Lux to Lumens Calculator

Lumens and lux measure different things in the same photometric system. The lumen is luminous flux, the total light emitted by a source weighted by the response of the human eye; formally it is one candela spread over one steradian of solid angle. The lux is illuminance, flux arriving per unit area, defined as exactly one lumen per square meter. That definition is the whole conversion: lumens = lux x area, and lux = lumens / area, with 1 foot-candle equal to 10.764 lux because a square foot is 0.0929 square meters. Because the weighting follows the CIE photopic curve, which peaks at 555 nm where 1 watt of radiant power equals 683 lumens, two lamps of equal wattage but different spectra can differ greatly in lumen output. Familiar reference points anchor the numbers: direct summer sunlight is around 100,000 lux, an overcast day 1,000 to 2,000 lux, a lit office 300 to 500 lux, a corridor near 100 lux, and full moonlight roughly 0.1 to 0.3 lux. The main caveat is that dividing lumens by floor area gives a perfectly uniform ideal. Real installations lose flux to walls, ceilings, and fixture optics, and lose more over time to lamp depreciation and dirt, so delivered illuminance is typically only 40 to 60 percent of the naive figure once a coefficient of utilization of about 0.5 and a light loss factor of about 0.8 are applied. Point sources also obey the inverse square law, E = I / d squared, so illuminance under a fixture is far higher than the room average this conversion reports.

Photometry long predates the SI units used here. Nineteenth century practice compared lamps against physical candle standards, including the British standard spermaceti candle, the French Carcel lamp, and the German Hefner lamp introduced in 1884, none of which reproduced well between laboratories. In 1881 Jules Violle proposed a standard based on the light from one square centimeter of platinum at its freezing point, a step toward a reproducible physical definition. The photometric quantities were placed on a common footing when the Commission Internationale de l Eclairage adopted the photopic luminous efficiency function V(lambda) in 1924, fixing how radiant power is weighted to produce lumens. The candela became one of the base units of the metric system in 1948, when the General Conference on Weights and Measures defined it from a blackbody radiator at the freezing temperature of platinum, and the lumen and lux were fixed as derived units at the same time. In 1979 the candela was redefined in purely physical terms, as the luminous intensity of a source emitting monochromatic radiation at 540 terahertz with a radiant intensity of 1/683 watt per steradian, which removed the need for any material artifact. The older CGS illuminance unit, the phot, equal to one lumen per square centimeter, fell out of use, but the foot-candle survived in North American codes and IES documents, which is why lux and foot-candles still appear side by side on professional light meters.