Illuminance measures the amount of luminous flux (light) incident on a surface per unit area. The SI unit is the lux (lx), defined as one lumen per square metre (lm/m²). The foot-candle (fc) is the imperial equivalent — one lumen per square foot — still widely used in North American architectural lighting standards, photography, and workplace safety codes.
Illuminance values span an enormous range: a clear sunny day outdoors reaches 100,000 lux (10,000 fc); a typical office is lit to 300–500 lux; a moonlit night is about 1 lux; and the threshold of human vision is around 0.001 lux. Photography and cinematography use lux and foot-candles to match artificial lighting to ambient conditions and to set correct camera exposures.
| Direct sunlight | ~100,000 lx |
| Overcast day | ~10,000 lx |
| Office lighting | 300–500 lx |
| 1 foot-candle | = 10.764 lx |
| 1 lux | = 0.0929 fc |
| Full moon | ~1 lx |