The vast majority of technically used light sources belong to the categories of gas discharge lamps, incandescent bulbs or light emitting diodes (LED). They release energy in the form of light. So they can, usually electrical energy must be supplied. The total amount of light emitted is also known as Luminous Flux and measured in Lumen (lm). A statement on the spatial distribution is not included herein, however, the Spektral Sensivity Function or Luminosity Function of the human eye has already been considered Therefore, the "lumen" is used only in the visible wavelength range. In the invisible infrared or ultraviolet region directly the power output in Watt (or Milliwatt) is used instead. The luminous flux is measured often with the aid of an integrating sphere. The luminous flux is a characteristic of light sources in the visible range, but contains no information about the spectral composition of the emitted light.
Typical values for the luminous flux of the standard low-voltage halogen lamps are (approximately):
- 950lm at 50W,
- 650lm at 35W and
- 350lm at 20W electric power supplied.
It should be noted that the light flux to a large extent depends on how the manufacturer has designed the lamp: A higher filament temperature produces more light, but causes durability problems.
Fluorescent lamps are much more efficient. They achieve a luminous flux of 3000lm at 40W supplied electrical energy.
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