A slide projector is an opto-mechanical device to view photographic slides. It has four main elements: a fan-cooled electric light bulb or other light source, a reflector and "condensing" lens to direct the light to the slide, a holder for the slide and a focusing lens. A flat piece of heat absorbing glass is often placed in the light path between the condensing lens and the slide, to avoid damaging the latter. This glass transmits visible wavelengths but absorbs infrared. Light passes through the transparent slide and lens, and the resulting image is enlarged and projected onto a perpendicular flat screen so the audience can view its reflection. Alternatively the image may be projected onto a translucent "rear projection" screen, often used for continuous automatic display for close viewing. This form of projection also avoids the audience's interrupting the light stream or bumping into the projector.
Slide projectors were common in the 1950s and 1960s as a form of entertainment; family members and friends would gather to view slideshows.
As of October 2004, Kodak no longer manufactures slide projectors. It is also increasingly difficult in some countries to locate photo processors who will process slide film.