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a device for forming an image of an opaque object on a screen as well as for projecting a transparent image, or diapositive, of an object onto a screen. An epidiascope is a projector whose optical system consists of a combination of an episcope and a diascope (see Figure 1).
In diascopic projection, the diapositive is illuminated by a light beam directed through it; thus, the image formed on the screen is sufficiently bright even with a weak light source and projection lenses with low transmission characteristics. Episcopic projection requires the use of light sources that are more powerful and high-transmission lenses.