Very often, different results are obtained when colored originals and/or reproductions under different so-called standard light sources are evaluated. A possible explanation for the different results is that not all admissible standard sources have the same spectral power distribution, either due to differences in manufacturing or due to aging. A possible solution to the problem would be to determine how the spectral power distribution of a given illuminant differs from the standard (D5000) distribution by visually comparing suitable metameric grays with a stable non-selective gray under the illuminant in question. This comparison would serve as a quick and inexpensive way of testing the light source. Two major questions are involved in this investigation: (1) Are the color differences obtainable with metamers large enough to be useful? (2) What part does the variation of color response of individual observers play in this system? The first question was investigated using two sets of metameric grays data: initially Wyszecki\u27s metamers and systematic changes in the D5000 power distribution; and then MacAdam\u27s metameric grays based on real pigments from spectrophotometric data. The results for Wyszecki\u27s metamers indicated that reasonable color differences occurred between non-selective gray and metameric grays with systematic changes in the spectral power distribution of the illuminant. Similar results were obtained for MacAdam\u27s metamers but with less sensitivity, that is, there were smaller differences between the non-selective gray and the metameric grays. It was found that deviations from the D5000 spectral composition can indeed be picked up and the method is sufficiently sensitive for the Standard Observer. The second question, the variation of individual observers, was investigated using information based on the measurements by Stiles and Burch, and also the Standard Deviate Observer determined by Eugene Allen. The investigation was done by obtaining the color differences relating to the individual observers, and comparing them to the Standard Observer when comparing the selective and non-selective grays to the standard D5000 sources. The result of this investigation indicated that the color differences due to the variations between observers were larger than the color differences between the non- selective gray and the metameric grays due to the changes in the power distribution of the source. The conclusion is that this system of determining the changes in the spectral power distribution of the light source by the color differences between non-selective and selective metameric grays is inadequate because of the variances in individual observers
