Illuminance Converter
Convert illuminance between lux, foot-candles, phots, and nox.
How to use this tool
- Enter illuminance and from unit in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your lux and the full breakdown beneath it.
Formula
All values convert to a base in lux first, then divide by the target factor:
base (lux) = input × factorfrom
Factors: 1 foot-candle = 10.76391 lux, 1 phot = 10 000 lux, 1 nox = 0.001 lux.
How it works
The converter multiplies the input by the source unit's lux-equivalent factor to reach a common base in lux (lumens per square metre), then divides by each target unit's factor to generate all outputs at once. Lux is the SI unit of illuminance and serves as the natural hub for all conversions in this domain.
Common mistake: Illuminance (lux) is often confused with luminance (candela per square metre, or nit). Illuminance measures how much light falls on a surface; luminance measures how much light leaves or reflects from it — they are fundamentally different quantities and cannot be directly converted into each other.
Worked example
Convert 1 foot-candle to lux, phots, and nox
- Start with input: 1 foot-candle.
- Convert to base: 1 × 10.76391 = 10.76391 lux.
- Divide by target factors: lux = 10.76391 / 1 = 10.7639; foot-candle = 10.76391 / 10.76391 = 1; phot = 10.76391 / 10000 = 0.001076; nox = 10.76391 / 0.001 = 10 763.91.
1 foot-candle = 10.7639 lux, 1 fc, 0.001076 phot, 10 763.91 nox.
Common mistakes to avoid
- Confusing illuminance (lux, how much light falls on a surface) with luminous intensity (candela, how bright a source is) or luminous flux (lumens, total light emitted) -- they measure related but distinct photometric quantities.
- Using foot-candles in a design spec intended for metric -- 1 fc = 10.764 lux; a workspace requirement of 50 fc is about 538 lux, not 50 lux.
- Ignoring distance when estimating lux -- illuminance follows the inverse-square law, so doubling the distance from a light source reduces lux to one quarter.
Key terms
- Lux (lx)
- The SI unit of illuminance, equal to one lumen per square metre. It quantifies how much visible light actually reaches a surface.
- Foot-candle (fc)
- An imperial unit of illuminance equal to one lumen per square foot, or approximately 10.764 lux. Still common in North American lighting and film/photography industries.
- Phot (ph)
- A CGS unit of illuminance equal to 10 000 lux. Rarely used in modern practice but appears in older photometric literature.
- Nox
- A unit of illuminance equal to 0.001 lux (1 millilux), historically used to measure very low light levels such as moonlight or twilight conditions.
Frequently asked questions
- What is the difference between lux and lumens?
- Lumens measure the total light output of a source (luminous flux). Lux measures how much of that light lands on a given surface area: 1 lux = 1 lumen per square metre. A single lumen spread over 1 m2 gives 1 lux; spread over 10 m2 it gives 0.1 lux.
- What lux level is recommended for office lighting?
- Standard recommendations (ISO 8995/EN 12464) suggest 500 lux for general office work and reading, 300 lux for corridors, and 1,000 lux or more for fine assembly or medical examination areas.
- What is a phot and when is it used?
- A phot (ph) is a CGS unit equal to 10,000 lux. It appears in older European photometric literature and some astronomical contexts, but is rarely used in modern engineering where lux is the standard.