What is illuminance?
Illuminance is the luminous flux received per unit area of a surface, measuring how brightly a surface is lit. Its SI unit is the lux (lx), equal to one lumen per square meter (lm/m2). This converter is a universal physics tool and does not depend on any country or region. It instantly expresses one illuminance value in thirteen common units, including lux, kilolux, phot, foot-candle, nox, and several lumen-per-area variants.
How to use it
Enter the illuminance value, choose the unit it is expressed in from the dropdown, and submit. The calculator shows the equivalent value in every supported unit at once. The headline figure is the value in lux, the international standard unit.
The formula explained
Each unit has a fixed "to-lux" factor. The input is first normalized to the SI base unit:
$$E_{\text{lux}} = \text{value} \times f_{\text{from}}$$Then every output is obtained by dividing the lux value by the target unit's factor:
$$E_{\text{target}} = \frac{E_{\text{lux}}}{f_{\text{target}}}$$Because each factor is strictly positive, no division-by-zero can occur. Note that several units are numerically identical: lux equals lm/m2, phot equals lm/cm2 (both 10000 lx), foot-candle equals lm/ft2 (about 10.764 lx), and nox equals millilux (0.001 lx). Area conversions use exact constants: 1 ft = 0.3048 m, 1 yd = 0.9144 m, 1 in = 0.0254 m.
Worked example
Suppose a surface receives 500 lux. The converter returns: 0.5 klx, 0.05 phot, 0.05 lm/cm2, about 46.45 lm/ft2, about 46.45 foot-candles, about 418.06 lm/yd2, about 0.3226 lm/in2, 500000 millilux, and 500000 nox. To confirm an area unit:
$$1\ \text{foot-candle} = 10.7639\ \text{lux}, \quad \text{since } 1\ \text{ft}^2 = 0.09290304\ \text{m}^2$$FAQ
Is a foot-candle the same as lux? No. One foot-candle equals about 10.764 lux because it is one lumen per square foot rather than per square meter.
What is a phot? A phot is one lumen per square centimeter, equal to 10000 lux. It is a CGS-system unit.
What is a nox? A nox equals one millilux (0.001 lux), a unit historically used for very low light levels.
