Available Illuminance units
- lux conversions
Lux is the SI unit of illuminance, measuring luminous flux received per unit area. It is the standard unit for evaluating lighting conditions in workplaces, photography studios, building codes, and health and safety standards.
- lumen per square metre conversions
Lumen per square metre is a unit of illuminance numerically equivalent to lux. It expresses the amount of light falling on a surface and is used in lighting engineering and photometry to quantify light intensity at a specific location.
- foot-candle conversions
A foot-candle is an imperial illuminance unit equal to one lumen per square foot. It is used in the United States for stage and event lighting specifications, photographic exposure references, and building lighting compliance standards.
- phot conversions
A phot is a CGS system illuminance unit equal to 10,000 lux. It is now largely obsolete but may be encountered in older scientific literature and historical photometric measurements.
Full Illuminance conversion matrix
| Unit | Equivalent in lux | lx | lm/m² | fc | ph |
|---|---|---|---|---|---|
| lux | 1 lx | 1 | 1 | 0.09290304 | 0.0001 |
| lumen per square metre | 1 lx | 1 | 1 | 0.09290304 | 0.0001 |
| foot-candle | 10.76391042 lx | 10.76391042 | 10.76391042 | 1 | 0.00107639 |
| phot | 10,000 lx | 10,000 | 10,000 | 929.0304 | 1 |
About Illuminance
Illuminance and luminous emittance units
Real-world uses
Illuminance measurements guide lighting design in offices, hospitals, schools, and public spaces. Building codes specify minimum lux levels for workplaces — typically 300–500 lux for office work, 500–1,000 lux for precision tasks, and 10,000+ lux for surgical fields. Photographers and cinematographers use lux or foot-candles to calibrate artificial lighting. Horticulturalists measure PPFD (photosynthetic photon flux density) for crop lighting, which is closely related to illuminance.
History
The foot-candle was one of the earliest standardised photometric units, defined in the 19th century as the illumination produced by one candle at one foot. The lux (lx), defined as one lumen per square metre, was formalised with the International System of Units. The candela was one of the original SI base units established in 1960, enabling coherent derivation of lumens and lux.
Common mistakes
Illuminance (lux) and luminance (cd/m²) are often confused: lux measures light falling on a surface, while luminance measures light emitted or reflected from a surface, which the eye actually perceives. People also conflate lumens (total light output) with lux (light intensity at a point). Converting foot-candles to lux by a factor of 10 (rather than the correct 10.764) is a common approximation error.
Measurement standards
The lux is the SI derived unit of illuminance, defined as one lumen per square metre (lm/m²). The CIE (International Commission on Illumination) provides standard illuminance recommendations for various visual tasks, which most national standards bodies adopt directly.
Did you know?
Direct sunlight at noon on a clear day delivers roughly 100,000 lux, while a dimly lit room might have only 50 lux. The human eye can function across a range of over 10 billion to one from starlight to direct sun — one of the widest dynamic ranges of any biological sensor.
Common values
| lux | lumens per square metre | |
|---|---|---|
| Full moon, clear night | 0.1 lux | 0.1 lumens per square metre |
| Living room lighting | 100 lux | 100 lumens per square metre |
| Office lighting | 500 lux | 500 lumens per square metre |
| Overcast daylight | 1,000 lux | 1,000 lumens per square metre |
| Direct sunlight | 100,000 lux | 100,000 lumens per square metre |