Why are there TWO glass tubes on a bulb.
If you look at a bulb, you will notice the arc chamber is surrounded in glass, while there is a second glass tube covering the whole bulb. The reason is the two glasses do two different things. The inner glass is a type of glass that can handle the high temperatures and pressures generated by the arc, while letting light out. The outer glass acts as a UV filter to remove the large amounts of UV radiation produced my HID bulbs. This is why you should NEVER operate a bulb that has a cracked or missing outer glass cover. The UV radiation is so intense it can blind you and/or give you a horrible burn (equivalent to a sunburn, but can be as bad as 2nd and 3rd degree).
What are the electrodes in HID bulbs made out of?
In OEM bulbs the electrodes are made out of thoriated tungsten. Thoriated tungsten electrodes were introduced approximately fifty years ago as an alternative to the use of pure tungsten electrodes. Thoriated means that the electrodes contain 1-2% (by weight) of Thorium dioxide (ThO2). Thorium dioxide is added to the tungsten to promote electrode life, which helps the HID bulb last longer. The added thorium dioxide also promotes a more stable arc than an electrode made of pure tungsten.
How long do HID bulbs last?
Unlike halogen bulbs, which typically die when the filament snaps, HID bulbs have very few methods of mechanical failure. This results in bulbs can last an excessively long time (thousands and thousands of hours), but as time goes on the output of the bulb decreases. According to some spec sheets, at around 1500 hrs of use, an HID bulb is only emitting around 75% of its brightness. So assuming linear lifespan (which is likely not a valid assumption, but for clarities sake we will assume it is), at around 3000 hrs, the bulb is at 50% brightness. This means, a bulb may still be working long after its luminous output has fallen below that of a halogen bulb.