Illuminance converter

The lux is the reference point for every unit in this category: it is defined as one lumen of luminous flux spread over one square meter, which is why lux and lumen per square meter carry the exact same factor in the table above — they are simply two names for the same quantity. Everything else here is either a metric multiple of the lux (kilolux, millilux, nox) or a unit built on a different base area, like the foot-candle, which packs the same idea of "light per area" into square feet instead of square meters. Because the lumen itself is already weighted to how sensitive the average human eye is to different colors, illuminance is a photometric quantity, not a raw physical power measurement, which matters the moment you try to relate it to anything measured in watts.

To go from lux to foot-candle, multiply by 0.09290304:

fc = lx × 0.09290304

A typical open-plan office is lit to around 500 lux for desk work. Run that through the formula: 500 × 0.09290304 = 46.45 foot-candle, which lines up with the 40–50 fc that US lighting codes often recommend for office tasks. Going the other direction, a lighting designer working from an IES recommendation of 50 fc for a reading area multiplies by 10.76391042 to get roughly 538 lux on the metric fixture spec sheet. A dimmer hallway or stairwell target of around 100 lux, by contrast, works out to just under 9.3 fc — a useful reminder of how quickly these numbers fall as a space gets less critical to light well.

Illuminance numbers like these drive real decisions well beyond the lighting aisle. Photographers and cinematographers use light meters calibrated in lux or foot-candles to set exposure before a shot, and camera and security-camera spec sheets quote minimum usable illuminance (sometimes down in the millilux range for "starlight" night-vision sensors) to describe how little ambient light the sensor needs to produce a usable image. Horticulturists and indoor growers track lux levels under grow lights because most crops need a minimum daily light integral to flower or fruit, even though serious plant-science work often switches to a related but distinct measurement of photon flux rather than lux alone. Museums and archives cap gallery lighting around 50–200 lux specifically to slow the fading of pigments and dyes on sensitive artwork and textiles, and road-lighting standards specify lux targets as low as 10–30 lux for residential streets versus much higher levels at intersections and building entrances. Workplace safety codes in many countries also set minimum lux levels by task — fine assembly or inspection work needs far more light than a warehouse aisle or a loading dock.

One easy mix-up: the phot (an old CGS unit still seen in vintage photographic literature and some antique exposure meters) equals 10,000 lux, not 1, so dropping a decimal point when reading an old meter's scale will throw a result off by four orders of magnitude. At the opposite extreme, the nox is defined as one-thousandth of a lux and shows up mainly in night-vision and low-light military specifications — easy to confuse with the kilolux, which sits a million times higher on the same numeric scale despite the superficially similar-sounding name. And because lux already bakes in the eye's color sensitivity, converting it to a raw radiometric unit like watts per square meter isn't a fixed multiplication at all — it depends on the exact color (spectrum) of the light source, which is why no such conversion appears in this table.