JOURNAL OF THE OPTICAL SOCIETY OF AMERICA Color Difference Comparisons in Low Reflectance Regions (A Study of the Precision of Spectrophotometric and Colorimetric Comparisons) A. R. MACDONALDAND G. P. BENTLEY* Instrument Development Laboratories, Inc., 163 Highland Avenue, Need ham Heights, Massachusetts (Received April 2, 1953) A series of tests have been performed to evaluate the relative precision of the spectrophotometer and the differential filter colorimeter for color difference measurements at low reflectance levels. The conditions and techniques of measurement are described and the results are tabulated and discussed. The superior precision of the colorimetric measurements at reflectance levels of 6 percent and below is demonstrated. INTRODUCTION THE spectrophotometer is the fundamental instru- T ment for the definition of the physical reflectance or transmission properties of materials and when combined properly with the C.I.E. standard observer and illuminants, it is the basic instrument for the determination of absolute "color" in C.I.E. space. However, for an increasing number of industrial applications, the differential filter clorimeter is proving to be a rapid, versatile, and satisfactory instrument for production colorimetric comparison. One of the problems currently of interest in the color field, therefore, is the relative applicability of the spectrophotometer and the filter colorimeter in color and color difference measurements. In most commercial color measurements, the problem is really a twofold one: (a) The basic standard color reference must be maintained constant over long periods of time. (b) The commercial production must be held to within specified limits of the basic color, a problem in color difference measurement. Without doubt, in industries where requirement includes high accuracy in absolute C.I.E. terms access to a good spectrophotometer is a necessity. For close tolerance work, this implies careful sample preparation, accurate calibration of the instrument, a highly trained operator, and, particularly on low reflectance colors, application of statistical procedures. Where the require- ment of (a) implies constancy of reference (precision), without equivalent absolute accuracy, a stable com- parison colorimeter may suffice. The filter colorimeter is well suited to the satisfaction of requirement (b), although it must be remembered that sample prepara- tion, conditions of illumination and viewing, tempera- ture, and many other variables may affect the measure- ment. The present paper is primarily concerned with measurements within the scope of requirement (b), using both the spectrophotometer and clorimeter. LOW REFLECTANCE CONSIDERATIONS The accuracy of a spectrophotometer in good operating condition and operated by competent * Instrument Development Laboratories, Inc., personnel is probably in the vicinity of 0.1 percent, which is a high order of accuracy for any instrument. Since the instrument is a linear scale device, an accuracy of 0.1 percent on reflectances of 10 percent represents a comparative precision of (0.001/0.100) or 1 percent. Relative differences of 1 percent approach eye sensi- tivity so it is obvious that in low reflectance regions (less than 5 to 10 percent) the spectrophotometer precision to eye tolerance limits is marginal when the instrument is at top performance. Consequently, the determination of C.I.E. trichromatic coefficientsto the generally desired accuracy of one in the third place, using measurements valid to the second significant figure only (as are single spectrophotometric values below 10 percent), requires application of statistical procedures to a series of measurements. The achievement of the necessary precision in color difference measurements at low reflectance levels re- quires an instrument with a logarithmic response, similar to that of the human eye. The inherent error of such an instrument is substantially a constant percentage of level under measurement and not of full-scale (white). A computing filter clorimeter can be designed to operate on this basis, effectively eliminating degrada- tion of precision at any reflectance levels commonly encountered with commercial colors. Such an instru- ment may also measure difference directly, rather than as a small subtractive difference between two large quantities, the precision of which latter operation is notably poor. For these two reasons and barring noncommercial metamerics, it was proposed that a properly designed logarithmic difference colorimeter should be more precise than a spectrophotometer for many commercial measurements in either dark colors or highly saturated colors where reflectance on any one tristimulus filter may be low. THE TEST PROGRAM In an attempt to obtain preliminary quantitative evaluation of the above proposal, an experimental meas- urement program was undertaken by A. R. Macdonald with the able cooperation of several industrial labora- tories that were themselves interested in the problem. 366 VOLUME 45, NUMBER 5 MAY, 1955
Transcript
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA
Color Difference Comparisons in Low Reflectance Regions(A Study of the Precision of Spectrophotometric and
Colorimetric Comparisons)
A. R. MACDONALD AND G. P. BENTLEY*Instrument Development Laboratories, Inc., 163 Highland Avenue, Need ham Heights, Massachusetts
(Received April 2, 1953)
A series of tests have been performed to evaluate the relative precision of the spectrophotometer and thedifferential filter colorimeter for color difference measurements at low reflectance levels. The conditions andtechniques of measurement are described and the results are tabulated and discussed. The superior precisionof the colorimetric measurements at reflectance levels of 6 percent and below is demonstrated.
INTRODUCTION
THE spectrophotometer is the fundamental instru-T ment for the definition of the physical reflectanceor transmission properties of materials and whencombined properly with the C.I.E. standard observerand illuminants, it is the basic instrument for thedetermination of absolute "color" in C.I.E. space.However, for an increasing number of industrialapplications, the differential filter clorimeter isproving to be a rapid, versatile, and satisfactoryinstrument for production colorimetric comparison.
One of the problems currently of interest in the colorfield, therefore, is the relative applicability of thespectrophotometer and the filter colorimeter in colorand color difference measurements. In most commercialcolor measurements, the problem is really a twofold one:
(a) The basic standard color reference must bemaintained constant over long periods of time.
(b) The commercial production must be held towithin specified limits of the basic color, a problem incolor difference measurement.
Without doubt, in industries where requirementincludes high accuracy in absolute C.I.E. terms accessto a good spectrophotometer is a necessity. For closetolerance work, this implies careful sample preparation,accurate calibration of the instrument, a highly trainedoperator, and, particularly on low reflectance colors,application of statistical procedures. Where the require-ment of (a) implies constancy of reference (precision),without equivalent absolute accuracy, a stable com-parison colorimeter may suffice. The filter colorimeteris well suited to the satisfaction of requirement (b),although it must be remembered that sample prepara-tion, conditions of illumination and viewing, tempera-ture, and many other variables may affect the measure-ment. The present paper is primarily concerned withmeasurements within the scope of requirement (b),using both the spectrophotometer and clorimeter.
LOW REFLECTANCE CONSIDERATIONS
The accuracy of a spectrophotometer in goodoperating condition and operated by competent
* Instrument Development Laboratories, Inc.,
personnel is probably in the vicinity of 0.1 percent,which is a high order of accuracy for any instrument.Since the instrument is a linear scale device, an accuracyof 0.1 percent on reflectances of 10 percent represents acomparative precision of (0.001/0.100) or 1 percent.Relative differences of 1 percent approach eye sensi-tivity so it is obvious that in low reflectance regions(less than 5 to 10 percent) the spectrophotometerprecision to eye tolerance limits is marginal when theinstrument is at top performance. Consequently, thedetermination of C.I.E. trichromatic coefficients to thegenerally desired accuracy of one in the third place,using measurements valid to the second significantfigure only (as are single spectrophotometric valuesbelow 10 percent), requires application of statisticalprocedures to a series of measurements.
The achievement of the necessary precision in colordifference measurements at low reflectance levels re-quires an instrument with a logarithmic response,similar to that of the human eye. The inherent errorof such an instrument is substantially a constantpercentage of level under measurement and not offull-scale (white).
A computing filter clorimeter can be designed tooperate on this basis, effectively eliminating degrada-tion of precision at any reflectance levels commonlyencountered with commercial colors. Such an instru-ment may also measure difference directly, rather thanas a small subtractive difference between two largequantities, the precision of which latter operation isnotably poor. For these two reasons and barringnoncommercial metamerics, it was proposed that aproperly designed logarithmic difference colorimetershould be more precise than a spectrophotometer formany commercial measurements in either dark colorsor highly saturated colors where reflectance on any onetristimulus filter may be low.
THE TEST PROGRAM
In an attempt to obtain preliminary quantitativeevaluation of the above proposal, an experimental meas-urement program was undertaken by A. R. Macdonaldwith the able cooperation of several industrial labora-tories that were themselves interested in the problem.
366
VOLUME 45, NUMBER 5 MAY, 1955
COLOR DIFFERENCE COMPARISONS
TABLE I. Description of sample glass tiles (mean spectrophotometric values of reference standard tiles).
Max diff MeanNickerson Mean percent
Color Set no. No. tiles x y Y units X purity
Dark blue 1 11 0.214 0.183 0.73 percent 5.3 469 52 percentDark red 2 12 0.566 0.321 2.14 percent 19.4 614 69 percentDark green 3 10 0.241 0.365 6.11 percent 4.7 499 23 percent
The results of the test series, which consumed the (3) Measurements on both types of instrumentsbetter part of a year, are reported herein, together with would be made in commercial plants on instruments inthe authors' qualitative conclusions therefrom. everyday use by skilled operators, but on a basis
Since it was not feasible to command the time and comparable to the commercial use of the instrumentspersonnel required for a complete statistical comparison for color difference work. Normal operational checksof color difference measurements, it was first necessary and careful calibration would be permitted, butto establish the following conditions of measurement: statistical averaging would not be utilized on individual
instrument measurements.(1) The color differences measured would be con- (4.n asc cm ut.
fine toa cmmerialrane ofdiferece mgniude (4) No statistical computation would be made untifined oua cmme difference magnitude all series of measurements were completed so no
*butwol i. .nlud adn h knowledge of differences measured by other instrumentsattributes, i.e inxy ndY could unconsciously enter the picture.
(2) Reflectances would be in the region of 6 percentand below to emphasize differences between the linear In brief, this was to be a practical operational com-spectrophotometer and the logarithmic difference parison of the precision of a recording spectrophotometercolorimeter so that a limited number of statistical and a computing logarithmic colorimeter on as near ameasurements would reveal definite trends. However, commercial basis as possible, but on commercial colorsit should be emphasized that reflectances below 10 of low reflectance where it was expected maximumpercent are very common in commercial practice and differences in precision would occur.represent an important practical measurement problem. Selection of suitable samples was a problem, but
TABLE II. Trichromatic coefficients of each tile (spectrophotometer).
a Readings with A106 as standard made using 10 percent filter.b Readings A17 to A23 inclusive made with duo dial.
through cooperation of the Research Laboratories were preserved in good ondition throughout the seriesof the Pittsburgh Plate Glass Company, three sets of of tests.aged uniformly polished colored plate glass samples SPECTROPHOTOMETRIC MEASUREMENTSwere procured. These sets are described in Table I.
A special carrying case was made to permit shipment Measurements on four spectrophotometers werebetween participating plants and, except for one made independently at laboratories of as many majoraccident which damaged a few tiles slightly, the samples industrial concerns. All participants were skilled in the
TABLE IV. Trichromatic coefficients of each tile (colorimeter).
use of the instrument. Except for the broad restrictionsof reasonably practical commercial operation, detailedmeasurement technique was up to the participatingactivity.
Spectrophotometric measurements were reduced byeach laboratory to C.I.E. tristimulus values, X, Y, Zfor illuminant "C," based on MgO reference standards.Two of the instruments were equipped with tristimuluscomputers and computer values were used. The thirdand fourth series of measurements required manualintegration. Thus, the final values include instrumentand computation errors.
C.I.E. trichromatic coefficients x, y, Y were computedfrom the tristimulus values and are presented inTable II.
COLORIMETRIC MEASUREMENTS
Six series of colorimetric measurements were made,with series "A" and "B" on the same instrumentseveral days apart, and series "C," "D," "E," and"F" on four other instruments that had seen consider-able service, but on which routine calibration checkswere made. Tiles A-1, A-12, and A-24 were defined asreference standards and were measured against awhite calibrated vitrolite tile included in the sampleset. All other tiles were measured against the referencestandard in the same color group on a direct differencebasis.
Actual colorimetric values of (sample/standard) X,Y, Z are given in Table III, together with clorimetricmeasurements of each reference standard against thewhite vitrolite tile.
Colorimetric data of the type obtained may beutilized in two ways. The preferred method for makingC.I.E. comparisons between instruments on a differencebasis is as follows: (a) Calibrate the reference standardin C.I.E. terms by use of a spectrophotometer andstatistical procedures to establish the position of thestandard in C.I.E. space to the best obtainable accuracy.(b) Make direct colorimetric measurements of (sample/standard) and refer all computation to the calibratedstandard. The clorimetric data of Table III has beenreduced by this method, using the average spectro-photometric values of the reference standards (TableI). The resultant trichromatic coefficients are shown inTable IV.
An alternate method of treating original clorimetricdata gives maximum emphasis at low reflectance tothe minor spectral response differences between filtercolorimeters. This method consists of measuring thereference standards against white vitrolite and com-puting their positions in space without reference tothe spectrophotometer. These standards are then usedfor direct comparison measurements of the sample tilesas before. This method is entirely adequate for differ-ence measurements on a single instrument where
369May 1955
A. R. MAcDONALD AND G. P. BENTLEY
* Spectrophotormeters (4)
Each Plot Represents Measurementson One Tile _ I
. A-2 I _ I
14
_ ____ _ ---a7 _
Vh
160 180
160 ISO 200x
+Colorimeters
x Colorlmeters
I au
160
yI40
120
100
160 180
220
200
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y
160
240 160 180 200x
200 220 240x'I1 Josef
A -5 9~~~~~~~~~~~~,
220 240
200
y
180
200 220 240 260 180x
200 220 240 260
Y-73%
FIG. 1. Comparative C.I.E. plots (blue series).
approximate position of the standard is all that isrequired to determine permissible tolerance ellipses.
The second method of computation was used, inconjunction with the Adams-Nickerson formula, toexpress the colorimetric results as single vector differ-ences in Table V. Consequently, this table provides adirect juxtaposition of spectrophotometric and colorim-etric results.'
1 Complete tables of data computed for dominant wavelengths,percent purity, and Munsell HV/C are available on requestfrom the authors for the benefit of those more familiar with thesemethods of color notation.
SUMMARY AND CONCLUSIONS
For improved visualization of the comparativeresults, the data of Table II (spectrophotometer) andTable IV (calorimeter) have been plotted in Figs. 1, 2,and 3. Each series of plots shows a small section of thex, y-plane with data for a single tile as measured byfour spectrophotometers and six series of colorimetrictests reduced by the preferred procedure. On Fig. 1,diagonal crosses represent plots of the six series ofcalorimetric data reduced by the alternate procedure.
(6) referred to calibratedstandard
(6) not referred to calibratedstandard
A-3 I
-7?l_1
180
160
y 140
120
100
180
y
160
140
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y 180
160
I48
180
A-9
-I
Standard Tile: x-.214 y-. 183
370 Vol. 45
COLOR DIFFERENCE COMPARISONS
* Spectrophotometers (4) +Colorlmeterm (6)
Each Plot Represents Measurements on One Tile
500 520x
540 56
340
y 320
3000 46 480 500
x520 540
340
500 520 540 560 32
300-500 520 540 560 460
-i 2 _
~~~~~~~~ - _---A C_
480 500 520 540x
560 580 600 620x
Standard Tile: x-.566
340
y 320
300
561 I 620
560 580 600 620
V-.321 Y- 2.1 4%
FIG. 2. Comparative C.I.E. plots (red series).
Study of the data and plots leads to the followingconclusions:
(1) Precision of measurement of the spectro-photometer between instruments decreases with de-creasing reflectance, becoming very poor in the regionof less than 5 percent reflectance.
(2) Precision of measurement of the spectro-photometer in the region of 6 percent reflectance isabout it3 in the third place for x and y.
(3) Precision of measurement of the colorimeterbetween instruments decreases slightly with lowerreflectance but is superior to that of the spectro-photometer by approximately 3 to 1 in the region of 6percent and by approximately 5 to 1 in the region of1 or 2 percent reflectances.
(4) For the "A" and "B" series of clorimetric
measurements, precision of measurement on the singleinstrument was about one unit in the third place.
(5) For color differences of normal commercialimportance (less than 10 Nickerson units) the colorim-eter precision is consistently superior to that of thespectrophotometer in this low reflectance region.Stated differently, the spectrophotometer errors mayfar exceed the tolerance involved in difference measure-ment below 5 or 6 percent reflectance.
(6) The data confirms the value of the logarithmiccalorimeter combined with a statistical spectro-photometric calibration of the standard as the mostprecise available method of color difference evaluationin the low reflectance regions. It further confirms thatmeasurements can be reliably compared betweeninstruments on a difference basis.
(7) Study of the computation shows that due to high
A-l9 | -l' I
1\V__ -4
340
y 320
300
A-21 I I
++_
340
y 320
300
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y 320
300
371May 1955
60
A. R. MACDONALD AND G. P. BENTLEY
*Spectrophotrometers (4) +Colorimeters (6)
Each Plot Represents Measurements on One Tile
220 240 260x
A-2
IzI I - -
220K
:
240
220 240x
400
y 380
360
400
y 380
260
260
200 220 240 260x
A- 2!
__-Eds360 _
200
400
y 30
360
220 240x
260
2 I I I I I 2 6
200 220 240 260x
Standard Tile, x-.241 y-.365 Y-6.11%
FIG. 3. Comparative C.I.E. plots (green series).
sensitivity of a logarithmic instrument, noticeabledifferences in comparison meter readings betweeninstruments, or in a given instrument, represent verysmall changes in the x, y-plane.
It should be emphasized that the present discussionrelates to precision and not accuracy. Precision is ofgreat importance in difference measurements, as theprecision must be greater than that of the eye formaximum instrument utility. Although most peopledo not realize it, the eye is an amazingly precise differ-
ence calorimeter but a very poor device as far asaccuracy is concerned.
RECOMMENDATIONS FOR FURTHER STUDY
It is believed that, although the precision of the*spectrophotometer will improve at higher reflectances,there may be considerably greater difference in com-
mercial use of the instrument than is realized. It issuggested, therefore, that a study be made of precisionof measurement at higher reflectances, with a spectro-photometer and a logarithmic difference colorimeter,together with a set of samples covering a wide colorrange and reflectances from 0.5 percent to 90 percent.Reduction of such data to the form of Figs. 1, 2, and 3should be most helpful in evaluating the current statusof precision of measurement by the instrumentsconcerned.
Notes: (1) The instruments used in the test programdescribed consisted of four G.E. spectrophotometersand five Color-Eye electronic difference colorimeters.
(2) The conclusions reported herein are those of theauthors and are not intended to imply either approvalor dissention by those other activities participating inthis study.
