Why is light created


The light visible to humans covers the range from 380 nm to 780 nm. It is precisely this range that is uniformly available on earth as solar radiation. The eye is extremely adaptable. It can recognize and differentiate between objects in moonlight with 0.1 lx to approx. 0.3 lx and in glaring sunlight with an illuminance of approx. 100,000 lx.

Fire, or more precisely the self-luminous flame of fire, was the first artificial light source for humans. As with sunlight, the glowing carbon particles in the flame produce light with a continuous spectrum. From the torch, the pine chip to the candle and the oil lamp to the gas light, the light generation worked on this principle. In the second half of the 19th century, with the invention of the incandescent mantle for gas lighting, the principle of the self-illuminating flame became obsolete. Almost simultaneously, the electric arc lamps and the incandescent lamps developed. The first discharge lamps soon followed. The beginning of the 1960s is considered to be the hour of birth of the first industrially manufactured LEDs, which, however, only began to triumph in the 21st millennium.

Electric light sources
All light sources today are based on the conversion of electrical energy into light using different processes. There are three main groups:

  • Thermal radiators (incandescent lamps, halogen lamps)
  • Discharge lamps (low pressure, high pressure discharge lamps)
  • Electroluminescence from semiconductor crystals (LED)

Temperature radiator
The thermal radiators essentially include incandescent lamps and halogen lamps in different versions (low-voltage halogen lamps, high-voltage halogen lamps). What they all have in common is that a metal coil - the filament - starts to glow when an electrical current flows through it. The higher the temperature, the shorter the wavelengths - from the red glow of the filament (mostly made of tungsten) to the typically warm white light of the incandescent lamp. According to this principle, most of the electrical energy is converted into heat (95%) and only 5% into light. At higher temperatures, the tungsten begins to evaporate faster and faster, which leads to the blackening of the glass bulb and sooner or later to the filament burning through (average service life 1,000 h). In order to stop the loss of material from the filament, the idea of ​​adding halogens to the gas filling of the lamp came up. The resulting halogen cycle prevents tungsten atoms from depositing on the bulb wall. This results in a longer service life, a higher light output and a brighter, more brilliant light. The end of the incandescent lamp began in Germany in September 2009, since then different types have not been manufactured and taken off the market. Since September 2012, the manufacture and sale of all incandescent lamps is no longer permitted.

Discharge lamps
With the discharge lamp, the light is created by exciting noble gases or metal vapors. In a gas-filled discharge vessel (e.g. a fluorescent tube), a voltage between two electrodes generates a current of electrons. On the way through the discharge vessel, the electrons collide with gas atoms, which are excited to emit radiation when the speed is sufficient. Depending on the gas used, a light with a characteristic wavelength is created. In the case of fluorescent lamps, the inside of the discharge tube is coated with phosphors that convert the UV radiation from the lamp into visible light by means of fluorescence. There are essentially two groups:

  • Low-pressure discharge lamps (fluorescent lamps, compact fluorescent lamps, low-pressure sodium vapor lamps)
  • High pressure discharge lamps (mercury vapor lamps, metal halide lamps, high pressure sodium vapor lamps)

An LED is a light-emitting diode (light-emitting diode). The name arose from the abbreviation of the English term Light Emitting Diode. The light is created here by electroluminescence from semiconductor crystals. From an electrical point of view, an LED is a pn junction, i.e. a diode that is operated in the forward direction. Visible light is produced during the electron transfer. The light color depends on the doping of the pn junction with other elements. The most difficult thing to do is to make white LEDs. It is possible to mix several monochrome chips with the basic colors red, green and blue in the correct ratio or to stimulate phosphors with a yellow-green spectral range with blue LEDs. In both cases, white is created.

The light output has been increased considerably in recent years. While it was around 40 lm / W in 2005, more than 100 lm / W is already achieved today with series-produced LEDs. Values ​​of up to 250 lm / W can arise under laboratory conditions. This makes them particularly energy efficient.

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