Illuminance indicates the amount of luminous flux from a light source falling on a given surface. It is the ratio of the luminous flux to the area of this surface. [2]

Title	Symbol	Unit
illuminance	E	lux [lx]

Illuminance can be defined as the ratio of the luminous flux incident on a surface to the area of that surface.

E illuminance [lx] Φ luminous flux [lm] A area [m²]

The illuminance is independent of the direction from which the luminous flux reaches the surface. [4]