How this instrument works
Illuminance is how much visible light lands on a surface, measured as luminous flux per unit area. Lux is lumens per square meter; the footcandle is the older imperial version, lumens per square foot. Both weight the light the way a human eye does, through the CIE photopic luminosity function, so a lux meter and a footcandle meter pointed at the same desk are reading the identical physical quantity, just carved up by a different unit of area.
The formula fc = lux ⁄ 10.764 looks like an empirical fudge factor, but it is not one — it falls straight out of the definition of the foot. A foot has equaled exactly 0.3048 meters since the 1959 international yard-and-pound agreement, so a square foot is exactly 0.09290304 square meters, and its reciprocal, 10.76391, is how many lux pack into the same light that fills one footcandle's area. Nothing about the light changes in the conversion; only the size of the square you measure it over does.
Because the constant is definitional rather than measured, the conversion is exact and linear at every scale — there is no threshold above which it drifts, unlike temperature scales with their offsets. The real limit is conceptual: illuminance says nothing about the light's color, direction, or how it looks reflecting off a surface. A plant physiologist reading grow-light output in PPFD, or a projectionist reading a screen in foot-lamberts, needs a different instrument entirely — footcandles and lux only describe how much photopic light falls on the plane you hold the meter against.
- Type the metric reading into Illuminance, lux — leave the default of 500 to see a bright-office example, or enter your own meter or spec value.
- Illuminance, footcandles updates immediately, computed as the lux value divided by 10.76391.
- Use the step control, which moves in increments of 10 lux, to nudge the input and watch the footcandle figure scale linearly with it.
- Compare the footcandle result against your local code's minimum, since US lighting standards are usually written in footcandles rather than lux.
Worked example — converting a 500 lux office spec to footcandles
A European fixture data sheet specifies 500 lux for general office lighting, matching the EN 12464-1 recommendation, but the US installer's checklist is written in footcandles. Dividing gives 500 ⁄ 10.76391 = 46.451522 fc, the exact figure this instrument returns at its six-decimal precision — comfortably inside the roughly 30 to 50 fc that IES guidance recommends for office tasks.
The relationship is strictly linear, which doubles as a sanity check: 1000 lux, roughly what overcast daylight delivers at ground level, converts to 92.903044 fc — exactly twice the office figure, because doubling the input simply doubles the result through the same fixed divisor.
Questions
Why is the conversion factor 10.764 instead of a round number?
Because it comes from the exact definition of the foot, not a measurement. A foot has equaled precisely 0.3048 meters since the 1959 international yard-and-pound agreement, so a square foot is exactly 0.09290304 square meters, and its reciprocal — 10.76391 lux per footcandle — is fixed by geometry alone, not something anyone had to measure in a lab.
Are lux and footcandles measuring the same kind of light?
Yes — both are illuminance, luminous flux landing on a surface, weighted by the CIE photopic luminosity function to match human eye sensitivity. Lux divides that flux by square meters and footcandles divide it by square feet; nothing about the light changes, only the unit of area underneath it. A radiometric sensor reading watts per square meter measures something related but different, since it carries no eye-response weighting at all.
Is a footcandle the same thing as a foot-lambert?
No, and confusing them is a common mistake. A footcandle measures illuminance — light falling onto a surface, which is what this converter handles. A foot-lambert measures luminance — light leaving or reflecting off a surface, the brightness a viewer actually perceives, used for things like projector-screen ratings. The two only coincide numerically for a perfectly reflecting, perfectly diffuse surface, which real materials never quite are.
What footcandle level counts as well lit?
For general office work, IES guidance typically calls for roughly 30 to 50 footcandles, which is exactly the range the golden example lands in: 500 lux converts to about 46.45 fc. Hallways and storage areas often target 5 to 10 fc, while detailed inspection or surgical tasks can call for 100 fc or more — the right figure always depends on the specific task, not one universal number.
Can this instrument convert footcandles back to lux?
Not directly — it is built for the lux-to-footcandle direction only, computing fc = lux ⁄ 10.76391. To go the other way, multiply your footcandle figure by 10.76391 lux per footcandle; 46.451522 fc, for instance, returns to exactly 500 lux, since the two operations are exact inverses of the same fixed constant.