Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/01—Arrangements or apparatus for facilitating the optical investigation
  • G01N21/03—Cuvette constructions
  • G01N21/05—Flow-through cuvettes
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/01—Arrangements or apparatus for facilitating the optical investigation
  • G01N21/03—Cuvette constructions
  • G01N2021/0346—Capillary cells; Microcells
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/01—Arrangements or apparatus for facilitating the optical investigation
  • G01N21/03—Cuvette constructions
  • G01N21/0317—High pressure cuvettes

Definitions

• This invention relates to a flow cell for measuring the color properties of a liquid, such as paint, having a transparent partition spaced from the window of the cell.
• Pigment dispersions and tints are widely used in formulating high performance liquid coating compositions. Such compositions are used, for example, as exterior finish paints for automobiles and trucks.
• Dry color measurement of such liquid compositions is believed to be the most accurate indication of the composition's color properties.
• Such measurement is usually made manually by taking an aliquot of the composition being prepared. The composition is sprayed as a coating onto a panel and the panel is baked and dried.
• One or more color properties of the dried coating may be measured against a reference using a colorimeter or spectrophotometer. Based upon the measurement the batch under preparation is adjusted in an effort to obtain a closer match to the reference.
• Manual color measurements are very time consuming, primarily due to the long preparation and drying times. Also, the procedure may have to be repeated numerous times before the desired color property is achieved.
• the instrument described in the last referenced patent employs a o variable pathlength measurement cell to measure properties of liquids, including color.
• the instrument employs a closed path for the flow of the liquid to be measured, thus allowing it to be placed in hazardous classification areas within a manufacturing plant environment.
• this particular instrument has multiple moving parts which are part of the 5 liquid 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 proper readings.
• the instrument can measure in both reflectance and transmission modes, it employs 0/0 geometry for each. As a result, in transmission mode no o information is provided about scattered light from the fluid being analyzed. In reflection mode unmitigated backscattered light from the source washes out the color sensitivity.
• the single most significant issue to overcome in the measurement of the color of a liquid in intimate contact with the window of the flow cell is the disruption of light on its way back to the detector that occurs because of the presence of the window itself.
• causes of such disruption of the light include, but are not limited to, reflection, refraction, total internal reflection, and loss or escape of said light with reference to the various surfaces of the window.
• the light ultimately either never reaches the detector or is modified by the surfaces of the window with which it interacts, such that spectral information presented to the detector is no longer truly representative of the sample being measured.
• a liquid in intimate contact with a viewing window looks different to the human eye when viewed through that window than the color of the same liquid when viewed in a free surface fashion, i.e., with nothing between the eye and the 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 form part of a flow cell or a probe.
• the liquid L is flowing past the window in a flow direction G at some 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 that has an index of refraction close to the index of refraction of the window material.
• the light ray R' that enters the liquid and strikes a suspended pigment particle is both specularly reflected and diffusely scattered into a solid hemisphere of 2 ⁇ radians emanating from a scatter site X.
• the scattered specular rays e.g., the ray S, impinges against the window surface E at an angle I 5 (measured with respect to a normal to that surface) that is less than the critical angle ⁇ c of the window/medium interface.
• the secondary scattering impact at site X J itself produces specular and diffuse scatterings. Such a scenario is repeated several times within the window material. At each scattering impact some of the light is reflected at angles which would render its direction at the window surface E greater than the critical angle for the window/air interface while some of the light is reflected at angles which would render its direction at the window surface E less than the critical angle for the window/air interface.
• ⁇ u is the angle that the diffusely scattered ray U makes with the normal to the surface E.
• the window must be thick enough to withstand the pressure of the sample stream it may be the case that there is insufficient lateral distance available for a diffusely scattered ray U to undergo a statistically significant number of secondary impacts before being scattered at an angle with respect to the normal to the surface E that is less than the critical angle for the window/air interface. In that case the ray U is more likely to exit through the peripheral surface P of the window W, as indicated at point Z. This energy is outside of the field of view F and is lost to the detector.
• the apparatus and method be able to operate in the environment of a pressurized liquid without alteration of the color measurement.
• the present invention is directed toward a method for measuring a color property of a pressurized flowing liquid under test in a way that mitigates the disruption of light.
• a liquid under test is contacted against a transparent partition that is spaced a predetermined distance from a transparent window.
• the partition has a predetermined index of refraction and has a thickness dimension that is less than that of the window.
• a ray of interrogating radiation having a wavelength within a predetermined range of wavelengths is directed through both the transparent window and the partition into the liquid. At least some of the radiation reflected from the liquid undergoes total internal reflection within the partition while, simultaneously, evanescent coupling of that reflected radiation into the material of the window is prevented.
• the prevention of evanescent coupling into the material of the window is accomplished by: i) disposing a medium having an index of refraction less than that of the partition between the window and the partition, and ii) maintaining the spacing between the window and the partition to a distance not less than three (3) times the wavelength of the interrogating radiation.
• the present invention is directed to color measurement apparatus in the form of a flow cell and to a system incorporating the same for measuring the color properties of a liquid flowing through the flow cell using interrogating radiation at a wavelength within a predetermined range of wavelengths.
• the flow cell comprises a base and a cover.
• the cover has a window transparent to interrogating radiation.
• a thin partition that is also transparent to the interrogating radiation is mounted within the flow cell in spaced relationship between both the window and the base.
• the partition is preferably formed from a flexible polymer membrane having a first surface and a second surface thereon.
• the partition has a predetermined index of refraction and has a thickness dimension that is less 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 spacing between the partition and the window is such that evanescent coupling of radiation reflected from the liquid into the material of the window is prevented.
• this spacing is a distance not less than three (3) times the predetermined maximum wavelength in the wavelength range of interrogating radiation.
• the partition affords sufficient lateral distance for the reflected radiation to undergo a statistically significant number of reflections before being scattered into an angle less than the critical angle for the partition/air cavity interface. In this way, substantially all of the radiation reflected from the liquid would then traverse the air cavity, enter the window, traverse the window and then exit the window on the side toward the detector, with little disruption to the light and loss of chroma.
• a plurality of spacer elements may be disposed in the air cavity to maintain the spaced relationship between the partition and the window.
• the spacers take the form of either cylindrical pole-like features or irregular-shaped nodular features formed on the surface of the window. Each such feature thereby defining a spacer element extends from the window toward the partition.
• the average dimension of each feature measured is approximately one (1 ) mil (0.001 inch) or twenty-five (25) microns. Each feature is separated from an adjacent feature by an average distance of not less than ten (10) times the average feature dimension.
• the spacers may be formed on the first surface of the partition (the surface confronting the window). If the first surface of the partition is a roughened surface, then the irregular roughened features on the partition may serve as the spacer elements.
• the spacers may take the form of members confined within air cavity that are unattached either to the window or to the partition.
• 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 at any location therealong exhibits substantially the same cross-sectional area.
• a system utilizing the flow cell of the present invention includes a reflectance mode spectrophotometer positioned with respect to the flow cell and a pump for pumping a liquid sample therethrough.
• the spectrophotometer is directs interrogating radiation toward a liquid flowing through the sample chamber and responds to interrogating radiation reflected from the liquid to produce an electrical signal representative of a color property thereof.
• the cover of the flow cell has a pressurized fluid inflow channel and a pressurized fluid outflow channel formed therein.
• Each of the inflow and outflow 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, in use, the spaced relationship between the partition and the window is maintained by pressurized fluid 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.
• the present invention may be implemented in the form of a probe for measuring a property of a liquid under test using interrogating radiation at a predetermined wavelength.
• the probe comprises 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 has a first surface and a second surface thereon, with the first surface of the partition confronting the window.
• the partition being disposed such that the first surface of the partition 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.
• Figure 1 is a stylized representation of the optical effects at the interface between a window of the flow cell and a liquid in contact with the window of a flow cell of the Prior Art;
• Figure 2 is an exploded, side elevational view, entirely 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 view lines 3-3 therein;
• Figure 4 is an enlarged side elevational view, entirely in section, showing details of the flow cell of the present invention, and in particular, the mounting of partition in spaced relationship between the base and the cover of the cell;
• Figure 5 is a plan view of the interior surface of the window of the assembled cover of the flow cell of Figure 2, taken along view lines 5-5 in Figure 2, illustrating the array of pole-like features disposed on the window;
• Figure 6 is a plan view similar to Figure 5 showing the interior surface of the window of the cover of the flow cell of Figure 2 and illustrating an array of nodular features disposed on the window;
• Figure 7 is a side elevational view, entirely in section, taken along section lines 7-7 in Figure 6;
• Figures 8, 9, 10 and 11 are sectional views, taken along correspondingly numbered section lines in Figures 3 and 4, illustrating the configuration of the flow path of a fluid through the flow cell;
• Figure 12 is a schematic representation of a measurement system incorporating a flow cell in accordance with the present invention
• Figure 13A and 13B are stylized representations, similar to Figure 1 , showing the optical interactions occurring within a flow cell of the present invention
• Figure 14 is an enlarged side elevational illustrating an alternate embodiment of the flow cell of the present invention in which the cover of the flow cell has a pressurized fluid inflow channel and a pressurized fluid outflow channel formed therein;
• Figure 15A is a side elevational of a probe implementation of the present invention, while Figure 15B is an enlarged view of the end of the probe of Figure 15A;
• Figure 16 is a plot of the reflectance versus wavelength for Example Sample 1 as measured with each instrument discussed in the Example.
• Figure 2 is an exploded side elevational view, entirely 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 effected 5 by a spectrophotometer 118 ( Figure 12; operating, e.g., in the reflectance mode) using interrogating radiation in a predetermined wavelength range.
• a suitable interrogating wavelength range is four hundred to seven hundred (700) nanometers.
• a reference axis 10A extends through the cell 10. It should be understood that although the description herein is cast in o terms of the measurement of one or more color properties of liquid paint, the flow cell 10 may 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 from conjoinable first and second housing members 14, 16.
• 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 instance, has a window 20 mounted therein.
• the window 20 is optically transparent to the interrogating radiation. Liquid under analysis is introduced into the cell 10 through the base 14. However, it should be understood that, if desired, the described arrangement of the parts may be reversed, in which case the window would be disposed 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. being 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 extending therethrough.
• the respective axes 18A, 19A of the respective liquid supply passage 18 and the liquid removal passage 19 define respective angles 18L, 19L ( Figure 1) with respect to the reference axis 10A.
• the angles 18L, 19L lie within a range from thirty to forty-five degrees (30° to 45°).
• the body 14B is relieved around its periphery to define a mounting boss 14S having external threads 14T ( Figure 2).
• An upstanding sealing lip 14L is formed on the top 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 measurement surface 14M and associated transition surfaces 141, 14J.
• the measurement surface 14M is a generally planar surface that is oriented perpendicular to the axis 10A.
• the measurement surface 14M occupies the major portion of the liquid flow area 14F.
• the measurement surface 14M may be defined by the exposed upper surface of a ceramic insert 14C ( Figure 2) that is cemented into a recess 14R formed in the surface of the body 14B.
• the ceramic has a 5 glassy surface (preferably white in color) having a reflectivity greater than eighty-five percent (85%).
• the transition surfaces 141, 14J incline from opposed edges of the measurement surface 14M toward the mouths 18M, 19M of the liquid supply passage 18 and the liquid removal passage 19, respectively.
• the base 14 is counterbored to accept respective liquid supply and liquid removal fittings 18F, 19F.
• the fittings 18F, 19F receive respective supply and removal lines 110, 112 ( Figure 12) whereby the flow cell 10 may be connected into a liquid flow circuit.
• the transition surfaces 141, 14J, the s 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 on the order of 0.002 to 0.005 inches (0.0051 to 0.0127 cm). Any suitable fluoropolymer material may o be used, provided only that at least the portion 26' of the layer 26 overlying a significant portion of the surface of ceramic insert 14C (if one is provided) is optically clear.
• a suitable fluoropolymer material for the layer 26 is that fluoropolymer material manufactured by E. I.
• the 5 optically clear layer 26' may be implemented using that fluoropolymer material manufactured by E. I. du Pont de Nemours and Company, Inc., and sold as Teflon ® AF.
• the cover 16 includes an outer rim 30 and an annular support ring 32.
• the support ring 32 o receives the generally disc-shaped transparent window 20.
• the rim 30 includes an annular disc portion 3OD from which a flange 3OF depends. Threads 3OT are disposed on the interior peripheral surface of the flange 3OF.
• the main body portion 32B of the support ring 32 has an inwardly extending lip 32L (i.e., extending toward the axis 10A) and an outwardly extending sealing shoulder 32S.
• the surface of the main body portion 32B beneath the lip 32L defines an annular support surface 32M.
• the window 20 includes a main body portion 2OB having generally parallel exterior and interior surfaces 2OE, 2Ol , respectively.
• the window 20 may be formed of quartz, sapphire, or synthetic material such as fused quartz, fused silica or borosilcate. Such materials have an index of refraction on the order of approximately 1.50. This index of refraction is close to the index of refraction of solvents used in the manufacture of liquid paint whose color properties may be measured using the flow cell 10.
• the peripheral bounding surface 2OP of the window 20 is configured to match the support surface 32M on the ring 32.
• the threads 3OT on the rim 30 are sized to engage the exterior peripheral threads 14T on the mounting boss 14M so that the cover 16 may be removably connected to the base 14.
• the window 20 is supported in a position overlying the liquid measurement surface 14M.
• the window 20 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 peripheral bounding surface 2OP of the window 20 mates against the support surface 32M on the ring 32.
• the exterior surface 2OE 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 support surface 32M are selected such that a clearance space 40 is defined between the exterior surface 2OE of the window 20 and the undersurface of the lip 32L. The space 40 minimizes the possibility of fracture of the window 20 when the cover 16 is treaded onto the base 14.
• the disc portion 3OD of the rim 30 is sized to overlap and act against the sealing shoulder 32S on the support ring 32 as the cover 16 is threaded onto the base 14.
• the annular gap 42 between the body 32 and the disc portion 3OD facilitates threading of the rim 30 to the boss 14S without the occurrence of binding between the rim 30 and the support ring 32.
• a transparent partition generally indicated by reference character 50 is mounted within the flow cell 12 in spaced relationship between the window 20 and the base 14.
• the partition 50 serves to subdivide the enclosed interior volume 48 into a cavity 54 ( Figure 4) and a liquid sample chamber 58.
• 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 insure the sealed integrity of this annular interface a gasket 60 may be provided between the partition 50 and the lip 14L.
• the body portion 5OP of the partition 50 may be formed from any material that is optically transparent to interrogating radiation at the predetermined wavelength and physically able to confine a pressurized flowing liquid within the liquid sample chamber 58.
• the partition has an index of refraction on the order of (1.3) to (1.7).
• the partition is formed from a flexible polymer material, such as a fluoropolymer or polyester. If the partition is formed from a material other than a fluoropolymer, it may, if desired, be coated with a thin layer 5OL of an optically clear fluoropolymer material, such as the fluoropolymer material used for the portion 26' of the coating 26.
• the index of refraction of the layer 5OL is close to that of the body portion 5OP of the partition 50.
• the partition 50 has a first surface 5OA and an opposed second surface 5OB thereon.
• the cavity 54 defines a region adjacent to the interior surface 201 of the window 20 able to receive a material that has an index of refraction that is different (on the order of about 0.2) from that of the partition and the window.
• the partition is a relatively thin member as compared to the thickness dimension of the window 20.
• the partition has a thickness "t" (see also, Figures 13A, 13B) in the range from 0.005 to 0.010 inches. (0.0127 to 0.0254 cm).
• the cavity 54 communicates with the atmosphere so that, in use, the material within the cavity is air.
• air would be the material disposed on both sides of the window 20 and refractive effects with reference to the incident radiation would be minimized.
• a spacing, or gap is defined between the second surface 5OB of the partition 50 and the interior surface 201.
• the dimension of gap between the second surface 5OB of the partition 50 and the window 20 is indicated by the reference character 54D.
• the magnitude of the dimension 54D is important.
• the dimension 54D of the gap should be, at a minimum, not less than three (3) times the maximum wavelength of the radiation used to interrogate a liquid sample under test.
• the dimension 54D should be in the range 2.1 to 3 microns.
• the liquid sample chamber 58 is defined between the second surface 5OB of the partition 50 and the confrontationally disposed liquid flow area 14F on the base 14. The inside 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 is 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 64I, through a measurement region 64M, and through an outlet transition region 64J ( Figure 14) to the mouth 19M of the removal passage 20.
• the inlet transition region 64I is defined between the transition surface 141 and the surface 5OB of the partition 50.
• the measurement surface 14M and the surface 5OB of the partition 50 cooperate to define the measurement region 64M.
• the outlet transition region 64 J is defined between the transition surface 14J and the surface 5OB of the partition 50.
• the dimension 64D of the measurement region 64M (measured in a direction parallel to the reference axis 10A) is sized to maintain laminar flow as liquid passes over the measurement surface 14M. Typically, this dimension 64D is on the order of 0.010 inches (0.0254 cm).
• the dimension 54D of the gap between the second surface 5OB of the partition 50 and the interior surface 2Ol of the window 20 is maintained and flexure or buckling of the partition 50 is simultaneously prevented by the disposition within the air cavity 54 of one or more spacer elements, generally indicated by the reference character 68.
• the spacer elements 68 may be preferably integrally formed on the interior surface of the body portion 2OB of the window 20. It also lies within the contemplation of the invention that the spacers may be formed on the surface 5OB of the partition 50 or otherwise physically confined within the air cavity 54 without attachment to either the window or the partition.
• the spacers 68 take the form of pole-like members 68P that are integrally formed on the interior surface of the body of the window 20.
• the pole-like members 68P have generally flattened ends.
• the members 68P project from the interior surface 201 into the air cavity 54 for a distance sufficient to maintain the predetermined gap dimension 54D of the air cavity 54. Accordingly, consistent with the minimum dimension 54D of the gap the axial length dimension of the members 68P is at least 2.1 to three microns microns.
• the array of pole-like members 68P prevent buckling or bulging of partition 50, thus serving to maintain the optical length of the liquid sample chamber 58 constant throughout the measurement region 64M.
• the pole-like members 68P are generally circular in their cross section, having an average diameter on the order of approximately one (1 ) mil (0.001 inch) [twenty-five (25) microns].
• Each pole-like member 68P is separated from an adjacent member by an average distance 68D of not less than about ten (10) times the transverse dimension (e.g., diameter) of the member.
• 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 one (1) mil (0.001 inch) [twenty-five (25) microns] and a height dimension consistent with the minimum dimension 54D of the gap.
• Each nodule 68N is separated from an adjacent nodule by an average distance of not less than ten (10) times the average transverse dimension (e.g., diameter) of the particle.
• the spacers 68 should not cover more than three percent to ten percent (3% to 10%) of the area of the interior surface 2Ol of the window 20. Preferably, the spacers 68 should not cover more than about five percent (5%) of the surface 201.
• the spacers 68 may be formed into a regular formation (as illustrated in the case of the pole-like members 68P) or as a randomly disposed array (as illustrated in the case of the nodules 68N).
• the pole-like members 68P or the nodules 68N are preferably formed on the body of the window using photolithographic techniques.
• a photolithographic technique involves deposition of a layer of a polymeric photoresist material the inner surface of the window 20.
• a photomask having a desired pattern of regular or random features is laid over the photoresist.
• the photomask may be created by using the nodular surface on one side of the ink jet printer transparency available from Hewlett-Packard Inc. and sold as model HP C3834A Premium InkJet Transparency Film as the template for the photomask.
• the photoresist is exposed to actinic radiation with the mask in place, resulting in the production of polymerized and non-polymerized areas in the polymer layer. Unwanted material in the pattern is chemically dissolved from the photopolymer layer, leaving the resulting pattern of spacers.
• a fused silica disk used for the window is subjected to a modified "RCA-type" cleaning in a wet cleaning station to remove organics and metal contamination.
• "RCA clean” is an industry standard developed by RCA Company for removing contaminants from wafers.
• the silica disk is dipped for ten (10) minutes into a 65°C bath containing N H 4 O H: H 2 O 2 : H 2 O in a 1 :1 :6 ratio.
• the photoresist is applied to the surface of the disk. Spin conditions are determined by the desired height of the spacer.
• the resist is soft-baked using a two-step hotplate bake at temperatures of 65 0 C and 95 0 C, respectively. Bake time is dependant on the resist thickness.
• the cooled disks are then imaged on a UV exposure unit such as that available from Optical Associates Inc., San Jose, California as the OAI HybralignTM Series 500 Mask Alignment and Exposure System.
• the UV is 365 nm 1-line UV. Power level for is 5mW/cm2; exposure time is dependant on resist thickness.
• a post-exposure bake follows. This is a two-step hotplate bake, 65 0 C and 95 0 C respectively. Bake time is dependant on the resist thickness.
• the disks are allowed to cool slowly and are immersion- developed in an SU8 Developer available from Microchem Incorporated. This developer is a solvent, PGMEA (Propylene Glycol Monomethyl Ether Acetate).
• the patterned disks are hard baked in a laboratory oven. The temperature is ramped up to 175 0 C, held for two (2) hours and ramped down to ambient.
• the spacers may also be formed on the surface of the window using any other suitable microfabrication process.
• the spacer elements may be integrally formed on the second surface of the partition.
• the opposite surface of the sheet may exhibit a nodular surface sufficient to maintain the spacing of the partition from the window.
• the ink jet printer transparency available from Hewlett-Packard Inc. and sold as model HP C3834A Premium InkJet Transparency Film is useful for this purpose.
• the spacer elements may be disposed within the cavity 54 unattached to either the window or to the partition.
• liquid supply passage 18, the liquid removal passage 19, the inlet transition region 64I, the measurement region 64M, and the outlet transition region 64J are all configured such that any cross section taken in a plane substantially perpendicular to the liquid flow path at any location therealong exhibits substantially the same area.
• the liquid supply passage 18 and the liquid removal passage 19 are each formed as substantially circular bores extending through the body 14B.
• the cross sections through the passages are circular in shape.
• the cross sections at the mouth 18M, 19M of the respective passages 18, 19, in the transition regions 64I, 64J, and in the measurement region 64M are substantially rectangular in shape (e.g., Figures 9 through 11 ).
• the geometry of the cell is such that the areas of these cross sectional planes are substantially equal.
• a liquid encounters no flow discontinuity as it is pumped along the flow path 62.
• each passage 18 and the liquid removal passage 19 may each be alternately configured as rectangular in shape.
• each passage may be formed from confronting pairs of substantially planar walls. The walls in at least one confronting pair of planar walls converge toward the passage axis over the length of the passage such that a uniform cross sectional area in a plane perpendicular to the passage axis is maintained at each point therealong.
• Figure 12 is a schematic representation showing the flow cell 10 in accordance with the present invention as utilized within a spectrophotometric system generally indicated by the reference character 100 for measuring a property of a pressurized flowing fluid.
• the fluid could be any liquid or gaseous fluid whose properties it is desirable to ascertain and to monitor. In the present discussion it is assumed that the color properties of liquid paint or tint are being ascertained and monitored.
• the components of the liquid material are metered into a vessel
• the liquid material is circulated by a pump 108 through a recycling flow path defined by a piping loop 106.
• a pressurized fluid e.g., pressurized air
• the flow path 106 may have one or more mounting openings 108A, 108B provided in predetermined locations along the flow path for purposes to be described.
• the flow cell 10 is connected into the recycle loop 106 by an inlet connection line 110 and an outlet connection line 112.
• the connection lines 110, 112 are respectively received by the fittings 18F, 19F provided in the cell 10 ( Figure 1 ).
• Respective pressure sensors 114, 116 may be provided to monitor the pressure in the connection lines 1 10, 1 12.
• a spectrophotometer 1 18 As the liquid flows through the liquid sample chamber 64 it is interrogated by a spectrophotometer 1 18.
• the spectrophotometer is operative to direct interrogating radiation toward the fluid flowing through the sample chamber of the cell and to respond to interrogating radiation reflected from a fluid to produce an electrical signal representative of a property thereof.
• the spectrophotometer may be arranged in a manner that utilized three directions of measurement, as disclosed in U.S. Patent 4,479,718 (Alman), assigned to the assignee of the present invention.
• the particular spectrophotometer utilized is dependant upon the nature of the liquid sample being measured.
• the preferred spectrophotometer may be arranged in a manner such that several (two or more) detectors are positioned at multiple respective angles with respect to the specularly reflected ray. Each detector is positioned either:
• the plane of illumination within the plane defined by the illuminating ray and the specularly reflected ray (hereafter referred to as the plane of illumination);
• the spectrophotometer would be a goniospectrophotometer.
• a spectrophotometer which has detectors at three directions of measurement as above, as disclosed in U.S. Patent 4,479,718 (Alman), assigned to the assignee of the present invention, may be utilized. Further color information may be obtained by orienting the flow cell 10, herein described, such that measurements may be made, wherein the flow direction through the cell is inclined at any arbitrary azimuthal angle with respect to the plane of illumination described above. It is also assumed that the spectrophotometer 118 has been calibrated either by a suitable off-line calibration procedure or by interrogating the surface of the measurement plaque (if one is provided).
• Figure 13A 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 toward the exterior surface 2OE of the window 20.
• the material of the window 20 has an index of refraction that is greater than the index of the medium surrounding the cell.
• the refracted ray R' propagates through the window until it encounters the interior surface 201 of the window.
• the disparity in indices of refraction between the material of the window and the material within the cavity 54 again causes the ray to be refracted.
• the resulting refracted ray then propagates toward 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 toward the surface 5OA of the partition 50.
• the ray R is refracted by the material of the partition 50.
• the refracted ray R" exits the surface 5OB and interacts with the liquid material in the sample chamber 58.
• the ray R" encounters a pigment particle or other scattering entity in the liquid the ray R" will be specularly reflected and diffusely scattered, similar to the interaction occurring at scatter site X in Figure 1.
• the specularly reflected radiation will exit the upper surface 5OA of the partition and propagate through the cavity 54 toward the window 20.
• the dimension of the cavity 54 is sized so as to prevent diffusely scattered radiation from evanescently coupling into the window 2 the diffusely scattered radiation will undergo total internal reflection in the partition. Owing 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 undergo a statistically significant number of secondary scatterings. The likelihood that the radiation will be re-scattered at an angle less than the critical angle of the interface between the partition and the cavity material is increased. Thus, the probability that a larger proportion of the totally internally reflected energy will exit the window 20 is enhanced.
• This latter effect is in reality leakage of the electric field of the radiation being totally internally reflected in the partition into the window material, and occurs when the two materials with similar indices of refraction are in close-to- intimate contact, to the extent that their respective juxtaposed surfaces are separated by a distance less than a small multiple of the penetration depth, /, of the radiation into the rarer medium (in this case the gap between the partition and the window), or the distance required for the evanescent wave amplitude to drop to 1/e of its value in the rarer medium.
• This penetration depth, T is governed by the relation:
• ⁇ is the maximum wavelength of light
• n P artition is the index of refraction of the partition ri g
• a p is the index of refraction of the gap between the partition and the window
• ⁇ u is the angle of incidence of the totally internally reflecting light rays within the partition with respect to the normal to the interface between the partition and the gap.
• a general rule of thumb for guaranteeing that a sufficient distance is maintained between two dense media separated by a rarer medium, so as to prevent frustrated total internal reflection, is to separate the two dense media by a dimension 54D not less than three (3) times the maximum interrogating wavelength.
• the thickness dimension "t" of the partition 50 it is important that this thickness dimension be fairly thin. To answer the question of how thin it should be, it is important to recall the issue of why a relatively thick window with an index of refraction close to the index of the material being measured disrupts the light so that the detector misrepresents the true color of the material, which it would see if no window were present and if it were viewing the free surface of the material. As noted above in connection with the discussion of Figure 1 , the reasons are that:
• the window glows due to stray scattering from the window edges, thus raising the background or baseline of the reflectance spectrum detected.
• the total distance D traversed by a diffusely scattered and totally internally reflected ray is comprised of several segments, di, d 2 and etc., or dj in general, due to the fact that said ray can scatter at different angles 0 u i, 0 U2 , and etc., or ⁇ u ⁇ in general, at surface 5OB at the different points of contact with the material being measured.
• the angles ⁇ u ⁇ are the scattering angles of a ray which scatters in an angular direction greater than the critical angle ⁇ c with respect to the system normal for the partition/air cavity interface at the surface 5OA, as the ray being considered is assumed to be totally internally reflecting.
• ⁇ O ⁇ the critical angle for the partition/air cavity interface, is defined as follows:
• the total distance D traveled in the transverse direction along the lateral dimension of the partition is given by:
• the dj up to but not including d m can be calculated from the partition thickness dimension t and the scattering angle 0 U i as:
• d m Assuming that, after the m th bounce, the ray re-emerges through surface 5OA, d m therefore has a minimum value of 0, and a maximum value given by:
• the criterion for the thickness of the partition can now be set as:
• the maximum thickness t of the partition may be found by minimizing the right hand side of the inequality and, therefore, maximizing the denominator of the above expression.
• the denominator tends to infinity, and t goes to 0, which just says that a free surface measurement would capture all of the light possible.
• the question may be case in terms of the percentage of the diffusely scattered light desired to be captured. It is presumed that specularly scattered light will re-emerge from surface 5OA after the first scattering encounter, since its angle of scatter is Q x , the refracted angle in the partition, which is by definition less than ⁇ c .
• the probability of a light ray being emergent at surface 5OA of the partition after m scattering events is just the cumulative combined probabilities of:
• Figure 14 illustrates still another alternative embodiment of a flow cell in accordance with the invention.
• the cover 16 is provided with at least one pressurized fluid inflow channel 70 and at least one pressurized fluid outflow channel 72.
• the inflow and outflow channels each communicate with the air cavity 54.
• a pump 80 is connected in a fluid circuit with both the inflow and outflow channels 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 the connection lines 110, 112 ( Figure 11) as monitored by the pressure sensors 114, 116.
• the inflow and outflow channels 70, 72, respectively, are sized to pass a pressurized fluid through the air cavity 54 such that, in use, the spaced relationship between the partition and the window is maintained.
• the window may be omitted and the lens of the spectrophotometer may effectively serve as the upper boundary of the cavity 54.
• a suitable expedient is provided to mount the photometer to the body of the flow cell.
• the present invention may also be implemented in the form of a probe apparatus 150, as compared to the flow cell described earlier.
• the probe 150 in accordance with this aspect of the present invention comprises a housing member 154 having a window 20 transparent to interrogating radiation mounted at a first end thereof.
• 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 at 159, whereby the probe 150 may be mounted within the mounting openings 108A, 108B ( Figure 12). Other appropriate mounting arrangements may be used.
• Interrogating radiation is conducted toward the window and reflected radiation exiting from the window by one or more fiber bundles 156A through 156D.
• the fiber 156D extends through the center of the housing 154 while the fibers 156A through 156C are arrayed about the interior of the housing. Other suitable arrangements may also be used.
• Each fiber may be secured within the housing 154 by a suitable clamp 158.
• Alternative arrangements for conducting radiation to and from the window, such as interior mirrors, may be provided.
• a partition 50 that is transparent to interrogating radiation is mounted at the end of the housing 154 in spaced relationship to the window 20.
• the partition 50 has a first surface and a second surface thereon.
• the first surface 5OA 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 radiation reflected from a liquid disposed in contact with the second surface 5OB of the partition is prevented from evanescently coupling into the window such that the reflected radiation undergoes total internal reflection in the partition 50 rather than in the window.
• the probe 150 may be mounted into the openings 108A and/or 108B (or at any other convenient locations within the flow path) using the external threads 159.
• interrogating radiation from a suitable source is conducted toward the window.
• the incident radiation is conducted to a reflectance mode spectrophotometer.
• Sample 1 was an orange tint available from E.I. du Pont de
• Tint 853 J mixed with a suitable amount of white mixing base to give full spectral information.
• Sample 2 was the same orange tint doped with 0.32% of a desaturating black colorant available from E.I. du Pont de Nemours and Co., Wilmington Delaware as Tint 806J.
• Reflectance versus wavelength measurements for the two liquid samples Sample 1 and Sample 2 were made using each of three instruments, the Reference Instrument, the Prior Art Comparison Instrument, and the Invention Instrument.
• the Reference Instrument was a rotating disc system generally as described in German Patent DE 25 25 701. Liquid Samples 1 and 2 were separately applied using a slotted container onto the surface of a rotating disc and free surface measurements of the reflectance were made.
• the reflectance measurements from this instrument were selected as the reference standard since they most closely present the color appearance of the sample as seen by the human eye. Wet free surface measurements approximate those available using a dry free surface measurement technique described in the Background portion of this application.
• the Prior Art Comparison Instrument was a closed flow cell system generally as described in U.S. Patent 4,511 ,251 (Falcoff et al.). Liquid Samples 1 and 2 were pumped through the flow cell. Owing to the construction of the cell each liquid sample was in intimate contact with the window of the cell as the sample passed therethrough.
• the Invention Instrument was a closed flow cell having a partition in accordance with the present invention, substantially as described herein and as illustrated in Figures 2-7.
• the Prior Art Comparison Instrument shows elevated reflectance values as compared to both the Reference Instrument and the Invention Instrument. Conversely, in the red region of the measurement spectrum (600-700 nm) the values produced by the Prior Art Comparison Instrument were below those of both the Reference Instrument and the Invention Instrument.
• the increased reflectance baseline in the blue region and the decreased reflectance peak in the red region are believed attributable to the disruption and loss of light energy from the window, as discussed in the Background.
• the chroma value for the Reference Instrument was 58.10, while the chroma value for the Prior Art Comparison Instrument was 46.81 and the chroma value for the Invention Instrument was 53.62.
• the difference between the chroma measured with the Prior Art Instrument and the reference instrument is 1 1.29.
• the difference between the chroma measured with the Invention Instrument 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 / 1 1.29, or ⁇ 60%.
• the Invention Instrument provided a significant improvement over the Prior Art Comparison Instrument.
• the Prior Art Instrument recorded ⁇ a of -1.05 and ⁇ b of -0.62, while the Invention Instrument recorded ⁇ a of -1.52 and ⁇ b of -0.85.
• To determine the sensitivity of the Invention and Prior Art Instruments it is only necessary to calculate the percentage of the total change recorded by the Reference Instrument by both the Prior Art Instrument and the Invention instrument. This can be done by forming the ratios of the ⁇ a and ⁇ b for each instrument with respect to the Reference Instrument. To wit,
• the Invention Instrument recorded approximately 90% of the sensitivity to color change of the free surface measurement, while the Prior Art Comparison Instrument exhibited under o 63% and 66%% of the sensitivity, respectively.
• the present invention avoids the measurement problem presented when a window of the cell is in intimate contact with the liquid under test.
• a partition which is sufficiently thin to mitigate the disruption of o light and attendant loss of chroma to confine the pressurized liquid sample
• the present invention facilitates color measurement via reflectance spectroscopy of wet liquids in a closed system that produces acceptably consistent results and predicts with confidence that the wet readings will also match the standard in the dry. 5
• the presence of the spacers or the pressurized fluid behind the partition provides sufficient strength to prevent bowing which may occur when the sample is under pressure.
• the present invention solves the seemingly contradictory problem of strength (thickness) versus chroma loss that attends the use of a windowed system.
• the flow cell embodiment or the probe embodiment of the present invention can be interposed in the flow path of a pressurized liquid, delivery of a sample under test can be accomplished quickly and easily. This permits measurements of color can be made rapidly;
• the flow cell or probe can operate within the confines of a closed system, the cell and probe may be placed on a plant floor in an environment that may contain an explosive atmosphere.

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