A slide projector is an opto-mechanical device to view photographic slides. It 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,
• Holder for the slide and
• 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 absorbsinfrared. 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 shadow on the projection or by bumping into the projector.