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 Halogen headlights ? Basically it is a way to make electric lights appear brighter by tinkering with the spectrum of the light, by adjusting the chemical composition of the filament and gas in the bulb. In a rainbow, some colors seem less intense than others because our eyes have more photo-receptors for some colors, by adjusting the color of a lamp to what we see more easily then we see more clearly and what we see appears to be brighter. 

halogen |?hal?j?n|
noun Chemistry
any of the elements fluorine, chlorine, bromine, iodine, and astatine, occupying group VIIA (17) of the periodic table. They are reactive nonmetallic elements that form strongly acidic compounds with hydrogen, from which simple salts can be made.
• [as adj. ] denoting lamps and radiant heat sources using a filament surrounded by the vapor of iodine or another halogen : halogen headlights.
DERIVATIVES
halogenic |?hal??jenik| adjective
ORIGIN mid 19th cent.: from Greek hals, halo- ‘salt’ + -gen .