One way to deal with the concern of whether a given light source is capable of rendering a great number of widely different colors is to devise a measure such that with it people can tell exactly which one is the first among several sorts of light sources, which one is the next, and which one is the last according to the number of the colors which can be rendered by the source. Using a special arithmetic, people are able to calculate the maximum range or area occupied by the chromaticities which can be produced with a specified luminance factor Y under a given light source. All of these areas, each being associated with one specified luminance factor Y between 0.0 and 1.0, can be piled up to form a solid in a uniform color space. The volume of this color solid corresponds to the total numb