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 8 in × 10 in (20 cm × 25 cm) card and one 4 in × 5 in (10 cm × 13 cm) card, each with an 18% reflectance across the visible spectrum, and a white reverse side with a 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).