Standard Viewing Conditions

The observed colour of a surface depends on the spectral quality of the illumination, the direction of illumination, the direction of viewing, the surround or background, the nature of any light that might be reflected from the surface and the nature and state of adaptation of the eyes of the observer. The is standard practice to view specimens illuminated by daylight from a lightly overcast north sky (south sky in the southern hemisphere) or the artificial equivalent of this illumination (1)65 daylight source). Viewing booths that provide controlled artificial daylight and other common illuminates are available from Macbeth. Their use is recommended because the light is much more reproducible than natural daylight, they provide standard viewing conditions at any hour, and, in interior rooms they provide a neutral ambient surround and they exclude extraneous light. Specimens should be viewed along their normal (the line of sight perpendicular to the surface) and illuminated at 45° to the normal. equivalent results can be obtained with reversed geometric arrange-illuminating normally and viewing at 45°. These conditions are described in a standard from the American Society for Testing and Materials (ASTM): D1729 Standard Practice for Visual Evaluation of Colour Differences of Opaque Materials.