Illuminance Converter

Six units of illuminance: millilux, lux, kilolux, foot-candle, milliphot and phot. Illuminance is the light that lands on a surface, lumens per unit area, which is the quantity a light meter reads and the one building codes specify. It is not the same as luminance, which measures light leaving a surface towards your eye — a sheet of white paper and a sheet of black paper under the same lamp have identical illuminance and very different luminance, and mixing the two up is the usual reason a lighting calculation comes out absurd.

The conversions are pure area changes, and the only awkward number is the foot-candle: one lumen per square foot, and since a square foot is exactly 0.09290304 m², that is 10.7639 lux. American lighting practice still runs on foot-candles while the rest of the world uses lux, so the factor of about 10.8 is the one to keep in your head — a US office specified at 50 fc and a European one specified at 500 lx are asking for almost the same thing.

The phot and milliphot are the CGS members of the family, lumens per square centimetre, which makes a phot 10,000 lux. They survive mostly in older photometric literature.

What makes the unit worth a tool is the sheer range daylight covers. Full sunlight is on the order of 100,000 lx, an overcast day around 1,000, a well-lit office 500, a corridor perhaps 100, and a full moon on a clear night about 0.25 — five orders of magnitude between the brightest and the dimmest thing a person reads by, which is why the human visual system's adaptation is more impressive than any camera's. Values under 0.0001 are shown in exponential form, and everything else to eight significant digits.