Projector Brightness Calculator
Convert projector lumens into screen luminance in foot-lamberts and nits (cd/m²) for any screen size and gain, or work backwards from a target such as the DCI 14 fL to the lumens you need. Includes optical losses and the effect of ambient light on contrast.
Screen
Projector & Light Path
Losses are optional; leave them at 0 (or 100 % for output and 3D transmission) when you don't know them. Use measured or manufacturer figures where you can.
fL = lumens × gain ÷ area (ft²) · cd/m² = fL × 3.426 = lumens × gain ÷ (π × area m²)
Reverse: Target Luminance → Lumens Needed
Uses the screen, gain and light-path losses above.
Ambient Light & Contrast
Light from the room that lands on the screen is reflected along with the image and lifts the blacks. Measure the illuminance with a lux meter at the screen, facing the audience, with the projector off (or showing black).
Contrast = (image + ambient) ÷ ambient, where reflected ambient (cd/m²) ≈ lux × gain ÷ π. The projector's own black level is ignored, so the real figure is lower. Treating ambient light as on-axis is conservative for high-gain and ambient-light-rejecting screens, which reflect less off-axis light back to the audience. AVIXA's categories are defined with a specific checkerboard measurement method; this is an estimate.
Reference Luminance Targets
| Application | cd/m² | fL | Tolerance / notes |
|---|---|---|---|
| Digital cinema 2D (DCI, SMPTE ST 431-1, RP 431-2) | 48 | 14 | Centre of screen. Theatre ±10.2 cd/m² (±3 fL); review room ±3.5 cd/m² (±1 fL). Sides 75–90 % of centre in theatres (80–90 % in review rooms). |
| High-brightness 3D (DCI recommendation, 2015) | 48 | 14 | ±6.9 cd/m² (±2 fL), measured at screen centre through all filters and glasses. Aimed at RGB laser and multi-projector 3D. |
| Legacy 3D (common practice) | ≈ 12–19 | ≈ 3.5–5.5 | Commonly reported practice, not a standard. Many single-projector polarised or active 3D systems delivered only a few fL through the glasses. |
| 35 mm film (SMPTE 196M) | 55 | 16 | Open gate (no film), ±2 fL (±7 cd/m²). |
| Dolby Cinema 2D (informational) | ≈ 106–108 | ≈ 31 | Manufacturer-quoted peak white for Dolby Vision content; 3D is quoted at up to 14 fL. |
| DCI HDR (informational) | 300 | ≈ 88 | Peak white in the DCI HDR addendum, aimed at direct-view (LED) cinema displays. |
Notes & References
- Units. One foot-lambert is 1/π candela per square foot, so 1 fL = 3.4263 cd/m² (and 1 cd/m² = 0.2919 fL). One foot-candle is 1 lm/ft² = 10.764 lx. For a perfect diffuser (gain 1.0) the luminance in fL equals the illuminance in foot-candles.
- Lumens. Use the white light output measured to ISO 21118 (or the older ANSI IT7.228 method). Single-chip DLP and other colour-sequential projectors can have a lower colour brightness (colour light output, measured to IDMS 15.4) than their white rating, so saturated colours look dimmer than white.
- Gain is the peak, on-axis value relative to a matt white reference. Off-axis the luminance falls; the half-gain viewing angle on the screen datasheet is where it drops to half the peak. This calculator gives the centre, on-axis figure: corners and off-axis seats will be lower.
- Projector rating vs. reality. The datasheet lumens are measured with a specific lens and mode. Colour calibration, lens choice, zoom position and light source age all reduce the light reaching the screen. Measure with a spot photometer at the reference seat to confirm.
- SMPTE ST 431-1:2006, D-Cinema Quality – Screen Luminance Level, Chromaticity and Uniformity (Table 1). SMPTE RP 431-2:2011, D-Cinema Quality – Reference Projector and Environment. DCI Digital Cinema System Specification.
- SMPTE 196M, Motion-Picture Film – Indoor Theater and Review Room Projection – Screen Luminance and Viewing Conditions, 16 ± 2 fL open gate as cited in SMPTE RP 12 and RP 51.
- DCI high-brightness 3D recommendation (July 2015), as reported by M. Karagosian, mkpeReport. Dolby Cinema figures: Christie / Dolby launch statements. DCI HDR: DCI High Dynamic Range D-Cinema Addendum.
- ANSI/INFOCOMM 3M-2011, Projected Image System Contrast Ratio: passive viewing 7:1, basic decision making 15:1, analytical decision making 50:1, full-motion video 80:1. It has since been revised as ANSI/AVIXA V201.01:2021, Image System Contrast Ratio; check the current edition for design work.