What is lux?
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.
Real-world uses
Lux is the SI unit of illuminance used in architectural lighting design, workplace safety regulations, and photography. Office lighting standards typically require 300–500 lux, hospital operating rooms need about 10,000 lux, and direct sunlight provides roughly 100,000 lux.
History
The lux was adopted as the SI unit of illuminance, derived from the Latin word "lux" meaning light. It replaced older units like the foot-candle and phot. One lux equals one lumen per square metre, standardized with the SI system in 1960.
Common mistakes
Confusing lux (illuminance on a surface) with lumens (total light output from a source). A 800-lumen bulb produces different lux readings depending on the distance and angle. Also, lux does not account for light colour or quality.
What is phot?
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.
Real-world uses
The phot is a CGS unit of illuminance rarely used in modern practice. It occasionally appears in older scientific literature, particularly in French and German optics texts. One phot (10,000 lux) approximates the illuminance of a heavily overcast day or well-lit interior.
History
The phot was part of the centimetre-gram-second (CGS) system, defined as one lumen per square centimetre. It derives from the Greek "phos" meaning light. The phot fell out of common use when the SI system replaced CGS, with the lux becoming the standard illuminance unit.
Common mistakes
Using phots in modern work where lux is expected. Since 1 phot = 10,000 lux, the numerical values are very different from lux readings. Also, confusing the phot with the photon, which is a quantum of light, not a unit of illuminance.
When is this conversion used?
Converting lux to phot is useful in the illuminance domain when comparing values across different measurement standards or applying formulas that require a specific unit.
Worked examples
1 lux = 0.0001 phot
1 phot = 10,000 lux
How to convert lux to phot
To convert lux to phot, multiply the value by 0.0001.
To convert phot back to lux, multiply by 10,000.
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.
Quick reference: lux to phot
| lux | phot |
|---|---|
| 0.1 | 0.00001 |
| 0.5 | 0.00005 |
| 1 | 0.0001 |
| 2 | 0.0002 |
| 5 | 0.0005 |
| 10 | 0.001 |
| 25 | 0.0025 |
| 50 | 0.005 |
| 100 | 0.01 |
| 250 | 0.025 |
| 500 | 0.05 |
| 1,000 | 0.1 |