Gray card

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Gray cards are used, together with reflective light meters, as a way to produce consistent images in film and photography.

[edit] Description

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 2 8x10" cards 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 appearing neutral; this flatness ensures that the card appears neutral under any illuminant (see Metamerism (color)).

[edit] Application

Reflected light meters are calibrated to give an accurate exposure when pointed at subject with reflectivity somewhere near 18%; the exact value varies and the details are complex (see the Light meter article)). 18% corresponds, by definition, to Ansel Adam's Zone V in the Zone System. By placing a gray card in the scene to be photographed and taking a reading off of it with a reflected light meter, the photographer can be assured of consistent exposures across their photographs. This technique is similar to using an incident meter, as it depends on the illuminance but not the reflectivity of the subject.

In addition to providing a means for measuring exposure, a gray card provides a reference for white balance. By taking a photo of the gray card in the scene, the photographer can later adjust the white balance of the prints so that the gray card appears neutral.

[edit] Limitations

A gray card is not fully satisfactory for color control, because it has uniform spectral reflectance, unlike colored objects. That is, it can only be used to directly control the color of spectrally-neutral gray objects. For more color control, a color chart such as the GretagMacbeth ColorChecker can be used.