A unit of light is a measure of the amount of light that is emitted from a source. The most common units of light are lumens (5 letters), lux (3 letters), rays (3 letters) and phot (4 letters). Lumens measure the total amount of visible light emitted from a source, while lux measures the intensity of light on a surface. Rays measure the amount of light energy that is emitted from a source, and phot measures the amount of light energy that is absorbed by a surface.