Opto-mechanical device for showing photographic slides
A slide projector is an optical device for
projecting enlarged images of
photographic slides onto a
screen. Many projectors have mechanical arrangements to show a series of slides loaded into a special tray sequentially.
35 mm slide projectors, direct descendants of the larger-format
magic lantern, first came into widespread use during the 1950s for
slide shows as home entertainment, and for use by educational and other institutes.
Reversal film created a small positive projectable image rather than the
negatives used since the early days of photography; photography now produced 35mm directly viewable small colour slides, rather than large monochrome negatives. The slide images were too small for unaided viewing, and required enlargement by a projector or enlarging viewer.
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.[citation needed]
An
incandescent lamp bulb is used, usually specially designed to have a small, bright
filament to produce a sharp and bright image. For example, the
Leitz Pradovit RC uses a special 24V 150W quartz lamp, with provision to center it, required for best performance. This projector had provision to reduce lamp power by 20% to double its life (50 hours at full brightness).[3]
Types
Slide projectors proper:
Straight-tray slide projectors have a straight magazine holding several slides.[citation needed]
Stereo slide projectors project two slides simultaneously with different
polarizations, making slides appear as three-dimensional to viewers wearing polarizing glasses[citation needed]
^Sloane, T. O'Conor. Facts Worth Knowing Selected Mainly from the Scientific American for Household, Workshop, and Farm Embracing Practical and Useful Information for Every Branch of Industry. Hartford: S. S. Scranton & Co. 1895.