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An Overhead Projector
An overhead projector is a type of slide projector that is commonly used to show images or data to an audience. Overhead projectors used to be common fixtures in teaching because it allowed the teacher to directly write upon a trasparency, hence creating a more hands-on approach to communicating with students. Using a generally simple concept, overhead projectors magnify and send images upon a screen using lenses, mirrors, and a light source.
The Gist
In an overhead projector, light finds its way from the lamp to the screen, in which the image from the transparency eventually appears enlarged and clear. An overhead projector consists of mainly two parts, the large metal box, which acts as the base unit, and the upper apparatus. Each of these two parts contains smaller components that make projecting an image upon a screen possible. The path of light begins in the large metal box where the lamp of the projector can be found. The lamp is the main light source of the projector. The light emanated from the lamp is then reflected by a reflector onto a mirror. In the light's path also lies a condenser that focuses the light onto the mirror within the metal box. The mirror within the base unit is more or less laying in a 45 degree angle. The mirror reflects and sends the light upwards through the projection stage, or Fresnel lens, and the transparency. Once past the Fresnel lens and the transparency, light then continues on to the upper apparatus. It first encounters the objective lens once past the transparency. The objective lens then sends the light to the mirror above it. This angled mirror continues on to direct the light onto the screen where the image is finally projected.
Lenses in Use
The Path of Light
Objective Lens: •Converging lens •Found in the upper apparatus •Changes the projected image’s size and its clarity •Focuses the image onto the screen •Utilized using the knob found on the upper apparatus •Closer to the transparency = smaller image (and vice versa) Fresnel Lens: •Converging lens •Essentially the top cover of the base unit •Rectangular and is encased in metal, keeping it hinged to the base unit •Magnifies and focuses the image on the transparency upward •Has a blurred, hazy appearance due to the concentric rings found on its surface
Bianca Eyales
