Lux (lx) is the base unit defined in the International System of Units (SI) for illuminance, which is the light energy incident on a body. The illuminance on a surface is then 1 lux (lx) when a luminous flux of 1 lumen (lm) falls perpendicularly on a surface of 1sqm.
The beam angle and the distance of the light source from the illuminated surface determine the size of the illuminated surface. If the distance is reduced by half for the same solid angle, only a quarter of the area is illuminated, whereas the illuminance increases many times over.
Illuminance is calculated by dividing the luminous flux (F) by the illuminated area (A). If lighting is provided at 1 lux for a given time, the exposure is expressed in lux seconds (lxs).
An artificially lit room has about 20 lx to 100 lx; outdoors, values can range from 3,000 lx in the shade to 100,000 lx in the sun.