Why the luminous flux decreases?

With in the first hundred hours burning time the luminous flux of a HID xenon bulb decreases faster, then there after the decrease of lumens is slightly reduced. This behaviour of the HID bulb indicates that we have more than one process of bulb aging involved. At the first burning hours it is very unlikely event that burned off material from the electrodes is condensed on the quarz arc burning compartment and is responsible for the fast light output reduction. Inside the burner compartment there are alkali metal salts with a defined ratio dazugefügt especially the yellow emitting sodium Na + . At high burning temperatures a certain amount of sodium will penetrating inside the surface of the quarz and starts acting as a filter (phase a of first 100 h). There will be an equilibrium of condensing and vaporising however the coating thickness will grow by penetration slightly (phase b after 500h). The following pictures shall explain involved process:

Quarz glas is basically a tetraeder structure of SiO2. However there is no periodically structure inside a glas at all (right picture b). Glas is a kind of fluid with a randomly arrangement of ground structure which has an extrem high viscosity when it is condensed. Bild 4 taken out of "Ch. Kittel: Festkörperphysik.

Quarz glas has even more imperfect areas of wider space compared to a perfect lattice structure shown in above picture 30.6 in such areas easily a small Na + ion can jump. At high temperatures you can doping glas or coating the surface. Picture taken out of "Ashcroft Mermin: Solid State Physics". Inside the host lattice now there are more positive (Na+) ions averaged and they can attract and bind higly localised electrodes which are optical active. Depending of the geometric structure in detail there are different kinds of bond electrodes, they are named "Farbzentrum" or F-center (color center). The F-center can absorp light and energy and emmit again light. The emitted energy must not have the same amount and wave lenght of light, because the host lattice can be stimulated to vibrations that the flux of originale wavelenght is decreasing.

Picture taken out of 30.5 "Ashcroft Mermin: Solid State Physics" shows broadband absorption of visible light inside a host lattice. Because glas has a random structure the kind of absorption might be even more of broadband nature.

Picture taken out of "Ch. Kittel: Festkörperphysik" shows different ways of a diffusion process indide a solid state. Inside a glas the process is named hopping. The diffusion is faster at higher temperatures or activated and it is a randomly statistical process to obtain finaly an equal concentration.

The diffusion of positive charged Na+ ions must be slowed down to avoid further decreasing and absorption of light to maintain transparency of the glas for a longer life time.

  • Avoid thermal oveloading of bulb by too high electrical power - 50W electrical power shall be supplied to 50W bulbs only like the extra bigger designed DL50.
  • An external applied averaged electrical field in between the two bulb electrodes and the surrounding potential given by a small metal reflector or other conducting grounded parts shall attract the positive ions in the direction of the arc center and slowing down the diffusion process. When HID was new the bulb manufacturers demands it - mean while no one takes care about, because the higher system cost and the involved effect is too small to do it.

The above mentioned aging process is the same kind within LED emitters. The crystal defect inside the LED will not cause a diode to fail electrically, it becomes just less effective in emitting light. The LED is degrading more and more to a NON EMITTING DIODE NED. This starts already from 80°C There fore LED-TEMPERATURE-PROTECTION is the only answer to this type of vehicle lighting !