STANDARD AUTOMOTIVE HALOGEN BULBS

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Halogen bulbs are standard light bulbs used in most automotive applications. Halogen light bulbs have very nice nature light. They provides light quality which is very close to natural light. Halogen light buts uses iodine from halogen group to prolong the lamp life and prevent is from blackening. The halogen iodine provides the cyclic process in which the light bulb can provide good efficiency at higher temperature with prolonged life. Due to this halogen in light bulb the bulb is not blackening with time and use.
Halogen bulbs are available in two basic types which are most commonly used in automotive. One type of light bulbs have single filament in them. They can only be used as low or high beam at a time. These bulbs have type number like H1,H3, H7, H9, H11, HB4 and HB3. There light bulbs had smaller glass tube and they are consuming less power. There are other type with high efficiency they are H7 and H1 bulbs. Where are the other type or the second type that is H4 light bulb which has two elements. One element is used at a time. These both elements could be used both for low and high beam in one single tube. There glass tube is a little bit thicker in diameter. There bulbs were very common a few decades ago in cars, trucks and buses.

Halogen bulb cyclic process
Supply of electronic energy makes the filament glow and metal evaporates off the filament. A halogen filling (iodine or bromine) inside the bulb lets the filament temperature rise to near the melting point of the tungsten (around 3 400°C) and this causes the high light output. In the immediate vicinity of the hot bulb wall, the evaporated tungsten combines with the filling gas (tungsten halogenide) to form a gas which is transparent.

However, once the gas gets back near the filament, it disintegrates due to the high filament temperature and forms an even tungsten layer. For the cyclic process to stop, the outer temperature of the bulb must be 300°C. The bulb made of quartz glass must closely enclose the filament. 
Another advantage is that a higher filling pressure can be used to counteract the evaporation of the tungsten. The gas composition in the bulb is also a major factor controlling the light output. Inclusion of a small amount of a noble gas such as xenon reduces the heat transfer away from the filament. However, despite the regeneration process inside the bulb, the tungsten filament is slowly consumed and thus service life is limited.