Illuminance is a major factor determining how swiftly, reliably and easily our eyes are able to perform a visual task, e.g. reading or working at a computer. Illuminance – like brightness distribution – is important for visual performance.

Illuminance (symbol: E) is measured in lux (lx) and indicates the amount of luminous flux from a light source falling on a given surface.

Illuminance is one lux where an area of one square metre is uniformly illuminated by one lumen luminous flux.

Illuminance is measured on horizontal and vertical surfaces, using a luxmeter. For good recognition of vertical surfaces and three-dimensional forms in a room – especially faces – the yardstick used is cylindrical illuminance. This is the average vertical illuminance on the surface of a cylinder.

Given the same lighting, a white room appears brighter than a room with darker surfaces: it reflects the light better. So the lower the reflectance and the more difficult the visual task, the higher the illuminance needs to be.

Uniform distribution of brightness

Uniform distribution of brightness facilitates the performance of visual tasks. For the working world, standards set out minimum levels for the average illuminance that needs to be maintained, e.g. 500 lux for office work, 300 lux for rough work at machines and 500 lux for precision machine work.

Illuminance may naturally be higher than the values stipulated in standards. Human beings are programmed to live in daylight, where illuminance reaches 100,000 lux on a cloudless summer's day and 20,000 lux when the sky is overcast.