Gray card
Reflectance reference used in photography / From Wikipedia, the free encyclopedia
A gray card is a middle gray reference, typically used together with a reflective light meter, as a way to produce consistent image exposure and/or color in video production, film and photography.
A gray card is a flat object of a neutral gray color that derives from a flat reflectance spectrum. A typical example is the Kodak R-27 set, which contains one 8x10" card and one 4x5" card which have 18% reflectance across the visible spectrum, and a white reverse side which has 90% reflectance. Note that flat spectral reflectance is a stronger condition than simply appearing neutral; this flatness ensures that the card always has the same color as its illuminant (see metamerism).