Press release

Glossary: Technical terms relating to lighting.


  • Headlamp technology in detail: halogen, xenon and LED
  • Kelvin, lumen, lux, etc.: the unusual units employed to classify lamps

Halogen, LED and xenon headlamps – how do they differ in terms of technology and luminous intensity? Engine output is specified in kilowatts and horsepower, luminous intensity in candela.

Candela – the measure of light intensity
The unit candela (cd) defines the light flux which is emitted from a light source in a specific direction.

Colour rendition index
Defines how well or faithfully an illuminated colour is rendered. Colours are rendered differently by different lamps, even by lamps with an identical colour temperature. The colour rendition index is specified in Ra or CRI. An Ra value of 100 corresponds to the best possible colour rendition, with the colours rendered faithfully.

Halogen headlamps: standard in vehicle manufacturing
Halogen headlamps are standard on most vehicles today. The first single-filament halogen headlamps were launched in the mid-1960s, followed just under ten years later by the two-filament halogen lamp H4 for combined low and high beam in a single lamp. Halogen lamps have a whiter light than the previously employed bulbs and a noticeably higher light output. Halogens are gaseous substances in the bulb.

Halogen H4 lamps are two-filament lamps (bilux bulbs) incorporating low beam and high beam in a single housing. Halogen H7 headlamps are single-filament lamps in separate housings for low and high beam.

Kelvin – the colour temperature
The colour of white lamps can vary greatly. It is specified in kelvin (K). The spectrum is vast: lamps with a reddish-yellow light output are perceived as being warm, while lamps emitting a blueish light are perceived as cold. Lamps are warm white up to 3300 kelvin, neutral white up to 5300 kelvin, daylight white above 5300 kelvin. Halogen headlamps from Setra and Mercedes-Benz lie in the range of 3200 kelvin, xenon headlamps in the 4000 kelvin range and LED headlamps in the 5500 kelvin range.

LED headlamps: the new state of the art
LED headlamps (LED = light-emitting diode) currently represent the state of the art for series production vehicles. They require only a fraction of the energy consumed by other lamps and their service life surpasses that of conventional headlamps many times over. Their price is currently roughly on a par with that of xenon headlamps, and they also attain the latters' luminous power.

LED lamps are still in their infancy in the field of vehicle manufacturing. Initially approved for rear lights only, they have been available as headlamps for almost ten years now, after first of all requiring a special licence. Due to the limited light flux, several LEDs are bundled together in a headlamp. Multibeam LED headlamps are the latest development. In these headlamps the LEDs are controlled individually, enabling the light from the headlamp to be modelled as required.

LEDs consist of several layers of semi-conductor compounds.The type and composition of the semi-conductors determine the LED's light colour and light output. The LED is encased in plastic, which defines the radiating characteristic. When current flows in the direction from the anode to the cathode, light is generated.As individual LEDs generate a comparatively low light flux, they are deployed in bundles for vehicle headlamps, for example.

Luminous intensity distribution curve
This defines how much light is transmitted by a lamp at a certain angle. On the basis of this information it can be established whether the light cone of a lamp is narrow or wide and how much light is emitted in a certain direction.

Lumen – light flux
The light flux of a light source is defined in lumen (lm). As such, this unit specifies the luminous power which is generated by a light source.

Lux – illumination value
The illumination of a lamp is measured in lux (lx). This specifies how much light from a light source arrives at a specific surface. Both the distance from the light source and the angle are defined. The calculation method is simple: luminous intensity (candela) divided by distance in metres squared.

Xenon headlamps: the state of the art in lighting technology
Xenon headlamps have been used in passenger cars since the beginning of the 1990s and defined the state of the art until recently. As they are highly complex and thus expensive, they only come as standard on top-class passenger cars and buses.

Xenon is a highly condensed noble gas contained in a very small combustion chamber made of fused quartz glass. This gas is responsible for the high light flux. In place of the filament in a halogen lamp or classic light bulb, this type of lamp operates by means of an arc between two electrodes. For the purpose of ignition, one ballast is required per lamp, generating a high voltage in the order of 25 000 V.

In order to prevent the strong light emitted by xenon headlamps from dazzling oncoming traffic, a headlamp washer system is mandatory from a luminous intensity of 2000 lumen, as well as automatic range adjustment – on omnibuses this function is performed by the electronic level control system of the air suspension. Bi-xenon headlamps combine low and high beam in a single lamp, switching between the two by means of a screen which is operated via a magnetic motor.