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).
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.
When is this conversion used?
Converting between lumen per square metre and foot-candle 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 lumen per square metre = 0.09290304 foot-candle
1 foot-candle = 10.76391042 lux
How to convert lumen per square metre to foot-candle
To convert lumen per square metre to foot-candle, multiply the value by 0.09290304.
To convert foot-candle back to lumen per square metre, multiply by 10.76391042.
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: lumen per square metre to foot-candle
| lumen per square metre | foot-candle |
|---|---|
| 0.1 | 0.0092903 |
| 0.5 | 0.04645152 |
| 1 | 0.09290304 |
| 2 | 0.18580608 |
| 5 | 0.4645152 |
| 10 | 0.9290304 |
| 25 | 2.322576 |
| 50 | 4.645152 |
| 100 | 9.290304 |
| 250 | 23.22576 |
| 500 | 46.45152 |
| 1,000 | 92.90304 |