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.

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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.

Photographic film slides and projectors have been replaced by image files on digital storage media shown on a projection screen by using a video projector, or displayed on a large-screen video monitor.

A continuous-slide lantern was patented in 1881.^[1] It included a dissolving views apparatus.^[2]

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^{[citation needed]}

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]

Slide projectors proper:

• Straight-tray slide projectors have a straight magazine holding several slides.^{[citation needed]}
• Carousel slide projectors have a circular magazine holding several slides.
• Stack-loader slide projectors^{[citation needed]}
• Bell & Howell Slide Cube Projector had a cube-shaped magazine holding several slides.
• Dual slide projectors^{[citation needed]}
• Single slide projectors project a single slide at a time, changed by the operator.^{[citation needed]}
• Dissolve projectors^{[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]}
• Medium-format slide projectors, for medium-format slides larger than 35mm.^{[citation needed]}
• Specialised large-format slide projectors for large transparencies of 18 × 18 cm (7.1 × 7.1") or larger.^{[citation needed]}

Related devices:

• Overhead projectors, for page-sized transparencies
• Slide viewer, for direct viewing of a magnified image of a slide

List of known manufacturers of slide projectors:

• Agfa Gevaert, Germany (–1984) → Reflecta (1984–)
• Bauer [de], Germany → Bosch; ceased production^{[citation needed]}
• Bausch & Lomb; ceased production
• Bell & Howell / TDC, US: "Headliner"; ceased production
• Braun AG, Germany: "D", "PA"; ceased production
• Braun Foto Technik, Germany: "Paximat", "Multimag" → Reflecta
• VEB DEFA, Germany: "Filius"→ VEB Gerätewerk Friedrichshagen: "Filius"; ceased production
• Eastman Kodak (–2004): "Carousel-S", "Ektagraphic", "Ektapro" → Leica^{[citation needed]}
• Elmo, Japan
• Enna, Germany; ceased production
• Erno Photo, Germany; ceased production
• VEB Feinmess, Germany; ceased production
• Filmoli, Germany → Gebr. Martin, Germany; ceased production
• Foto Quelle, Germany: "Revue"; ceased distribution
• GAF, US; ceased distribution
• Götschmann, Germany (1978–2009) → Gecko-Cam (2009–)^{[citation needed]}
• Hasselblad, Sweden; ceased production
• HASPE, Germany; ceased production
• Hähnel, Germany; ceased production
• Inox, France: "Prestige" → Prestinox
• Kindermann [de], Germany: "Diafocus" → Leica^{[citation needed]}
• Leitz, Germany (1958–): "Prado" → Leica Projektion GmbH Zett Gerätewerk, Germany (1990–2004): "Pradovit", "Pradovit RT" → Leica Camera, Germany (2004–2006): "Pradovit"; ceased production^{[citation needed]}
• Liesegang [de], Germany: "Fantax", "Diafant", "Fantimat"; ceased production
• Malinski, Germany: "Prokyon", "Malicolor" → Pentacon
• Minolta, Japan; ceased production
• Minox, Germany: "Minomat"; ceased production
• Navitar, US^{[citation needed]}
• Nikon, Japan; ceased production
• Ernst Plank, Germany: "Noris", "Trumpf"; ceased production
• Pentacon, Germany: "Aspectar", "Malicolor"; ceased production
• Asahi Pentax, Japan; ceased production
• Prestinox, France → Plawa Condor (1969–?); ceased production^{[citation needed]}
• Pouva, Germany; ceased production
• RBT, Germany
• Queen, Germany: "Automat"; ceased distribution
• Reflecta, Germany: "Multimag"^{[citation needed]}
• Rollei, Germany (1960–2007): "Rolleiscop", "Rolleivision" → Franke & Heidecke, Germany (2007–2009): "Rolleivision" → DHW Fototechnik, Germany (2009–2015): "Rolleivision"; ceased production
• Royal, Germany?; ceased distribution
• Sankyo, Japan; ceased production
• Sawyer's, US; company sold to GAF
• Silma [it], Italy → Bauer and Rollei; ceased production^{[citation needed]}
• TAV Simda [fr]^{[citation needed]}
• Vicom^{[citation needed]}
• Vivitar, US
• Voigtländer, Germany: "Perkeo" → Zett
• Zeiss Ikon, Germany (1964/1969–): "Ikolux" → Zett
• Zeiss Jena, Germany → Pentacon, Germany^{[citation needed]}
• Zett, Germany (1928–1989): "Fafix", "Zett", "Zettomat", "Perkeo" → Leica Projektion GmbH Zett Gerätewerk, Germany (1990–2004)
• CBИTЯ3ъ, Russia: "ABTO"; ceased production^{[citation needed]}

• Slide viewer
• Carousel slide projector
• Presentation slide