Editing Slide projector (section)

==Design==
[[File:Diaprojektor beleuchtungssystem IMGP1044.jpg|thumb|right|upright=1.5|Slide projector light generation and collimation system: from left to right, [[Parabolic reflector|mirror / reflector]], [[Halogen lamp|bulb]], [[Condenser (optics)|condenser lens]] 1, green-tinted [[Hot mirror|heat-absorbing glass]], condenser lens 2; a [[centrifugal fan]], used to cool the bulb, is visible at the top of the picture.]]
A projector has three main optical elements:<ref name=PopPhoto-9010>{{cite magazine |url=https://books.google.com/books?id=qOIgD4S4RCQC&pg=PA38 |title=Try Projection! |first=Robert |last=Beeler |date=October 1990 |magazine=Popular Photography |pages=38–47;160–161 |access-date=2 September 2024}}</ref>{{rp|39–40}}
* high [[luminous flux]] electric [[incandescent light bulb]] or other light source, usually fan-cooled
* reflector and [[Condenser (optics)|condenser lens system]] to direct the light through the slide
* focusing projection [[lens (optics)|lens]]

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

===Light source and collimation===
[[File:Braun Camera-Werk Novamat M 330 - Philips halogen lamp-2203.jpg|thumb|left|[[Philips]] [[halogen lamp]] used in [[Carl Braun Camera-Werk]] Novamat M 330 slide projector]]
An [[incandescent lamp bulb]] is used to generate light, usually specially designed to have a small, bright [[Electrical filament|filament]] to produce a sharp and bright image. For example, the [[Ernst Leitz GmbH|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).<ref>{{Cite web|url=http://www.pradoseum.eu/Literatur/pradovit-rc-74.pdf|title=Pradovit RV-74 projector manual|publisher=Leitz|date=September 1972}}</ref>

Light from the bulb is modified from a point source by reflection and passing through condenser lens(es) to generate a [[collimated 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 absorbs [[infrared]].<ref name=Kodak-E2>{{cite web |url=https://archive.org/details/kodak-ektagraphic-e-2-slide-projector-manual-1970s/ |title=Kodak Ektagraphic Slide Projector, Model E-2 |publisher=Eastman Kodak |date=May 1975 |access-date=30 August 2024}}</ref>{{rp|10}} Some slide projectors used bulbs that had an integral [[multifaceted 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.<ref name=PopPhoto-9010/>{{rp|43}}

===Projection===
[[File:Zeiss Ikon Slide projectors Royal AF selektiv AV Diaprojektor Duo 03.jpg|thumb|right|upright=1.3|This slide projector, sold under the [[Zeiss Ikon]] brand, uses a straight-through tray; a thin arm grabs slides from a tray and puts them into the light path; the tray is advanced along its length. At the front of the projector, the protruding projection lens features rings to control focal length and focus.]]
After the collimated beam passes through the [[transparent materials|transparent]] slide, it is enlarged by a projection lens onto a flat [[projection 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.<ref name=Kodak-E2/>{{rp|3}}<ref>{{cite web |url=https://archive.org/details/cbpf_000213/page/n17/mode/2up |title=GK Delineascope Reference Manual |publisher=American Optical Corporation |date=September 1968 |access-date=30 August 2024}}</ref>

Standard [[focal length]]s for projector lenses are approximately twice that of a [[normal lens]] for that [[film format]], which allows the projector to be located behind the audience for an average screen size. For example, with [[135 film]], a standard projector lens focal length would be around 100&nbsp;mm.<ref name=PopPhoto-9010/>{{rp|41}} Starting in the 1970s, some slide projectors were offered with [[autofocus]] to compensate for differences in slide mount thickness or warping.<ref name=Rollei78>{{cite web |url=https://www.pacificrimcamera.com/rl/03480/03480.pdf |title=Rollei slide projectors (dealer catalog) |date=1978 |publisher=Rollei |access-date=2 September 2024 |via=Pacific Rim Camera, Reference Library}}</ref>

Alternatively, the image may be projected onto a translucent "rear projection" screen, often used for continuous automatic display for close viewing. For example, the [[Singer Corporation|Singer]] ''Caramate'' and [[Bell & Howell]] ''RingMaster'' projected slides onto a flat screen approximately the size and shape of a small CRT TV;<ref>{{cite web |url=https://www.retrothing.com/2007/08/singer-caramate.html |title=Singer Caramate II SP: Multimedia Slide Projector |author=bohus |date=August 23, 2007 |website=Retro Thing |access-date=2 September 2024}}</ref><ref>{{cite web |url=https://www.radiomuseum.org/r/bell_howel_ringmaster_soundslide_projector_797c.html |title=RingMaster |website=Radio Museum |access-date=30 August 2024}}</ref> the RingMaster also could be used for front projection for larger audiences.<ref>{{cite magazine |url=https://books.google.com/books?id=xB0DAAAAMBAJ&pg=PA24 |title=Get the picture two ways with one sound-slide projector [advertisement] |date=May 1978 |page=24 |magazine=Cincinnati Magazine |access-date=30 August 2024}}</ref> 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.{{fact|date=September 2022}}

===Mechanical===
[[File:60 CARRUSEL.jpg|thumb|left|[[Kodak]] [[Carousel slide projector|Carousel S-AV 1050]], which features a horizontal rotary slide tray]]
Some slide projectors required users to manually place each slide that was being shown.<ref>{{cite web |url=https://archive.org/details/pp-46-9-23 |title=To do a slide justice ... show it with an Argus projector [advertisement] |publisher=Argus Camera Corporation |date=September 1946 |access-date=30 August 2024}}</ref> 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,<ref>{{cite news |url=https://www.nytimes.com/2013/01/21/arts/design/its-a-spaceship-no-its-a-time-machine.html |title=It's a Spaceship! No, It's a Time Machine |first=Alice |last=Rawsthron |date=January 20, 2013 |work=The New York Times |access-date=30 August 2024}}</ref> which were popular in Europe; these use an arm, moving laterally, to extract a slide from the cartridge for projection.<ref name=PopPhoto-9010/>{{rp|42}} Some straight-tray machines could also accept vertical rotary cartridges, which resemble a [[Ferris wheel]].<ref name=PopPhoto-9010/>{{rp|42}} In the United States, Eastman Kodak introduced [[Carousel slide projector]]s in 1961, which used a horizontal rotary cartridge like the namesake [[merry-go-round]], and the format soon became ubiquitous there.<ref>{{cite magazine |url=https://edtechmagazine.com/higher/article/2016/09/kodak-carousel-projectors-revolutionized-lecture |title=Kodak Carousel Projectors Revolutionized the Lecture |first=Meghan Bogardus |last=Cortez |date=September 26, 2016 |magazine=EdTech |access-date=30 August 2024}}</ref> 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;<ref name=PopPhoto-9010/>{{rp|41}} however, the circular trays tended to be bulkier and more expensive.<ref name=PopPhoto-9010/>{{rp|42}}

Sophisticated [[film transition|transition effects between images]], such as [[Dissolve (filmmaking)|dissolve]]s, can be performed with multiple projectors; [[Rollei]] introduced the P3800 at [[photokina]] 1976, which had two lenses and could perform without an additional unit.<ref name=Rollei78/> By 1990, [[Rollei]] marketed the Rolleivision 35 Twin Digital, an upgraded P3800 which added a personal computer interface to control its operation.<ref name=PopPhoto-9010/>{{rp|47}} However, competing technologies which used images directly displayed from computers, such as [[projection panel]]s and [[video projector]]s, largely displaced film slide projectors by the mid-1990s.