This article needs additional citations for verification. (February 2012) (Learn how and when to remove this template message)
A projection clock (also called ceiling clock) is an analog or digital clock equipped with a projector that creates an enlarged image of the clock face or display on any surface usable as a projection screen, most often the ceiling.
The clock can be placed almost anywhere, if only the projected image must be seen. The image generated by most projection clocks is large enough that a nearsighted person can see it from a distance without glasses or contact lenses. Clocks usually have a conventional display on their body, in addition to the projector,
Projection clocks are also used in advertising and merchandising. High-brightness analog projection clocks can superimpose a business' logo on top of the clock face, while there are low-brightness projection clocks designed for home use that project for example a logo in addition to the time.
Projection clocks were patented at least twice: once in 1909, and another time in 1940. Both patents have expired.
Early projection clocks were universally analog but with the widespread adoption of digital clocks, digital projection clocks became the standard.
There are low-brightness and high-brightness clocks. While the projection created by low-brightness clocks can be viewed only in a darkened room, high-brightness ones can also be viewed at bright light or daylight.
Low-brightness projection clocks
Most modern projection clocks have a red LED-based projector. Additional optional features not specific to projection clocks are the inclusion of a LED or LCD display in addition to the projector, an alarm function, and synchronisation to a broadcast time standard.
High-brightness projection clocks
Modern high-brightness projection clocks are in most cases analog, and have a halogen bulb backlight. In most cases they use a set of rotating and fixed transparent discs with hands and a face. An LCD is integrated into some clocks to combine analog and digital information on the projected image.
Projectors used in projection clocks are similar to other projectors, and use the same optical principles. They usually use lenses although some projectors use the principle of shadow theater[dubious ], vector or raster scanning.