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A Shabbat lamp is a special lamp that has movable parts to expose or block out its light so it can be turned "on" or "off" while its power physically remains on. This enables the lamp to be used by Shabbat observant Jews to make a room dark or light during Shabbat without actually switching the electrical power on or off, an act that is prohibited during Shabbat.
One company manufacturing such lamps is Kosher Innovations, a Toronto-based company, based on a US patent application registered to Shmuel Veffer in 2004. Tens of thousands of units have been sold to observant Jews.
Halakha of the Shabbat lamp
There are various Jewish laws governing Shabbat lamps that allow them to be used during Shabbat.
According to some authorities, the lamp itself cannot be moved from its location due to the laws of muktzah. However, all authorities agree the component that is used to open or close the light is not attached, and under Jewish law, has the status of a lamp shade, which can be moved during Shabbat in order to control the amount of light that is exposed.
In addition, it is problematic in halacha to move a light because, historically, the most common lights were candles or other flames which could easily go out if moved. (This applies only to Shabbat; during a Yom Tov that does not coincide with Shabbat, the lamp may be moved if it is not unplugged, as moving a candle on such a day is permitted.) The restriction on moving candles may also apply to incandescent lights, which are generally treated in halacha as similar to fires. However, Shabbat lamps are lit by and only accept a compact fluorescent lamp. These bulbs do not contain electrical filaments like those found in incandescent light bulbs, and therefore, according to some rabbinical authorities, do not constitute a fire, but rather, an electrical appliance that is permissible to move on Shabbat.
Practically speaking, a normal lamp could be moved to a corner (according to some opinions) or covered on Shabbat in order to minimize its light. However, the Shabbat lamp has a close-fitting cover which is much more effective and safe at producing total darkness.
The compact fluorescent lamp that must be used also has the benefit of using a minimal amount of energy, as the lamp must remain on for a period of at least 25 hours.