Our guide to the growing number of headlight options
Headlamps come in halogen, xenon, bi-xenon, LED and laser form and each has its pros and cons.
Halogen bulbs represent the lower end of the market. They incorporate a quartz envelope that contains gas from the halogen group. There is also a filament. When the filament receives an electric charge it gets hot enough to produces light. The process is called incandescence. The light is then projected via reflectors and a lens onto the road.
Halogen bulbs are compact, lightweight, easy to manufacturer, available with a wide range of power outputs, and produce their maximum intensity within a short period. Within a headlight assembly, they can serve as both the low beam and high beam bulb.
The downside is that they generate a tremendous amount of heat and – as their purpose is to produce light – this energy is wasted. The heat is enough to burn a motorist's hand.
Xenon headlamps are also known as high-intensity discharge headlamps. They typically produce more light, consume less energy, generate less heat and illuminate a larger section of road than halogens. Life expectancy is also longer which minimises time off the road, and repair costs. A downside is that they are more likely to dazzle oncoming drivers.
Light is produced by an electric arc that forms between two electrodes. These are encased in a compact quartz envelope that is filled with xenon gas. As with halogen lamps, they can be used in a headlight assembly as both the low and high beam bulb.
Bi-Xenon headlamps incorporate one bulb that produces low and high beam light. When low is selected by the motorist, the beam is partially shielded by a shutter. Switching to high slides the shutter out of the beam's path to increase the amount of light on the road.
LED headlamps incorporate a number of small LEDs. These require very little power and there is no filament to degrade over time and burn out (as with halogen, xenon and bi-xenon). LEDs are illuminated by the movement of electrons within a semiconductor material. They also produce a strong beam of light and should last the life of the vehicle.
The BMW i8 will soon be available with optional lases lights. This system - that produces more light than LED equivalents and requires very little power - incorporates three blue lasers positioned at the rear of the assembly. These fire through a prism and emerge as a single beam. It then passes through a lens that creates white light which is safer for the human eye.
BMW claims that its laser-powered headlamps save energy compared to older systems. They are also more compact which creates space under the bonnet for other components.
Laser lights could be common in the future, subject to legislation. Whereas BMW's are legal in some countries – including the UK - the law would have to be changed in the USA.