Matching Color Differences*

A new visual colorimeter has been designed and built to be used primarily for matching color differences. The colorimeter provides seven visual fields, arranged in a cluster of seven regular hexagons, one in the center and six adjacent to it. Both eyes of the observer see each field uniformly filled with light. The color of each field is a mixture of three primary lights (red, green, blue) and can be controlled by the observer independent of the color of the other fields. Depending on the nature of the experiment, the observer can be presented with all seven fields in operation or with some fields switched off. The colorimeter is calibrated to convert instrumental readings to CIE color coordinates. The first experiment performed with the colorimeter was to assess quantitatively the ability of the observer to make matches of color difference in an array of three adjacent fields of constant brightness. Fixing the colors of two fields, the observer was required to produce a color in the third field so as to form a perceptually equilateral triangle of colors of constant brightness. The repeatability of such matches is presented graphically in the 1960 CIE–UCS diagram.