This is called the luminance , or brightness , of the surface. Note that a large wall and a small face can have the same brightness, therefore we do not want to measure the total amount of light reflected from the wall, but the amount of light reflected from a standard measuring area so that we can compare the brighness of different surfaces regardless of size. Note also that this is an inside-out measuring situation. Previously we were using a card of a standard area to count rays diverging from a point source. We sit in the auditorium and look at the stage.
Rays fall on a uniformly painted flat and are scattered by it.
Some go one way, some another, and from each square foot of the flat, some number of rays come to our eye and are counted. This is the brightness of the flat in foot-lamberts . If we count the number of rays from one square meter we are measuring the brightness in meter-lamberts , also known as nits . The number of rays reflected from an area one square centimeter is lamberts . Of course this is a simplified case.