Illuminance is incident luminous flux per receiving area, measured in lux. It is the visually weighted counterpart of irradiance. Uniform illumination of an aperture of area gives collected luminous flux . An aperture measurement alone does not give a source's total emitted luminous flux.[ref]
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Illuminance is a measure of the amount of light incident on a surface per unit area. It quantifies how much luminous flux (measured in lumens) is spread over a given area (measured in square meters). The unit of measurement for illuminance is the lux (lx), where 1 lux equals 1 lumen per square meter.