EFFICIENCY OF LIGHT SOURCES

Since the majority of light sources (natural or artificial) are blackbody radiators, this discussion would not be complete without a discussion of efficiency of various sources. Most practical sources of light involve a heated medium, and as such, most of the radiation emitted is in the infrared region of the spectrum at wavelengths too long to be usable by the eye as light. Some sources are more efficient than others at emitting light at visible wavelengths. To compare light output by various sources, we use a unit called a lumen, which represents the power of light emitted by a source. In terms of a practical light source, we measure efficiency as a function of light output to power input using the unit lumens per watt, which measures the number of lumens a light source emits for 1 W of input power. In theory, a perfect light source emitting at the peak sensitivity of the human eye could deliver 622 lm/W. Practical light sources fall far below this figure. Consider an oil lamp in which kerosene is drawn up through a wick and burned. This is an incandescent system in which heat from the flame causes particles of carbon to burn brightly in the flame. Although kerosene contains a large amount of energy in a given volume, it burns inefficiently, with an efficiency of about 0.1 lm/W. The vast majority of energy in the kerosene is converted to heat rather than light. Electric incandescent lamps with a tungsten filament fare better, with an efficiency of up to 20 lm/W. This still represents an enormous waste of energy, though, since over 90% of the emission from such a lamp is wasted as heat. Other artificial lamp technologies such as fluorescent lamps, provide higher efficiencies of about 90 lm/W, and discharge lamps such as the sodium lamp (often used for street lights and easily identified by the yellow color of the light emitted) have the highest efficiencies, at about 150 lm/W. Bear in mind that these last two technologies (fluorescent and sodium lamps) are not thermal light sources at all but atomic emission sources in which electrical energy is converted directly to light, not heat, as in the incandescent lamp.