How to make overhead projector transparencies


projector, Projection apparatus, optical instrument for generating a real, enlarged image of a flat, transparent object on a screen or a wall. The main performance requirements of the projector depend on its intended use. Image projectors are required to have high illuminance and measurement projectors to be free from distortion in the image plane (imaging errors). A distinction is made between different types of projection, depending on the type of lighting of the object and how the image is viewed. 1) If lighting and viewing take place in transmitted light, one speaks of a Slide projector (Diascope), as it is used e.g. for the projection of slides (slides). Slide or transparent originals do not have a scattering effect, and the course of the radiation coming from the light source is not changed, but only weakened by partial absorption. The lighting device includes the light source and the condenser, which images the lamp filament of the light source into the entrance pupil of the objective and must be of such a diameter that it fully illuminates the transparent object. The enlarged image takes place through the lens onto a screen. The mapping equation is a = f (1 – β), in which a the projection distance between the object and the screen, f the lens focal length and β is the reproduction scale. To reduce aberrations that are strongly noticeable when imaging through the lens, one uses multi-level Condensers. The projection lenses consist of 3 to 5 lenses, and with the help of varifocal lenses (pancratic system) the image size and sharpness can be continuously adjusted without changing the projection distance. The structure of the writing or overhead projector corresponds to the slide projector (see Fig. 1). The projection is carried out with a powerful halogen lamp, and a Fresnel lens serves as a condenser directly below the writing surface. The class of slide projectors also includes the reader for reading microfilms. 2) For the front-light projection of opaque objects (pictures, books) in DIN A5 format and larger, one is used Epiprojector (Episcope). A lens with a relatively large focal ratio images the object illuminated with particularly strong light sources via a plane mirror on a screen, so that the projected image is oriented the right way round (see Fig. 2). 3) Under one Epidiascope one understands a device for slide and epiprojection. 4) As Movie projector This is an electromechanical optical device for reproducing phase images of a movement process photographically stored on a perforated film. It is similar in structure and functionality to the camera.

Projector 1: Overhead projector or slide projector (1 deflecting mirror, 2 lens, 3 transparent template, 4 Fresnel lens, 5 lamp).

Projector 2: Functionality and beam path of the epiprojector.