The overhead projector features a set of major components. As with any machine or device, these components work together to allow the overhead projector to perform its intended function (projecting an image onto a screen). Major parts are held together by minor parts such as screws and nuts and bolts.
Lamp and Reflector
Situated in the base unit of the overhead projector is a lamp that serves as a light source. Behind the lamp is a reflector to direct the light forward toward a mirror. Without this reflector, the light would be scattered inside the base, making for a very dim projected image. Between the lamp and the mirror in the base of the unit is a condenser. It focuses or condenses the light onto the mirror.
Since the lamp in the base unit generates heat, it is necessary to have a cooling fan to dissipate this heat. The base unit houses a blower that serves this purpose. The blower is powered by an electric motor. Connected to the motor's shaft is a fan that circulates cooling air within the base of the unit.
Also inside the base unit is a mirror. This mirror, mounted at more or less a 45-degree angle, serves to change the angle of the light emitted from the lamp and reflector from its original horizontal direction. The mirror reflects the light and directs it upward through the projector's stage.
The overhead projector's stage is a glass surface upon which transparencies are placed for projection. The projection stage includes a so-called Fresnel lens for magnifying and focusing the image upward. The Fresnel lens is a material with a surface of concentric rings, giving it a hazy or fuzzy appearance.
Above the stage is another set of components that includes a focusing knob, an objective lens that receives the light sent through the Fresnel lens and another mirror to direct the image forward onto a projection screen. Operating the focusing knob moves the upper unit up or down in order to focus the image on the projection screen.