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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 on any suitable projection screen, most often the ceiling.
The advantage of a projection clock is that the "clock" can be placed almost anywhere. Depending on the projector used, the image can be in monochrome or color. The image generated by most projection clocks is large enough that a nearsighted person can see it from distance without glasses or contact lenses.
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 may be connected to and synchronize with an Atomic clock to self adjust their time accurately.
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 daylight or in illuminated indoor places.
Low-brightness projection clocks
Most modern projection clocks use a red LED-based projector, and also an ordinary LED or LCD display to enable time reading when ambient light is too bright for the projection to be seen. They are often integrated into alarm clocks.
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. Sometimes, an LCD is integrated into this set 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.