Practical tools for measured work
Projector Brightness & Foot-Lamberts Calculator
Estimate foot-lamberts from delivered lumens, screen size and gain, or reverse-calculate required output.
How it works
How to use it: enter screen size, rated lumens, an estimated delivered-output percentage, screen gain and your comparison target. This keeps marketing output separate from the lumens assumed to reach the screen.
Formula. Foot-lamberts = delivered lumens x gain / screen area in square feet. Approximate cd/m² = fL x 3.426. Reverse rated lumens also accounts for the entered delivery percentage.
Example. A 2500-lumen projector modeled at 70% output delivers 1750 lumens before screen gain and area are applied.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- Screen diagonal
- 120 in
- Aspect width
- 16
- Aspect height
- 9
- Rated lumens
- 2500
- Estimated delivered output
- 70 %
- Screen gain
- 1
- Target brightness
- 16 fL
Primary result: Estimated screen brightness: 41.0 fL.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
This is an idealized uniform-screen estimate. Calibration mode, zoom, lamp/laser age, optical losses, screen directionality, ambient light, contrast and measurement position affect the real image. Measure the installed system when brightness matters.
Frequently asked questions
Why include delivered-output percentage?
Rated maximum output may not match the calibrated picture mode you use. The editable percentage makes that uncertainty explicit.
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