Overhead projector
Overhead projectors (OHP) are used to project and enlarge an image contained on a transparent sheet onto a remote screen for easy viewing. Overhead projectors are old in the art but continue to enjoy widespread use in the fields of education and marketing. An overhead projector projects enlarged images onto a screen from a transparent original such as a transparency or a liquid crystal panel. An overhead projector is provided with a projection lens for projecting images onto a screen from a transparent original, and a reflection mirror for reflecting the light from a light source toward the screen. Overhead projectors produce magnified images on a screen or wall of indicia-bearing transparencies, typically clear plastic sheets including textual and/or graphical information. The head assembly includes a lens system and a mirror for changing the direction of the path of the light and for projecting the image onto the remote screen. Transmissive overhead projectors are generally comprised of a base having a transparent stage area, a light source inside the base, a projection head mounted above the stage, and a condensing lens system located near the stage for collecting and directing the light towards the projector head. In a transmission type overhead projector, a stage is constituted by a Fresnel lens. To project the image, light emitted by the light source is passed through the Fresnel lens, and through the original material. The light of the image of the original material passes through an optical system in the projecting head, and proceeds to the screen. In a reflection type overhead projector, the stage is constituted by a Fresnel mirror consisting of a Fresnel plate and a reflective layer formed thereon. Light emitted by the light source is directed to the Fresnel mirror. Transmitted through the original material, the light is reflected upward by the Fresnel mirror to illuminate the original material from below. The light of the image of the original material passes through the optical system inclusive of a projector lens and a projector mirror, and proceeds to the screen. In general, these projectors have a light source which projects light towards a fresnel lens which supports the transparency. After passing through the fresnel lens and the transparency, the light containing the image is focused and projected by a projection head assembly mounted some distance above the top surface of the fresnel lens.
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