The routine visual evaluation of color differences for the matching of similar materials may often be carried out satisfactorily under one light source and one set of illuminating and viewing conditions. However, the consumer of colored materials is likely to be more satisfied if the color match is specified for and actually made under the illuminating and viewing conditions of ultimate use. When materials are likely to be viewed under a variety of light sources or directions of illumination and view, the color match should be both specified and verified for a wide range of both. The variety of light sources available today makes it imperative to evaluate general-purpose color matches under more than one light source. For many purposes, evaluating under daylight and incandescent lamp light will be adequate. Natural daylight has long been the favorite source, but because of the variability of natural daylight in both quantity and quality, artificial daylight of specified intensity and spectral composition is preferable. In unusual circumstances a minus red light source as represented by production type cool white fluorescent lamp could be used in addition to the two sources specified above.