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 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.
When is this conversion used?
Converting between foot-candle and lux 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 = 10.76391042 lux
1 lux = 1 lux
How to convert foot-candle to lux
To convert foot-candle to lux, multiply the value by 10.76391042.
To convert lux back to foot-candle, multiply by 0.09290304.
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 lux
| foot-candle | lux |
|---|---|
| 0.1 | 1.07639104 |
| 0.5 | 5.38195521 |
| 1 | 10.76391042 |
| 2 | 21.52782083 |
| 5 | 53.81955208 |
| 10 | 107.63910417 |
| 25 | 269.09776042 |
| 50 | 538.19552084 |
| 100 | 1,076.39104167 |
| 250 | 2,690.97760418 |
| 500 | 5,381.95520835 |
| 1,000 | 10,763.9104167 |