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 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 lumen per square metre to lux is useful in the illuminance domain when comparing values across different measurement standards or applying formulas that require a specific unit.
Worked examples
1 lumen per square metre = 1 lux
1 lux = 1 lux
How to convert lumen per square metre to lux
To convert lumen per square metre to lux, multiply the value by 1.
To convert lux back to lumen per square metre, multiply by 1.
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 lux
| lumen per square metre | lux |
|---|---|
| 0.1 | 0.1 |
| 0.5 | 0.5 |
| 1 | 1 |
| 2 | 2 |
| 5 | 5 |
| 10 | 10 |
| 25 | 25 |
| 50 | 50 |
| 100 | 100 |
| 250 | 250 |
| 500 | 500 |
| 1,000 | 1,000 |