Foot Lambert Calculator: Brightness Made Simple

A foot lambert (fL) is a unit of luminance that tells you how much light a projector screen actually reflects toward your eyes. This calculator converts between foot lamberts and the two other units you’re most likely to see on spec sheets — nits (candelas per square meter) and lux — so you can compare projectors, TVs, and screens without doing the math by hand.

Formula

The core conversion is simple: 1 foot lambert = 3.426 nits (exactly 1/0.2919). To convert from foot lamberts to nits, multiply by 3.426. To convert from foot lamberts to lux (illuminance on the screen), multiply by 10.7639 (since 1 foot-candle = 10.7639 lux, and 1 fL = 1 foot-candle × screen reflectance).

• fL – foot lamberts, the luminance unit used mainly in North America for projection.
• nits – candelas per square meter (cd/m²), the standard SI unit for display brightness.
• lux – lumens per square meter, a measure of illuminance (light falling on a surface), not luminance (light leaving it).

Worked Example

Using the default 16 fL (a common target for a dark home theater):

1. Convert to nits: 16 × 3.426 = 54.8 nits.
2. Convert to lux (assuming a perfect 100% reflective screen): 16 × 10.7639 = 172 lux.

That 54.8 nits is surprisingly low compared to a typical TV, which often exceeds 300 nits. But in a dark room, 16 fL is the SMPTE (Society of Motion Picture and Television Engineers) recommended reference for theatrical projection.

Why Foot Lamberts Matter

Foot lamberts are the standard way to measure how bright a projected image appears to the viewer. Unlike projector lumens (which measure raw light output), foot lamberts account for screen size, screen gain, and distance — they tell you what your eyes actually perceive. A projector with 3,000 lumens can look dim on a 150-inch screen but bright on a 100-inch screen; foot lamberts quantify that difference.

For reference, here are typical target values:

• Dark home theater (lights off): 16–20 fL
• Living room with some ambient light: 30–40 fL
• Bright office or classroom: 50+ fL
• Standard TV (self-lit): 100–300 fL (300–1000 nits)

How to Measure or Estimate Foot Lamberts

You don’t need a dedicated luminance meter for a rough estimate. If you know the projector’s lumens, screen gain, and screen area, use this formula:

fL = (Projector Lumens × Screen Gain) / (Screen Area in square feet)

For example, a 2,500-lumen projector on a 1.0-gain screen that is 8 feet wide and 4.5 feet tall (36 sq ft) gives:

2500 × 1.0 / 36 = 69 fL — plenty bright for a living room.

If you don’t know screen gain, assume 1.0 for a standard matte white screen. If you’re using a gray screen (gain 0.8) or a high-gain screen (gain 1.3), adjust accordingly.

Common Mistakes and Misconceptions

The biggest mistake is confusing lumens with foot lamberts. Lumens measure light output from the projector; foot lamberts measure light reflected from the screen. A projector with more lumens is not always brighter in practice if the screen is huge or the gain is low.

Another common error is using the wrong conversion factor. Some sources say 1 fL = 3.426 nits, others say 3.43 — the difference is negligible. But if you accidentally use 10.7639 (the lux conversion) for nits, you’ll be off by a factor of ~3.14.

Also, remember that lux is a measure of incident light, not reflected light. If you measure 200 lux falling on a screen with 50% reflectance, the luminance is only 100 lux-equivalent (or about 9.3 fL).

When to Use This Calculator

Use this calculator whenever you need to compare a projector spec (often given in nits or lux) to the fL targets used in home theater forums. It’s also handy if you’re calibrating a projector with a meter that reads in nits, or if you’re trying to match a TV’s brightness to a projector screen.

For example, if a projector review says it hits 100 nits on a 120-inch screen, you can convert to fL: 100 / 3.426 = 29 fL — which is bright enough for a dark room with some ambient light.

Limits of the Foot Lambert Unit

Foot lamberts are a photometric unit, meaning they are weighted to the human eye’s sensitivity. They do not account for color accuracy, contrast ratio, or screen uniformity. A screen can have a high fL reading but poor contrast if ambient light washes out the image.

Also, the fL value is an average across the screen. Most projectors are brighter in the center and dimmer at the edges. A 16 fL reading at the center might drop to 12 fL at the corners. For critical viewing, measure at multiple points.

Frequently Asked Questions

What’s the difference between foot lamberts and nits?

They measure the same thing — luminance — but use different scales. One foot lambert equals 3.426 nits. Nits are the metric standard used by TV manufacturers; foot lamberts are traditional in cinema and projection in the US.

How many foot lamberts do I need for a home theater?

For a dark, dedicated home theater, 16 fL is the SMPTE reference. Many enthusiasts prefer 20–30 fL for HDR content. If you have ambient light, aim for 30–40 fL.

Is a brighter screen always better?

No. Too much brightness can cause eye strain and wash out blacks, especially in a dark room. Projectors are often calibrated to a specific fL target for the room’s lighting. Brighter is only better if the room has significant ambient light.

How do I convert fL to lux?

Multiply fL by 10.7639 to get lux, but only if the screen is 100% reflective. Real screens reflect less, so the actual lux falling on the screen is higher than the fL value. For a 1.0 gain screen, lux = fL × 10.7639.

Can I measure fL with a smartphone app?

Some light meter apps can estimate lux, but they are not accurate for luminance. You can use the lux reading with a known screen gain to estimate fL, but the result will be rough. For critical calibration, use a dedicated colorimeter or lux meter.

Why do projectors list lumens instead of fL?

Lumens are a measure of the projector’s light output, independent of screen size. It’s a standardized spec that lets you compare projectors. Foot lamberts depend on your specific setup, so manufacturers can’t list them without knowing your screen.

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in fL | Foot lamberts | fL | 16 | 0
out nits | Nits (cd/m²) | cd/m² | fL * 3.426 | 1
out lux | Lux (lm/m²) | lux | fL * 10.7639 | 0