Illumination Converter
Convert illuminance (the luminous flux per unit area) between lux, foot-candles, phot, nox and other SI / imperial units.
Written by Rahman · Last reviewed
Quick answer
Illuminance is light falling on a surface, measured in lux, one lumen per square metre. One foot-candle is 10.764 lux and one phot is 10,000. For reference, full daylight is around 10,000 lux, an overcast day about 1,000, and typical office lighting 300 to 500.
| Lux (lx) | 1 |
| Lumen per square metre (lm/m²) | 1 |
| Lumen per square centimetre (lm/cm²) | 0.0001 |
| Foot-candle (fc) | 0.09290304 |
| Phot (ph) | 0.0001 |
| Nox (nx) | 1000 |
About the Illumination Converter
Illumination, or illuminance, measures the amount of light falling on a surface. The SI unit is the lux; the foot-candle is common in the United States.
It is used in lighting design for offices, homes, streets and photography.
Lumens and lux answer different questions
A lamp is sold in lumens; a room is specified in lux. They are not alternative units for the same thing. Lumens measure the total light a source emits in every direction: a property of the bulb. Lux measures how much of that light lands on a surface: a property of the bulb, the distance and the geometry together.
One lux is one lumen spread over one square metre. The same 800-lumen bulb produces high lux on a desk directly beneath it and very little on a far wall, because the light is spread over a much larger area. This is why a lighting specification that names only lumens does not tell you whether a space will be adequately lit.
- Illuminance falls off with the square of distance: doubling the distance from a point source quarters the lux.
- Foot-candles are the same idea in imperial units: one lumen per square foot. Because a square metre is 10.7639 square feet, one foot-candle is 10.7639 lux.
- A 500 lux office target is about 46.5 foot-candles.
What illuminance does not tell you
Illuminance describes how much light arrives, and nothing about its quality. Two rooms at identical lux can look completely different depending on the colour temperature of the source, its colour rendering, and whether the light arrives as harsh glare or diffuse fill.
Luminance is the related measure that is frequently confused with it: illuminance is light falling on a surface, luminance is light leaving one toward the eye. A screen's brightness in nits is luminance; the reading on a light meter pointed at a desk is illuminance.
- Colour temperature in kelvin describes the appearance of the light, not its quantity, and is independent of the lux figure.
- Colour rendering index describes how faithfully colours appear under the source, and is likewise independent.
- Photographers meter incident light in lux or foot-candles and reflected light as luminance; the two produce different exposures for the same scene.
Frequently Asked Questions
What is lux?
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Lux is the SI unit of illuminance: one lumen of light spread over one square metre.
How many lux are in a foot-candle?
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One foot-candle equals about 10.764 lux.
How much light is recommended for an office?
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General office work typically calls for around 300 to 500 lux on the working surface.