ANSI Lumen Ratings
The light output of a projector is usually quoted in ANSI lumens; the higher the ANSI lumen rating, the brighter the projector. (ANSI = American National Standards Institute). The lowest rated product in this category is the Compaq MP-1600 at 600 ANSI lumens. The brightest is the newly announced 7 lb. Mitsubishi X70 at a remarkable (for its size) 1200 ANSI lumens.
Several notes on ANSI lumen ratings. First, ANSI lumens are often over-emphasized as a factor in comparing one machine to another. When two projectors, one rated at 600 lumens and the other rated at 700, are projected side-by-side, it is difficult to see a difference in brightness. Comparing a 600 lumen unit to an 800 lumen unit, there may be a small difference in apparent luminosity, but most viewers would not consider it to be a difference worth paying a premium for.
In general, there needs to be about a 30% differential in the ANSI lumen rating before the human eye can really notice an appreciable difference in brightness when two projectors are shown side by side. In this category, most of the products rated 600, 650, or even up to 800, will have roughly the same apparent brightness when compared with one another. Most of the products rated 1000 to 1200 lumens will also look about the same. However, those rated 1000 to 1200 lumens will produce noticeably brighter pictures than those in the 600 to 700 range.
Another point to keep in mind is that the ANSI lumen rating applies to a *new* lamp only. A lamp will generally degrade to about half of its original light output over the course of its life. So a 1000 ANSI lumen projector will probably average no more than 750 to 800 lumens over its life unless you change the lamp frequently.
How much lumen output you need depends upon five factors, (1) the level of ambient room light in your normal presenting environment, (2) the size of your typical audience, (3) the size of your projected image, (4) the quality of the projection screen, and (5) the amount of detail in your presentation material (e.g., do you normally present "bullet-item" type sales presentations, or financial data on spreadsheets?). The more lumens (and contrast) your projector has, the greater its ability to deal with these conditions.
In general, most mobile presenters can function very well with 600 to 700 lumen machines. However, projectors with higher light output can give you more flexibility to throw brighter images on larger screens.