Aslide projector is an optical device forprojecting enlarged images ofphotographic slides onto ascreen. 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-formatmagic lantern, first came into widespread use during the 1950s forslide shows as home entertainment, and for use by educational and other institutes.Reversal film created a small positive projectable image rather than thenegatives 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 byimage files ondigital storage media shown on a projection screen by using avideo projector, or displayed on a large-screen videomonitor.

•  
  Continuous-Slide Lantern,c. 1881
•  
  AKodak Carousel model 4400 slide projector, first sold in the mid-1980s
•  
  Self-contained slide projector with rear-projection screen and carousel tray

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

A projector has three main optical elements:^{[3]: 39–40}

• highluminous flux electricincandescent light bulb or other light source, usually fan-cooled
• reflector andcondenser lens system to direct the light through the slide
• focusing projectionlens

Most slide projectors have a mechanism to hold slides in place during projection; many feature automated or mechanized slide advance and return to facilitateslideshows with multiple images.

Anincandescent lamp bulb is used to generate light, usually specially designed to have a small, brightfilament to produce a sharp and bright image. For example, theLeitz 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).^[4]

Light from the bulb is modified from a point source by reflection and passing through condenser lens(es) to generate acollimated beam, which is then directed through the slide. A flat piece of heat-absorbing glass is usually placed in the light path before the slide, to avoid damaging the latter. This glass transmits visible wavelengths but absorbsinfrared.^[5]: 10 Some slide projectors used bulbs that had an integralmultifaceted reflector, which were more expensive than bare bulbs, but were more convenient to use, as the reflector did not have to be adjusted after bulb changes.^[3]: 43

After the collimated beam passes through thetransparent slide, it is enlarged by a projection lens onto a flatprojection screen so the audience can view the reflected image. For some slide projectors, the projection lens is removable and may be swapped with alternative lens(es) to change the brightness or focal length(s), which may affect potential size of the image or the distance from the screen required for a certain projected image size, respectively.^{[5]: 3 [6]}

Standardfocal lengths for projector lenses are approximately twice that of anormal lens for thatfilm format, which allows the projector to be located behind the audience for an average screen size. For example, with135 film, a standard projector lens focal length would be around 100 mm.^[3]: 41 Starting in the 1970s, some slide projectors were offered withautofocus to compensate for differences in slide mount thickness or warping.^[7]

Alternatively, the image may be projected onto a translucent "rear projection" screen, often used for continuous automatic display for close viewing. For example, theSingerCaramate andBell & HowellRingMaster projected slides onto a flat screen approximately the size and shape of a small CRT TV;^[8][9] the RingMaster also could be used for front projection for larger audiences.^[10] This form of projection also avoids the potential of the audience interrupting the projected light beam by casting their shadows on the projection or by bumping into the projector.^{[citation needed]}

Some slide projectors required users to manually place each slide that was being shown.^[11] Starting in the 1950s, manufacturers introduced slide projectors with mechanisms which handle slides preloaded into cartridges, moving individual slides into and out of the light path in sequence. One of the primary differentiators between slide projectors was the form factor of the cartridges used to hold and, in many cases, store slides. Some automated slide projectors offered slide trays with straight, rectangular shapes,^[12] which were popular in Europe; these use an arm, moving laterally, to extract a slide from the cartridge for projection.^[3]: 42 Some straight-tray machines could also accept vertical rotary cartridges, which resemble aFerris wheel.^[3]: 42 In the United States, Eastman Kodak introducedCarousel slide projectors in 1961, which used a horizontal rotary cartridge like the namesakemerry-go-round, and the format soon became ubiquitous there.^[13] Advantages of Carousel-type slide projectors include a simpler, gravity-fed mechanism, and the ability to automatically repeat the sequence of images without having to reload;^[3]: 41 however, the circular trays tended to be bulkier and more expensive.^[3]: 42

Sophisticatedtransition effects between images, such asdissolves, can be performed with multiple projectors;Rollei introduced the P3800 atphotokina 1976, which had two lenses and could perform without an additional unit.^[7] By 1990,Rollei marketed the Rolleivision 35 Twin Digital, an upgraded P3800 which added a personal computer interface to control its operation.^[3]: 47 However, competing technologies which used images directly displayed from computers, such asprojection panels andvideo projectors, largely displaced film slide projectors by the mid-1990s.

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 & HowellSlide 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 differentpolarizations, making slides appear as three-dimensional to viewers wearing polarizing glasses^{[citation needed]}
• Medium-format slide projectors, formedium-format slides larger than 35mm.^{[citation needed]}
• Specialisedlarge-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

• Murphy, Burt (February 1973).Slide projectors get smarter all the time.Popular Mechanics. Retrieved2011-10-22.