When light shines on an object, we say that the surface of the object is illuminated. The degree to which the surface of a light-receiving object is illuminated by light can be expressed by the photometric quantity of illuminance. The quantity of illuminance is defined as the luminous flux falling on a surface unit divided by the area of the surface unit.
The unit of illumination is called "lux" (lx). When a luminous flux of 1lm is uniformly illuminated on an area of 1m2, the illumination on this area is equal to 1lx, that is, 1lx=1lm/1m2.
