Overhead projectors generally include a light source, one or more lenses and reflectors, and a stage upon which a transparency is placed for projection. Overhead projectors generally comprise a glass table or horizontal support with a light underneath, and with an overhead optical system including a mirror and a projection lens. A transparency is laid on the glass table, and an enlarged image of the transparency is projected onto a vertical display screen or the like. A projection head is provided for projecting a magnified image on the transparency onto a remote viewing surface such as a screen and includes a focusing lens and a reflecting mirror for focusing and directing the image from the transparency to the screen. 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. An exhaust fan is incorporated in the projector housing for exhausting air inside the housing through the air outlet. A projection light source is also provided in the housing for illuminating the transparency placed on the stage. Light from the source is directed through the transparency, to the projection lens and is reflected by the mirror to a vertical surface for viewing by an audience. Overhead projector lenses have taken many forms, and the most conventional form in use today is a two-element projection lens utilizing two positive spaced meniscus lenses, one to direct light toward a reflector and the other to receive light from the reflector to project an image from a stage to the screen. The image is focused by movement of the head toward and away from the stage upon which the transparency is placed. A Fresnel lens is disposed inside the stage for converging light from the light source on the projection lens. 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. Conventional inks for use on overhead projector (OHP) film fall into two categories: permanent OHP inks and erasable OHP inks. Most erasable OHP inks are washable. This can make removal of the markings messy and inconvenient, and also can make it difficult to erase a small portion of a marking.

There are three general types of overhead projectors: transmissive, reflective, and opaque. 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 directing the light towards the projector head. The base includes a light source and optical element designed to direct light from the source through a transparent stage defining the upper surface of the base and to the projection head. The post is pivotable between the illustrated upright projection position and a storage position wherein the post is rotated forward and down to engage a clip. Above the base is disposed a projection head having lenses and a mirror which function to gather light from the projector base and redirect it to the screen. The projection head is supported by a post extending upwardly from the base. It is necessary that the distance between the projection head and the base be adjustable so that the image projected on the screen may be brought into focus. A transparent film having translucent or opaque indicia may be placed on the stage and an image of the indicia projected onto a distant vertical surface by the projection head. In a transmission type of overhead projector, a stage is constituted by a Fresnel lens. The condensing lens system often takes the form of a Fresnel lens or a two-element Fresnel lens combination. Transmissive projectors have the light source below the stage and an overhead arm carrying a projection lens and a mirror. Light is focused and transmitted through the transparency on the stage toward the projection lens which is located by the arm over the stage and reflected by the mirror. The image of the transparency is thereby projected onto a large surface.

A reflective overhead projector is used for presentation of an image to a considerable number of viewers. The light source of the reflection type is arranged in a unit incorporating the projector head, and the stage has a Fresnel mirror that supports the original material. The projector body has a top face as a stage, where a transparent sheet or original is placed. When the overhead projector is used, a projector head is set above the stage and at the top of a stay. The projector head protrudes over the projector body in such an extended manner that it is likely to obstruct a view of some of the viewers. In reflective overhead projectors, the stage is constituted by a Fresnel mirror consisting of a Fresnel plate and a reflective layer formed thereon. Reflective projectors carry the light source in the overhead arm and have a reflective surface below the stage. Light shines down onto the stage and is reflected back up through the material which is placed on the stage. A light source is included in the projecting head. To project the image on a screen remote from the apparatus, 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. Opaque projectors are similar to reflective projectors but have the ability to project images of opaque materials and three dimensional objects in addition to transparencies. Of the three types of projectors, the transmissive type is generally the largest and the heaviest.

Computer addressable projection systems have become available in recent years, whereby a computer generated image may be projected onto a wall or screen. With this technology, a presenter can create a more dynamic presentation using video, graphic animation, enhanced colors, programmable slide timing, and sound synchronization. The presenter can also control the presentation from a remote location using a remote control device, freeing the presenter to walk away from the projector. Flat panel displays or liquid crystal displays (LCDs) are popular display devices for conveying information generated by a computer system. The decreased weight and size of a flat panel display greatly increases its versatility over a cathode ray tube (CRT) display. An overhead projector projects enlarged images onto a screen from a transparent original such as a transparency or a liquid crystal panel. The projector display panels have a flat LCD (liquid crystal display) panel which is transparent and open on both sides to pass light from the projector. The panel is framed within an enclosure which houses electronics for interfacing the panel to a computer. The LCD panels can be coupled to a video source or a portable computer so that a slide or video presentation can be projected onto a large surface. The use of LCD panels with overhead projectors has become quite widespread in recent years. In order to project the image created by an LCD panel, a relatively bright transmissive projector is required since LCD panels can absorb the light passing through them. The LCD panel projects a video image, such as a motion picture image, to a screen, and also smoothes an operation of setting one image after another.

The feature of interacting with the display is considered one of the advantages of the overhead projector, where it is simple to use a pointer while facing the audience and not moving from the podium. Presentation techniques such as image revelation are more easily done with an overhead projector. Overhead projectors are in widespread use because they provide an economical and efficient method of displaying and presenting information to a large number of spectators. Overhead projectors are in common use in schools, and businesses, and at conferences, and in fact, are used more frequently than slide projectors nowadays. The projection and resultant enlargement of these images onto the screen allows image presentation to a large audience. The presenter stands near one side of the overhead projector and places a series of transparencies on the top transparent stage of the light box forming the base of the projector. Images carried by the transparencies are reflected by the elevated projection head through its lenses and mirror onto a suitable remote projection screen.