The Lux is used as a measure of illuminance - the intensity, as perceived by the human eye, of light that hits or passes through a surface. Illuminance is a measure of how much luminous flux is spread over a given area. We can think of luminous flux (measured in lumens) as a measure of the total "amount" of visible light present, and the illuminance as a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance is inversely proportional to area. One lux is = equal to one lumen per square metre.