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 film and photography.

A gray card is a flat object of a neutral gray color that derives from a flat reflectance spectrum. A typical examples is the Kodak R-27 set, which contains two 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 simply appearing neutral; this flatness ensures that the card appears neutral under any illuminant (see metamerism).

A major use of gray cards is to provide a standard reference object for exposure determination in photography. A gray card is an (approximate) realisation of a Lambertian scatterer; its apparent brightness (and exposure determination) therefore does not depend on its orientation relative to the light source. By placing a gray card in the scene to be photographed, oriented at a defined angle relative to the direction of the incident light, and taking a reading from 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 convenient reference for white balance, or color balance, allowing the camera to compensate for the illuminant color in a scene.

Gray cards can be used for in-camera white balance or post-processing white balance. Many digital cameras have a custom white balance feature. A photo of the gray card is taken and used to set white balace for a sequence of photos. For post-processing white balance, a photo of the gray card in the scene is taken, and the image processing software uses the data from the pixels in the gray card area of the photo to set the white balance point for the whole image.

Most digital cameras do a reasonable job of controlling color. For the casual user, a gray card is unnecessary. Many serious photographers or hobbyists consider gray cards an essential part of the digital photography process.

Gray cards are made of a variety of materials including plastic, paper, and foam. Some photographers hold[citation needed] that any neutral white or grey surface, such as a white piece of paper, a concrete or stone wall, or a white shirt are suitable substitutes for a gray card; however, since bright white papers and clothing washed in typical detergents contain fluorescent whitening agents, they tend to not be very spectrally neutral.[1] Gray cards specially made to be spectrally flat are therefore more suitable to the purpose than surfaces that happen to be available.