Luminous flux arriving per unit area of a surface, measured with a cosine-corrected illuminance meter. It is the main design target in most codes and guides: roughly 300 to 500 lux on office desks, 50 to 100 lux in corridors, around 100,000 lux in full sun. Illuminance describes light arriving, not how bright the surface looks, which also depends on its reflectance.
Illuminance on the work plane averages 420 lux, but the corners near the windows drop to 180 at night.
Also heard as
- light level
- E
Do not mix it up with
- Luminance: Luminance is the light leaving a surface toward the eye, the brightness you actually see; illuminance is light landing on it.
Related terms
Lux
The metric measure of light landing on a surface: one lumen falling evenly on each square meter.
Foot-candle
The North American working unit of light on a surface: one lumen falling evenly on each square foot.
Luminance
The brightness of a surface or source as seen from a particular direction, measured in candelas per square meter.
Illuminance meter
A handheld meter that reads the light falling on a surface in lux or foot-candles, commonly called a light meter.
Uniformity
How evenly light is spread over an area, expressed as a ratio such as minimum to average or maximum to minimum.