Uses of Overhead Projectors
Overhead projectors are familiar features in many classrooms and conference rooms. While the LCD projector is becoming more common, many public speaking venues still offer overhead projectors to illustrate key points in presentations. Because this form of technology is readily available, overhead projectors are quite inexpensive to operate. Many educational institutions have overheads in each classroom because of the low cost and the adaptability of overheads to any discipline.
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Function
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An overhead projector is a metal box with mirrors, lenses and a bright light. The bright light from a bulb reflects on the Fresnel lens inside the overhead. The light then bounces to a reflecting mirror under a stage where you set the overhead transparency. This image is in reverse form. A convex mirror then reflects an enlarged image with the proper orientation from the overhead transparency to a screen behind the speaker and the overhead.
Types
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The three types of overhead projectors are the transmissive, reflective and opaque. The transmissive overhead projector is the most commonly used type. It relies on the Fresnel lens to reflect the image from the overhead transparency to the screen. Reflective overhead projectors work from an overhead light source rather than one underneath the presenter's hand as in a transmissive overhead projector. This type of overhead projector gets the most use in large auditoriums. An opaque overhead projector allows you to show three-dimensional objects as well as overhead transparencies.
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Size
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One key component of the size of an overhead projector is the lumen output from the bulb. A higher lumen projector is visible in a brightly lit room. A lower lumen projector requires dimming the room lighting to be clearly visible. The size of the stage is also another element of the size of an overhead. The stage of a projector should be adequate to hold a standard size sheet of paper, which is also the standard size of an overhead transparency.
Features
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An overhead projector should have an on/off switch. This switch should automatically allow the cooling fan to continue running even when the switch is off so that the bulb can cool properly. Overheads also allow you to get the best view of your overhead transparency by offering different methods of adjusting the focus. You can raise and lower the head on many models with a knob or lever. Other models have a keystone feature that allows you to adjust the image if it's not projecting square with the screen. Some models of overheads may have bulb changers that make it easy to swap out used bulbs.
Warning
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Remember to always keep a spare bulb for the overhead projector nearby. While newer models have two bulbs that you can easily switch during a presentation, older models don't. Always allow the fan to cool the bulb properly before shutting off the projector. Turning off the bulb before it has time to cool can lead to damage. Handle bulbs with care. They can be extremely hot.
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