Phot to Lux Converter

How it is calculated

Illuminance is converted with fixed factors: lux = phot × 10,000 and foot-candles = phot × 929.0304. A phot is one lumen per square centimetre, a lux is one lumen per square metre, and a square metre holds 10,000 square centimetres. A foot-candle is one lumen per square foot, and 929.0304 is the number of square centimetres in a square foot. Both results are shown with two decimals. The conversion is a straight multiplication, so any number is accepted, including zero and, mathematically, negative values, although a negative illuminance has no physical meaning. It converts the unit only; it does not say how bright a room feels, which depends on the surfaces and the eye.

Common mistakes

• Mixing up phot with lumen or lux. A phot is a very large unit, equal to 10,000 lux, so values in phot are usually small decimals.
• Using the factor 10.76 (lux per foot-candle) in the wrong direction. Here you start from phot, and the foot-candle value is 929.0304 times the phot value.
• Confusing illuminance with luminance. Phot and lux describe light arriving on a surface; luminance (stilb, nit) describes light leaving it.
• Typing a lux reading into the phot field. A lux meter reading of 500 entered here would be treated as 500 phot, giving 5,000,000 lx.