Gray card

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 colour as its illuminant (see metamerism).

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