phot (ph)
1 phot = 10,000 lux
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.
Notes: CGS illuminance unit
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 phot to lux is useful in the illuminance domain when comparing values across different measurement standards or applying formulas that require a specific unit.
How to convert phot to lux
To convert phot to lux, multiply the value by 10,000.
Conversion formula (from base unit)
x * 10000.0
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.
| phots | lux |
|---|---|
| 0.1 | 1,000 |
| 0.5 | 5,000 |
| 1 | 10,000 |
| 2 | 20,000 |
| 5 | 50,000 |
| 10 | 100,000 |
| 25 | 250,000 |
| 50 | 500,000 |
| 100 | 1,000,000 |
| 250 | 2,500,000 |
| 500 | 5,000,000 |
| 1,000 | 10,000,000 |
Common values
| phots | lux | |
|---|---|---|
| Full moon, clear night | 0.00001 phots | 0.1 lux |
| Living room lighting | 0.01 phots | 100 lux |
| Office lighting | 0.05 phots | 500 lux |
| Overcast daylight | 0.1 phots | 1,000 lux |
| Direct sunlight | 10 phots | 100,000 lux |
Full phot conversion table
| Unit | Value | Factor |
|---|---|---|
| lux | 1 ph = 10,000 lx | × 10,000 |
| lumen per square metre | 1 ph = 10,000 lm/m² | × 10,000 |
| foot-candle | 1 ph = 929.0304 fc | × 929.0304 |