The Illuminant in Color Matching and Discrimination

A study of the part played by the illuminant in color discrimination may be divided into two broad sections. In one the chief concern is to find an illuminant under which color differences will surely be evident. The single illuminant most satisfactory for this purpose will depend upon the reflectance curve of the samples to be examined. In the other, the choice is limited to an illuminant under which an observer may see the colors with which he is concerned in the same relation to each other as he would if they were observed under an illuminant to which he has become previously accustomed, the most usual example being the selection of an artificial daylight in substitution for natural daylight. Results of studies made in the color-measurements laboratory of Agricultural Marketing Service regarding this latter choice are presented in charts and table form. They include studies of 18 illuminants, actual and theoretical, several pairs of samples expected to show large color differences under a change in illuminant, and 30 samples of cotton, the product with which this laboratory is chiefly concerned. The final results are summarized in a table which gives a relative rating of illuminants as substitutes for each other.