An instrumental measurement of the changes in reflectance in terms of CIE tristimulus value Y, the luminous reflectance, is employed as an objective measure of the rate of fading of the British Standard BS1006:1971 (ISO R105) blue-wool standards 1 through 5. A tentative relation of the change in Y value (DELTAY) with steps in the standard geometric grey scale (BS2662) is noted. In addition, the relative rates of fading of the blue-cloths in a carbon-arc and a xenon-arc fadeometer are reported in terms of the visually-rated geometric grey scale steps; the rates of fading in the xenon-arc fadeometer and under daylight fluorescent lamps are reported in terms of instrumentally measured changes in reflectance. Such exposure data are useful for establishing the photochemical stability class of an artist's or conservator's material during accelerated aging tests. One of the principal objectives of this research is to recommend that the blue-wool standards be used as inexpensive monitors for the exposure of materials to light in museum galleries and showcases.