HID (High Intensity Discharge)
In HID headlights, a mixture of rare metals and gases are heated to produce a bright white glow. HIDs are around two to three times brighter than halogen lamps and their glare can be particularly annoying for other road users.
Despite their brighter output, HID lamps require less power to operate. They generally draw about 35 watts and are said to be good for around 2000 hours of use.
In cars HID lights contain a trace amount of the inert noble gas xenon, hence their common name: xenon headlights. Xenon gas prevents automotive HID lights from flickering when they’re switched on and ensure that an adequate amount of illumination is provided before the headlights reach maximum brightness, usually within a few minutes.
Automotive HID lamps are easy to distinguish as their cool white glow is accompanied by a distinctive blue tinge around the edges. Some cars feature only HID lighting for their low beam, with high beam provided by a separate set of halogen lamps. Bi-xenon setups, however, are able to provide both high and low beam from the same set of HID lights.
LEDs (Light Emitting Diodes)
The working principle of an LED is quite hard to explain, but in short words, they rely on negative electrons moving against positive “holes” across a semiconductor. When a free electron falls into a hole that sits on a lower energy level, it will loose its energy which is released as a photon (the tiniest fraction of light) in a process called electroluminescence.
Although LED headlights currently fall a little short of the brightness achieved by HIDs, they hit maximum brightness within a millionth of a second compared with the half second required by incandescent and halogen lights. When LEDs are used in brake and indicator lights they’re said to improve the reaction time of other road users by around 30 per cent.
Touch a halogen bulb or xenon globe when it’s on and you’ll likely recoil in pain from the intense heat, but with LED headlights you’ll struggle to break a sweat. Therein lies their great advantage, they convert most of the energy directed to them into light not heat. In fact most of the heat generated by LEDs is at their electrical base, not on the bulb surface.