Exposure

All visible objects reflect a certain amount of light; We wouldn't be able to see them if they didn't. The term incident light refers to the percentage of original light that falls on an item. The resulting percentage is called the item's reflection factor or diffuse reflectance.

This may sound confusing, so here's an example: White objects have high reflection factors because they reflect the majority of the light (usually at least 90%) that falls onto them. Black objects on the other hand absorb most of the light that falls on them (hence why it's better to wear a white t-shirt on a hot and sunny day than a black one) and have a low reflection factor - usually less than 5%. If you shine 100 foot-candles of light onto white chalk, 96% of the light will be reflected; This is expressed as 96 footlamberts. If you shine the same 100 foot-candles on black velvet only 2% of the light (2 footlamberts) would be reflected.

The average diffuse reflectance throughout nature is 18%. Despite being far from a 50% midpoint that people might expect, 18% is the mid-reflection point for all things.

Here is a list of typical reflection factors:

White Chalk: 96%
Off-Whites: 80%
Light Grays: 70%
Caucasian Face: 36%
Midgray: 18%
Brown Face: 16%
Green Leaves: 14%
Black Face: 10%
Black Velvet: 2%

:: :: :: :: :: :: :: :: :: :: ::