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.
A gray card is a flat object of a neutral gray color that derives from a flat reflectance spectrum. Typical examples are 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, the Digital Grey Kard set of three 2x3" cards, white, black, and neutral gray, and others. 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).
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[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. 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. White balance, or color balance, refers to the setting that the camera uses to compensate for the illuminant color in the scene. Gray cards are useful for white balance since their reflectance is not as intense as that of a white object, which reduces the possibility of clipping, or exceeding the maximum possible reflectance value that the camera's sensor can record.
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. For the serious photographer or hobbyist, gray cards and color control are 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.
[edit] Limitations
A gray card is only useful for setting or correcting the balance of neutral colors. 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.[2]
[edit] References
- ^ North Carolina State University (Department of Wood and Paper Science). FLUORESCENT WHITENING AGENTS (FWAs). Mini-Encyclopedia of Papermaking Wet-End Chemistry.
- ^ Freeman, Michael [2005]. The Digital SLR Handbook. Ilex, 69. ISBN 1904705367.
[edit] External links
- Contrary to common belief, light meters are engineered to expose for 12% grey. See the article "Meters Don't See 18% Gray" for more.
