Editing Overhead projector (section)

==Optical system==
[[File:Zrcalni grafoskop.jpg|thumb|Mirror and lens]]
An overhead projector works on the same principle as a slide projector, in which a focusing lens projects light from an illuminated slide onto a [[projection screen]] where a [[real image]] is formed. However some differences are necessitated by the much larger size of the [[transparency (overhead projector)|transparencies]] used (generally the size of a printed page), and the requirement that the transparency be placed face up (and readable to the presenter). For the latter purpose, the projector includes a mirror just before or after the focusing lens to fold the optical system toward the horizontal. That mirror also accomplishes a reversal of the image in order that the image projected onto the screen corresponds to that of the slide as seen by the presenter looking down at it, rather than a [[mirror image]] thereof. Therefore, the transparency is placed face up (toward the mirror and focusing lens), in contrast with a 35mm slide projector or [[film projector]] (which lack such a mirror) where the slide's image is non-reversed on the side ''opposite'' the focusing lens.

A related invention for enlarging transparent images is the solar camera. The opaque projector, or [[Opaque projector|episcope]] is a device which displays opaque materials by shining a bright lamp onto the object from above. The episcope must be distinguished from the [[Slide projector|diascope]], which is a projector used for projecting images of transparent objects (such as films or slides), and from the [[epidiascope]], which is capable of projecting images of both opaque and transparent objects.

===Condenser===
Because the focusing lens (typically less than {{convert|10|cm|disp=sqbr|abbr=on}} in diameter) is much smaller than the transparency, a crucial role is played by the optical [[Condenser (optics)|condenser]] which illuminates the transparency.<ref name="Young">{{cite book |url=https://books.google.com/books?id=l8AZCQAAQBAJ&pg=PA36 |pages=36–37 |title=Optics and Lasers: Including Fibers and Optical Waveguides |last=Young |first=Matt |edition=4th |publisher=Springer Science & Business Media |year=2013 |isbn=978-3-662-02699-1}}</ref> Since this requires a large optical lens (at least the size of the transparency) but may be of poor optical quality (since the sharpness of the image does not depend on it), a [[Fresnel lens]] is employed. The Fresnel lens is located at (or is part of) the glass plate on which the transparency is placed, and serves to redirect most of the light hitting it into a converging cone toward the focusing lens.<ref name="Gayeski">{{cite book |url=https://books.google.com/books?id=_3jsj-DuBLEC&pg=PA86 |title=Designing Communication and Learning Environments |editor-last=Gayeski |editor-first=Diane |publisher=Educational Technology |year=1995 |page=86 |isbn=0-87778-285-7}}</ref> Without such a [[Condenser (optics)|condenser]] at that point, most of the light would miss the focusing lens (or else the focusing lens would have to be very large and prohibitively expensive). Additionally, mirrors or other condensing elements below the Fresnel lens serve to increase the portion of the light bulb's output which reaches the Fresnel lens in the first place. In order to provide sufficient light on the screen, a high intensity bulb is used which often requires [[Forced convection|fan cooling]].

===Focus adjustment===
Overhead projectors normally include a manual focusing mechanism which raises and lowers the position of the focusing lens (including the folding mirror) in order to adjust the object distance (optical distance between the slide and the lens) to [[thin lens#Image formation|focus]] at the chosen image distance (distance to the projection screen) given the fixed [[focal length]] of the focusing lens. This permits a range of projection distances.

Increasing (or decreasing) the projection distance increases (or decreases) the focusing system's [[Magnification#Size ratio (optical magnification)|magnification]] in order to fit the projection screen in use (or sometimes just to accommodate the room setup). Increasing the projection distance also means that the same amount of light is spread over a larger screen, resulting in a dimmer image. With a change in the projection distance, the focusing must be readjusted for a sharp image. However, the condensing optics (Fresnel lens) is optimized for one particular vertical position of the lens, corresponding to one projection distance. Therefore, when it is focused for a greatly different projection distance, part of the light cone projected by the Fresnel lens towards the focusing lens misses that lens. This has the greatest effect towards the outer edges of the projected image, so that one typically sees either blue or brown fringing at the edge of the screen when the focus is towards an extreme. Using the projector near its recommended projection distance allows a focusing position where this is avoided and the intensity across the screen is approximately uniform.

===Source of illumination===
The lamp technology of an overhead projector is typically very simple compared to a modern [[LCD projector|LCD]] or [[Digital Light Processing|DLP]] video projector. Most overheads use an extremely high-power halogen lamp that may consume up to 750 or 1000 watts.<ref name="Mangal">{{cite book |url=https://books.google.com/books?id=lioOxagHliIC&pg=PA225 |page=225 |title=Teaching of Social Studies |last1=Mangal |first1=S.K. |last2=Mangal |first2=Uma |publisher=PHI Learning |year=2008 |isbn=978-81-203-3534-9}}</ref> A high-flow blower is required to keep the bulb from melting due to the heat generated, and this blower is often on a timer that keeps it running for a period after the light is extinguished.

Further, the intense heat accelerates failure of the high intensity lamp, often burning out in less than 100 hours, requiring replacement, which is often the most expensive part of owning a projector.<ref name="Scott">{{cite web |url=https://www.outdoormoviehq.com/projector-lamp-guide/ |title=Classroom Projector Lamp Life Guide |last=Scott |first=Steve |website=outdoormoviehq.com |date=July 10, 2019 |access-date=November 2, 2019}}</ref> In contrast, a modern LCD or DLP projector often uses an [[Ultra-high-performance lamp]] which has a higher [[luminous efficacy]] and lasts for thousands of hours.<ref name="Derra">{{cite journal |url=https://www.researchgate.net/publication/238963512 |title=UHP lamp systems for projection applications |last1=Derra |first1=Guenther |last2=Moench |first2=Holger |last3=Fischer |first3=Ernst |last4=Giese |first4=Hermann |journal=[[Journal of Physics D|Journal of Physics D: Applied Physics]] |volume=38 |issue=15 |pages=2995–3010 |year=2005 |doi=10.1088/0022-3727/38/17/R01|s2cid=73578905 }}</ref> A drawback of that technology is the warm up time required for such lamps.

Older overhead projectors used a tubular quartz bulb which was mounted above a bowl-shaped polished reflector. However, because the lamp was suspended above and outside the reflector, a large amount of light was cast to the sides inside the projector body that was wasted, thus requiring a higher power lamp for sufficient screen illumination. More modern overhead projectors use an integrated lamp and conical reflector assembly, allowing the lamp to be located deep within the reflector and sending a greater portion of its light towards the Fresnel lens; this permits using a lower power lamp for the same screen illumination.

A useful innovation for overhead projectors with integrated lamps/reflectors is the quick-swap dual-lamp control, allowing two lamps to be installed in the projector in movable sockets. If one lamp fails during a presentation the presenter can merely move a lever to slide the spare into position and continue with the presentation, without needing to open the projection unit or waiting for the failed bulb to cool before replacing it.