This practice is concerned with the spectral characteristics of light-transmitting, -diffusing, or -reflecting nonluminescent materials: reflectance, transmittance, and color, in the CIE system. Measurement of color difference is not of direct concern, but see Footnote 4. The colors of luminescent (phosphorescent or fluorescent) materials are frequently commercially important enough to require measurement; specialized techniques have been developed for this purpose, but they are beyond the scope of this recommended practice. Due to the frequent occurence of low levels of fluorescence, and of its effect on measurements, general principles for treating this factor are included. The materials are without limitation as to composition but must be homogeneous to the unaided eye over portions large enough for measurement.