What is foot-candle?
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.
Real-world uses
Foot-candles are used in US lighting design, building codes, and OSHA workplace safety standards. The Illuminating Engineering Society (IES) recommends 30–50 fc for office work, 5–10 fc for corridors, and 75–100 fc for detailed tasks. US photography light meters often display in fc.
History
The foot-candle was defined as the illumination on a surface one foot away from a standard candle. It has been used in American lighting engineering since the 19th century. While most countries use lux, the foot-candle remains the standard in US building codes and IES guidelines.
Common mistakes
Assuming 1 foot-candle equals 1 lux. Actually, 1 fc ≈ 10.764 lux, so a 50 fc requirement equals about 538 lux. Also, foot-candles should not be confused with candlepower or candela, which measure luminous intensity, not illuminance.
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 between foot-candle and phot is common when working across metric and imperial systems, such as international trade, travel between countries with different measurement standards, or following instructions from a different region.
Worked examples
1 foot-candle = 0.00107639 phot
1 phot = 10,000 lux
How to convert foot-candle to phot
To convert foot-candle to phot, multiply the value by 0.00107639.
To convert phot back to foot-candle, multiply by 929.0304.
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: foot-candle to phot
| foot-candle | phot |
|---|---|
| 0.1 | 0.00010764 |
| 0.5 | 0.0005382 |
| 1 | 0.00107639 |
| 2 | 0.00215278 |
| 5 | 0.00538196 |
| 10 | 0.01076391 |
| 25 | 0.02690978 |
| 50 | 0.05381955 |
| 100 | 0.1076391 |
| 250 | 0.26909776 |
| 500 | 0.53819552 |
| 1,000 | 1.07639104 |