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 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) realization of a Lambertian scatterer; its apparent brightness (and exposure determination) therefore depends only on its orientation relative to the light source. By placing a gray card in the scene to be photographed, oriented toward 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. (Of course taking photographs with side lighting or back lighting implies that the gray card should be oriented toward the camera instead.)
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 balance 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.
Gray cards are made of a variety of materials including plastic, paper, and foam. Some photographers hold that any neutral white or gray 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. Gray cards specially made to be spectrally flat are therefore more suitable to the purpose than surfaces that happen to be available.
A gray card is useful for setting or correcting the balance of neutral colors, as well as for exposure. Other charts, such as various color charts, provide standard reference patterns with calibrated reflectance spectrum and color coordinates, for use in adjusting color rendering in a larger range of situations.