Editing Overhead projector (section)

===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.