Symbol E. The energy in the form of visible radiation reaching a surface per unit area in unit time; i.e. the luminous flux per unit time. It is measured in lux (lumens per square metre).
Astronomy
The energy in the form of visible radiation that falls on a surface in a given time. It is measured in lux (lumens per square metre). The energy incident at all wavelengths, not the visible, is termed irradiance.