A slide projector is an opto-mechanical device to view photographic slides. Slide projectors became common since the 1950s as a form of entertainment; family members and friends would gather to view slide shows. In-home photographic slides and slide projectors have largely been replaced by low cost paper prints, digital cameras, DVD media, video display monitors, and video projectors.
A projector has four main elements:
- electric incandescent light bulb or other light source (usually fan-cooled)
- reflector and "condensing" lens to direct the light to the slide
- slide holder
- 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 interrupting the light stream by casting their shadows on the projection or by bumping into the projector.
It is increasingly difficult in some countries to locate photo processors who will process slide film. Several manufacturers have stopped production of slide projectors.
Types of projector 
- Carousel slide projectors
- Straight-tray slide projectors
- Dual slide projectors
- Overhead projectors
- Single slide projectors (manual form)
- Viewer slide projectors
- Slide Cube projectors
- Stereo slide projectors project two slides simultaneously with different polarizations, making slides appear as three-dimensional to viewers wearing polarizing glasses
- Large Format Slide Projector for use on stages, at large events, or for architectural and advertising installations where high light output is needed.
See also 
- Burt Murphy (February 1973). "Slide projectors get smarter all the time". Popular Mechanics. Retrieved 2011-10-22.
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