Editing Incandescent light bulb (section)

==Light output and lifetime==
{{See also|Lamp rerating}}
Incandescent lamps are very sensitive to changes in the supply voltage. These characteristics are of great practical and economic importance.

For a supply voltage ''V'' near the rated voltage of the lamp:

* ''Light'' output is approximately proportional to ''V''<sup> 3.4</sup>
* ''Power'' consumption is approximately proportional to ''V''<sup> 1.6</sup>
* ''Lifetime'' is approximately proportional to ''V''<sup> −16</sup>
* ''Color temperature'' is approximately proportional to ''V''<sup> 0.42</sup><ref name="StdHdbk11">[[Donald G. Fink]] and H. Wayne Beaty, ''Standard Handbook for Electrical Engineers, Eleventh Edition'', McGraw-Hill, New York, 1978, {{ISBN|0-07-020974-X}}, pg 22–8</ref>

A 5% reduction in voltage will double the life of the bulb, but reduce its light output by about 16%. Long-life bulbs take advantage of this trade-off in applications such as traffic signal lamps. Since electric energy they use costs more than the cost of the bulb, general service lamps emphasize efficiency over long operating life. The objective is to minimize the cost of light, not the cost of lamps.<ref name="GETP110" /> Early bulbs had a life of up to 2500 hours, but in 1924 the [[Phoebus cartel]] agreed to limit life to 1000 hours.<ref>{{cite web |title=The Great Lightbulb Conspiracy |url=https://spectrum.ieee.org/the-great-lightbulb-conspiracy |work=IEEE Spectrum |publisher=IEEE |first=Markus |last=Krajewski |date=24 September 2014|url-status=live |archive-url=https://web.archive.org/web/20141106040631/https://spectrum.ieee.org/geek-life/history/the-great-lightbulb-conspiracy |archive-date=6 November 2014}}</ref> When this was exposed in 1953, General Electric and other leading American manufacturers were banned from limiting the life.<ref>{{cite web|url=http://www.mercurynews.com/ci_17287091|title=Tests shine light on the secret of the Livermore light bulb|date=6 February 2011|url-status=live|archive-url=https://web.archive.org/web/20120310100846/http://www.mercurynews.com//ci_17287091|archive-date=10 March 2012}}</ref>

The relationships above are valid for only a few percent change of voltage around standard rated conditions, but they indicate that a lamp operated at low voltage could last much longer than at rated voltage, albeit with greatly reduced light output. The "[[Centennial Light]]" is a light bulb that is accepted by the ''[[Guinness Book of World Records]]'' as having been burning almost continuously at a [[fire station]] in [[Livermore, California]], since 1901. However, the bulb emits the equivalent light of a four watt bulb. A similar story can be told of a 40-watt bulb in Texas that has been illuminated since 21 September 1908. It once resided in an [[opera house]] where notable celebrities stopped to take in its glow, and was moved to an area museum in 1977.<ref>{{cite web|url=http://www.homelighting.com/article.cfm?intarticleID=880 |title=Watts Up? – A parting look at lighting|archive-url=https://web.archive.org/web/20090207211654/http://www.homelighting.com/article.cfm?intarticleID=880 |archive-date=7 February 2009}}</ref>

[[Photoflood|Photoflood lamps]] used for photographic lighting favor light output over life, with some lasting only two hours. The upper temperature limit for the filament is the melting point of the metal. Tungsten is the metal with the highest melting point, {{convert|3695|K|°C °F}}. A 50-hour-life projection bulb, for instance, is designed to operate only {{convert|50|°C|°F}} below that melting point. Such a lamp may achieve up to 22 lumens per watt, compared with 17.5 for a 750-hour general service lamp.<ref name="GETP110" />

Lamps of the same power rating but designed for different voltages have different luminous efficacy. For example, a 100-watt, 1000 hour, 120-volt lamp will produce about 17.1 lumens per watt. A similar lamp designed for 230&nbsp;V would produce only around 12.8 lumens per watt, and one designed for 30 volts (train lighting) would produce as much as 19.8 lumens per watt.<ref name="GETP110" /> Lower voltage lamps have a thicker filament, for the same power rating. They can run hotter for the same lifetime before the filament evaporates.

The wires used to support the filament make it mechanically stronger, but remove heat, creating another tradeoff between efficiency and long life. Many general-service 120-volt lamps use no additional support wires, but lamps designed for "[[rough service]]" or "vibration service" may have as many as five. Low-voltage lamps have filaments made of heavier wire and do not require additional support wires.

Very low voltages are inefficient since the lead wires would conduct too much heat away from the filament, so the practical lower limit for incandescent lamps is 1.5 volts. Very long filaments for high voltages are fragile, and lamp bases become more difficult to insulate, so lamps for illumination are not made with rated voltages over 300 volts.<ref name="GETP110" /> Some infrared heating elements are made for higher voltages, but these use tubular bulbs with widely separated terminals.

<gallery widths="200" heights="200">
File:Livermore Centennial Light Bulb.jpg|The [[Centennial Light]] is the longest-lasting light bulb in the world.
File:Various lighting spectrums - Flurescent incandescent diode and candle.jpg|Various lighting spectra as viewed in a [[diffraction grating]]. Upper left: fluorescent lamp, upper right: incandescent bulb, lower left: white LED, lower right: candle flame.
</gallery>