Light Bulb Efficiency Comparisons
Approximately 90% of the power consumed by an incandescent light bulb is emitted as heat, rather than as visible light.
The effectiveness of an electric lighting source is determined by two factors – the relative visibility of electromagnetic radiation, and the rate at which the source converts electric power into electromagnetic radiation.
Luminous efficacy of a light source is a ratio of the visible light energy emitted ( the luminous flux) to the total power input to the 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 is a measure of how well the distribution of energy matches the perception of the eye. The maximum efficacy possible is 683 lm/W for monochromatic green light at 555 nanometres wavelength, the peak sensitivity of the human eye. For white light, the maximum luminous efficacy is around 240 lumens/watt, but the exact value is not unique because the human eye can perceive many different mixtures of visible light as “white”.
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. A similar chart in the article on luminous efficacy compares a broader array of light sources to one another.
| Type | Overall luminous efficiency | Overall luminous efficacy (lm/W) |
|---|---|---|
| 40 W tungsten incandescent | 1.9% | 12.6 |
| 60 W tungsten incandescent | 2.1% | 14.5 |
| 100 W tungsten incandescent | 2.6% | 17.5 |
| glass halogen | 2.3% | 16 |
| quartz halogen | 3.5% | 24 |
| high-temperature incandescent | 5.1% | 35 |
| ideal black-body radiator at 4000 K | 7.0% | 47.5 |
| ideal black-body radiator at 7000 K | 14% | 95 |
| ideal monochromatic 555 nm (green) source | 100% | 683 |
Unfortunately, the spectrum emitted by a blackbody radiator does not match the sensitivity characteristics of the human eye. Tungsten filaments radiate mostly infrared radiation at temperatures where they remain solid (below 3683 kelvins / 3410°C / 6,170°F). Donald L. Klipstein explains it this way: “An ideal thermal radiator produces visible light most efficiently at temperatures around 6300 °C (6600 K or 11,500 °F). 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.” No known material can be used as a filament at this ideal temperature, which is hotter than the sun’s surface. An upper limit for incandescent lamp luminous efficacy is around 52 lumens per watt, the theoretical value emitted by tungsten at its melting point.
For a given quantity of light, an incandescent light bulb produces more heat (and consumes more power) than a fluorescent lamp. Incandescent lamps’ heat output increases load on air conditioning in the summer, but the heat from lighting can contribute to building heating in cold weather.
High-quality halogen incandescent lamps have higher efficacy, which will allow a 60 W bulb to provide nearly as much light as a non-halogen 100 W. Also, a lower-wattage halogen lamp can be designed to produce the same amount of light as a 60 W 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 produced by incandescent emitters. By careful selection of which electron energy level transitions are used, the spectrum emitted can be tuned to either mimic the appearance of incandescent sources or else produce different color temperatures of white for visible light.