Projection clock
From Wikipedia, the free encyclopedia
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. They are not very complex, and can be built by hobbyists.
The advantage of a projection clock is that the "clock" can be placed almost anywhere, even on surfaces difficult to install traditional clocks on. They can also be moved easily, and 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 a distance without glasses or contact lenses, while an ordinary clock of similar size would be costly and impractical.
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 Ferrari fans that project a Ferrari logo in addition to the time.
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[edit] History
Projection clocks were patented at least twice: once in 1909, and another time in 1940 [1]. Both patents have expired by now, so any company or hobbyist can build projection clocks.
Previously, projection clocks, like other clocks of that period, were universally analog. With the widespread adoption of digital clocks, digital projection clocks have also appeared.
[edit] Technology
A projection clock usually needs a backlight, as most clock displays do not radiate their own light. The backlight can use different lighting technologies, from incandescent bulbs to LEDs.
Projection clocks can be divided into two groups: low-brightness and high-brightness. The image created by low-brightness projection clocks can be viewed only in a darkened room, while high-brightness ones create an image that can be viewed even in lighted rooms, but are more power-hungry.
[edit] Low-brightness projection clocks
Most modern projection clocks use a red LED-based projector, and also an ordinary LED or LCD display. That is convenient, since these clocks are low-brightness, and the conventional display can be read when the room is not darkened and the image generated by projector is unreadable. Such projection clocks are often integrated into clock radios and home weather stations. Clock radios equipped with projectors are only slightly more expensive.
[edit] 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. They are more expensive than low-brightness ones.
Projectors used in projection clocks are similar to other projectors, and use the same optical principles. Most of them use lenses, but some use the principle of shadow theater, vector scanning or raster scanning.
[edit] Safety notes
Most projection clocks use mains power, and some also use other potentially dangerous technologies, including but not limited to halogen bulbs, lasers, etc. One should observe what potentially dangerous technologies are used in a projection clock he or she is going to build, buy or repair, and find and follow safety guidelines devoted to those technologies.
Using halogen bulbs in homemade projection clocks in not advised.
Projection clocks should be avoided by people having photosensitive epilepsy.
