Basic principles of car lighting technology

Basic principles of vehicle interior lighting technology

Lighting technology plays a very important role in motor vehicles with regard to the safety of vehicle occupants and that of other road users. On this page, we will explain to you the basic principles of automotive lighting technology and show you the design and function of the most common light sources. You will also find the causes for failure of light sources, as well as practical tips for their replacement.

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 suitably qualified personnel only.

Cycle process in a halogen bulb

The tungsten filament is made to glow by the addition of electrical energy. This leads to metal evaporating from the filament. Thanks to a halogen filling (iodine or bromine) in the light, the filament temperatures increase to almost the melting point of tungsten (approx. 3400 °C).

This results in high light output. In the direct vicinity of the hot bulb wall, the evaporated tungsten combines with the filling gas to form a translucent gas (tungsten halide). If the gas approaches the filament again, it breaks down due to the high filament temperature and forms a homogenous tungsten layer.

To keep this cycle going, the outside temperature of the light bulb must be 300 °C. To achieve this, the quartz glass bulb has to fit closely round the filament.
A further advantage is that a higher filling pressure can be used, thus combating tungsten evaporation.

The gas composition in the bulb is decisive for the luminous efficiency. The addition of small amounts of inert gases such as xenon reduces heat dissipation from the filament.

Despite regeneration within the bulb, the tungsten wire gradually becomes worn, thus limiting the service life.

Factors that influence a vehicle interior light source

Filling pressure
Filling gas

Positive factors of influence

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.

Factors of influence: Service life vs. voltage increase diagram

For this reason, series resistors are used in some vehicle types to prevent the supply voltage from exceeding 13.2 V. In today's modern vehicles, the voltage is adjusted through pulse-width modulation.

In the case of undervoltage, e.g. if the alternator is faulty, the opposite is the case. The light now has a significantly higher red share and the luminous efficiency is correspondingly lower.