An overhead projector (OHP) is a display system that is used to display images to an audience. An overhead projector typically consists of a large box containing a very bright lamp and a fan to cool it off, on top of which a large lens is that, collimates the light. Above the box, typically on a long arm, is a mirror that redirects the light forward instead of up. Overhead Projectors come in many configurations which lend themselves to different performance and cost criteria. The majority of the applications are for displaying images in rooms where the distance from the projector to the screen is 6-12 feet.
For the conventional projection distances, an overhead projector can be found in the following optical designs:
Direct Optics Configuration - in this Overhead Projector format the light source and the associated optics are positioned on a straight line known as an optical axis. The light source is mounted in lamp housing in conjunction with a spherical reflector located immediately under the optical center point of the Fresnel condensing lens.
Folded Optics - Also know as "Chamber Optics" is a method of interposing a front surface mirror between the Light source and the Fresnel lens. The focal length of the objective side of the Fresnel lens (the side facing the lamp) is increased for this purpose. The light source is typically a lamp mounted into an Elliptical faceted reflector.
Reflected Optics - The two previous designs are typically table top models. Reflected optics is a configuration expressly for the purpose of portability. In this design the light source, cooling device and head projection optics are housed in a single unit positioned above a projection stage via a supporting arm or arms. |
