Overheadprojektorer

Overheadprojektorer, also known as transparency projectors, are optical devices that project images from a transparent sheet onto a screen. They are widely used in educational settings, business meetings, and theatrical presentations. The basic principle involves placing a bright light source—originally incandescent bulbs—illuminating a transparency, and then focusing the light through a single lens to project the image onto a surface.

Historical development began in the late 19th century with the invention of the first flash projector. The design evolved rapidly during the 20th century, especially in the 1960s and 1970s when colored light sources and more robust lenses were introduced. The 1980s and 1990s saw the introduction of high‑intensity tungsten lamps and the use of ergonomically designed cases, making the devices more portable and reliable. In recent years, digital technology has largely displaced mechanical overhead projectors, but they remain valuable for their simplicity, low cost, and reliability under limited power supplies.

Modern overheadprojectorer models typically include a tungsten halogen bulb rated between 250 and 400 watts, a 10‑inch convex projection lens, and a 10‑inch tripod head with adjustable height and tilt. The unit is mounted on a stand that provides quick positioning and rotation for easier alignment. Some models incorporate a projector focus knob and a cam holder for rotary transparency. The design can be further modified by incorporating cool‑tungsten bulbs or LED LEDs to reduce power consumption and darkroom compatibility.

While they have largely been supplanted by laptop projectors and interactive displays, overhead projectors continue to find use in environments such as workshops, resource caravans, and areas where digital infrastructure is limited. They offer a low‑maintenance solution that can be used with simple transparency slides, making them useful for low‑budget educational contexts and emergency backup systems.