Illuminance (symbol: E, unit: lux) indicates how much light – more specifically how much luminous flux (in lumens) – from a light source falls on a given surface. Where an area of one square metre is uniformly illuminated by one lumen of luminous flux, illuminance is one lux. For example, an ordinary candle flame generates around one lux of illuminance at a distance of one metre.

Illuminance is measured on horizontal and vertical surfaces using a luxmeter. However, it is not a precise measure of the brightness of a room, which depends crucially on the reflectance of the room surfaces. Under the same lighting, a white room appears brighter than a room with darker surfaces. See also semi-cylindrical illuminance.

Because the pattern of light distributed by ordinary lighting is not absolutely uniform, standards generally refer to average illuminance. This takes mathematical account of irregularities: average illuminance is the weighted arithmetic mean of all illuminances in the room.