On the Relative Spectral Energy Distribution of CIE Standard Source A

WURZBURG, DIBERNARDO, AND CLEARY

Since the elements of A' are functions only of theconstant k and of the chromaticity coordinates of theilluminant and the reference color, they can be pre-computed for any point in the CIE chromaticitydiagram for a given illuminant. Figures 1-4 show loci ofconstant all', a, a21

1, a22' (each multiplied by 100)for CIE illuminant C and with k equal to 0.2 and can beused as nomograms for determining the coefficients inEqs. (13) for any reference color whose chromaticitycoordinates are known. With the aid of these nomo-grams it is possible to determine directly A~x and Ay bya simple linear transformation of differences in ratios asread directly from a clorimeter, whenever it is per-

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

missible to take the ratio lo/2 equal to unity. If theratios, RA, RG, RB employed in Eqs. (10) and (11) areexpressed as percentages, and if the ak"s are expressedin hundreds (as shown in Figs. 1-4), then the unit valueof Avx and Ay in Eqs. (13) will be one in the fourth placeof decimals and only integral values will normally besignificant.

ACKNOWLEDGMENTS

Finally, we would like to acknowledge the helpfulsuggestions and criticisms made by Professor Arthur C.Hardy and Dr. David L. MacAdam in the developmentof the concepts described here.

VOLUME 49, NUMBER 10 OCTOBER 1959

On the Relative Spectral Energy Distribution of CIE Standard Source A

HILTON WRIGHTDivision of Applied Physics, National Research Council, Ottawa, Canada

(Received March 23, 1959)

The relative spectral energy distribution of a tungsten ribbon filament lamp calibrated at 2854 0K(C2 = 14 380) differs from that of a coiled filament type lamp operating at the same color temperature. Thereis reason to believe that the relative spectral energy distribution of the latter resembles more closely therelative spectral energy distribution tabulated by the CIE for that source and, consequently, lamps of thecoiled filament type have been recommended for use as the standard. It has been shown theoretically thatthe difference between the two sources causes discrepancies in the chromaticities of certain test colors and itfollows that the chromaticity coordinates of samples obtained by using the coiled filament lamp should benearer the true values. The following paper describes an experiment in which ribbon and coiled filamentlamps calibrated at 2854 0K were subjected to visual and photoelectric tests to determine the significance ofof the effect of the different energy distributions on selected test colors. An analysis of the results showsthat small changes in chromaticity do occur, but for practical clorimetric purposes these differences cansafely be ignored.

INTRODUCTION

IT is known that the relative spectral energy distribu-tion of a tungsten ribbon filament lamp calibrated

as CIE source A deviates from that of a coiled filamentlamp operating at the same color temperature and,consequently, deviates from the corresponding Planck-ian distribution for that source. In a recent paper byHisdall the effect of the different energy distributionson certain test colors was shown by computing andfinding a discrepancy in the chromaticity coordinatesof the various samples and from the magnitude of thediscrepancies it was concluded that these differencesshould not be neglected in all cases. The purpose ofthis paper is to investigate the significance of the differ-ences in spectral energy distributions from the experi-mental point of view.

FIRST EXPERIMENT

A tungsten ribbon filament lamp (6V-18A) and atungsten coiled filament lamp (500W) were chosen as

I B, Hisdal, J. Opt. Soc. Am. 48, 608 (1958).

test sources and calibrated in the usual photoelectricway to give a color temperature of 28540 K (C2= 14 380)corresponding to the color temperature of CIE sourceA. The precision of the color temperature calibrationfor both lamps was 480 K. Each source illuminated inturn four colored test filters which could be matched bya mixture of three primaries on a calibrated Donaldsoncalorimeter. The field of view subtended an angle ofapproximately 10 degrees. The observer made thirtymatches with each lamp and filter, thus providingeight sets of thirty matches in terms of the instrumentprimaries. Each setting was transformed into CIE(xy) chromaticity coordinates. The results were plottedin the CIE chromaticity diagram of which Figs. 1 to 4show appropriate sections. The open and solid circlesrepresent the color matches obtained with the coiledand ribbon filament sources, respectively. The corre-sponding mean chromaticities are shown by the openand solid squares. The open and solid triangles representthe chromaticity points computed for each test coloron the basis of the CIE 2 standard observer data andof the actual energy distributions for the two sources.

980 Vol. 49

October 1959

y

400

395

A

A

.450

155

.150

y

.145

.160

X

FIG. 1.

.455

I I , ,.165

X

FIG. 2.

CIE STANDARD SOURCE A

.

430

A25

460 .235

.270

.260

/ y2S

I ,,.170

I .I , I.375 .380

981

.240

X

FIG. 3.

X

FIG. 4.

I8385

245

.J.390

FIGS. 1-4. Portions of the CIE chromaticity diagram showing results of color matches of four test samples. The chromaticity coordi-nates (xy) of the color matches are represented as open and solid circles which correspond to the coiled- and ribbon-filament illuminants,respectively. Open and solid squares are the observed means of the color matches; open and solid triangles denote theoretical means.Ellipses are 95% confidence regions of the color matches derived by statistical methods. Broken and closed line ellipses are related to thecoiled and ribbon filament sources, respectively.

Factors such as the transmittance of the glass andblackening of the lamp envelopes may cause the energydistributions of the sources to alter slightly from thetheoretical distributions.' In the case of this experi-ment, however, the glass envelopes were considered tobe nonselective, and because neither of the lamps wassubjected to prolonged use, the effect of blackening wasnot appreciable. In particular the energy distributionof the coiled filament source was assumed to be that ofthe Planckian distribution at 28540 K, while the energydistribution of the ribbon filament source was takenfrom Hisdal's table given in reference 1. Computed and

observed mean chromaticities for each test color andsource are given in Table I.

In addition 95% confidence ellipsoids2 were calculatedgiving the regions of scatter in the color matches foreach test sample. Each ellipsoid is shown in its projec-tion on the chromaticity diagram (Figs. 1 to 4). Thecontinuous and dashed line ellipses relate to the ribbonand coiled filament sources, respectively, and their

2 The computation procedure for such ellipsoids may be foundin detail in the paper by Gnter Wyszecki, "The specification ofcolor rendering properties of light sources," CIE Proceedings ofthe Brussels Meeting, 1959 (to be'published).

I . . .

HILTON WRIGHT

TABLE I. Computed and observed mean chromaticity coordi-nates for four test colors, illuminated by ribbon and coiled filamentsources.

Test Observed mean Computed meancolor Source A x y x y

1 ribbon filament 0.4539 0.4013 0.4480 0.4086coiled filament 0.4539 0.4023 0.4476 0.4075




centers coincide with the corresponding means of thecolor matches (solid and open squares).

From the diagrams it is evident that the differencesbetween matches due to the two sources are not sig-nificant. For each test color the regions of experimentalscatter overlap sufficiently well to justify the conclusionthat not only the coiled filament source but also theribbon filament source may be used as a good CIEstandard source A. It is also evident that the differencesbetween observed means (solid and open squares) andthe differences between the computed means (solid andopen triangles) do not correlate with each other. If anysignificant difference between the two sources is presentat all much more precise observations would be re-quired in order to detect them. Differences between theobserved mean chromaticities and computed chro-maticities are attributed mostly to the difference inthe color-matching functions of the actual observerwho made the color matches on a 10° calorimeter fieldand the color-matching functions of the 1931 CIEstandard observer which is based on 2 observations.

SECOND EXPERIMENT

The same test sources were employed in conjunctionwith eleven colored glass filters of quite different spec-tral transmittances. The relative responses (R.,) of aGillod-Boutry photocell with known spectral sensi-tivity were obtained for each source and filter andcompared with the corresponding calculated responses(RC) obtained as follows

0 00RC= t (i)H(S(xd / H(x)S)dX.

In the formula t(x) M (i=1, 2, 3 . 11) is the spectraltransmittance of test filter i at wavelength , I(x)(r)is the relative spectral energy distribution of the ribbonor coiled filament source, and S(x) is the relative spec-tral sensitivity of the photocell. The results obtainedfor RMf and Re are compiled in Table II.

There is slight statistical evidence to indicate thatthe two lamps are significantly different; however, ifconsideration is given to the scatter in the individuallymeasured responses, this difference may be considerednegligible. It was noticed that in general the scatter inthe measured responses obtained with the ribbon fila-ment source was somewhat greater than that experi-enced with the coiled filament source. This effect isexplained by the fact that the radiation from lamps ofcoiled filament structure is more stable than that fromribbon sources and, consequently, the smaller variationin the light output from the former results in betterreproducibility of measurements.

In summarizing the results obtained in the experi-ment described it may be stated that the effect of the

TABLE II. Calculated and measured relative responses of aphotocell for eleven colored glass filters.

123456789

1011

Ribbonfil. source

1.893.634.184.51

15.919.114.27

13.164.481.040.18

Coiledfil. source

1.913.674.204.52

15.909.064.25

13.114.471.050.18

Rm ..Ribbon Coiled

fil. source fil. source

1.89 1.913.64 3.654.17 4.234.50 4.49

15.95 15.909.15 9.054.30 4.27

13.30 13.244.52 4.511.05 1.070.18 0.18

difference in spectral energy distribution between coiledand ribbon filament sources, both calibrated at thecolor temperature of 28540 K, is very small for visualwork and the discrepancies which may be expected cansafely be ignored. A similar statement may be madeabout the photoelectric experiment, but here preferenceshould be given to the coiled filament lamp as the betterstandard since greater precision of measurement willresult for this source. It may be added that althoughthe analysis of the two experiments is based on theresults of only a few test colors and is by no means anexhaustive study, it may be expected that the differ-ences encountered in a similar investigation using othercolors, and possibly other observers and photoelectricreceivers, would be of the same order.

ACKNOWLEDGMENT

The author is gratefully indebted to Dr. G. Wyszeckifor his advice on all the theoretical and technicalmatters of the experiment.

982 Vol. 49

On the Relative Spectral Energy Distribution of CIE Standard Source A

Documents

Transcript of On the Relative Spectral Energy Distribution of CIE Standard Source A