Brightness or Lumens
Light output can vary greatly from one model of projector to another. ANSI lumens are a standard for measuring this light output.
When there's light in the room, or when you're trying to project onto a wall or other atypical surface, you need a strong light source.
In general, the higher the lumens rating, the more light you've got—and the better the projected image.
Less than 1000 Lumens
The lowest output projectors are typically the least expensive—but under non-ideal conditions the image may not be satisfactory.
Low light output means that presentations will need to be made in a dark or dimly lit room so that the image on the screen is not washed out by ambient light.
1000 - 2000 Lumens
Most standard-use projectors, suitable for normal business conference room and small classroom use, fall into this category.
Although a totally dark or dimly lit room is usually not necessary, presenters should still expect to reduce room lighting somewhat for best screen viewing.
2000 - 3000 Lumens
These projectors are typically found in the high-performance range of the portable and semi-portable projectors, and are suitable for large conference rooms and classrooms.
They offer more flexibility in terms of ambient lighting, since the image is bright and less likely to wash out.
They also offer more flexibility in terms of audience size, since they can illuminate a larger screen without much loss of image quality.
More than 3000 Lumens
Ultra-bright projectors produce more than 3000 lumens and are typically expensive.
They are used in a variety of large venue applications, including very large board rooms, conference rooms, training rooms, auditoriums, churches, concerts, and nightclubs.