Vehicle lighting technology variables
Here you will find useful basic information and handy tips relating to lighting technology and light sources in vehicles.
Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for use by suitably qualified personnel only.
Below you will find a summary of the most important basic terms in lighting technology and the respective units of measure for the evaluation of the properties of bulbs and lamps:
Unit: lumen [lm]
Luminous flux F is the term used to describe the complete light output radiated from a light source.
Unit: candela [cd]
The luminous intensity is the portion of the luminous flux radiating in a specific direction.
Unit: lux [lx]
Illuminance E specifies the ratio of the impinging luminous flux to the illuminated surface.
Illuminance is 1 lx when a luminous flux of 1 lm impinges an area of 1 m² evenly.
Unit: candela per square meter [cd/m2]
Luminance L is the impression of brightness the eye has from a luminous or illuminated surface.
Unit: lumen per watt [lm/W]
Luminous efficiency h specifies the rate of efficiency with which the consumed electrical power is transformed into light.
Unit: Kelvin [K]
Kelvin is the unit for colour temperature. The higher the temperature of a light source, the greater the proportion of blue and the lower the proportion of red is in the colour spectrum.
A bulb with warm white light has a colour temperature of approx. 2700 K. However, at 4250 K, a gas discharge lamp (D2S) has a cool white light that is more similar to daylight (approx. 5600 K).
Light sources are thermal radiators that produce light through thermal energy. This means the more strongly a light source is heated up, the higher its luminous intensity will be.
The low efficiency of the thermal radiator (8 % light radiation) only allows a relatively low luminous efficiency in comparison with gas discharge lamps (28 % light radiation). In recent years, LEDs have been used more and more as the light source in motor vehicles.
Bulbs (vacuum incandescent lights) are temperature radiators, since the tungsten filament is made to glow by the addition of electrical energy.
As already mentioned, the light output of a standard bulb is comparatively low. In addition, the vaporised tungsten particles that can be seen clearly as black marks on the bulb reduce all the technical lighting values, and the service life of such bulbs is relatively short.
The halogen bulb offers significant advantages in comparison to classic bulbs. Adding small quantities of halogen atoms, e.g. iodine, can reduce the blackening of the light bulb.
Thanks to the so-called "cycle process", halogen bulbs can be operated at higher temperatures with the same service life and thus offer greater efficiency.
There are two different types of halogen bulb available. The types H1, H3, H7, H9, H11 and HB3 only have one filament. They are used for low beam and high beam. The H4 bulb has two filaments, one for low beam and one for high beam.
The filament for low beam is fitted with a cover cap. This has the task of covering the dazzling share of the light and producing the cut-off line.
H1+30/50/90 and H4+30/50/90 are advanced developments of conventional H1 or H4 bulbs with an inert gas filling.
H7 bulbs have a higher luminance, lower power consumption, and better light quality in comparison to H1 bulbs. These are also available as H7+30/50/90.
Halogen bulbs with a blue finish have also been available for some time now. In contrast to conventional halogen bulbs, these bulbs produce a bluish-white light (up to 4000 K) and are thus closer to the colour of daylight. The light appears brighter and more contrasting to the eye. This should help drivers to drive for longer without fatigue. This impression is subjective, however.
Those who want maximum light output are better served by the +30/50/90 bulbs.
Up to now, lights with amber-coloured glass bulbs have been used as direction indicators. Magic Star indicator bulbs are also available for design-oriented drivers. When switched off, they are barely visible in the silver reflector. The characteristic amber light is only radiated at the usual brightness once they are switched on.
The application of several interference layers on the glass bulb of the light quenches certain shares of the light spectrum radiated by the filament. Only the amber share penetrates the layers and then becomes visible.
Gas discharge lamps generate light according to the physical principle of electrical discharge. Through the application of an ignition voltage from the ballast (up to 23 KV in 3rd generation HELLA ballasts), the gas between the lamp electrodes (filled with the inert gas xenon and a mixture of metals and metal halides) is ionized and made to glow with the aid of a light arc.
During the controlled feeding of alternating current (at approx. 400 Hz) the liquid and solid substances evaporate due to the high temperatures. The lamp only achieves its full brightness after a few seconds when all the components have been ionized.
To prevent destruction of the lamp through uncontrolled increases in current, the current is limited by a ballast. Once the full light output has been reached, an operating voltage (not the ignition voltage) of only 85 V is necessary to keep up the physical process. Luminous flux, luminous efficiency, luminance, and service life are significantly better than with halogen bulbs.
Gas discharge lamps are categorized according to their respective development version: D1, D2, D3, and D4. The "D" stands for "discharge". There are some major differences between the generations. The D1 lamps, for instance – the original xenon bulbs – have an integrated ignition section. D2 lamps, on the other hand, only consist of the socketed burner itself and, unlike all other development versions of automotive gas discharge lamps, have no exterior protective glass bulb around the discharge tube. All further developments have a UV protection bulb and are much more robust in their design.
The old D1 is often mistaken for the current D1 S/R lamp with integrated ignition module. Further developments of the D1 and D2 lamps, the D3 and D4 lamps, are more environmentally compatible, as they use no mercury. Due to different electrical parameters (42 V instead of 85 V arc voltage, with identical output), the D3 and D4 lamps cannot be used with the control units for D1 or D2 lamps.
|Halogen bulb (H7)||Gas discharge lamp|
|Light source||Filament||Light arc|
|Luminance||1450 cd/m2||3000 cd/m2|
|Power||55 W||35 W|
|Energy balance||8 % light radiation |
92 % heat radiation
|28 % light radiation |
58 % heat radiation
14 % UV radiation
|Service life||approx. 500 h||2500 h|
|Vibration-proof||to a certain extent||yes|
|Ignition voltage||no||yes 23,000 V (3rd generation)|
Despite regeneration within the bulb, the tungsten wire gradually becomes worn, thus limiting the service life.
The service life and the luminous efficiency depend to a large extent on the existing supply voltage, among other factors.
As a rule of thumb it can be said: If the supply voltage of a light is increased by 5%, the luminous flux increases by 20% but at the same time the service life is cut by half.
Our bulb configuration tool helps you find the right bulb quickly and easily. In order to find out which bulbs are compatible with your vehicle, select the vehicle make, vehicle class, and year of construction.
This way to the bulb configuration tool.