 14B is in relief around its periphery to define a mounting enhancement 14S having external threads 14T (Figure 2). An upstanding sealing lip 14L is formed on the upper surface of the base 14 and encloses a liquid flow area generally indicated by the reference character 14F (Figure 3). The liquid flow area 14F includes a liquid measuring surface 14M and associated transition surfaces 141, 14J. The measuring surface 14M is a generally planar surface which is oriented perpendicular to the axis 10A. The measuring surface 14M occupies the largest portion of the liquid flow area 14F. In the preferred case, the measuring surface 14M can be defined by the exposed upper surface of a ceramic insert 14C (FIG. 2) which is cemented to a depression 14R formed on the surface of the body 14B. The ceramic has a vitreous surface (preferably white in color) that has a reflectivity greater than eighty-five percent (85%). The transition surfaces 141, 14J are tilted from opposite edges of the measurement surface 14M to the ports 18M, 19M of the liquid supply passage 18 and the liquid removal passage 19, respectively. Base 14 is widened to accept respective liquid supply and fluid removal accessories 18F, 19F. The fittings 18F, 19F receive respective supply and removal lines 110, 112 (Fig. 12) so that the flow cell 10 can be connected to a liquid flow circuit. In the preferred implementation, the transition surfaces 141, 14J, the measurement surface 14M, the interior surface of both the liquid supply passage 18 and the liquid removal passage 19, and the lip 14L are all coated with a thin layer 26 (figure 4) of a fluoropolymer material. The layer 26 preferably has a uniform thickness of the order of 0.0051 to 0.0127 cm. Any suitable fluoropolymer material can be used, provided that at least the portion 26 'of the layer 26 which overlaps a significant portion of the ceramic insert surface 14C (if one is provided) is optimally clear. A suitable fluoropolymer material for layer 26 is the fluoropolymer material manufactured by E. I. du Pont de Nemours and Company, Inc., and sold as Teflon®.

Silverstone The single optically flat layer 26 '(if used) can be implemented with the use of the fluoropolymer material manufactured by E. I. du Pont de Nemours and Company, Inc., and sold as Teflon® AF. Structurally, as illustrated in Figures 2 and 4, the cover 16 includes an outer flange 30 and an annular support ring 32. The support ring 32 receives the generally disk-like transparent window 20. The flange 30 includes a portion of annular disc 30D from which a 30F flange hangs. Threads 30T are disposed on the inner peripheral surface of the flange 30F. The main body portion 32B of the support ring 32 has an inwardly extending lip 32L (i.e. extending to the axis 10A) and a sealant extending outwardly 32S. The surface of the main body portion 32B below the lip 32L defines an annular support surface 32M. The window 20 includes a main body portion 20B having generally parallel outer and inner surfaces 20E, 201, respectively. The window 20 can be formed of quartz, sapphire or synthetic material such as fused quartz, fused silica or borosilicate. Said materials have a refractive index of the order of approximately 1.50. This refractive index is near the refractive index of solvents used in the manufacture of liquid paint whose color properties can be measured by use of the flow cell 10. The peripheral junction surface 20P of the window 20 is configured to equalize the support surface 32M in the ring 32. The threads 30T in the flange 30 are sized to engage the outer peripheral threads 14T in the mounting flange 14M so that the cover 16 can be removably connected to the base 14. When the cover 16 is threaded onto the 14, the window 20 is supported in a position that overlaps the measuring surface of the liquid 14M. As best seen in Figure 4, when the cover 16 is assembled and connected to the base 14, the window 20 is telescopically received by the support ring 32 such that the peripheral joining surface 20P of the window 20 engages against the support surface 32M in the ring 32. The outer surface 20E of the window 20 confronts the undersurface of the lip 32L of the ring 32. The thickness of the window 20 and the height of the supporting surface 32M are selected in such a way that a clearance space 40 is defined between the outer surface 20E of the window 20 and the sub-surface of the lip 32L. The space 40 minimizes the possibility of fracture of the window 20 when the cover 16 is threaded onto the base 14. The disk portion 30D of the flange 30 is dimensioned to overlap and act against the sealing shoulder 32S in the support ring 32 as the cover 16 is screwed onto the base 14. The annular space 42 between the body 32 and the disk portion 30D facilitates the screwing of the flange 30 to the embossment 14S without the appearance of connection between the flange 30 and the support ring 32. When the base 14 and the cover 16 are completely joined each other, the inner surface 201 of the window 20 and the upper surface of the base 14 cooperate to define an enclosed inner volume 48. In accordance with the present invention, a transparent partition generally indicated by reference character 50 is mounted within the flow cell 12 in separate relation between window 20 and base 14. Partition 50 serves to subdivide the enclosed inner volume 48 into a cavity 54 (figure 4) and a chamber liquid sample 58. Perhaps as best seen in Figure 4, the partition 50 is held in place within the flow cell 10 by the clamping action of the mounting shoulder 32S acting against the mounting lip 14L. If desired, to further ensure the sealed integrity of this annular interface, a package 60 can be provided between the partition 50 and lip 14L. The body portion 50P of the partition 50 may be formed of any material that is optically transparent to the interrogating radiation at a predetermined wavelength and physically capable of confining a liquid in pressurized flow within the liquid sample chamber 58. The partition it has a refractive index of the order of (1.3) to (1.7). In practice, the partition is formed of a flexible polymer material, such as a fluoropolymer or polyester. If the partition is formed of a material other than a fluoropolymer, if desired, it can be coated with a thin layer 50L of an optically clear fluoropolymer material, such as the fluoropolymer material used for the portion 26 'of the coating 26. The refractive index of the layer 50L is close to that of the body portion 50P of the partition 50. The partition 50 has a first surface 50A and a second opposing surface 50B therein. The inner surface 201 of the window 20 together with a portion of the engaging surface 32M in the support ring 32 cooperates with the first surface 50A of the partition 50 to define the cavity 54. The cavity 54 defines a region adjacent to the surface interior 201 of the window 20 capable of receiving a material having a refractive index that is different (in the order of about 0.2) from that of the partition and the window. As will be described more fully here, the partition is a relatively thin member compared to the thickness dimension of the window 20. In practice, the partition has a thickness "t" (see also, Figures 13A, 13B) in the interval from 0.0127 to 0.0254 cm). In the simplest implementation, the cavity 54 communicates with the atmosphere so that, during use, the material within the cavity is air. Therefore, if the flow cell 10 were operated in the open atmosphere, the air would be the material disposed on both sides of the window 20 and the refractive effects with reference to the incident radiation would be minimized. However, if any refractive effects are assumed to be accommodated, it is contemplated in the present invention to provide within the cavity 54 a material that is different from the atmosphere in which the cell is used. It should be noted that the cell can be operated in an atmosphere other than ambient air. With the partition 50 secured in its position, a partition, or space, is defined between the second surface 50B of the partition 50 and the inner surface 201. The space dimension between the second surface 50B of the partition 50 and the window 20 ( measured in a direction parallel to the axis 10A) is indicated by the reference character 54D. The magnitude of dimension 54D is important. For reasons explained more fully here in connection with Figures 13A and 13B the dimension 54D of the space (measured in a direction parallel to the reference axis 10A) would be, at least, not less than three (3) times the length Maximum wavelength of radiation used to interrogate a sample of liquid under test. By way of example, if a maximum wavelength of the interrogating radiation is seven hundred (700) nanometers, dimension 54D would be in the range of 2.1 to 3 microns. The liquid sample chamber 58 is defined between the second surface 50B of the partition 50 and the confrontationally arranged liquid flow area 14F in the base 14. The inner surface of the lip 14L serves as the peripheral boundary of the sample chamber 58 The liquid sample chamber 58 confines a liquid sample as it flows, under pressure, along a flow path 62 extending from the liquid supply passage 18, through the sample chamber 58. to the liquid removal passage 19. The liquid sample flows from the mouth 18M of the supply passage 18, through an inlet transition region 641, through a 64M measurement region, and through a transition region outlet 64J (FIG. 14) to the mouth 19M of the removal passage 20. The entry transition region 641 is defined between the transition surface 141 and the surface 50B of the partition 50. The measuring surface 14M and the surface 50B of the partition 50 cooperate to define the measuring region 64M. The output transition region 64J is defined between the transition surface 14J and the surface 50B of the partition 50. The dimension 64D of the measurement region 64M (measured in a direction parallel to the reference axis 10A) is dimensioned to maintain flow laminate as the liquid passes over the measuring surface 14M. Typically, this dimension 64D is of the order of 0.0254 cm. In the preferred implementation, the dimension 54D of the space between the second surface 50B of the partition 50 and the inner surface 201 of the window 20 is maintained and the bending or buckling of the partition 50 is prevented simultaneously by the arrangement within the cavity of air 54 of one or more spacer elements, generally indicated by the reference character 68. The spacer elements 68 may preferably be integrally formed on the inner surface of the body portion 20B of the window 20. It is also contemplated in the invention that the spacers can be formed on the surface 50B of the partition 50 or otherwise physically confined within the air cavity 54 without joining either the window or the partition. In the embodiment illustrated in Figures 3, 4 and 5, the spacers 68 take the form of post-shaped members 68P that are integrally formed on the inner surface of the window body 20. The post-shaped members 68P have generally flattened ends. The members 68P project from the inner surface 201 towards the air cavity 54 for a distance sufficient to maintain the predetermined space dimension 54D of the air cavity 54. Accordingly, consistent with the minimum dimension 54D of the space, the dimension of Axial length of the 68P members is at least 2.1 to three microns. In addition to maintaining the dimension 54D of the air cavity 54, the arrangement of the post-shaped members 68P prevents buckling or bulging of the partition 50, which serves to maintain the optical length of the liquid sample chamber 58 constant throughout the 64M measurement region. (It should be noted that in Figure 4, the flattened ends of the members 68P are shown as being slightly separated from the partition 50 only for purposes of clarity of illustration.) As best illustrated in FIG. 5, the members in the shape of a post 68P are generally circular in cross section, having an average diameter of the order of about 0.00254 cm [twenty-five (25) microns]. Each member in the form of a post 68P is separated from an adjacent member by an average distance 68D not less than approximately ten (10) times the transverse dimension (e.g., diameter) of the member. In an alternative embodiment, illustrated in Figures 6 and 7, the spacer elements 68 take the form of generally circular granular nodules 68N. Each nodule 68N is a generally rounded feature that has an average diameter of approximately 0.00254 cm [twenty-five (25) microns] and a height dimension consistent with the minimum dimension 54D of the space. Each nodule 68N is separated from an adjacent nodule by an average distance not less than ten (10) times the transverse dimension (e.g., diameter) of the particle. Whether implemented in the form of members in the form of a 68P post or in the form of nodules 68N, the spacers 68 should not cover more than three percent to ten percent (3% to 10%) of the interior surface area. of the window 20. Preferably, the spacers 68 should not cover more than about five percent (5%) of the surface 201. The spacers 68 can be formed in a regular array (as illustrated in the case of the shaped members). of post 68P) or as a randomly disposed arrangement (as illustrated in the case of nodules 68N). The members in the form of a 68P post or 68N nodes are preferably formed in the body of the window by the use of photolithographic techniques. In general, Una Photolithographic technique involves the deposition of a layer of a polymeric photoresist material on the inner surface of the window 20. A photomask having a desired pattern of regular or random characteristics is laid on the photoresist. For example, the photomask can be created by using the nodular surface on one side of the inkjet printer transparency available from Hewlett-Packard Inc. And sold as Premium InkJet Transparency Film model HP C3834A as the template for the photomask . The photoresist is exposed to actinic radiation with the mask in place, which results in the production of polymerized and unpolymerized areas in the polymer layer. Unwanted material in the pattern is chemically dissolved from the photopolymer layer, which leaves the resulting separator pattern. In a particular manufacturing technique, a fused silica disc used for the window is subjected to a modified "RCA-type" cleaning in a wet cleaning station to remove contamination with organic and metal compounds. "RCA cleaning" is an industry standard developed by the RCA Company to remove contaminants from wafers. The silica disc is immersed for ten (10) minutes in a 65 ° C bath containing NH4OH: H202: H20 in a ratio of 1: 1: 6. After the disc is rinsed for (10) minutes with deionized water It is immersed for ten (10) minutes in a bath at 85 ° C containing ninety-five percent (95%) of H2SO4. It is rinsed for fifteen (15) minutes with deionized water and dried by blowing with nitrogen. The disk is then dehydrated under vacuum and heated and cooled in a dry nitrogen atmosphere to prepare for film deposition. The pole separators are formed by the use of a photoresist and a photo-tool. A suitable photoresist is that available from Microchem Incorporated, Newton, Massachusetts as NANO ™ SU-8 2000 Negative Tone Photoresist. This epoxy based resistance is available in various viscosities to rotate at different thickness ranges. Basically, the percentage of solvent (cyclopentanone) is adjusted to achieve the correct viscosity. This photoresist contains a photoinitiator and sensitizer that is "marked" to a UV of line 1 of 365 nm. With the use of a rotation apparatus such as that available from Headway Research, Inc., Garland, Texas, photoresist is applied to the surface of the disc. The conditions of rotation are determined by the desired height of the separator. The resistance is baked smoothly with the use of a two-step hot plate oven at temperatures of 65 ° C and 95 ° C, respectively. The time of Baking depends on the thickness of the resistance. The cooled discs are then imaged in a UV exposure unit such as those available from Optical Associates Inc., San Jose, California as the OAI Hybralign ™ 500 series Mask Alignment and Exposure System. The UV is Uv of line 1 of 365 nm. The power level is 5 mW / cm2; The exposure time depends on the thickness of the resistance. Follow a post-exposure bake. This is a two-step hot plate baking, 65 ° C and 95 ° C respectively. The baking time depends on the thickness of the resistance. The discs are allowed to cool slowly and are revealed by immersion in an SU8 developer available from Microchem Incorporated. This developer is a solvent, PGMEA (propylene glycol monomethyl ether acetate). After inspection, the patterned discs are hard baked in a laboratory oven. The temperature rises to 175 ° C, sustained for two (2) hours and decreased to room temperature. The spacers can also be formed on the surface of the window with the use of any other suitable microfabrication process. In an alternative embodiment, the separating elements can be formed integrally on the second surface of the partition. For example, if a sheet of The polyester base of an inkjet printer transparency (with any adhesive coating removed from the front surface) is used to implement the partition, the opposite surface of the sheet may have a sufficient nodular surface to maintain the separation of the partition. window. The inkjet printer transparency available from Hewlett-Packard Inc. And sold as a model HP C3834A Premium InkJet Transparency Film is useful for this purpose. In another alternative embodiment, the spacer elements may be disposed within the cavity 54 not attached either to the window or to the partition. In order to maintain a laminar flow of the liquid through the sample chamber 58 it is important that there is no flow disturbance to a liquid as it progresses along the flow path 62. For this purpose, the passage of liquid supply 18, liquid removal passage 19, inlet transition region 641, measurement region 64M, and exit transition region 64J are all configured in such a way that no cross section taken in a plane substantially perpendicular to the liquid flow path anywhere along it presents substantially the same area. This construction is illustrated in the series of views in elevation shown in Figs. 8 to 11. These various views illustrate the configuration of the liquid supply passage 18 within the body 14B of the flow cell (Fig. 8), in the mouth 18M of the liquid supply passage 18 (Fig. 9), in the input transition region 641 (FIG. 10), and in the measurement region 64M '(FIG. 11). Since the construction of the cell 10 is symmetrical, the configuration of the flow path 62 in the outlet transition region 64J, in the mouth 19M of the liquid removal passage 19, and in the liquid removal passage 19 are identical to the configurations shown in figure 10, figure 9 and figure 8, respectively. In the preferred case, the liquid supply passage 18 and the liquid removal passage 19 are each formed as substantially circular holes extending through the body 14B. Therefore, the cross sections through the passages (e.g., figure 8) are circular in shape. Due to the geometry of the cell 10, the cross-sections in the mouth 18M, 19M of the respective passages 18, 19, in the transition regions 641, 64J, and in the measurement region 64M, are substantially rectangular in shape (v .gr., figures 9 to 11). The geometry of the cell is such that the areas of these planes of cross section are substantially equal. Therefore, a liquid does not find flow discontinuity as it is pumped along the flow path 62. It is also contemplated in this invention that the liquid supply passage 18 and the liquid removal passage 19 may each be alternately configured as rectangular in shape. In this arrangement, each passage can be formed of confronting pairs of substantially flat walls. The walls in at least one confronting pair of flat walls converge towards the axis of the passage in the length of the passage such that an area of uniform cross section in a plane perpendicular to the axis of the passage is maintained at each point along the length of the passage. same. Figure 12 is a schematic representation showing the flow cell 10 according to the present invention as used within a spectrophotometric system generally indicated by the reference character 100 for measuring a property of a pressurized flow fluid. The fluid could be any liquid or gaseous fluid whose properties it is desirable to investigate and monitor. In the present discussion, it is assumed that the color properties of paint or liquid ink are investigated and monitored. The components of the liquid material are measured in a container 102 and are combined by the mixing action imparted by a mixing blade 104. The liquid material is circulated by a pump 108 through a recycle flow path defined by a pipe loop 106. Instead of a pump, a pressurized fluid (e.g., pressurized air) can be used to move liquid from a closed container along the flow path 106 The flow path 106 may have one or more mounting openings 108A, 108B provided at predetermined locations along the flow path for the purposes to be described. In one arrangement, the flow cell 10 is connected in the recycling loop 106 by an input connection line 110 and an output connection line 112. The connection lines 110, 112 are respectively received by the accessories 18F, 19F provided in cell 10 (figure 1). The respective pressure sensors 114, 116 can be provided to monitor the pressure in the connecting lines 110, 112. As the liquid flows through the liquid sample chamber 64, it is interrogated by a spectrophotometer 118. The spectrophotometer operates to direct interrogating radiation to the fluid flowing through the sample chamber of the cell and to respond to questionable radiation reflected from a fluid to produce an electrical signal representative of a property thereof. If desired, the spectrophotometer can be arranged in a manner that uses three measurement directions, as described in the U.S. patent. 4,479,718 (Alman), assigned to the assignee of the present invention. The particular spectrophotometer used depends on the nature of the liquid sample being measured. For the color measurement of liquids containing effect pigments, the preferred spectrophotometer can be arranged in such a way that several (two or more) detectors are located at multiple respective angles with respect to the specularly reflected beam. Each detector is located either: 1) within the plane defined by the illumination beam and the specularly reflected beam (hereinafter referred to as the illumination plane); or 2) out of the plane in multiple azimuthal directions relative to the plane, and at multiple predetermined respective angles of declination with respect to the sample flow plane through the sample flow chamber. In the latter case, the spectrophotometer would be a gonioespectrophotometer. As an example of the first, in the measurement of liquids containing metallic pigments, a spectrophotometer having detectors in three directions of measurement as before, as described in the patent of E.U.A. 4,479,718 (Alman), assigned to the assignee of the present invention, can be used. Additional color information can be obtained at orienting the flow cell 10, described here, in such a way that measurements can be made, wherein the direction of flow through the cell is inclined at any arbitrary azimuthal angle with respect to the illumination plane described above. It can also be assumed that the spectrophotometer 118 has been calibrated either by an appropriate off-line calibration procedure or by interrogation of the surface of the measuring plate (if one is provided). Figure 13 is a ray diagram, similar to Figure 1, illustrating the optical operation of the flow cell of the present invention. An incident ray R of interrogating radiation at a predetermined wavelength propagates towards the outer surface 20E of the window 20. The material of the window 20 has a refractive index that is greater than the index of the medium surrounding the cell. Upon striking the surface 20E, the disparity in refractive indices between the medium above the window and the window material produces a refracted ray R '. The refracted ray R 'propagates through the window until it meets the interior surface 201 of the window. As the beam leaves the window, the disparity in refractive indexes between the material of the window and the material within the cavity 54 again causes the beam to be refracted. To minimize refractive effects, it is preferable that the medium M and the material within the cavity 54 are the same (e.g., ambient air). The resulting refracted beam then propagates to the partition 50 with the same angle of inclination to the axis 10A as the ray R. The ray R propagates through the cavity 54 towards the surface 50A of the partition 50. The ray R is refracted by the material of the partition 50. The refracted ray R "leaves the surface 50B and interacts with the liquid material in the sample chamber 58. If the ray R" encounters a pigment particle or other dispersion entity in the liquid, the ray R "will reflect specularly and diffuse diffusely, similar to the interaction that occurs at the dispersion site X in figure 1. The specularly reflected radiation will leave the upper surface 50A of the partition and will propagate through the cavity 54 towards the window 20. If the dimension of the cavity 54 is dimensioned to prevent the diffusely dispersed radiation of evanescent coupling towards the window 2, the diffusely dispersed radiation will suffer total internal reflection in the partition. Due to the thickness "t" of the partition (relative to the thickness of the window) there is sufficient lateral distance D along the plane of the partition for the internally reflected radiation to pass through a number statistically significant secondary dispersions. The probability that the radiation will be re-dispersed at an angle less than the critical angle of the interface between the partition and the cavity material is increased. Therefore, the probability that a larger proportion of the energy fully reflected internally comes out of the window 20 increases. The proper selection of the dimension 54D of the space between the window 20 and the partition 50 to prevent light in the partition from evanescently coupling to the window therefore increases the amount of diffusely dispersed radiation that will be harvested by the detector. The window and partition must be kept separated by a sufficient dimension 54D to avoid frustration of the total internal reflection that occurs within the partition. This last effect, called total frustrated internal reflectance, is actually leakage from the electric field of radiation that is totally internally reflected in the partition towards the window material, and occurs when the two materials with similar refractive indices are in close contact close, to the extent that their respective juxtaposed surfaces are separated by a smaller distance than a small multiple of the penetration depth, I, of the radiation to a more rare medium (in this case the space between the partition and the window), wave The distance required for the evanescent wave amplitude falls to 1 / e of its value in the rarer medium. This depth of penetration, I, is governed by the relationship: where ? it is the maximum wavelength of light; nparClC? ón is the index of refraction of the partition nespacio is the index of refraction of the space between the partition and the window, and? u is the angle of incidence of the rays of light totally internally reflecting inside the partition with respect to the normal to the interface between the partition and space. A general rule to ensure that sufficient distance is maintained between two dense media separated by a more rare medium, to avoid total frustrated internal reflection, is to separate the two dense media by one dimension 54D not less than three (3) times the maximum questionable wavelength. In addressing now the thickness dimension "t" of the partition 50, it is important that this thickness dimension be very thin. To answer the question of how thin It would be important to remember the problem of why a relatively thick window with a refractive index close to the index of the material being measured disturbs the light so that the detector misrepresents the true color of the material, which would be seen if it were not present the window and if you see the free surface of the material. As noted earlier in connection with the discussion of Figure 1, the reasons are that: 1) some light escapes through the edges of the window, thereby reducing the lightness of the object as seen by the detector and that some radiation never reaches it, and 2) the window shines due to direct dispersion from the edges of the window, which raises the background or baseline of the detected reflectance spectrum. These two phenomena are mitigated if a partition is interposed between the window and the material being measured, and if within the partition, something is prevented that the light escapes from the detector's F display field, as shown in FIG. 13A. In the discussion that follows, it is assumed that the lateral dimension of the display field F of the detector is smaller than the lateral dimension of the partition, Dp. To achieve this, it is established that some of the diffusely scattered light of the material being measured undergoes total internal reflection within the partition, it must be ensured that the The lateral distance D traversed by any scattered ray given within the partition before re-emerging, defined as in Figure 13A, is less than F / 2. With reference to Figure 13B, it should be noted that the total distance D traversed by a diffusely scattered beam and totally internally reflected is composed of several segments, dx, d2, etc., or dx in general, due to the fact that the beam is Can you disperse at different angles? ui /? u2 etc., or? ul? in general, on the surface 50B at the different points of contact with the material being measured. The angles TU1, as noted above, are the scattering angles of a beam that is dispersed in an angular direction greater than the critical angle Tc with respect to the normal system for the air partition / cavity interface on the 50A surface, and that the considered ray is supposed to be totally internally reflected. Now, Tc, the critical angle for the partition / air cavity interface, is defined as follows: sin 0c = - ^ =! - with? c =? ui = * / fl partition If the beam does rebound within the partition, which is fully internally reflected in each bounce except for the month:? Mo, after which it re-emerges through the upper surface 50A of partition 50, the Total distance D traveling in the transverse direction along the lateral dimension of the partition is given by: D d] + d7 + ... + dm =? D¡ From the geometric considerations, the d? up to but not including dm, it can be calculated from the partition thickness dimension t and the scattering angle? ul as: dt = 2t tan? ul Suppose that, after the bounce, the ray emerges again through of the surface 50A, dm therefore has a minimum value of 0, and a maximum value given by: dx = t tan? c Therefore, the criterion for the thickness of the partition can now be set as: (- \ D = 2í Ttan01, / \ + ttan? M = F v, i F / t = (m- \ 2? Tan 0", + tan? 9" V i Assume that the display field F is set by the manufacturer of the spectrophotometer, the maximum thickness t of the partition can be found by minimizing the right side of the inequality and, therefore, by maximizing the denominator of the previous expression. Obviously, if all TU1 to p / 2, and? M =? C, the denominator tends to infinity, and t goes to 0, which states that a free surface measurement would capture as much light as possible. However, in practical terms, if it is desired to contain the sample in a closed system, the question may be the case in terms of the percentage of the diffusely dispersed light that it is desired to capture. It is assumed that the specularly scattered light will re-emerge from the surface 50A after the first dispersion encounter, since its scattering angle is Qx, the angle refracted in the partition, which is by definition less than? C. For diffusely scattered light, therefore, if the measured sample is assumed to be a Lambertian scatter, and therefore, all scattering angles are equally likely, and in a near-worst-case scenario in which the distance maximum lateral, dlf per scattering encounter with the surface 50B is suffered by the internally reflective / scattering beam, from a practical point of view therefore, when setting TU1 all equal to a high percentage of its maximum possible value of 11/2 , that here is denominated? umáx, and when fixing Tm = Tc, to increase to the maximum the denominator of the inequality for "t" previous, but does not have infinity of reach. The expression for "t" then becomes returns : (lim - 1) tan 0"nux + tan fl) or, when using the definition of the critical angle, where p is a percentage close to 90% -100%. To determine what value of m, the number of dispersions within the partition, should be used in the previous expression, it is necessary to consider the probability of a ray, once an encounter is made with a scattering center in the limit 50B, which scattered at an angle greater than the critical angle versus the probability of the beam being scattered at an angle less than the critical angle. Again, if the measured material is assumed to be the Lambertian disperser, the diffuse scattering must be isotropic, and therefore all angles equally likely. Since this is the case, therefore, the probability, P (? Ul &? C), of a beam that is scattered at an angle greater than the critical angle for the air par tition / air interface in a scattering center i is given by: Similarly, the probability P (? Ul =? C) of a ray that is scattered at an angle less than the critical angle as before is: Therefore, the probability that a ray of light that is emergent on the 50A surface of the partition after m scattering events is just the combined cumulative probabilities of: 1) the probability of the ray spreading at a greater angle than the critical angle for m-1 dispersion events, and 2) the probability of the ray that is dispersed at an angle smaller than the critical angle over the month or dispersion event, or mathematically: m •) When performing the addition, this is then reduced to: This expression can then be inverted to solve for m, given a desired percentage of light, Pra (? Ui =? C), which re-emerges after m scattering events and collected by the detector. You have: From these expressions, then, the maximum thickness "t" of the partition can be estimated by making certain assumptions. For example, if the space is first assumed to be composed of air, then n = = 1- In addition, if a polyester material is assumed for the partition, npartition = 1.65, and? C then it becomes 37.3 °. Finally, if it is desired to collect 90% of the light, Pm = 0.9, and m, the number of rebounds needed to achieve this becomes: where it is recognized that the factors that start with 2 / p are actually angle relationships and where the angles have been converted to units of degrees. A common display field for reflectance spectrophotometers is F = 1.27 cm. Also, suppose that in each scattering event the scattering angle is 90% of p / 2 for maximum lateral distance traversed by scattering event, this result can be used in the previous expression instead of t, and get: Figure 14 illustrates a further alternative embodiment of a flow cell according to the invention. In this embodiment, the cover 16 is provided with at least one inlet flow channel of pressurized fluid 70 and at least one outlet flow channel of pressurized fluid 72. The incoming and outgoing flow channels communicate each with the air cavity 54. A pump 80 is connected in a fluid circuit to both the incoming flow channel and the projection 70, 72, respectively. The pump 80 is controlled by a pump controller 82. The controller 82 generates a pump control signal in accordance with the pressure values in connection lines 110, 112 (figure 11) as monitored by pressure sensors 114, 116. Incoming and outgoing flow channels 70, 72, respectively, are dimensioned to pass a pressurized fluid through the air cavity 54, such that, during use, the separated relationship between the partition and the window is maintained. It should also be appreciated that, in an alternative to this embodiment, the window may be omitted and the spectrophotometer lenses may effectively serve as the upper limit of the cavity 54. In this event, an adequate record is provided to mount the photometer to the body of the flow cell. The present invention can also be implemented in the form of a probe apparatus 150, as compared to the flow cell described in the beginning. As shown in Figures 15A and 15B, the probe 150 in accordance with this aspect of the present invention comprises a housing member 154 having a window 20 transparent to the questioning radiation mounted on a first end of it. The housing 154 preferably takes the form of a generally elongated tubular member. The cross section of the housing can assume any convenient configuration. The exterior of the housing is threaded over a portion of its length, as in 159, whereby the probe 150 can be mounted within the mounting openings 108A, 108B (Figure 12). Other suitable mounting arrangements can be used. The interrogation radiation is conducted to the window and reflected radiation exiting the window by one or more fiber bundles 156A to 156D. (In the drawings, fiber 156D extends through the center of housing 154 while fibers 156A to 156C are disposed approximately in the interior of the housing Other suitable arrangements can also be used.) Each fiber can be secured within the housing 154 by a suitable clamp 158. Alternative arrangements for conducting radiation to and from the window, such as internal mirrors, can be provided. A partition 50 that is transparent to the questioning radiation is mounted at the end of the housing 154 in separate relation to the window 20. The partition 50 has a first surface and a second surface thereon. The first surface 50A of the partition confronts and cooperates with the window 20 to define a cavity therebetween 54. The spacing between the partition 50 and the window 20 is such that the reflected radiation of a liquid disposed in contact with the second surface 50B the partition is prevented from evanescently coupling to the window in such a way that the reflected radiation suffers total internal reflection in the partition 50 rather than in the window. During use, with reference again to Figure 12, the probe 150 can be mounted on the openings 108A and / or 108B (or at any other convenient locations within the flow path) by the use of the external threads 159. As in the case of the flow cell, the questionable radiation from a suitable source is conducted to the window. The incident radiation is conducted to a reflectance mode spectrophotometer.

Example The prevention of disturbance of light and the corresponding improvements in chroma and The color sensitivity provided by a flow cell according to the present invention can be understood from the following example. Sample 1 was an orange ink available from E.l. du Pont de Nemours and Co. , Wilmington Delaware as Tint 853 J mixed with an adequate amount of white blend base to give full spectral information. Sample 2 was the same orange ink doped with 0.32% of a denaturing black colorant available from E.l. du Pont de Nemours and Co. , Wilmington Delaware as Tint 806J. The reflectance measurements versus wavelength for the two liquid samples, sample 1 and sample 2, were made by using each of the three instruments, the reference instrument, the comparison instrument of the prior art and the instrument of the invention. The reference instrument was a rotary disc system generally as described in German patent DE 25 25 701. Samples of liquid 1 and 2 were applied separately by using a slotted container on the surface of a rotating disc and they made free surface measurements of the reflectance. The measures The reflectance of this instrument was selected as the reference standard since they present very closely the appearance of color of the sample as seen with the naked eye. The wet free surface measurements approximate those available by using a dry free surface measurement technique described in the background portion of this application. The comparison instrument of the prior art was a closed flow cell system generally as described in the US patent. 4,511,251 (Falcoff et al.). Samples of liquid 1 and 2 were pumped through the flow cell. Due to the construction of the cell, each liquid sample was in close contact with the window of the cell as the sample passed through it. The instrument of the invention was a closed flow cell having a partition in accordance with the present invention, substantially as described here and as illustrated in Figures 2-7. For each instrument, the reflectance values for each liquid sample were measured by using a model spectrophotometer MA90BR available from X-Rite, Incorporated, Grandville, Michigan. The L, a, b values of CIEI_ab76 for each set of Measurements were calculated by using reflectance spectra. The chroma (C * ab) was calculated using the CIELab76 formalism: (C * ^ = [a * 2 + b * 2] 1/2). The changes? L,? A and? B between the reflectance values measured for samples 1 and 2 with each instrument were also calculated. All the measured and calculated results are shown in the following table. A graph of reflectance versus wavelength for sample 1 as measured with each instrument is illustrated graphically in Figure 16.

Table Discussion. In the region of the blue of the measuring spectrum (400-500 nm), the comparison instrument of the prior art shows high reflectance values in comparison with both the reference instrument and the instrument of the invention. On the contrary, in the red region of the measurement spectrum (600-700 nm) the values produced by the comparison instrument of the prior art were below both the reference instrument and the instrument of the invention. The baseline reflectance increased in the blue region and the reflectance peak decreased in the red region is believed to be attributable to the disturbance and loss of light energy of the window, as described in the background. The chroma value for the reference instrument was 58.10, while the chroma value for the comparison instrument of the prior art was 46.81 and the chroma value for the instrument of the invention was 53.62. With reference to the table, the difference between the chroma measured with the prior art instrument and the reference instrument is 11.29. The difference between the chroma measured with the instrument of the invention and the reference instrument is 4.48. The improvement can be measured by taking the difference of the two differences, which is 6.81. Therefore, the relative improvement is just 6.81 / 11.29, or -60%. Therefore, the instrument of the invention provided a significant improvement on the comparison instrument of the prior art. A comparison of the changes? A and? B reveals that the instrument of the invention also provides a significantly better color sensitivity compared to the prior art matching instrument. The reference instrument registered changes? A and? B between sample 1 and sample 2 of -1.66 and -0.95, respectively. The prior art instrument recorded a? Of -1.05 and? B of -0.62, while the instrument of the invention recorded a? A of -1.52 and? B of -0.85. To determine the sensitivity of the instruments of the invention and of the prior art, it is only necessary to calculate the percentage of the total change registered by the reference instrument both by the prior art instrument and the instrument of the invention. This can be done by forming the relationships of the a and b for each instrument with respect to the reference instrument. Namely, a prior art instrument 1.05 / 1.66 = 63% instrument of the invention 1.52 / 1.66 = 91% ? b prior art instrument .62 / .95 = 66% instrument of the invention .85 / .95 = 89% For both a? A and? B, the instrument of the invention recorded approximately 90% sensitivity to color change of the free surface measurement, while the comparison instrument of the prior art recorded below 63% and 66 %% sensitivity, respectively. From the foregoing, it can be appreciated that the flow cell of the present invention provides significant advantages over prior art systems. The present invention avoids the measurement problem presented when a window of the cell is in close contact with the liquid under test. By using a partition that is thin enough to mitigate the light disturbance and the consequent loss of chroma to confine the pressurized liquid sample, the present invention facilitates color measurement by reflectance spectroscopy of wet liquids in a closed system that produces acceptably consistent results and reliably predicts that wet readings also equal the dry standard. The presence of the separators or the pressurized fluid behind the partition provides sufficient strength to prevent buckling occurring when the sample is under pressure. Therefore, the present invention solves the apparently contradictory problem of resistance (thickness) versus chroma loss that accompanies the use of a window system.

By providing the fluoropolymer material coatings the cell is able to be cleaned quickly [within one to two (1 or 2) minutes] so the cycle time of the measurement is extremely small compared to the process changes. Since the flow cell embodiment or the cell embodiment of the present invention can be interposed in the flow path of a pressurized liquid, the supply of a sample under test can be achieved quickly and easily. This allows color measurements to be made quickly. Moreover, since the flow cell or probe can operate within the confines of a closed system, the cell and probe can be placed on a plant floor in an environment that may contain an explosive atmosphere. Those skilled in the art, who have the benefit of the teachings of the present invention, as set forth above, may make numerous modifications thereto. These modifications are to be considered as contemplated in the present invention as defined in the appended claims. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (15)

Claims Having described the invention as above, the content of the following claims is claimed as property:

1. A probe for measuring a property of a liquid under test by using questionable radiation at a predetermined wavelength, characterized in that it comprises: a housing member having a transparent window with questionable radiation mounted on a first end thereof; a partition transparent to the questioning radiation mounted in separate relation to the window, the partition has a first surface and a second surface on it, the first surface of the partition confronts the window, the partition is arranged in such a way that the first The surface of the partition and the window cooperate to define a cavity between them, the separation between the partition and the window is such that the reflected radiation of a liquid disposed in contact with the second surface of the partition is prevented from evanescently coupling towards the partition. window in such a way that the reflected radiation goes through reflection internal total in the partition more than in the window.

2. The probe according to claim 1, characterized in that the separation is not less than three times the predetermined wavelength of the interrogating radiation. The probe according to claim 1, characterized in that the probe further comprises a plurality of spacer elements arranged in the cavity and extending between the second surface of the partition and the window, the spacer elements are sized to maintain the separation between the partition and the window. 4. The probe according to claim 3, characterized in that the separating elements are attached to the window. The probe according to claim 3, characterized in that the window has a plurality of post-shaped features formed on the surface of the same, each feature in the form of a pole extends from the window towards the partition with what defines thus a separating element, each feature in the form of a pole has an axis through it, the average dimension of each characteristic in the form of a pole measured in a plane perpendicular to its axis is approximately 0.00254 cm, each pole-shaped feature is separated from a feature in the form of an adjacent pole by an average distance not less than ten times the average dimension. The probe according to claim 5, characterized in that the post-shaped features form a regular arrangement on the surface of the window. The probe according to claim 3, characterized in that the window has a plurality of nodular characteristics formed on the surface of the same, each nodular feature extends from the window towards the partition, which defines a separating element, each characteristic Nodular has an average dimension of approximately 0.00254 cm, each nodular feature is separated from an adjacent nodular feature by an average distance not less than ten times the average dimension. The probe according to claim 3, characterized in that the nodular characteristics form a random arrangement on the surface of the window. The probe according to claim 3, characterized in that the partition is a substantially flat member having a maximum thickness, the thickness The maximum is such that the propagation of radiation due to the total internal reflection within the partition is minimized, whereby substantially all of the radiation reflected from the fluid being measured leaves the partition within a predetermined lateral distance throughout. of the plane of the partition. The probe according to claim 9, characterized in that the partition has a maximum thickness in the range of 0.0127 to 0.0254 cm. 11. The probe according to claim 1, characterized in that the partition has a maximum thickness in the range of 0.0127 to 0.0254 cm. 12. The probe according to claim 9, characterized in that the first surface of the partition has irregular rough characteristics thereon, the irregular rough characteristics on the partition extend towards the window to define the separating elements. 1

3. The probe in accordance with the claim 1, characterized in that the partition is a flexible polymer membrane. 1

4. The probe in accordance with the claim 4, characterized in that the window has a surface on it having a predetermined area, and wherein the Separator elements cover no more than three percent to ten percent (3% to 10%) of the surface area of the window. 1

5. The probe according to claim 14, characterized in that the spacer elements cover no more than about five percent (5%) of the surface area of the window.