The greater its value, the greater the luminous flux that can be used by the eye for a given power consumption of the lamp.
The light output of a lamp is made up of two factors: the radiation output of the lamp (proportion of the power consumed that is emitted as radiation) and the photometric radiation equivalent of the radiation emitted (sensitivity of the eye to this radiation):
The English term luminous efficacy can denote (luminous efficacy of radiation) or (overall luminous efficacy) depending on the context .
The greater this number, the greater the proportion of the power consumed that is emitted as electromagnetic radiation. Usually only part of the emitted radiation power is in the visible spectral range and can therefore be used as "light" by the eye.
Photometric radiation equivalent
The human eye is differently sensitive depending on the wavelength of the light. In order to describe the extent to which electromagnetic radiation can be used as visible light, the radiation power measured in watts is multiplied by a factor that describes the sensitivity of the eye and is strongly dependent on the wavelength . This factor is the photometric radiation equivalent. The result is the luminous flux , which is specified in the SI unit of lumen :
The greater K , the greater the luminous flux that can be used by the eye for a given radiant power of a light source. The eye is most sensitive to green light with a wavelength of 555 nm; For monochromatic light of this wavelength, K has its maximum possible value of 683 lm / W. Usually, however, light is a mixture of electromagnetic radiation of different wavelengths. K is then the weighted mean (“average”) of the photometric radiation equivalent of the individual wavelengths.
Light output of some light sources
An incandescent lamp converts the electrical power consumed almost completely into electromagnetic radiation. It can approximately be regarded as a Planck radiator . In this case, the photometric radiation equivalent depends strongly on the temperature of the radiator. Only when the red heat begins , part of the radiation is perceived as visible light, but it is still at the red wavelengths, to which the eye is less sensitive. At a temperature of 2800 K (the filament temperature of an incandescent lamp) the Planckian radiator has a radiation equivalent of 15 lm / W, with 6% of the radiation being emitted in the visible range from 400 to 700 nm. At a temperature of 6640 K, the Planck radiator reaches 96.1 lm / W, the maximum possible photometric radiation equivalent for Planck radiation.
Since most of the radiation emitted lies outside the visible spectral range, thermal emitters generally have only a low photometric radiation equivalent and, despite the high radiation yield, only achieve a low light yield. The light output can be increased through higher temperatures, but for this advantage you have to accept other disadvantages. In the case of incandescent lamps, for example, an increase in the operating voltage by 1% leads to an increase in output by 1.5 to 1.6% and the luminous flux by 3.4 to 4% (i.e. a better light yield), but also to a reduction in service life by 12 to 16%. An overvoltage of around 10% reduces the service life to around 50%.
In the case of some incandescent lamps that are operated briefly, a significantly shorter service life is accepted in order to achieve the highest possible light output. While a normal general-purpose incandescent lamp (100 W) achieves around 14 lm / W with a service life of 1000 hours, cinema projection lamps achieve 27 lm / W, but only have a service life of 100 hours. Narrow film lamps achieve 27.7 lm / W, but their service life is limited to 25 hours. The upper limit of the light output that can be achieved with incandescent lamps is around 40 lm / W.
Light sources such as fluorescent lamps or LED lamps achieve significantly lower radiation yields because of the lossy ballast electronics required, as well as the light generation, conversion and internal absorption losses. However, they emit a large part of the radiation generated in the visible range and therefore achieve significantly better light yields than incandescent lamps. The highest known luminous efficacy is achieved with 200 lm / W from light-emitting diodes and low-pressure sodium vapor lamps . The disadvantage of the latter, however, is their poor color rendering.
The energy efficiency class of the EU energy label provides information on the respective luminous efficacy of incandescent lamps, fluorescent lamps and halogen lamps for orientation when buying light sources . The energy efficiency class A stands for products with high luminous efficiency.
|Lamp type||Light output||Power consumption for 700 lumens|
|Lightbulb||10 to lm / W30||60 W|
|Energy saving lamp||50 to lm / W80||12 W.|
|Led lamp||60 to 160 lm / W||8 W.|
There is an extensive table with the light output in the article Light source # examples .
- At very low brightness the eye has a different sensitivity curve. The photometric radiation equivalent for night vision is denoted by K ' .
- The arbitrarily determined numerical value 683 lm / W results from the definition of the unit "lumen" from 1979. It was chosen so that the photometric units of measurement corresponded as closely as possible to their definition valid until 1979.
- International Electrotechnical Commission (IEC): International Electrotechnical Vocabulary , ref. 845-01-55, Luminous efficacy of a source - luminous efficacy of a radiation source (accessed on February 24, 2015)
- Terminology of the International Commission on Illumination
- International Electrotechnical Commission (IEC): International Electrotechnical Vocabulary , ref. 845-01-54, Radiant efficiency (of a source of radiation) (accessed February 24, 2015)
- T.W. Murphy, Jr .: Maximum Spectral Luminous Efficacy of White Light. Journal of Applied Physics 111 (2012), 104909 doi : 10.1063 / 1.4721897
- H.-J. Hentschel: Light and Lighting - Theory and Practice of Lighting Technology. 4th edition, Hüthig Buch, Heidelberg 1994, ISBN 3-7785-2184-5 , p. 129
- H.-J. Hentschel: Light and Lighting - Theory and Practice of Lighting Technology. 4th edition, Hüthig Buch, Heidelberg 1994, ISBN 3-7785-2184-5 , p. 128
- Carsten Meyer: Power LEDs with 200 lumens per watt. In: Heise online . December 31, 2012 . Retrieved November 30, 2015.
- H.-J. Hentschel: Light and Lighting - Theory and Practice of Lighting Technology. 4th edition, Hüthig Buch, Heidelberg 1994, ISBN 3-7785-2184-5 , p. 137