How is illuminance defined? Illuminance indicates how much luminous flux (lumen) hits a certain surface. How is illuminance measured? The lux (lx) is used as the unit of measurement. For an illuminance of one lux, there is a luminous flux of 1 Lumen evenly covers an area of 1 m², as is the case, for example, with candlelight from a distance of one meter.
It is important to understand that the illuminance does not reflect the impression of brightness, as this depends largely on the properties of the illuminated material. With the same illuminance, dark walls appear much darker than light ones.