Lumens are a measurement of light output commonly used to compare the brightness of data projectors. The American National Standards Institute have a commonly used standard for measuring light output referred to as ANSI lumens. The standard requires an average of several measurements to be taken across the face of the light source being measured.

Factors affecting required brightness (lumens):

• Ambient lighting level
• Projected image size – brightness drops with increased image size (projector output remains static). Increasing a projected images size from 50 inches to 100 inches (diagonal) decreases the image brightness by 75% (aspect ratio of 4:3 assumed)
• Aspect Ratio
• Projector calibration. Projectors should be calibrated to D6500 which is an agreed-upon standard for “white” based on certain proportions of red, green, and blue light
• Projection surface gain (measurement of reflectivity)

There are some rough guides to brightness (projector lumens) required for certain room types and ambient lighting conditions which make assumptions for the other factors involved. For a professional result the required brightness should be calculated professionally using the correct units.

A general brightness guide is as follows:

• Smaller rooms with low ambient light require between 1,000 and 1,500 ANSI lumens
• Medium sized rooms with some ambient light require between 1,500 and 3,000 ANSI lumens
• Large rooms with high ambient lighting (eg. conference room) require over 3,000 ANSI lumens

The Projector People website has a lumens guide to help determine how bright a projector is needed for business, home theater and house of worship environments.

An excellent brightness calculator is available from Digital Projection’s websiteIt allows one to experiment with different screen gains, sizes, aspect ratios, projector lumens and venue ambient light.