Foot-Lambert calculator

Calculate screen luminance in foot-lamberts (ftL) from projector lumens and screen size, or convert between luminance units like nits, candela/m², and exposure values.

Input

is a unit of luminance (the intensity of light emitted from a surface). It is the luminance of a perfectly diffusing surface emitting or reflecting 1 lumen per square foot. This unit is used in the US and Canada, and worldwide in the cinema business.
     To calculate from screen dimensions, click here

Luminance conversion

Nit
2.5 cd/m2
Candela per square meter
2.5 cd/m2
Candela per square foot
0.2323 cd/sq.ft.
Stilb (CGS)
0.00025 Stilb
Apostilb (blondel)
0.8 Apostilb
Lambert
8.0E-5 Lambert
Foot-Lambert
0.7297 ftL
Exposure Value
4.32 EV

Screen luminance: EV4.32

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starlight to dim ambient lightmoon eclipse to night home interiorneon lights to landscape after sunsetbright light to weak sunbright daylight to direct sunlight

Explanation

A foot-lambert or footlambert (ftL, sometimes fl or ft-L) is a unit of luminance. For screens that are projected upon (like cinema, home cinema), it quantifies how much light is reflected (to the center of the room). This depends on the amount of light produced (lumens), the size of the screen and the screen gain.

Examples

Cinema: 32000 lumen on 23x10m screen

References

Foot-lambert
Lambert
Nit
Screen gain
Stilb

Frequently Asked Questions

What is a foot-lambert?

A foot-lambert (ftL) is a unit of luminance measuring light intensity emitted from a surface. It equals 1 lumen per square foot from a perfectly diffusing surface. It's widely used in cinema projection.

What is the ideal foot-lambert for cinema?

Cinema screens typically aim for 14-16 foot-lamberts for 2D content and 3.5-6 ftL for 3D content. Home theaters often target 12-22 ftL depending on room lighting conditions.

What is screen gain?

Screen gain measures how much light a screen reflects compared to a reference white surface. Gain > 1 means brighter center viewing but narrower viewing angle. Gain < 1 (gray screens) improves contrast in rooms with ambient light.

How do nits relate to foot-lamberts?

One foot-lambert equals approximately 3.426 candelas per square meter (nits). Nits are the SI unit preferred internationally, while foot-lamberts remain common in US/Canadian cinema.