JOURNAL OF THE OPTICAL SOCIETY OF AMERICA
Photoelectric Color Difference Meter*
RICHARD S. HUNTER
Hunter Associates Laboratory, Inc., McLean, Virginia (Received April 14, 1958)
The color difference meter has three photodetectors, each with a separate tristimulus filter, and each receiving some of the light reflected from the specimen. Signals from the photodetectors are measured by analog circuits that give rectangular coordinates for surface colors in close correspondence to their positions in uniform color space. The first model described in 1948 uses barrier-layer photocells and three tristimulus filters. Recently, a model employing vacuum phototubes and four filters has been built. Use of vacuum phototubes makes it possible to substitute a dc amplifier and pivot meter for the suspension galvanometer necessary with barrier-layer photocells. By thermostatting the phototube chamber, excellent stability is obtained. A light pipe in the viewing beam provides a more stable and efficient mixer of light to the different photodetectors than the white-lined sphere used previously.
I. INTRODUCTION
THE
producers of many of the colored materials of commerce have long felt the need for simple physical instruments not dependent on visual judg- ments for the evaluation of colors and color differences.The need is greatest in the manufacture of paints, papers, plastics, ceramics, textiles, soaps, foods, and other materials which have diffusely reflecting surfaces.
The color difference meters described herein were designed to meet the needs of these industries for instru- mentation to be used in quality control of color during manufacture, in measurements of color difference from standard, and in investigations of color change with processing variables, composition, exposure, use, etc.
An instrument using barrier-layer photocells and mechanical standardization was developed at the Henry A. Gardner Laboratory, Inc., in the period 1948-1950.1 A second instrument with vacuum tubes and electrical standardization has been developed at the Hunter Associates Laboratory, Inc., during the past two years. Both are intended to measure the colors of surfaces as they appear to a normal observer under daylight, 45°0 observing conditions.
The forerunners of the color difference meters are the various photoelectric tristimulus reflectometers, each designed to give tristimulus values of reflectance directly. The unique feature of a tristimulus reflectome- ter is the spectral character of its filters. Each is designed so that, with the light source and photo- detector in the instrument, it has a spectral distribution close to one of the tristimulus functions of the CIE Standard Observer for Colorimetry.2 In both, the sample illuminant is incandescent lamp light and the tristimulus filters are bluer than they otherwise would
* Combination of papers given before the Optical Society of America, March, 1948 and October, 1957.
' R. S. Hunter, "Color and color difference meter, U. S. Patent 2,574,264" (November 6, 1951), Henry A. Gardner Laboratory, Inc., Bethesda, Maryland; "Description and instructions for Hunter color and color-difference meter" (June, 1950); and Van den Akker, Aprison, and Olson, Tappi 34, 143-58A (1951).
2 Optical Society of America, Committee on Colorimetry, The Science of Color (Thomas Y. Crowell Company, New York, 1953).
be so as to give results as if the illuminant were day- light. The author's Multipurpose Reflectometer was the first of several instruments of this type.3 Other tri- stimulus reflectometers are the Photovolt Reflection Meter (made by Photovolt Corporation, 95 Madison Avenue, New York City), the Color Eye (made by Instrument Development Laboratories, Inc., Needham Massachusetts), and the Colormaster Colorimeter4 (made by Manufacturer's Engineering and Equipment Company, Hatboro, Pennsylvania). As has been shown by the author and others,5'6 the accuracy of color measurements with any photoelectric tristimulus instru- ment is limited by the accuracy with which the spectral tristimulus functions of the standard observer are matched by the source-filter-photodetector combina- tions of the instrument.
A primary objective in planning the color difference meter was a precision photoelectric tristimulus instru- ment which would give direct values of color on uniform, visually meaningful scales. To be useful, such an instru- ment should measure colors of surfaces on scales giving approximately the spacing of the Munsell Color System, and should have precision equal to, or better than, that of the eye trained to detect commercial color differences.
The lightness dimension of the psychological surface- color solid shown in Fig. 1 varies solely with Y tri- stimulus reflectance, but the horizontal chromatic dimensions of color depend upon intercomparisons of the three tristimulus reflectances. A tristimulus re- flectometer will usually detect luminous reflectance with a precision which is considerably better than the eye's ability to see the same differences. However, the normal eye is more sensitive to chromatic (horizontal), then to luminous differences in color. Conversely, the tri- stimulus reflectometer is less sensitive to chromatic I R. S. Hunter, J. Opt. Soc. Am. 30, 536 (1940); also J. Research Natl. Bur. Standards 25, 581 (1940), NBS RP1345.
4 L. G. Glasser and D. J. Troy, J. Opt. Soc. Am. 42, 652 (1952).
' K. S. Gibson, Instruments 9, 309 and 335 (1936); also J. A.
Van den Akker, J. Opt. Soc. Am. 27, 401 (1937).
6R. S. Hunter, J. Opt. Soc. Am. 32, 509 (1942); also Natl. Bur Standards (U.S.) Circ. No. C429 (1942).
985
VOLUME 48, NUMBER 12 DECEMBER, 98
RICHARD S. HUNTER
FIG. 1. The psychological surface-color solid showing the dimensions of hue, saturation, and lightness.
than luminous differences in color because each chro- matic value of color computed from tristimulus re- flectances contains the uncertainties of two or three instrumental determinations.
The color difference meter was planned to have three photodetectors, each with a different filter and each receiving specimen-reflected light simultaneously. Meas- uring circuits were sought by which intercomparisons of these photodetector signals would give visually intelligible, chromatic dimensions of color, each with the manipulative uncertainties of only one observation.
I. COLOR DIFFERENCE METER SCALES The work during the past twenty years by Judd, Hunter, Scofield, and Adams on uniform color scales is the basis for the color scales of the new apparatus.
Judd was the first to propose uniform color scales based on physical measurements. He proposed a transforma- tion of the CIE chromaticity diagram giving nearly uniform measurements of chromaticity.7 Shortly there- after, he derived an equation for computing the approxi- mate magnitude of difference between any two sur- face colors from their reflectance and chromaticity differences.8
From Judd's uniform chromaticity triangle, Hunter devised a rectangular chromaticity diagram for colors under daylight illumination.' It has approximately the same spacings as Judd's but was altered so that magnitudes of surface-color difference could be readily computed from data obtained with Hunter's Multi- purpose Reflectometer.3
Scofield simplified Hunter's methods without mate- rially changing the good correlation between computed and perceived color differences of Judd and Hunter.
Scofield prepared separate equations for each of the
7 D. B. Judd, J. Opt. Soc. Am. 25, 24 (1935); also J. Research Natl. Bur. Standards 14, 41 (1935), NBS RP756.
8 D. B. Judd, Am. J. Psychol. 52, 418 (1939).
components of chromaticity.' He, therefore, had three approximately uniform, rectangular scales of color on which he could identify colors and measure color difference. These three scales are related to tristimulus values of reflectance G, A, and B obtained with Hunter's green, amber, and blue filters, respectively, by
L= 1000 a= 700GI
A+2G+B/
b= 280GI G B (A+2G+B
The L, a, and b rectangular surface-color solid is shown in Fig. 2.
The unit of color measurement for Scofield's three scales was kept as near as possible to the NBS unit of color difference devised by Judd.8 This unit was designed to have approximate perceptual uniformity throughout the color solid. Judd adjusted its magnitude to make the color difference of one unit the maximum difference tolerable in the average commercial color match. It is two to four times the minimum difference detected by the trained observer.
In 1942 Adams0 devised a rectangular surface-color solid in which the spacings of surface colors are quite close to those of the Munsell Color System. His chro- matic scales, now quite widely used, are customarily' represented by
aA = 40(Vx- Vy),
bAe= 16 (Vz - VA
where the V's are the Munsell value-function equiva- lents of X, Y and Z. Because the value function introduces much the same change of scale intervals with reflectance as does G' in the Scofield equations above, the two sets of chromatic dimensions are similar.
However, between the Scofield and Adams solids, 100 HITE
/I
IL
I , / /* r/ to /
-em~~~~ -m / o_
-6'O -T0 0 _ A.5 #/00
0-BLACK
FIG. 2. The rectangular surface-color solid with dimensions L, a, and b.
° F. Scofield, Natl. Paint, Varnish Lacquer Assoc., Sci. Sect.
Circ. No. 664 (1943).
10 E. Q. Adams, J. Opt. Soc. Am. 32, 168 (1942).
"lD. Nickerson, Am. Dyestuff Reptr. (Aug. 21, 1950); G. L.
Buc, Am. Dyestuff Reptr. (June 9, 1952).
986 Vol. 48
I
PHOTOELECTRIC COLOR DIFFERENCE METER there is one significant spectral difference. The Hunter
source-filter-photocell combinations were designed to make G similar to Y, B similar to Z, and A similar to that part of the X distribution above 500 my.. As is well known, the CIE X distribution possesses a short wave component having a maximum at 442 m/u that is lacking in the A combination. Thus the Adams scales for chromaticity are spectrally similar to those in the equa- tions in reference 9 except for the use of X with its double-humped spectral distribution instead of the single-humped A distribution.
In Fig. 3 are compared hue circles obtained with the equations in reference 9 and those obtained when the double-humped X, distribution is substituted for A in the equation for a. The specimens represented are ten Munsell colors at 5 value, 8 chroma. CIE coordinates for adjusted Munsell colors derived by way of re- notations12 were used to compute the values of a and b plotted in Fig. 3. Since the ten Munsell colors repre- sented are of equal value and chroma, they should-if the new solid is to duplicate the spacing of the Munsell color solid-arrange themselves in a chromaticity diagram at the vertices of an equilateral decagon having its center at the gray point. It will be seen that the ten points obtained when Xc is used in the equation for a approach this arrangement more closely than those obtained when A is used. For this reason, an a chro- maticity scale using Xc instead of A is to be preferred.
III. THE ANALOG CIRCUITS OF THE COLOR DIFFERENCE METER
To devise analog circuits with which photosignals proportional to illuminant C reflectances, Xc, Y and Ze, can be converted to readings of surface color on
+50
-50
a
+50
0
-50
FIG. 3. Plot showing Munsell 5/8 hue circles obtained when a=fy(X,-Y) (solid line), and when a=fy(A-G) (dotted line).
12Newhall, Nicherson, and Judd, J. Opt. Soc. Am. 33, 385 (1943).
BARRIER-LAYER
CURRENT DIFFERENCE
VOLTAGE DIFFERENCE
E
RLOADR _
iRESISTORSi
G
VACUUM
FIG. 4. Current-difference (series) and voltage-difference (parallel) circuits for barrier-layer and vacuum photodetectors.
visually uniform scales, it is necessary to take the following three steps:
(1) Obtain the luminous reflectance Y, and the reflectance differences, Xc- Y and Y-Zc.
(2) Adjust the magnitudes of these differences for specimen reflectance as is required by data on uniform- surface-color scales.
(3) Adjust the three components by the known uni- form-color-scale factors, of proportionality (1.0 for L, 1.75 for a, and 0.70 for b).
Analog circuits have been devised for both barrier- layer and vacuum photodetectors. There are available both current-difference and voltage-difference X0- Y and Y-Ze circuits for each photodetector, as is shown by the block diagrams in Fig. 4. In the current- difference type, the exact differences in Xc, Y and Zc photocurrents are measured. Therefore, the Xc, Y, and Zc photocurrents for neutral white have to be equal. In the other type, differences in the voltages across load resistors are used. When Xc, Y and Zc photocurrents for neutral white are different, load re- sistors are adjusted to compensate so that voltages are equal.
Because photocurrent is a function of light signal, there is in the current-difference instruments a mechani- cal method for adjusting light directed to the different photodetectors. For example, the standardizing device used in the Gardner current-difference, barrier-layer models of the color difference meter is a screw-actuated arm on which one the photocells involved in each dif- ference has been mounted for movement into, or away from, the center of light distribution. In the voltage-
GY - --
RO
-7 GR I
…-/- ---o BG I 1--X
~~~RP0
4- ---2.5 (A-Y) 1- 2.5(X-Y)
. . . I . . . . December 1958 987
c
RICHARD S. HUNTER
FIG. 5. Factors with which signals should be multiplied (dotted lines) and are multiplied (solid lines) to obtain uniform scales of surface color.
difference type of circuit, electrical standardization is possible by adjustments of the load resistors across which voltages are compared. Thus, standardizing potentiometers are used in the new Hunterlab voltage- difference, vacuum-tube model.
The difference signals obtained from the circuits shown in Fig. 4 have to be adjusted for specimen luminous reflectance, or the chromatic measures of dark colors will be visually too small relative to those for light colors. The nature of the adjustment needed is indicated by the curves of Fig. 5. Here are shown the fy multiplying factors represented by the Munsell value function (.IV/Y) and, by dotted lines, those computed for four Munsell Renotation"2 hues, each at constant chroma, but varying value. Also plotted are the two functions that have been devised as Ohm's law analogs for use in the color difference meter.
The fy multiplier in the color difference meter is actuated by the reflectance (or lightness) dial which, therefore, must be set first. The mechanism used for fy is to reduce with reduced luminous reflectance the reference current against which the difference signals are measured. With this reduced reference current, it is necessary to move each scale potentiometer further to obtain a balance for a given difference signal. The two circuits used for this adjustment of reference current with Y signal are shown in Fig. 6. In the first, the Y
signal is read as percent luminous reflectance Rd. In the second, it is read as lightness [L= 10Rd ].
The corresponding fy multiplying functions are, for the Rd circuit, fy= 0.51 (21+20Y)/(1+20Y); for the L circuit, fy=1/(Y)l (where Y varies from 0 to 1.0).
Both circuits require that two potentiometers be actuated by the singal Y dial.
The rectangular, uniform, surface-color scales achieved by those analog circuits are related to CIE illuminant C, X, Y, and Z by the equations in Table I.
The constants 1.02 and 0.847 which appear in front of X and Z, respectively, are for converting Illuminant C
=1Rd
~
0 d
L
b
(n
~~~Y
Xc -Y)
FIG. 6. The Rd and L multiplying circuits for fy.
tristimulus values to reflectances relative to MgO.3 That is, if XC and Zc are the Illuminant C reflectances for which MgO= 1.0,
Xc= 1.02X, Zc= 0.847Z.
The multiplying constants in front of the equations for
Rd or L, a, and b are obtained by selection of resistances in the respective networks used for their measurement.
TABLE I.
Rd scales
Color difference meter from CIE
CIE from color difference meter
Rd= 100Y
a= 175fy(1.02X- Y) b= 70fy(Y-0.847Z) vhere
/21+20Y\
fY=0.51
(
1+20Y/Y=O.OlRd
a
X=0.9804 0.01R+) 175fy
b
Z= 1.1811 0.01Rd- 70fyJ
L scales
L= 100Yi
a= 175 Y-i(1.02X- Y) b= 70Y-i(Y-0.847Z)
Y= (O.O1L)2
l9 aL) X=0.9804 Y+__
175
/bL\
Z= 1.181 _ 70
988 Vol. 48
PHOTOELECTRIC COLOR DIFFERENCE METER
FIG. 7. Circuit for barrier- layer, current difference instrument.
L MODIFICATION
Thus, in the barrier-layer, current-balancing circuit of Fig. 7, these multiplying constants are incorporated in
the series resistors for control of balancing current.
IV. PHOTODETECTOR AND FILTER STUDIES
It has been repeatedly shown' 6 that the colorimetric accuracy of any photoelectric tristimulus instrument is
20000
18 EITBskB
15000 wU)
z0
CE
10000
W ;~~~~~~~~~~~~EC
I-
limited to the spectral accuracy of the source-filter- photocell combinations used in it. The filters used in the Gardner instrument were the three originally designed for the Multipurpose Reflectometer3 and described by the writer in 1942.6 In some Gardner instruments, the blue filter has been altered so that it passes more of the shorter wavelengths. This alteration
400 450 500 550 600 650 700
WAVELENGTH - MILLIMICRONS
FIG. 8. Spectral curves for barrier-layer source-filter-photocell combinations (heavy lines) and the CIE illuminant C spectral distribution functions they are designed to duplicate (thin lines) (from National Bureau of Standards Circular C429).
December 1958 989
RICHARD S. HUNTERV
20000 1.18EI TBSKE
I E,
5 000 1 \
5000 0.8EI T3XSKBX
10 000 yEc IGK
I
0) I i~~~~I
10 °° l . 80 El TA SKA
W5 00 ECd It _ ,
0400 450 500 550 600 650 700 750 WAVELENGTH, mu
FIG. 9. Spectral curves for S-4 source-filter-vacuum-phototube combinations (heavy dotted lines) and the CIE illuminant C functions they are designed to duplicate.
was made as the result of Jacobsen's finding'3 that some actual observers responded more strongly to the 400- 430-mu region of the spectrum than does the CIE Standard Observer. Figure 8 is the figure previously published which compares the xEc, Ec, and Ec standard functions with those of the source-filter- photocell combinations acutally used in the Gardner barrier-layer instrument. The light source used for these computations was 3100'K, the receptor was a typical General Electric Company barrier-layer photo- cell. There is no separate x-blue filter for the short-
1.40
1.30
LX
U)1.20
0 0.~~~~Q
WAVELENGTH, mu
FIG. 10. Spectral responses of 12 brown-cathode P39 photo- tubes relative to that of a selected tube; all responses adjusted to 1.0 at 620 my.
13 A. E. Jacobsen, J. Opt. Soc. Am. 38, 442 (1948).
wave end of the MEc function; a piece of the z-blue filter is used for the purpose.
For the new vacuum-phototube model color difference meters, the S-4 photosurface was selected even though it is undesirably weak in the region of the spectrum centering at 620 mya. Tubes with this photosurface have good sensitivity and pass small dark currents. It was for
these same reasons that Glasser and Troy used them in their tristimulus reflectometer.4 There are two different types of S-4 photocathodes found in P39 phototubes, one bluish in color, the other dark brown (called black in the trade). The brown-cathode tubes tend to be relatively more red sensitive than the blue type and accordingly, they were selected for the X and Y
tristimulus functions. In Fig. 9, the standard CIE functions are compared with source-filter-phototube
540 WAVELENGTH, my
FIG. 11. Reciprocals of the spectral transmittances relative to that at 620 mu of filters available for trimming.
functions computed for a specific brown-cathode tube.
A separate x-blue filter was designed for this set. All the filters are of glass except the x-red one. Dyed gelatine was used here because the available amber glasses all have absorption curves that are too steep, or not steep enough.
It will be seen by comparing Figs. 8 and 9 that the barrier-layer combinations tend to be spectrally weak in the blue, the new S-4 combinations tend to be spectrally weak in the red.
Before the new color difference meter was designed, a Bausch & Lomb grating monochrometer was combined with a standard strip-chart potentiometer to build a recording spectrometer which could be used to study the responses of photodetectors with and without filters. To attain the highest precision, it is necessary to
90 Vol. 48
PHOTOELECTRIC COLOR DIFFERENCE METER thermostat the photodetectors. Photodetectors generally
shift spectral response toward the red as they warm up.
For a temperature change from 75 to 125F, the re- sponse curves of two P39 phototubes, and a GE barrier-layer photocell each moved, on the average 3 mg toward the red. However, the peak response of the 1P39 tubes remained nearly constant, while that for the barrier-layer photocells dropped by several percent.
In another series of tests, it was found that even when the vacuum tubes are separated for blue- and red- sensitive photocathodes, vacuum tubes of the same manufacturer are spectrally less uniform than barrier- layer cells, Thus, the standard deviation for 620-mg response divided by 520-mg response for 50 red-sensitive phototubes was 26% of the mean ratio. For 27 GE barrier-layer photocells, the same standard deviation was 9% of the mean ratio.
To compensate for these spectral differences between individual 1P39 phototubes, trimming filters were used.
Since the 1P39 phototubes lack response in the 620-mg region, trimming filters were sought which would not absorb in this region. Figure 10 shows the sort of trans- mission curves needed in these trimming filters. Here are plotted ratios of the spectral response of each of 12 tubes divided by the response of the standard tube.
The values for each tube have been adjusted to unity at 620 mg. Figure 11 shows curves for filter materials which can be used with data of the type in Fig. 10 to select filter materials. In Fig. 11 are plotted the 620-mg transmittances of each of a number of filters divided by their respective transmittances at the wavelengths shown. The dotted curves represent dyed gelatines, most of them Wratten filters. The solid curves represent glasses. At least the gross phototube differences can be
compensated for by the use of trimming filters.
FIG. 12. Photograph of barrier-layer color difference meter built by the Gardner Laboratory.
Fr REFLECTING SPECIMEN I
FIG. 13. Block diagram of Gardner color difference meter.
V. BARRIER-LAYER INSTRUMENT WITH MECHANICAL STANDARDIZATION
Figure 12 is a photograph of the instrument developed in 19481; Fig. 13 is a block diagram of the same device.
It will be seen that the apparatus consists of two parts, and the separate spotlight galvanometer used with the current-balancing bridge. The upper part of the instrument is an exposure unit containing light source, window at the top where test specimens are placed, and the phototubes which view the specimens. Below is the measurement unit containing current-balancing bridge, potentiometer rheostats with dials from which values of color are read, switches, and other electrical
components.
In the exposure unit are an automobile spotlight lamp, and lenses and mirrors for projecting two beams from this lamp onto the test specimen mounted on top.
These beams strike the surface of this specimen from opposite directions along axes that are 450 from per- pendicular. Light reflected from the test specimen in perpendicular and near-perpendicular directions is measured. A fluted lens is used to collect this light from the specimen and direct it into the diffusing enclosure at the bottom of the exposure unit. The diffusing enclosure
December 1958 991
RICHARD S. HUNTER
apparatus is oval in shape, about two inches long and one and one-half inches wide. It is possible, however, to install lenses in the incident beams which confine the area to roughly three-eighths by one-quarter inch in diameter. Normally, the specimen window is a beveled round hole cut in a black Bakalite block, but colorless plate glass can be substituted when the specimens are powdered or liquid. Since the exposure unit is separate from the measurement unit, it can be moved to any place or position permitted by the several feet of cable which connects the two units. Another feature designed to help the instrument user is a specimen-viewing port through which the instrument user can identify and examine the specimen area under measurement. There is also an attachment for color by transmission of light through both clear and translucent materials.
VI. VACUUM-TUBE INSTRUMENT WITH ELECTRICAL STANDARDIZATION
Figure 14 is a photograph and Fig. 15 a block digaram of the new instrument. Like the earlier instrument, it is divided into an exposure unit and a measurement unit, but the galvanometer is a vacuum-tube device built into the measurement unit panel. As with the older in- strument, the exposure unit may be moved to any posi- tion within cable length of the measurement unit. In Fig. 14, the sample window faces downward and there
FIG. 14. Photograph of new vacuum-phototube color difference meter with electrical standardization.
is lined inside with magnesium oxide so that light reaching it is thoroughly diffused before it is directed through one of the windows to the photocells outside.
There are three photocells, each covered with a different filter, or in the case of the X photocell, with pieces of two filters. To simulate the two-component Ec func- tion shown in Fig. 8, roughly two-thirds of the cell is covered with an amber filter, one-third with a blue filter.
In the measurement unit, the currents and current differences from these photocells are measured with three separate 10-turn rheostats by reference to a photocurrent from a constantly illuminated comparison photocell. The measurement circuit of this instrument is shown in Fig. 7. The manner in which current-difference magnitudes are, in effect, multiplied by a luminous- reflectance (fy) function to obtain visually uniform, chromatic scales is described in the text and Figs. 5 and 6 above. On the instrument panel will be seen the three 10-turn dials, the scale selector switch, the galvanometer switch, and polarity switches immediately above the
a and b dials. These polarity switches are necessary
because, as shown above, a and b values change polarity with color.
The area of surface normally measured with this
/1
COLOR-MEASUREMENT CIRCUIT
THERMOSTATED BLOCK
) Xfi ^ 4,HOTOTUBES
¶
TRISTIMULUS FILTERSREFLECTING SPECIMEN
FIG. 15. Block diagram of new vacuum-phototube color difference meter.
oO
C) L;3V'10
Orentss &d4neis
< -b Cry6i Lat b
-j I
-
CLEAR METHACRYLATE
If MUT OIDi
\ t/\t/\t/
DIFFUSING FACE
I ) I
IS I I\\ I
992 Vol. 48
PHOTOELECTRIC COLOR DFFERENCE METER
is a scissors jack to lift the sample to measurement position.
The method of specimen illumination is much the same as in the previous instrument except that a 6-v instrument lamp is used. However, the light mixing device between specimen and photodetectors is a rectangular light pipe with prismatic diffusing face rather than the white-lined enclosure of the earlier device. In the new instrument, the phototubes are in an aluminum block, the temperature of which is held steady at slightly above room temperature with a thermostat and small resistance heater.
In Fig. 14, it can be seen that the dials are digital rather than ruled as in the older instrument. Each is operated by a finger-hole plastic knob. Instead of polarity switches for the a and b scales, there are separate dials, one for plus values and one for minus values of both a and b. Each of these four dials has a curtain that closes whenever the reading should be taken from its opposite number. These a and b double- digital dials minimize the possibility of errors in taking data. They eliminate the bother and the end coil errors introduced by throwing a polarity switch every time zero is passed. They also make it possible to have all dials turn in the same rotational sense as the galva- nometer pointer.
The top standardizing knob on the left of the panel adjusts reference current taken from the regulated power supply (lightness standardization). The two below adjust the load resistances (for a and b standardi- zation). The galvanometer in the top center of the panel is a zero-center microammeter operated by a high-u 12AX7 duo-triode. In addition to the polarity switch and thermostat pilot, there is a switch marked CTC (color-temperature check) which can be used to guide the adjustment of lamp voltage to the same color temperature each time the instrument is operated.
VII. PERFORMANCE STUDIES
In operation, a color difference meter is always adjusted to read a standard correctly before it is used to read unknown specimens. Because of errors arising from spectral inaccuracies of source-filter-photocell such as are shown in Figs. 8 and 9, it is recommended that the standard always be similar in spectral character to the specimens to be measured. This is sometimes called the "hitching-post technique." The standard which is similar to the specimens is the "hitching post."
Hunter showed6 that with the tristimulus filters he designed about 1940 and used in the barrier-layer model of the color difference meter, errors in a and b averaged about one-tenth the chromaticity difference between the standard and each specimen measured against it.
In Fig. 16 are points representing a and b values for a number of ceramic colored plaques each measured in three different manners: (1) by computation from spectrophotometric curves and calculations using the
*50
+40
+30
+20
410 b
-10
-20I
-301
-40
-20 -d0o 0 -_ +10 +20 +30 +40
a +50
FIG. 16. Graph showing a and b values from spectrophotometric curves of a number of ceramic panels and values for same panels observed with two types of color difference meters after each was standardized on white.
equations above, (2) by measurement with a barrier- layer instrument, and (3) by measurement with a new vacuum-tube instrument. Although points from the new instrument are, in general, closer to those calculated from spectrophotometric data than are the points from the barrier-layer instrument, the order-of- magnitude differences are still the same. Neither instrument will measure a colored specimen against a white standard with the accuracy necessary for most color specifications.
Although the instruments thus lack the accuracy needed for measurements of large color intervals, the high precision of the color difference meter makes it useful for many problems. In the study of small color difference measurements being conducted by the National Bureau of Standards and the Porcelain Enamel Institute,'4 the barrier-layer instrument was used to give ten different settings each on a porcelain enamel standard and a number of similarly colored porcelain enamel panels. A study of the repeatability of the color differences measured in this manner gave the following average standard deviations:
L=0.131, a= 0.148, b=0.132.
14 J. C. Richmond and W. N. Harrison, J. Am. Ceram. Soc. (to be published).
H~~~
I
L~~~~~~~~~~
0
,b co 0 0 0;
X FROM SPECTRAL CURVES 0 BARRIER-LAYER MODEL
O VACUUM-TUBE MODEL
lo
, 0
I , _
December 1958 993
RICHARD S. HUNTER
TABLE II. Correlation coefficients between magnitudes of color difference measured with each instrument model and visual estimates of difference magnitude by trained observers of 15 sets of panels of 12 color differences each, (PEI porcelain-enameled panels).
Barrier-layer Vacuum-phototube
instrument instrument
Average Average
differ- differ-
ence ence
(N BS Correlation (N BS Correlation Color units) with eye units) with eye
Red 3.7 0.72 3.1 0.90
Orange 3.4 0.98 3.8 0.99
Yellow 3.8 0.92 3.6 0.94
Lt. green 3.1 0.91 3.2 0.95
Dark green 2.1 0.77 2.6 0.77
Buff 2.1 0.54 1.9 0.67
Cream 1.9 0.63 1.7 0.45
Fawn 1.4 0.82 1.5 0.83
Purple 1.8 0.90 1.8 0.95
White 2.3 0.69 2.2 0.76
Black 0.6 0.85 0.7 0.74
Gray 1.6 0.70 2.0 0.72
Lt. blue 1.6 0.47 1.6 0.68
Brown 0.5 0.87 0.7 0.44
Olive 1.0 0.77 1.3 0.88
Average 2.1 0.77 2.1 0.78
In a soap manufacturing plant using one of the new vacuum-tube instruments, a series of colored ceramic panels was measured each day after standardization of the instrument on white. Standard deviations for this day-to-day repeatability averaged:
L= 0.067, a= 0.095, b= 0.075.
The new instrument is believed to be better in this day- to-day constancy than the old because of its phototube thermostatting and because of its color-temperature checking feature which permits adjustment of lamp voltage to compensate for lamp-aging and lamp-change alterations of color temperature.
Correlations have been obtained by comparing color differences derived from instrument readings and the formula,
AE= ((AL2)+ (Aa2)+ (Ab2))I
with visual estimates of the same color differences. The specimens used were the above mentioned Porcelain Enamel Institute sets of 13 panels of each of 15 different colors. In Table II are shown for each color and each instrument the average difference in NBS units between the central standard plaque and 12 other plaques of each color. Following this average difference is the product-moment correlation coefficient between differ- ence magnitudes measured with the instrument and the average visual estimate of 19 experienced observers.'4
Blame for failure of the correlation coefficients to be higher can be divided between three factors; (1) the above mentioned spectral errors of the instruments, (2) insufficient instrument precision, and (3) insufficient observer precision. That precision is a factor is suggested by the poorer correlations where the differences are smaller. The vacuum-tube instrument described herein was built especially for light colored soaps and soap powders; it performed quite poorly on the black and brown panels of the PEI test. Spectral errors can per- haps be blamed for the poor barrier-layer instrument showing on reds and yellows.
The average correlation coefficients for the 15 sets of panels are esentially the same; 0.77 and 0.78. There is under way a more detailed study of these plaques which involves character as well as magnitude of color dif- ference. The results of this study will provide further data for the evaluation of these instruments.1 4
There is at present widespread commercial interest in materials which are made to appear whiter than they otherwise would by the addition of ultraviolet-absorb- ing, blue-fluorescing dyes. The color difference meter is responsive to this sample fluorescence because the filters are between sample and receiver. However, the instrument illuminant is lamplight and the tristimulus filters are bluer than they would otherwise be so as to, in effect, change the color of the illuminant from yellow incandescent light to bluish daylight. It is not, therefore, possible to measure accurately color change arising from sample fluorescence in daylight because the illumina-
+2
+1 n
I-
-J
0
-
-I 0 +1
RELATIVE a +2
FIG. 17. Comparison of colors of three products of one stock (B,C,D) with a fourth product (A) to determine how much fluorescent brightener to add to a yellowish stock (C) to equal the whiteness of the target color.
YELLOW
4.'
L L89.o
.000%
-GREEN -- i
~
iAM~T -RED -BLUE
©)
.005%l l
994 Vol. 48
I I
. =89.7
tion on the sample does not have the spectral distri- bution of daylight. Nevertheless, the instrument appears to have some value for this application.
In Fig. 17 is plotted a comparison of the colors of three white products from one stock with that of a fourth product from an inherently whiter stock. Two of three products from the first stock have a fluorescent brightener. The instrument was not standardized before these measurements, since the only question to answer is which of the three products from the first stock is closest in color to product A. It will be seen that for the measurement conditions of the color difference meter, product B containing 0.00125% fluorescent
995 brightener, is the closest of the three to the target product A. Since the same decision was reached by visual examination, it would appear that the present instrument rates fluorescent brighteners in roughly the same manner as does the normal observer working under average lighting conditions.
ACKNOWLEDGMENT
The data on which Figs. 10 and 11 are based were obtained in the course of a subcontract for the Quarter- master Research and Development Command, Natick, Massachusetts.
December 1958 PHOTOELECTRIC COLOR DIFFERENCE METER
