Foot-Lambert
Calculate screen luminance for cinema projection
How to use Foot-Lambert
- 1Enter the brightness (fL or nits) measured.
- 2Select the direction of the conversion.
- 3Reads the equivalent under another unit.
foot-lambert and nits
1 fL = 3.426 nits（cd/m²）；1 nit = 0.2919 fLFoot-lambert is a unit of beauty for cinema and projection; Nits (Kendra/m2) is an international unit.
The typical dark cinema screens are about 14-16 fL, and bright living room televisions can exceed 500-1000 nits.
Foot-lambert (fL, foot Rambo) is the unit for the projection industry to measure ** the reflection brightness of the screen**: 1 fL = 1/ π Candra ≈ 3.426 nits (cd/m2) per square foot. The reason for using "reflection brightness" rather than projector luminaries is that ** The light that people see reflected from the screen depends on the projector brightness, the benefit of the screen and the size of the screen **. Formula: fL = projector flow x screen gain ÷ screen area (square feet).
For example: 3,000 luminous projector, shot at 120 inches 16:9 of the gain 1.0 (approx. 37.5 square feet, i.e. 121.8 cm x 68.5 cm converted), brightness = 3000 x 1.0 ÷ 37.5 = 80 fL - etc.), this result is high because it is actually multiplied by the coefficients associated with the thorium and takes into account the loss of light (light bulb decline, camera percelerity approximately 0.8). **Practical experience formula: fL ≈ gain x 0.85 ÷ area (square feet)**, the previous example of about 68 fL, still well above the cinema standard, indicates that family environment 1500-2000 fluency is usually sufficient; the luminous living room of the environment requires over 3000 fluency.
| scene | Centre Brightness | Edge evenness | Standard |
|---|---|---|---|
| Commercial cinema 2D | 14 fL | 75% centre | SMPTE 196M |
| Commercial cinema 3D | 4.5 – 7 fL | ≥ 75% | (3D loss of glasses) |
| Family cinema (shadow room) | 12 – 16 fL | — | Recommended value |
| Family cinema (environment light) | 20 – 30 fL | — | More. |
| HDR Peak | 100 – 1000+ nits | — | Different from SDR standard |
| SDR parent belt standard | 100 nits | — | BT.709 |
Standards for cinema and family projectivity
Frequently asked questions
Why do cinemas use fl?
It is the traditional unit of the United States market's projection screen.
14 fl?
About 48 Nits (14 x 3.426), a classic SMPTE cinema target.
Could HDR's nits match fL?
;HDR peak nits measure high light, not stable screen fL.
Is it like ANSI?
It's different. ** ANSI Fluorescent** is a good-spectrum (takes an average of 9 points) and is the most authoritative;** ISO Fluorescent** is a dailies (most of the daily plants are commercial) and the values are approximately the same as ANSI; and many low-priced projectors are called "light-sourced Fluorescent" "LED Fluorescent" (LED) is a theoretical value that has not been standardizedly measured and may be 2-4 times that of ANSI Fluent,** with no reference value at all**. How to judge: The 1,000-dollar LED projector says "5,000 Fluorescent" is almost certainly a false sign, and real ANSI is probably 300-800. Pre-purchase physical data from professional assessments.
Why are the projections dark in the daytime?
Because the projection is ** reflecting **, the environment will be diluted. Contrast = image brightness ÷ environmental light reflection, around 300-500 lux during the day, projection brightness may be only 50-100 lux, contrasting to extremely low, ash. Solutions: 1 luminous (high curtains with the most direct effect); 2 Increased projector brightness (marginal gains are decreasing and doubling can only raise one view); 3 ALR curtains, absorbing environmental light through reflections of special optical structures, with significant but high prices; 4 Acceptance of reality - projection is inherently suitable for the dark room and the bright environment should buy television.