Gray card

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. 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).

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 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 reference for white balance. White balance, or color balance, allows the camera to compensate for the illuminant color in a scene. In theory, a white object should be the ideal means of creating a white balance reference, but in practice the relatively high reflectance of white objects increases the risk of a white balance exposure exceeding the maximum possible reflectance value that the camera's sensor can record. This would produce an incorrect white balance value, and for this reason gray cards are preferred as a means of setting white balance.

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 holdFact|date=April 2007 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. [cite web | title = FLUORESCENT WHITENING AGENTS (FWAs) | url = http://www4.ncsu.edu/~hubbe/FWA.htm | author = North Carolina State University (Department of Wood and Paper Science)| work = Mini-Encyclopedia of Papermaking Wet-End Chemistry] Gray cards specially made to be spectrally flat are therefore more suitable to the purpose than surfaces that happen to be available.

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. [cite book
last = Freeman
first = Michael
title = The Digital SLR Handbook
origyear = 2005
url = http://www.ilex-press.com/publishing/product.php?productid=16145
publisher = Ilex
isbn = 1904705367
pages = 69 ]

References

External links

* Contrary to common belief, light meters are engineered to expose for 12% grey. See the article [http://www.bythom.com/graycards.htm "Meters Don't See 18% Gray"] for more.