18% gray
From The Glossary of Digital Photography
Refers to the amount of light reflected by a gray card. Also used as the name of the card: 18% gray card. 18% gray is referred to as the standard by which light meters are calibrated, but light meters are actually calibrated by reference to the ANSI standard, which some argue is more closely represented by a reflectance of 12%. If properly done, a light meter pointed at an 18% gray card will properly give a reflected light reading, representing the average light value in a scene. This is useful when photographing a scene that has a lot of white, such as sand or snow, or a scene that has a lot of black. By using a gray card in those cases, the exposure will be much more correct than trying to use a reflected light-meter reading of the scene. A 12% reflectance can be adjusted by opening up a stop. This can be tested by photographing an 18% gray card and looking at the camera histogram to see if the curve or line is centered. If it is off to one side, the card is either over- or under-exposed and the exposure
can be adjusted accordingly.
