A foot-lambert (abbreviated fL) is a non-SI unit of luminance , or the brightness of a surface that emits or reflects light. It is primarily used in the film, television, and cinema industries to measure the brightness of projection screens and displays .
The lambert is an obsolete unit of luminance, named after the 18th-century German physicist Johann Heinrich Lambert. It symbolizes the brightness of a perfectly diffusing surface that emits or reflects one lumen per square centimeter. In the International System of Units (SI), luminance is measured in candelas per square meter (cd/m²), also known as nits . Although the lambert is no longer officially used, the foot-lambert (fL), equal to 1/π candela per square foot, is still commonly used in the film and television industries to specify the brightness of projection screens and studio lighting.
1. How is a foot lambert defined?
A foot lambert is defined as:
whereby:
- arises from the definition of luminance for a perfectly diffuse surface.
- (candela per square meter) is the standard SI unit for luminance.
- stands for square foot, which is used in the English measurement system.
📌 So 1 fL is about 3.43 times less bright than 1 candela per square meter (nit).
2. Applications of the foot lambert
🎬 Cinemas and projection screens → Foot lamps are used to measure the brightness of film projections.
📺 Television & Studio Production → Helps calibrate studio monitors and screens.
🖥 Image quality in displays → Sometimes used in HDR (High Dynamic Range) measurements.
📌 In cinemas, the recommended brightness for a 2D projection is around 16 fL, while 3D projections are lower (4-6 fL) due to the use of 3D glasses that absorb light.
3. Voetlambert vs candela per square meter (nit)
| Unit | Definition | Usage |
|---|---|---|
| Voetlambert (fL) | Non-SI unit, used in the motion picture industry | Projection brightness in cinemas |
| Candela per square meter (cd/m² or nit) | SI unit for luminance | Standard for screens and lighting |
📌 Foot lamps are mainly used in the US, while cd/m² is the global standard for luminance.
💡 In short:
A foot-lambert (fL) is a unit of luminance used primarily in the film and cinema industry to measure the brightness of projection screens. 1 fL ≈ 3.426 cd/m² , and the recommended brightness for cinemas is 16 fL for 2D films .