Some of the more expensive projectors use metal halide bulbs which are brighter and last nearly 750 hours. An overhead projector should last at least five to 10 years with proper maintenance and cleaning. Reflective projectors house the light in the head of the unit so that the light is projected down onto the base, bounced off the reflector, and then back through the head onto the screen. Although reflective projectors are not as bright as comparably priced transmissive projectors, they are significantly easier to tote around. The key to projecting a sharp image is obtaining an overhead projector with the right combination of light output and focal length.
Light output affects how well an image can be viewed and is measured in lumens.
For a typical office, a projector's light output should be between 2000 to 4000 lumens. If the overhead projector will be used with an LCD panel, at least 4000 lumens is recommended. Since there is no standard measurement for lumens, it is always best to test the unit under the conditions you expect it to be used most often. Also think about the room in which the projector will be used. Focal length determines the size of the projected image.