As illustrated earlier, vision can only occur when all the necessary elements are present: light; an object; a receptor (the eye); and a decoder (the brain). We do not actually see footcandles or light striking an object (illuminance), but rather the resulting brightness from light transmitted or reflected by a surface. This type of brightness, called luminance, is measured in candelas per square meter (cd/sq.m.) There is always a subtractive interaction between a surface and the light falling on it.  Because of absorption, some light is always lost.