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.
What is lumen per square metre?
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.
Real-world uses
Lumen per square metre is the explicit form of the lux and is used interchangeably in lighting engineering specifications. It appears in technical datasheets where the relationship between luminous flux (lumens) and illuminated area needs to be made explicit.
History
Lumen per square metre is simply the expanded definition of the lux. The lumen itself was standardized in the early 20th century based on the candela (unit of luminous intensity). The notation lm/m² is used when dimensional clarity is needed in calculations.
Common mistakes
Treating lm/m² as a different unit from lux—they are identical (1 lm/m² = 1 lux exactly). Also, confusing luminous flux per unit area (lm/m², an illuminance) with luminance (cd/m², brightness perceived from a surface).
When is this conversion used?
Converting phot to lumen per square metre is useful in the illuminance domain when comparing values across different measurement standards or applying formulas that require a specific unit.
Worked examples
1 phot = 10,000 lumen per square metre
1 lumen per square metre = 1 lux
How to convert phot to lumen per square metre
To convert phot to lumen per square metre, multiply the value by 10,000.
To convert lumen per square metre back to phot, multiply by 0.0001.
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: phot to lumen per square metre
| phot | lumen per square metre |
|---|---|
| 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 |