How Does an Overhead Projector Work?

By Jack Gerard

Lighting and Projector Sheets

The main component of an overhead projector is the base. This base contains a clear glass top with an interior light. A small fan on the back of the unit removes heat that is produced by the light, preventing the overhead projector from overheating. When the light is turned on, the only portion of the base that lets light escape is the glass top; this causes a portion of the light to illuminate the glass as well as anything placed on top of it. Clear plastic projector sheets are placed on the glass with whatever image or text is to be displayed, letting the light shine through the sheet so that it can be projected.

Adjustable Arm and Head

Attached to the base of the overhead projector is an adjustable arm. The arm is attached to a head unit that contains a mirror. The head unit is generally square or triangular in shape; it features an open bottom and a front side that has a glass lens in it. The mirror within the head unit is positioned in such a way that it can reflect light that comes in through the open bottom out through the lens. When the light in the overhead projector's base is turned on, it causes whatever is on the projector sheet to be reflected by the mirror so that it can be displayed on whatever surface the lens is pointed at.

Mirror Positioning

In order to allow an overhead projector to be focused and the lens to be pointed at different angles, both the arm that holds up the projector head and the angle of the head itself can be adjusted. Moving the arm up or down will affect both the focus and the size of the image that is projected, while changing the angle of the projector head will allow the projected image to be moved as needed in order to better fit the surface that it is being projected on. It may take a few moments of adjustment to get a projected image in focus at the perfect size. However, considering the simplicity of its construction, there is actually quite a bit of customization that can be done to how the projected image is displayed.