[museav]

The projector brightness required will be based on the amount of ambient light hitting the screen, the size of the image and the desired result. The ambient light you may be able to control but that may also depend upon the room. The screen size will be dependent upon the viewing area and the type of media. For the same audience area you can typically get away with a smaller image for video than you can for most computer images but an image height (not diagonal) of 1/8 to 1/6 the distance to the furthest viewer might be typical. Note that the image size is dependent upon the distance to the viewers and not the number of viewers. The desired result is something only you can really define. I have seen many presentations with images that were less than ideal but were perfectly acceptable for the application. However, some people may have higher expectations. One thing to note is that projector brightness is not dependent on the throw distance (the distance from the projector to the screen) however it can be affected by the lens and using anything other than the standard lens may result in a lower brightness output.

The lens and functionalities such as lens shift, digital keystone correction, etc. may also be a consideration. Projectors with optional lenses, lens shift and a good range of keystone correction may cost a bit more but allow for much greater flexibility in projector placement.

You may also want to consider a multiple lamp projector. Some projectors use two or four lamps and these can be set to all be used at once for a brighter output or one at a time for redundancy. Being able to get a brighter image when necessary and having what is in effect an automatic lamp changeover if a lamp fails during an event can be very useful. If you do get single lamp projector then I would suggest carrying a spare lamp at all times.