Foot-Lambert Foot-lambert (fL, ftL)

What a foot-lambert is

A foot-lambert is a unit of luminance: how bright a surface looks, not how much light a source emits. One foot-lambert equals 1/π candela per square foot, which is 0.3183 cd/ft² or 3.426 candela per square meter (the SI unit, also called a nit). The unit is named after Johann Heinrich Lambert, an 18th-century Swiss-German physicist, and it survives mainly in the motion picture industry, where screen luminance has been specified in fL for decades. The same unit tracks a perfect diffuse reflector: a matte surface lit by 1 foot-candle of light shows 1 fL, which is why projector math in feet and lumens lands directly on it.

Converting is one multiplication. 14 fL is 48 nits, 16 fL is 54.8 nits, 31 fL is 106 nits, and a 100-nit SDR television is about 29 fL. If a spec sheet says nits, divide by 3.426 to get foot-lamberts; if it says foot-lamberts, multiply.

How to calculate foot-lamberts from lumens

For a projector, screen luminance is the lumens landing on the screen divided by the screen area in square feet, multiplied by the screen gain: fL = (lumens ÷ ft²) × gain. A matte white screen is gain 1.0. ProjectorScreen.com works the example of a 2,000-lumen projector on a 16 ft by 9 ft screen (144 ft²): 2,000 ÷ 144 gives 13.9 fL, just under the cinema target. Shrinking the image to 13 ft by 7.3 ft (95 ft²) raises it to 21 fL, and adding a 1.3-gain screen to the larger image gives 18.07 fL.

Run it backward to size a screen. A JVC DLA-NZ8 is rated at 2,500 lumens. At 16 fL on a gain 1.0 screen, 2,500 ÷ 16 is 156 ft², which is a 16:9 screen about 16.6 ft wide, roughly 229 inches diagonal. The rating is a peak-mode number, so a calibrated picture mode delivers less and the real screen size that hits target is smaller. For the lumens side of the equation see ANSI lumens and screen gain, and use the projector lumens tool to run the numbers for your room.

The cinema standards: SMPTE 196M, ST 431-1 and DCI

SMPTE 196M recommended 16 fL for film projection measured open gate, meaning with no film in the gate. Film base density of about 0.05 drops the on-screen peak white to roughly 14 fL, so the working target for a running film print became 14 fL. The current revision of 196M is written in SI and specifies 55 cd/m².

Digital cinema locked in the same number. SMPTE ST 431-1 sets the nominal screen luminance at 48.0 cd/m² (14.0 fL), measured at the center of the screen with 100 percent white. The allowed tolerance is plus or minus 3.5 cd/m² (1.00 fL) for a reference-class room and plus or minus 10.2 cd/m² (3.00 fL) for a theater, so a compliant theater can sit anywhere from 11 to 17 fL. Side luminance must be 85 percent of center in a reference room and 75 to 90 percent in a theater, and white chromaticity is fixed at x = .314, y = .351. Acoustic Frontiers summarizes the same target as 14 fL plus or minus 3 for the DCI specification, and the standard itself warns that screen luminance falls with lamp age and dirt on the optics or screen.

Foot-lamberts in premium large format

The 14 fL baseline is the floor that premium formats are measured against. Infinity Vision, the Disney certification announced April 16, 2026, requires a screen at least 45 feet wide, brightness reaching 14 fL in 2D or 6 fL in 3D, and 7.1 or immersive audio. The 3D figure of 6 fL is 20.6 nits, less than half the 2D requirement.

Dolby Cinema goes well past the baseline. A Christie product-management director said at the system's launch that the Dolby Vision laser projection would deliver up to 14 fL onscreen in 3D and up to 31 fL for 2D Dolby Vision content, which is about 106 nits and more than double the 2D cinema norm. See Dolby Cinema for the full format. I could not find a published IMAX screen-luminance figure on any page I loaded, so the IMAX brightness story stays unquantified here; the comparison page Infinity Vision vs IMAX vs Dolby Cinema covers what each format publishes.

The light budget at that scale is large. A 45-foot screen at roughly 1.9:1 is about 24 feet tall, or 1,080 ft². Holding 14 fL on a gain 1.0 surface takes about 15,100 screen lumens. Christie lists its RGB pure laser cinema projectors at 15,750 to 50,000 lumens, so a laser engine can supply that budget. The premium large format entry covers the category.

Foot-lamberts vs nits in a living room

Home theater rooms need a different target than a cinema because the screen competes with ambient light. In a light-controlled room the common practice is the cinema number: ProjectorScreen.com rates 16 to 26 fL as good for a dark home theater and anything under 16 fL in a completely dark room as not recommended. Its table steps up to 27 to 39 fL for low ambient light, 40 to 59 fL for medium ambient light such as a living room, and 60 fL and above for high ambient light. Digital Projection recommends 40 fL for some ambient light and 60 fL for high ambient light, based on holding a contrast ratio of at least 1,000:1 for movies.

Televisions live far above those figures. The ITU's BT.2408 puts HDR reference white at 203 cd/m², about 59 fL, against the 100 cd/m² nominal SDR peak white that equals 29 fL. A 1,000-nit HDR highlight is 292 fL. That gap is why a projector at 16 fL is judged in a dark room while a TV is judged in daylight; see nits (cd/m²) and HDR brightness windows for how TV peaks are measured.

Common confusions

Lumens are not foot-lamberts. Lumens measure light output; foot-lamberts measure what the screen returns per unit area, so the same projector gives 16 fL on a 100-inch screen and 7 fL on a 150-inch screen. Higher screen gain raises fL at the cost of a narrower viewing cone. And more than 16 fL in a dark room is not a bonus: the SMPTE and DCI targets exist so the picture matches what the colorist graded, and ProjectorScreen.com rates 27 fL and above as suited to rooms with ambient light, not dark ones.