When you make a presentation, it can be difficult to have more than a few people huddled around a piece of paper. On the other hand, distributing paper copies of your presentation can be risky because there are always those people who like to flip ahead. As a result, overhead projectors continue to be a staple for presenters everywhere. There are primarily two types of overhead projectors available. They can be distinguished from one another by the location of the light source used to project the image.
Transmissive projectors house the light source in the base of the machine under a clear surface.
Because light is projected directly from the base, a transmissive projector can project images in bright rooms quite effectively. If you plan to use an LCD panel, which is used to show computer-based presentations, a transmissive projector is a must have. When evaluating an overhead projector, compare transparencies of dark images with some of fine images to see how each one projects. Halogen light bulbs provide light for most overhead projectors. Some of the more expensive projectors use metal halide bulbs which are brighter and last nearly 750 hours.