::.. What makes bulbs 'chrome' over?

[Qube]

I was replacing the 'halo' bulbs few days ago with blue 'factory chrome'd' ones and noticed that the regular 'yellow' halogen ones (#194) had a chrome finish on them. Why do bulbs do that?

[Adnan]

Hi,

If I recall correctly, halogen bulbs get very hot, such that the filament actually "evaporates" and then redeposits onto the filament, resulting in a halogen cycle. Remember that the inside is evacuated, which allows the metal to evaporate at a lower temperature than you'd normally expect.

If the wall of the bulb is a bit cooler than ideal, some of the metal deposits itself onto the glass. Since it's a molecular layer of metal, it's very reflective.

Regards,
Adnan