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Luminous Efficiency in Light Bulbs

October 26, 2011   

In a 120 volt, 100 watt “standard” bulb with a rated light output of 1750 lumens, the efficiency is 17.5 lumens per watt. This compares poorly to an “ideal” of 242.5 lumens per watt for one idealized type of white light, or 683 lumens per watt ideally for the yellowish-green wavelength of light that the human eye is most sensitive to.

Other types of incandescent light bulbs have different efficiencies, but all generally have efficiencies near or below 35 lumens per watt. Most household incandescent bulbs have efficiencies from 8 to 21 lumens per watt. Higher efficiencies near 35 lumens per watt are only achieved with photographic and projection lamps with very high filament temperatures and short lifetimes of a few hours to around 40 hours.The reason for this poor efficiency is the fact that tungsten filaments radiate mostly infrared radiation at any temperature that they can withstand. An ideal thermal radiator produces visible light most efficiently at temperatures around 6300 Celsius (6600 Kelvin or 11,500 degrees Fahrenheit). Even at this high temperature, a lot of the radiation is either infrared or ultraviolet, and the theoretical luminous efficiency is 95 lumens per watt.Of course, nothing known to any humans is solid and usable as a light bulb filament at temperatures anywhere close to this. The surface of the sun is not quite that hot.

There are other ways to efficiently radiate thermal radiation using higher temperatures and/or substances that radiate better at visible wavelengths than invisible ones. This is covered by Part II of the Great Internet Light Bulb Book, Discharge Lamps. The efficiency of an incandescent bulb can be increased by increasing the filament temperature, which makes it burn out more quickly.

 

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