How can overhead projectors be used to enhance the quality of presentations

Overhead projectors (OHP's) 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. Overhead projectors have been popular as a means of projecting prepared transparencies or free-hand transparencies that are drawn in real-time, on to a display screen for viewing by a group of viewers. Overhead projectors enable a marking on a transparent film to be projected on a screen or wall by shining light through the transparent film. A speaker giving a presentation can write on the transparent film, and/or make additions or corrections to a preprinted image, while the film is on the overhead projector. The use and display of graphics and text in business presentations, lectures and the like often augments the effectiveness of the speaker and of the presentation. Transparencies, bearing graphics and/or text, are often exhibited to the viewers on a screen using an overhead projector. Transparencies allow the presenter to preview the presentation, easily sort through materials, point out specific features, and easily add additional comments or graphics. This allows the speaker a flexibility which would not be possible if transparencies or other audiovisual aids were used. Use of OHP's has recently expanded from the traditional projection of images printed on transparent films, to the projection of images which are stored in machine-readable form and which are presented by means of a light-transmissive liquid crystal display (LCD) panel which is laid on the stage of the OHP.

Here 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.