The Light
An overhead light projector is traditionally powered by a powerful incandescent light. An electric current runs through a small filament. The filament is a resistor, which is a device that resists the flow of electricity. The electric current is turned into heat by the resistor, making the filament hot enough to glow and generate light. Because the incandescent light creates so much heat, a fan is usually installed to blow heat on it.
The Fresnel Lens
The light shines through a Fresnel lens, which is a flat lens that uses small grooves to focus light. Because the Fresnel lens is flat, it creates a transparent surface that can be used to display things. Presenters typically use transparencies, which are sheets of clear plastic with diagrams, writing or other information, to display their information. The sheets are centered on the Fresnel lens so the light can shine through them, carrying the image to a wall or screen.
The Mirror
Above the Fresnel lens is a mirror angled at 45 degrees. The light from the projector goes straight up to this mirror and bounces off it toward a wall or screen. The screen displays the image. Many projectors also have a second lens in front of the mirror, which allows the presenter to focus the image. The size of the projected image depends on how close or far the projector is from the wall--the further away, the larger the image.
Tags: electric current, Fresnel lens, Fresnel lens, incandescent light, light Because