What's the difference between a white card and gray card?

[KmH]

Neutral gray has equal amounts of red, green and blue (RGB). None of the 3 colors dominates.

The density of neutral gray can range from White (R=255, G=255. Blue=255) to Black (R=0, G=0, B=0). R=100, G=100, B=100 is a neutral gray tone lighter than black but darker than white.

The percentage mentioned is the % of incident light that is reflected, and has nothing to do with color.

 

[TCampbell]

A gray card can *also* be used in conjunction with the built-in light meter of the camera as a way of doing incident metering because it has a "known" level of reflectivity.

If you could know precisely how much light a surface is going to reflect then a reflected meter reading is as good as an incident meter reading. The trouble is... you typically don't know how much light any given surface will reflect.

There is a catch, however... which is that most gray cards are 18% gray -- but few cameras are tuned to this. Most camera light meters are calibrated to some value in the 12% to 18% range so you'd have to know what your camera uses and match that to the gray card (or figure out how far off they are in terms of stops so you can meter the card and compensate. While you can technically do this... I don't know anybody who does at a matter of regular practice. When I want highly accurate metering, I grab my incident meter and skip the gray card.

 

[Biota]

Great, thanks for explaining it.