Calculating Screen Brightness
The total light output of a projector is measured in lumens.
The projector's light output is used to illuminate the screen (i.e. project an image on the screen). The amount of illumination is inversely proportional to the area illuminated, and is measured in footcandles = lumens of illuminating light / square foot.
The brightness of the illuminated screen is ultimately what matters, because that is what the eye sees. The brightness is measured in footlamberts = lumen of light output / square foot.*
If we assume the illuminated screen has a gain of 1, then brightness = illumination, i.e. footlamberts = footcandles.
* Another commonly used measure of brightness is cd/m2 = candela / square meter = footlamberts * 3.43. In the United States, candelas per square meter are sometimes called "nits".
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Example 1 Calculate screen brightness when a 1000 lumens projector is used to project on a 6ft wide, 16:9 screen with a gain of 1. The screen height is 9/16 * 6 = 3.375ft. The area of the screen is 3.375 * 6 = 20.25 sq. ft. The brightness can be estimated to be 1000 / 20.25 = 49.4 footlamberts. Is 49.4 footlamberts sufficiently bright? To see how "sufficient brightness" can be estimated, click here. Example 2 Assuming the 16:9 screen with a gain of 1, what is the screen size limit for a 1000 lumens projector? We want to achieve 16 footlamberts, i.e. 16 = 1000 / screen area. This implies that screen area = 1000/16 = 62.5 square foot. The width of a 16:9 screen = 1.33 * square root of area = 1.33 * 7.9 = approximately 10.5 feet. In other words, to achieve 16 footlamberts with a 1000 lumens projector the screen should be no wider than 10.5 feet (corresponding to a diagonal of approximately 12 feet = 124 inch). |
Other Projector Facts
Lens Shift
Keystone Correction
Throw Ratio and Zoom Range
Min and Max Throw Distance
Sufficient Brightness
Calculating Throw Ratio from Focal Length
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