Photographic Grey Cards

What is the reason behind a grey card? Nearly all hand-held light meters, as well as many meters built into cameras, are calibrated to provide information that will allow you to set your camera so that the film will record the scene as an "average" scene. And to a light meter, an "average" scene is one in which the tones when combined form middle grey - a tone that reflects 18% of the light falling on it (the same tone and reflectance of the grey card). That's fine if you are photographing an average scene or subject. However, when the subject is not average - for example, a white rabbit in the snow - then you need to make an adjustment to the setting your light meter tells you to make.And that's where a grey card comes in; it helps you make the proper exposure judgments. The white rabbit will still appear white, because it is brighter than 18% grey. Reflection meters tend to make all subjects that they are pointed at reproduce as 18% grey, regardless of their original colour.What is a Kodak grey card? It is a piece of sturdy 8 x 10 inch size cardboard that is neutral grey on one side and white on the other. The card is specially manufactured to photographic specifications. The grey side reflects 18%of the light falling on it, and the white side reflects about 90%. Also,the grey side has a protective lacquer overcoat that improves spectral reflectance and resists damage due to fading, fingerprints, soil, etc.