Foot-lamberts (fL) measure the amount of light reflected off a projection screen, representing the actual brightness seen by the audience. Unlike lumens, which measure a projector's total light output, foot-lamberts account for screen size and reflectivity
Foot-Lambert Recommended Scale
The ideal number of foot-lamberts depends on the ambient light in your viewing environment.
The ideal number of foot-lamberts depends on the ambient light in your viewing environment.
| Foot-Lamberts (fL) | Viewing Quality | Recommended Environment |
|---|---|---|
| 0 – 15 fL | Generally too dim | Specialized uses (e.g., flight sims) |
| 16 – 26 fL | Ideal cinematic experience | Dedicated Home Theater (dark room) |
| 27 – 39 fL | Good for low ambient light | Den, "Man Cave," or dimmed meeting room |
| 40 – 59 fL | Good for medium ambient light | Living rooms or classrooms |
| 60+ fL | High visibility in bright light | Lecture halls, outdoor movies, or offices |
- Commercial Standard: The Society of Motion Picture and Television Engineers (SMPTE) recommends 16 fL for commercial movie theaters in a dark room.
- Home Theater Standard: Most experts recommend a range between 12 and 22 fL for dedicated dark home theaters, with 16 fL being the "sweet spot".
- High Brightness: For rooms with windows or lights on, a target of 50 fL is often suggested to maintain a clear image.
How to Calculate Foot-Lamberts
To find the foot-lamberts for your specific setup, you need the projector's lumens, the screen's area, and the screen's gain.The Formula:
To find the foot-lamberts for your specific setup, you need the projector's lumens, the screen's area, and the screen's gain.The Formula:
fL = (Projector Lumens / Screen Area in Sq. Ft.) x Screen Gain
- Step 1: Find the screen area. For a 100-inch 16:9 screen, the area is roughly 30 square feet.
- Step 2: Divide the projector's lumens by the area. For a 1,000-lumen projector, this is
1000 / 30 = 33.3. - Step 3: Multiply by the screen gain. If your screen has a 1.0 gain, your final output is 33.3 fL.
Key Factors Impacting the Scale
Screen Gain: High-gain screens (e.g., 1.5 gain) reflect more light toward the viewer, increasing foot-lamberts without needing a brighter projector.
Screen Gain: High-gain screens (e.g., 1.5 gain) reflect more light toward the viewer, increasing foot-lamberts without needing a brighter projector.
- Lamp Aging: Traditional projector bulbs can lose 25% to 50% of their brightness over their lifespan. Designing for an initial 28 fL can ensure you still have at least 14 fL toward the end of the bulb's life.
- Eye Comfort: Extremely high foot-lambert levels (over 100 fL) in a completely dark room can be uncomfortable and cause eye strain.