Energy usage for different types of light bulbs operating at different light outputs. Points lower on the graph correspond to lower energy use.

Approximately 90% of the power consumed by an incandescent light bulb is emitted as heat, rather than as visible light.

Luminous efficacy of a light source may be defined in two ways. The radiant luminous efficacy (LER) is the ratio of the visible light flux emitted (the luminous flux) to the total power radiated over all wavelengths. The source luminous efficacy (LES) is the ratio of the visible light flux emitted (the luminous flux) to the total power input to the source, such as a lamp. Visible light is measured in lumens, a unit which is defined in part by the differing sensitivity of the human eye to different wavelengths of light. Not all wavelengths of visible electromagnetic energy are equally effective at stimulating the human eye; the luminous efficacy of radiant energy (LER) is a measure of how well the distribution of energy matches the perception of the eye. The units of luminous efficacy are "lumens per watt" (lpw). The maximum LER possible is 683 lm/W for monochromatic green light at 555 nanometres wavelength, the peak sensitivity of the human eye.

The luminous efficiency is defined as the ratio of the luminous efficacy to the theoretical maximum luminous efficacy of 683 lpw, and, as for luminous efficacy, is of two types, radiant luminous efficiency (LFR) and source luminous efficacy (LFS).

The chart below lists values of overall luminous efficacy and efficiency for several types of general service, 120-volt, 1000-hour lifespan incandescent bulb, and several idealized light sources. The values for the incandescent bulbs are source efficiencies and efficacies. The values for the ideal sources are radiant efficiencies and efficacies. A similar chart in the article on luminous efficacy compares a broader array of light sources to one another.

For a given quantity of light, an incandescent light bulb produces more heat (and thus consumes more power) than a fluorescent lamp. In buildings where air conditioning is used, incandescent lamps' heat output increases load on the air conditioning system. Heat from lights will displace heat required from a building's heating system; generally space heating energy is of lower cost than electricity.

High-quality halogen incandescent lamps have higher efficacy, which will allow a 60-watt bulb to provide nearly as much light as a non-halogen 100-watt bulb. Also, a lower-wattage halogen lamp can be designed to produce the same amount of light as a 60-watt non-halogen lamp, but with much longer life.

Many light sources, such as the fluorescent lamp, high-intensity discharge lamps and LED lamps offer higher efficiency, and some have been designed to be retrofitted in existing fixtures. These devices produce light by luminescence, instead of heating a filament to incandescence. These mechanisms produce discrete spectral lines and so don't have the broad "tail" of wasted invisible infrared emissions. By careful selection of which electron energy level transitions are used, the spectrum emitted can be tuned to mimic the appearance of incandescent sources, or other different color temperatures of white light.

An LED lamp (or LED light bulb) is a solid-state lamp that uses light-emitting diodes (LEDs) as the source of light. LED lamps offer long service life and high energy efficiency, but initial costs are higher than those of fluorescent and incandescent lamps. Chemical decomposition of LED chips reduces luminous flux over life cycle as with conventional lamps.

Because the eye's sensitivity changes with the wavelength, the output of lamps is commonly measured in lumens, a measure of the power of light perceived by the human eye. The luminous efficacy of lamps is the number of lumens produced for each watt of electrical power used. The luminous efficacy of a typical CFL (compact fluorescent lamp) is 50–70 lumens per watt (lm/W) and that of a typical incandescent lamp is 10–17 lm/W. Compared to a theoretical 100%-efficient lamp (680 lm/W), these lamps have lighting efficiency ranges of 9–11% for CFLs and 1.9–2.6%, for incandescents.

See also:
Incandescent Light Bulb
Luminous Efficacy
Compact Fluorescent Lamp
LED Lamp

Source: Wikipedia (All text is available under the terms of the Creative Commons Attribution-ShareAlike License)