AU614683B2

AU614683B2 – Portable colorimeter and method for characterization of a colored surface
– Google Patents

AU614683B2 – Portable colorimeter and method for characterization of a colored surface
– Google Patents
Portable colorimeter and method for characterization of a colored surface

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Publication number
AU614683B2

AU614683B2
AU46998/89A
AU4699889A
AU614683B2
AU 614683 B2
AU614683 B2
AU 614683B2
AU 46998/89 A
AU46998/89 A
AU 46998/89A
AU 4699889 A
AU4699889 A
AU 4699889A
AU 614683 B2
AU614683 B2
AU 614683B2
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AU
Australia
Prior art keywords
sample
colorimeter
color
measured
values
Prior art date
1988-12-20
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AU46998/89A
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AU4699889A
(en

Inventor
Larry E. Steenhoek
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EIDP Inc

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EI Du Pont de Nemours and Co
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1988-12-20
Filing date
1989-12-20
Publication date
1991-09-05

1989-12-20
Application filed by EI Du Pont de Nemours and Co
filed
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EI Du Pont de Nemours and Co

1990-06-28
Publication of AU4699889A
publication
Critical
patent/AU4699889A/en

1991-09-05
Application granted
granted
Critical

1991-09-05
Publication of AU614683B2
publication
Critical
patent/AU614683B2/en

2009-12-20
Anticipated expiration
legal-status
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Status
Ceased
legal-status
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Classifications

G—PHYSICS

G01—MEASURING; TESTING

G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRA-RED, VISIBLE OR ULTRA-VIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY

G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours

G01J3/46—Measurement of colour; Colour measuring devices, e.g. colorimeters

G01J3/50—Measurement of colour; Colour measuring devices, e.g. colorimeters using electric radiation detectors

G—PHYSICS

G01—MEASURING; TESTING

G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRA-RED, VISIBLE OR ULTRA-VIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY

G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours

G01J3/02—Details

G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows

G01J3/0251—Colorimeters making use of an integrating sphere

G—PHYSICS

G01—MEASURING; TESTING

G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRA-RED, VISIBLE OR ULTRA-VIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY

G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours

G01J3/02—Details

G01J3/10—Arrangements of light sources specially adapted for spectrometry or colorimetry

G—PHYSICS

G01—MEASURING; TESTING

G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRA-RED, VISIBLE OR ULTRA-VIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY

G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours

G01J3/46—Measurement of colour; Colour measuring devices, e.g. colorimeters

G01J3/50—Measurement of colour; Colour measuring devices, e.g. colorimeters using electric radiation detectors

G01J3/502—Measurement of colour; Colour measuring devices, e.g. colorimeters using electric radiation detectors using a dispersive element, e.g. grating, prism

G—PHYSICS

G01—MEASURING; TESTING

G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRA-RED, VISIBLE OR ULTRA-VIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY

G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours

G01J3/46—Measurement of colour; Colour measuring devices, e.g. colorimeters

G01J3/50—Measurement of colour; Colour measuring devices, e.g. colorimeters using electric radiation detectors

G01J3/504—Goniometric colour measurements, for example measurements of metallic or flake based paints

G—PHYSICS

G01—MEASURING; TESTING

G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRA-RED, VISIBLE OR ULTRA-VIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY

G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours

G01J3/46—Measurement of colour; Colour measuring devices, e.g. colorimeters

G01J3/52—Measurement of colour; Colour measuring devices, e.g. colorimeters using colour charts

G01J3/524—Calibration of colorimeters

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

G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated

G01N21/47—Scattering, i.e. diffuse reflection

G01N21/4738—Diffuse reflection, e.g. also for testing fluids, fibrous materials

G—PHYSICS

G01—MEASURING; TESTING

G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRA-RED, VISIBLE OR ULTRA-VIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY

G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours

G01J3/46—Measurement of colour; Colour measuring devices, e.g. colorimeters

G01J2003/466—Coded colour; Recognition of predetermined colour; Determining proximity to predetermined colour

G—PHYSICS

G01—MEASURING; TESTING

G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRA-RED, VISIBLE OR ULTRA-VIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY

G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours

G01J3/02—Details

G01J3/0256—Compact construction

G—PHYSICS

G01—MEASURING; TESTING

G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRA-RED, VISIBLE OR ULTRA-VIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY

G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours

G01J3/46—Measurement of colour; Colour measuring devices, e.g. colorimeters

G01J3/463—Colour matching

G—PHYSICS

G01—MEASURING; TESTING

G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRA-RED, VISIBLE OR ULTRA-VIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY

G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours

G01J3/46—Measurement of colour; Colour measuring devices, e.g. colorimeters

G01J3/465—Measurement of colour; Colour measuring devices, e.g. colorimeters taking into account the colour perception of the eye; using tristimulus detection

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

G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated

G01N21/47—Scattering, i.e. diffuse reflection

G01N2021/4704—Angular selective

G01N2021/4711—Multiangle measurement

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

G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated

G01N21/47—Scattering, i.e. diffuse reflection

G01N21/4738—Diffuse reflection, e.g. also for testing fluids, fibrous materials

G01N2021/4764—Special kinds of physical applications

G01N2021/4771—Matte surfaces with reflecting particles

Description

AUSTRALIA
Form PATENTS ACT 1952 COMPLETE SPECIFICATION 6146853
(ORIGINAL)
FOR OFFICE USE Short Title: Int. Cl: Application Number: Lodged: Complete Specification Lodged: Accepted: Lapsed: Published: Priority: Related Art: TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: Actual Inventors: Address for Service: E.I. DU PONT DE NEMOURS AND COMPANY a corporation organized and existing under the laws of the State of Delaware, of Wilmington, Delaware, 19898, United States of America Larry E. STEENHOEK CALLINAN LAWRIE, 278 High Street, Kew, 3101, Victoria, Australia Complete Specification for the invention entitled: «PORTABLE COLORIMETER AND METHOD FOR CHARACTERIZATION OF A COLORED SURFACE» The following statement is a full description of this inventon, including the best method of performing it known to me:- 1A FA-03C3 TITLE PORTABLE COLORIMETER AND METHOD FOR CHARACTERIZATION OF A COLORED SURFACE BACKGROUND OF THE INVENTION This invention is directed to a portable colorimeter and a method for the characterization of a colored surface and in particular a color surface containing metallic or pearlescent particles.
1C In the manufacture of pigmented finishes one rarely, if ever, achieves a satisfactory color match versus a color standard without an adjustment process known as shading. Shading usually involves a relatively minor but critical manipulation of the formula pigment composition, correcting for the cumulative effects of manufacturing variables on pigment dispersions.
Traditionally, the Lhading process has been carried out by highly skilled and treined personnel who require extensive on-the-job experience to achieve proficiency in their craft. Since visual shading at best is an art, effective administration of the process was difficult.
In more recent years, such visual shading has been supplemented by the use of apparatuses for instrumentally characterizing a paint or pigment composition. Colorimeters and spectrophotometers are well-known in the art and are used to measure certain optical properties of various paint films which have been coated over test panels. A typical spectrophotometer provides for the measurement of the amount of light reflected at varying light wavelength in the visible spectrum by a painted panel that is held at a given angle relative to the direction of an incident source of light. The reflectance factor of i i. 2 the paint enables paint chemists to calculate color values by which to characterize various paint colors.
For a paint containing no light-reflecting flakes or platelets non-metallic paints), the reflectance factor will not vary with the angle of the panel relative to the direction of incident light except at the gloss (specular) angle. Consequently, a single spectrophotometric reading at any specified angle will produce a reflectance value by which to accurately characterize the paint.
However, the paint industry often utilizes light-reflecting flakes in paints metallic paints) to obtain pleasing aesthetic effects. Paints containing light-reflecting flakes of such materials as aluminum, bronze, coated mica and the like are characterized by a «two-tone» or «flip-flop» effect whereby the apparent color of the paint changes at different viewing angles. This effect is due to the orientation of the flakes in the paint film. Since the color of such metallic paints will apparently vary as a function of the angle of illumination and viewing, a single spectrophotometric reading is inadequate to accurately characterize the paint.
Although measurement studies have shown that visual color differences existing between two metallic paints were detectable at an infinite number of angles, it is obvious that practical reasons preclude the collection of reflectance factors for an infinite number of viewing angles. However, previous studies have also indicated that measurement of the optical properties of a metallic paint at only two or three specified angles can provide useful characterization. See, for example, U.S. Patent 3,690,771, issued September 12, 1972 to Armstrong, Jr., et al and U.S. Patent 4,479,718, issued October 30, 1984 to Alman, the 2 i x disclosures of which are incorporated herein by reference.
Instruments have also been devised wherein measurements are taken at a fixed angle by varying the angles of illumination. See, for example, U.S. Patent 4,583,858, issued April 22, 1986 to Leblin et al.
Various other devices and methods are disclosed in U.S. Patents 3,389,265; 3,885,878; 3,916,168; 3,999,864; 4,449,821; 4,669,880; 4,711,580.
However, there is a need in the automobile paint industry for a device which is portable, compact, and capable of measuring the color of automobile panels and the like, and especially metallic or pearlescent finishes.
OBJECTS AND SUMMARY OF THE INVENTION The principal object of the present invention is to provide a portable colorimeter for characterizing the optical properties of a color surface and in particular a colored surface containing metallic or pearlescent particles by using three multiangular spectrophotometric measurements to derive color constants for the sample surface.
An object of the present invention is to provide a portable colorimeter whi -h includes a compact integrated unit for housing irradiation, detection, control, analysis, and display means.
Another object of the present invention is to provide a portable colorimeter which employs three illumination angles preferably of and and one detection angle preferably of 45′, as measured from the sample normal.
Yet another object of the present invention is to provide a portable colorimeter which employs a silicon photo diode array detector comprising 10-16 1 detector elements for detection across the entire visible spectrum.
An additional object of the present invention is to provide a method for characterizing the optical properties of a surface containing metallic or pearlescent flakes by determining the tristimulus values (color constants X, Y, Z) from low resolution spectral reflectance data by correcting the tristimulus function curve representing sensitivity data of the human eye by multiplying it with the spectral power distribution curve of the illuminant, determining the spectral response curve of the detector elements represented as a series of generally triangular pass bands, and fitting the illuminant corrected tristimulus function curves with a multiple linear combination of the triangular pass bands representing the spectral response curve.
In summary, the main object of the present invention is to provide a portable compact colorimeter and a method for characterizing a colored surface in particular a colored surface containing metallic or pearlescent particles, which employs three illumination angles and one detection angle.
BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 is a schematic illustration of the portable colorimeter of the present invention; FIG 2 is a schematic for the control circuitry of the colorimeter; FIG 3 is a persnective partial view of the colorimeter showing the necessary parts only; FIG 4 is a representation of the operator key pad; FIG 5 is an illustration of the measuring surface of the colorimeter; 4 j FIG 6 is a view taken along line 6-6 of FIG. FIG 7 is an illustration of the measuring surface resting on the color sample; FIG 8 shows the tristimulus function curves representing the sensitivity data of the human eye; FIG 9 is the spectral power distribution curve for the illuminant; FIG 10 shows the illuminant corrected tristimulus function curves; FIG 11 shows the spectral response curves of the detector elements as represented by a series of triangular pass bands; FIG 12 shows the weighted detector response function curves; FIG 13 is a comparison of the invention with the prior art.
DETAILED DESCRIPTION OF THE INVENTION In optically characterizing surfaces containing metallic particles, such as metallic paints and films, it was recognized that directional reflectance had to be considered. Metallic paints contain light-reflecting flakes or platelets of such material as aluminum, bronze, coated mica and the like. These flakes or platelets function much like little mirrors, reflecting light directionally rather than in a diffuse manner. The directional reflectance characteristic of a metallic paint film results in a phenomenon known as goniochromatism, which is defined as the variation in color of a paint film as a function of the directions of illumination and viewing. This phenomenon is also sometimes described as «two-tone», «flop» «flip-flop», «flash», «side-tone», etc. In sum, the color of a metallic L. paint will appear different at different viewing and/or illumination angles.
To account for this directional or angular reflectance, goniochromatism, spectrophotometrically determined reflectance factors must be taken multiangularly. The reflectance factor of a paint film is the ratio of the light flux reflected from the film sample to the light flux reflected from a perfect reflecting diffuser when the sample and perfect diffuser are identically irradiated. A perfect white reflector has a value of 1. A perfect black nonreflector has a value of 0.
The reflectance factors are used to calculate color descriptor values used to specify color and color difference. The tristimulus values Y, Z) of a color are calculated by combining the reflectance factor data with data on the sensitivity of the human eye y, z) and the irradiance of a light source all as functions of wavelength in the visible spectrum. The defining equations for tristimulus values are: 830 X R E x dA 360 8Y R E Y dA 360 830 Z R E z dA 360 The tristimulus values can be used to calculate color descriptors which relate to visual perception of color and color difference. One of many sets of descriptors 6 J
L
which can be used is the CIELAB perceptual color scale recommended by the International Commission on Illumination («Recommendations on Uniform Color Spaces, Color Difference Equations, Psychometric Color Terms», Supplement No. 2 To CIE Publication No. (El.3.1) 1971/CT(1.3) 1978. Bureau Central De La CIE, 52 Boulevard Malesherbes 75008, Paris, France).
Transformations of the tristimulus values can be used to calculate perceptual color values describing lightness (L redness/greenness (a yellowness/blueness (b saturation or hue A color can be completely described by a set of L, a, b or L, C, h values. The following equations which have been specified by the International Committee on Illumination relate the tristimulus values to L a L =116(Y/Yo)1/316 a =500[(X/Xo)1/3 (Y/Yo) 1 /3] b*=200[(Y/Yo) 1 /3 (Z/Zo) 1 /3 where Xo, Yo and Zo are the tristimulus values of the perfect white for a given illuminant; X, Y and Z are the tristimulus values for the color.
The saturation and hue descriptors are related to the a and b values as follows: C=(a* 2 b*2)1/ 2 h=tan-l(b*/a*) Often it is necessary to compare a color, such as a sample batch of paint, to a standard color
G
.1 8 and determine the difference and then adjust the sample with appropriate additives to bring the sample within tolerance values of the standard. The difference in color between a color standard and a batch sample is described as follows: AL=L* (batch)-L (standard) Aa =a (batch)-a (standard) Ab*=b*(batch)-b*(standard) The resultant values agree with the visual assessments of differences in lightness (AL redness/greenness (Aa and yellowness/blueness (Ab Further discussion will employ the tristimulus values Y, Z) and perceptual color values (L a b C, h) to quantify the influence of changing conditions of illumination and viewing on measurement of goniochrom tic color. The specific color descriptors employed are only one of many possible choices of transformations of tristimulus values which could be employed in this task.
The method used in the portable three angle colorimeter of this invention to calculate color constants X, Y, 2 of a sample is different from that used in conventional filter colorimeters or spectrophotometers.
A Filter colorimeters utilize optical filters whose transmission spectra have been tailored such that the product of the spectral power distribution curve of the light source, the filter transmittance curve and the detector spectral response curve closely approximate the tristimulus response functions y, 8
-I
z response of the human eye) for a given illuminant.
The signal from each of three detectors (red, yellow-green, and blue) relative to a white standard gives a direct measurement of the color coordinates of a sample. To measure color under a different illuminant would require a different set of filters, (See, for example, U.S. Patent 4,711,581 to Venable).
Conventional spectrophotometers measure the reflectance of the sample at a series of evenly spaced non-overlapping intervals (typically 10 nm) across the visible portion of the optical spectrum. These reflectance values are then multiplied point by point by the tristimulus response functions y, z) corrected for the illuminant and/or observer of choice. Properly normalized the sum of these products yield the color coordinates for the sample. In typical spectrophotometers anywhere from 16 to 31 detectors are employed for the point by point measurement of the visible spectrum. Description of such conventional measurement can be found in Publication CIE No. 15 1971, COLORIMETRY.
In the method eiployed by the portable colorimeter described below, however, the sample reflectance spectrum is determined, preferably by using only twelve detector elements. The spectral sensitivity or response of each of the twelve detector elements is described by a generally triangular shape pass band which is a representation of the shape of the intensity envelope with respect to wavelength location. The illuminant corrected tristimulus function curve is then fit by a multiple linear combination of these triangular shape pass bands which when properly normalized yields color constants, i.e., tristimulus values X, Y, Z.
9 Relying on a conventional principle that three properly selected measurement angles are an or t!ii.d, selection to give maximum information on retallic color for minimum measurement effort, a portable instrument has been constructed. However, in order to minimize space requirements, the portable three angle colorimeter employs a reverse geometry.
The conventional method used multiangular spectrophotometric measurements taken at three specified angles, preferably 15″, 45*, and 110* as measured from the specular angle, with a single light source having an illumination angle of 450 relative to the metallic paint sample being measured (which is the same as saying the light reflected is detected at and 65° as measured from the sample normal), However, in the portable colorimeter of this invention, multiple light sources sequentially illuminate the sample at angles of about -35° to to +10° and 20° to 75″, preferably from 0°, and 65″ as measured from the sample normal, and light reflected from the sample is detected at a detection angle from about 35°-55″, preferably at 45″, as measured from the sample normal.
In addition, the portable instrument of this invention employs a different method for determining the tristimulus values X, Y, Z, of a paint sample by using low resolution spectral data obtained from a silicon photo diode array detector, preferably comprising only twelve elements for detection across the entire visible spectrum (380 nm 700 nm). By this method, the illuminant corrected tristimulus function curve is fit with a multiple linear combination of the triangular pass bands for each of the twelve elements.
11
COLORIMETER
As schmatically shown in Figure 1, the portable colorimeter 20 of this invention employs three sources of illumination, lamps lla, llb, and l1c. The output of these lamps is collimated by each achromatic source lens 12a, 12b, and 12c mounted at its focal. length form the lamp filament. Each lamp may be a 20 watt quartz halogen lamp, such as the lamp manufactured by Gilway Technical Lamp, Model Number L7404. In order for the measurement technique employed in this device to work properly it is necessary that the lamps operate at a fixed color temperature as will be discussed below. The lenses employed may be Model Number 01LAU004-006, manufactured by Melles Griot, The collection optics may include a single achromatic collection lens 13 (Melles Griot QILAU006-006) mounted at twice its focal length from the sample surface 14. A monochromator 9, comprising a diffraction grating 17 and a silicon diode array detector 18 is mounted opposite to the sample side of lens Entrance slit 15 to monochromator 19 is mounted at a distance of one focal length from lens 13. This arrangement permit5 only light 16 which is very nearly collimated to pass through entrance slit permitting only light scattered at or about 450 from the sample normal to enter the monochromator 19.
After passing through entrance slit light 16 diverges until it hits the diffraction grating 17 where it is dispersed and refocused onto a silicon diode array detector LE. with twelve detecting elements 21. The diffraction grating 17 may be Model Number #523-00-460 as manufactured by Instruments SA.
The array detector 18 may be Model Number L020-5, as i±zcui.Lion means, saia aetection means, said analysis means and said display means.
j 12 manufactured by Centronics. Preferably, the dimensions of the entrance slit are 0.9mm X The visible spectrum of light 16 is dispersed and refocused across array detector 18. As schematically shown in Figure 2, each of the elements 21 of the photodiode array detector 18 has an associated amplifier 24 which converts the diode current to a voltage signal. The twelve signals vre then multiplexed by multiplexer 27 and digitized by an analog to digital converter 28. The amplifier may be Model No. OPA2111 as manufactured by Burr-Brown. rie multiplexer 27 may be Model Number AD7506KN as Manufactured by Analog Devices. The analog to digital converter may be Model Number ADC71JG aF manufactured by Burr-Brown.
P11 of the functions are controlled by xicrocomputer 29, which may be an INTEL 8052 based computer with auxilliary I/O and memory card. The measurement data as will be described below derived from the portable instrument is displayed on an LCD display As can be seen by Figure 1, in portable colorimeter i_0, the sample is sequentially illuminated, preferably from and 65 as measured from the sample normal. Light reflected from the sample is detected, preferably at 450 as measured from the sample normal. It should be noted that the illumination and detection angles may be varied and the specific argles provided herein are merely optimum values.
As mentioned above, for proper operation cf the colorimeter, the illumination source lamps lla, llb, and llc operate at a fixed color temperature.
Since the lamps are turned on only for a few seconds each per measurement, time is insufficient to allow 13 the lamps to «warm up» t) equilibrium in order to achieve consistent color-temperature. Thus the lamps, as schematically shown in Figure 2, are contrciled by an active feedback circuit. Each source lamp lla llb, and lic is monitored by two photodiodes 22, A blue filter 23a is placed in front of one photodiode and a red filter 23b is placed in front of the other.
Each of these diodes produces a voltage signal which is proportional to the lamp emission in the blue and red regions of the spectrum, respectively. The control circuit as schematically designated by block 2 adjusts the lamp current to maintain a fixed ratio betwecn the output voltagas of the two diodes, thus maintaining a fixed color temp,.ratture.
Figure 3 shows a plan view of the int’ rior of portable colorimeter 10, illustrating ornlyth parts necessary for an understanding of the invention.
Schematically shown is the layout of the illumination sources as represented by illumination lenses 12a, .12b and 12c; collection lens 13.; lamp control circuit card 38 which comprises multiplexer 27 and analog to digital converter 28; detector card 39 which comptises elements of photodiode array 2?1 and ampli(.ier 24; computer’ control and analysis means 29; diffraction grating L7_; and LCD displaiy The instrument may be powered by a remote battery pack 31 which may be shoulder mounted by an .!pe.-ator.
Preferably, the instrument is of the approximate size 1/2″ x 8″ x 10″, approximate weight of 7 lbs, and has a flat -measuring surface of approximately two inches.
An interface plate 33 mounts over the lenses 12a 12c, and 13, and is affixed to mounting block 32 (Figure Referring to Fig. four magnetic feet 34 protrude through interface plate 33.
13 Each foot 34 is a rare earth magnet which is covered with neopreyne sheeting 35 of approximately 1/16″ thickness. The feet may be circular disc magnets of Sm/Co of approximately 1/2″ diameter and 3/8″ thickness, such as those manufactured by Crucible Magnetics. The feet 34 provide registration and resistance to slippage to a curved surface of an automobile panel to be measured, and the neoprene sheeting 35 provides protection to the car finish against, for example, surface scratching. In the center of the interface plate 33 is a donut shaped flexible magnet 36 which provides a light tight seal around measurement port 37. The spacing of the magnetic feet 34 and the distance that feet 34 protude define tht minimum radius of curvature of the surface which can be measured, approximately 24 inches. The operator key pad is shown in Fig. 4.
The instrument is provided with an internil temperature monitor (not shown) located near the detector elements 21. Because of the instrument’s portability, the temperature of the environment under which the instrument will be expected to operate may vary widely. To insure uniformity of results, temperature parameter limits are determined and preprogramed into the instrumentation. When such limits are exceeded, the operator is alerted and forced to recalibrate the instrunentation. The temperature sensing chip may be an Integrated Circuit Temperature Transducer AD592, METHOD FOR CALCULATION OF COLOR CONSTANTS Three factors are essential for the production, perception and measurement of color: The source of light, the illuminated object, and the detector. Each of these three is described, by an appropriate response curve plotted against wavelength: the light source, by its spectral power distribution curve; the object, by its spectral reflectance or transmittance curve; and the detector, by its spectral response curve. The combination of these curves provides the stimulus, or signal, which is represented as the numerical descriptors of color X, Y, Z the tristimulus values. Thus the tristimulus values (X, Y, Z) of a color are calculated by combining the reflectance factor data with data on the sensitivity of the human eye y, z) and the irradiance of a light source all as functions of wavelength in the visible spectrum, as described above.
Figure 8 shows the tristimulus response functions curves x, y, z as cited in «Principles of Color Technology», page 44, 2nd Edition, Billmeyer and Saltzman, John Wiley Sons (19E1).
Figure 9 shows the spectral power distribution curve for the illuminant used. In the present embodiment two different standard illuminants are used. Figure 9 shows the spectral power distribution for CIE Source D65 which is a representation of average natural daylight over the visible spectrum having a correlated color temperature of 6500°K. The other illuminant source is CIE Source A which is a tungsten-filament lamp operating at a color temperature of 2854°K. For most applications of the portable colorimeter, the taking of measurements using these two variant illuminants at the three stated angles should suffice. However, it is well within the scope of this invention to employ other standard illuminants for taking measurements. The values for the spectral power distribution curve shown in Figure 5, are cited in «Principles of Color I .1 44/,718, issued October 30, 1984 to Alman, the 2 16 Technology», pp. 36-37, 2nd Edition, Billmeyer and Saltzman, John Wiley Sons (1981).
By multiplying the tristimulus response curves (Fig 8) by the spectral curve for the illuminant (Fig 9) corrected response curves as shown in Figure 10, are produced.
Figure 11 shows the spectral response curves for the detector elements. The diagram represents data from photodiode detector array 18 which may be seen as a series of triangular pass bands 71 whose vertex is associated with wavelength 72. The spectral sensitivity of each of the twelve detector elements 21 is represented by triangular pass band 71 whose base width is equal to the portion of the spectrum subtended by two detector elements, 56-60 nm.
Each of the corrected response curves of Figure 10 is fit with a multiple linear combination of detector response triangles from Figure 11. The multiple linear combination used is the same as that cited in «Applied Regression Analysis», page 178, Draper and Smith, John Wiley Sons, Inc., NY (1966).
Figure 12 shows the result of this fit for the x tristimulus function of Figure 8, where each of the triangular pass bands has been weighted by the coefficients derived from the fit. A set of weighting coefficients for each tristimulus function curve and for each illuminant used may be derived. Thus, in the instrument of this invention three sets of weighting coefficients, one for each tristimulus function for illuminant A are derived and three sets of weighting coefficients for illuminant D65 are derived.
In the portable colorimeter 10, color coordinants are calculated in the following manner.
The instrument is first zeroed by measuring a black glass tile (not shown). These values are subtracted 17 from any future measurement. Then, the reflectance spectrum of a white calibration tile (not shown) is ii measured, and a series of gain coefficients is calculated to adjust numerically the response of each detector element 21 to be equal to the reflectance of the calibration tile at the appropriate wavelength.
Any subsequent detector readings are multiplied by these gain coefficients.
To measure a sar.ple panel, the colorimeter is first secured on the panel by magnetic feet 34, and lamps lla, llb and llc sequentially illuminate the sample surface at and 65″ as measured from the sample normal. The light reflected from the panel is collected by lens 13 at 45″ as measured from the sample normal, and is collimated to pass through entrance slit 15 to enter monochromator 19 (Fig. 1) Once in the monochromator, the collected light is detected by array detector 18 and ultimately converted to a voltage signal. The measurements taken are processed by microcomputer 29 and displayed on LCD display 30. The detector response for each of the twelve elements 21 is first multiplied by the appropriate gain coefficient and then by the appropriate weighting coefficient for the particular tristimulus value being calculated. The sum of these products is then scaled to correct for the X Y Z perfect white under the specific illumination conditions employed. These tristimulus values can then be converted into the desired coordinant system, *s for example, L a and b or L, C, and h.
EXAMPLE
Figure 13 shows a comparison of tristimulus values X, Y, Z derived by the system of this invention
IL.
18 versus the values obtained by a conventional laboratory spectrophotometric system.
The graph shows X, Y, Z color coordinants obtained for a set of twelve standard ceramic tiles, specifically Ceramic Colour Standards-Series II, as supplied by the British Ceramic Research Associated Ltd. For each tile, X, Y, Z, coordinants are obtained using first, the portable instrument of this invention and second, a conventional system. The values for the portable colorimeter of the invention are plotted along Y axis and the values for the prior art instrument are plotted along X axis. Linear least squares fitting of the X, Y, Z data show a slope of approximately 1 and low scatter about the line. The graph illustrates comparable performance of the two instruments.
The colorimeter of this invention can be used to characterize not only metallic paint films but any surface containing metallic particles, such as plastic containing reflective metallic flakes and also can be used on solid colors, colors iot containing metallic particles. The method is particularly useful in shading paint wherein the L*, b* a and b* values are determined for a standard. Then a batch of paint is manufactured according to a given formula; a painted panel of the batch is made and the L a and b values are determined. Often the batch of paint, even if carefully made, does not match the standard because of variations in pigments and color drift of pigment dispersions. The AL* A* and b* 30 rif ofpigentdisersons Th AL, Aa and Ab values of the batch are calculated and if outside of an acceptable tolerance value, calculations are made for the addition of pigments in the form of mill bases and the mill bases added to the batch and a second panel prepared and values are measured as above. The 18 process is repeated until there is an acceptance color match between the standard and the batch of paint.
While this invention has been described as having a preferred design, it will be understood that it is capable of further modification. This application, is therefore intended to cover any variations, uses or adaptations of the invention following the general principles thereof and including such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains, and as may be applied to the essential features hereinbefore set forth and fall within the scope of this invention or the limits of the claims appended hereto.
L

Claims (22)

1. A portable colorimeter for determining the optical properties of a color film sample comprising: an integrated unit housing irradiation means, detection means, control means, analysis means, and display means; said irradiation means including a plurality of illumination means for sequentially illuminating the sample from a plurality of corresponding illuminating angles; said detection means including an optical deitector means for sequentially detecting the light reflected from the sample at a detection angle different from any of said illuminating angles; whereby the light detected by said detector means is analyzed by said analysis means and converted into a signal displayed by said display means; and said control means controlling said irradiation means, said detection means, said analysis means and said display means.

2. The colorimeter of claim 1 wherein: three illumination means are used to illuminate the sample from three corresponding illuminating angles having values of about -35° to -10* to +10″ and 200 to 75*, as measured from the sample normal.

3. The colorimeter of Claim 2, wherein: the three illumination means illuminate the sample from three corresponding illuminating angles having values of about and 65°, as measured from the sample normal.

4. The colorimeter of claim 1, wherein: said detection angle having a value of about 35° to 55°, as measured from th sample normal.

Tlhe colorimeter of claim 4, wherein: said detection angle having a value of about 45″, as measured from the sample normal.

6. The colorimeter of claim 1, wherein: said optical detector means including an achromatic collection lens mounted at a distance of about twice its focal length from the sample, and a monochromator comprising a diffraction grating and an array of diode detectors; and said monochromator is mounted opposite to the sample side of said lens.

7. The colorimeter cf claim 6, wherein: an entrance slit to said monochromator is mounted at a distance of about one focal length from said lens.

8. The colorimeter of claim 1, wherein: said array of diode detectors including ten to sixteen detector elements.

9. The colorimeter of claim 1, wherein: said irradiation means comprising three l.l: tg «b’ said illuminating means and three corresponding lenses; and each of said lenses is mounted at a distance of about one focal length from the filament of corresponding illuminating means.

The colorimeter of claim 1, further comprising: an interface plate mounted over said irradiation means and said detection means; and said interface place including a center port for allowing the light to pass therethrough; and 21 A .J 22 means for releasably securing the colorimeter on the sample surface.

11. The colorimeter of claim 10, wherein, said securing means comprising a plurality of magnetic feet positioned around said center port and protruding through said interface plate.

12. The colorimeter of claim 11, further comprising:. a generally donut-shaped flexible magnet means positioned concentrically with said center port for providing a light tight seal thereabout; and a resilient sheet disposed between said interface plate and said flexible magnet means for protecting the sample surface.

13. The colorimeter of claim wherein: said irradiation means and said detection means being positioned in a semi-cirale around said center port.

14. In a colorimeter for determining the optical properties of a color film sample, a method of determining the tristimulus values from low resolution spectral reflectance data, comprising the steps of: determining for the illuminant a set of weighting coefficients for each tristimulus function curve x, y, and z by: correcting the tristimulus function curve representing sensitivity data of the human eye by multiplying it with the spectral power distribution curve of the illuminant; (ii) determining the spectral response curve of the detector elements represented as a series of generally triangular pass bands; and /22 (iii) and fitting the illuminant corrected tristimulus function curves with a multiple linear combination of the generally triangular pass bands representing the spectral response curve; illuminating the sample sequentially from a plurality of angles; detecting sequentially the light reflected from the sample at a detection angle different from any of said illuminating angles; determining sample reflectance response by a plurality of detector elements; and multiplying sample reflectance response of each detector element by corresponding weighting coefficient and adding them together to produce the tristimulus values for the color of the sample.

The method of claim 14, further comprising the step of: calculating gain coefficients to adjust numerically the response of each detector element to be equal to the reflectance of a white calibration tile at an appropriate wavelength.

16. The method of claim 15, comprising: multiplying sample reflectance response of each detector element first by corresponding gain coefficient and then by corresponding weighting coefficient; and adding the data obtained above for all detector elements and scaling to correct for the tristimulus values (Xo, Yo, Zo) for perfect white color for the specific illumination conditions employed.

17. The method of claim 14, comprising: illuminating the sample sequentially from three angles having values of about -35* to -10» to +10*iamd-20° to 75′, as measured from the sample normal. 24

18. The method of claim 17, comprising: illmintin the 0 illuinaingthesample suentliali1y from three angles having values of about 0* and as measured from the sample normal.

19. The method of claim 14, comprising: detecting sequentially the light reflected from the sample at said detection angle having a value of about 35* to 55* as measured from the sample normal.

20. The method of claim 19, comprising: detecting aequentially the light reflected from the saimple at said detection angle having a value of about 45*, as measured from the sample normal.

21. The method of claim 14, comprising: determining sample reflectarnce response by ten to sixteen detector elements.

22,* A colo,iiot~ o a’w Im(-tIthxd o)f dt u in tritrnultiq valuoms, subtantid11ly as; h-n’rinixi on,~~ori with reforenco to the oxnniple and ‘or di-awing&. DIVPED this daly of- I X( 0 )11~ T I 9H~ E. f 1)0 PONTIDPI NIN( Ui<8 AND' COMPANY b~y thoi r IPatont At r.oirnt vs CAiJINAN LAMPTI, 6hi AU46998/89A 1988-12-20 1989-12-20 Portable colorimeter and method for characterization of a colored surface Ceased AU614683B2 (en) Applications Claiming Priority (2) Application Number Priority Date Filing Date Title US07/286,785 US4917495A (en) 1988-12-20 1988-12-20 Portable colorimeter and method for characterization of a colored surface US286785 1988-12-20 Publications (2) Publication Number Publication Date AU4699889A AU4699889A (en) 1990-06-28 AU614683B2 true AU614683B2 (en) 1991-09-05 Family ID=23100156 Family Applications (1) Application Number Title Priority Date Filing Date AU46998/89A Ceased AU614683B2 (en) 1988-12-20 1989-12-20 Portable colorimeter and method for characterization of a colored surface Country Status (7) Country Link US (1) US4917495A (en) EP (1) EP0375317B1 (en) JP (1) JPH02245623A (en) AU (1) AU614683B2 (en) CA (1) CA2005367A1 (en) DE (1) DE68928401T2 (en) NZ (1) NZ231835A (en) Families Citing this family (56) * Cited by examiner, † Cited by third party Publication number Priority date Publication date Assignee Title GB8918605D0 (en) * 1989-08-15 1989-09-27 Mckeown Samuel T J Shade distinguishing device EP0444689B1 (en) * 1990-03-01 1995-12-20 X-Rite, Inc. 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