How to choose an overhead projector

The most important specifications

• Brightness. The overhead projectors United Visual carries range in brightness from 1,700 to 11,000 lumens. Typically, the brighter units are desirable if you are projecting in a large room with color transparencies or are using color LCD panels, which are quite a bit less transmissive than standard transparencies. Most LCD panel manufactures recommend you use an overhead of at least 4,000 lumens. In practice, a 2,500-lumen unit will look good in most classrooms with standard transparencies and a 60 - 96" screen, but you’ll want brighter projectors as your room and screen size increase, if you’re using an LCD panel, or as your ability (or desire) to darken the room decreases.

• Focal length determines the distance your projector will need to be from your projection screen, and thus the amount of space you’ll need to allow for the projector and screen. The following table shows the throw distance needed (from lens to screen) for various screens and focal length lenses. Note that these are for overheads with a 10" wide stage; larger stages will require less distance. Note, too, that an LCD panel or framed transparency with an opening smaller than 10" will require more distance to fill the screen. (You can calculate the exact distance, if you wish, with this formula: focal length x screen width / panel width = throw distance in inches.)

• Lamp type. Overhead projectors traditionally use a halogen lamp, which can provide very good brightness at an economical cost, but can last anywhere from 25-130 hours. Note that there’s a trade off between lamp life and brightness, with brighter units typically raising the voltage on a given lamp and thus cutting its lifespan. This is an area where there can be a big difference between projectors, with many economy projectors looking a lot less economical when lamp costs are factored in.

• A lamp changer is an important consideration with a halogen lamp, as it can burn out without warning, stopping a presentation cold.

• Metal halide lamps, while much more expensive than halogen, are much more reliable. First, these lamps tend to last much longer—500 hours or more. Second, it’s very rare for a lamp of this type to burn out suddenly; instead, a metal halide will grow gradually dimmer, giving you plenty of warning that it’s time for a replacement. Finally, the halide lamps have a bluer tone that most people find more pleasing than the yellowish halogen lamps.

• Stage size can be important for two reasons. If you’re using transparencies that take up a full 8.5 x 11" (most don’t, since they’ll have some margins), you’ll find a 10" x 10" or smaller stage inconvenient. If you’re using an LCD panel, you need to be sure your stage is large enough to hold it. While that shouldn’t be a problem with the overheads United carries, it can be with some brands.