OverheadProjektoren

OverheadProjektoren, commonly known as overhead projectors (OHPs) in English, are optical devices used to display content printed or handwritten on transparent sheets. The transparency, placed on a flat glass stage, is illuminated from beneath by a bright light source, usually a halogen lamp. The light passes through the transparency and is focused by a lens to form an enlarged image that is projected onto a screen in front of the presenter. A small mirror may be used to redirect the image toward the screen, depending on the model.

Users can write on the transparencies with dry-erase or other erasable markers, allowing on-the-fly annotations during a presentation. The projection size is adjustable by moving the lens assembly or using a different lens, and sharpness is controlled by focusing the lens and aligning the transparency. A heat management system helps dissipate heat from the lamp and stage.

Overhead projectors became common in classrooms and meeting rooms from the 1960s through the 1990s as a simple, affordable way to share printed material and notes. They have largely been superseded by digital projectors, which can display digital documents and multimedia content without a physical transparency, but they remain in use in some institutions for low-cost presentations, training, or in environments with limited access to digital equipment.

Safety and maintenance considerations include proper ventilation to dissipate heat from the lamp, careful handling of hot surfaces, and timely replacement of worn bulbs. See also transparency, projector, and slide projector.