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.

Vehicle lighting technology variables

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:

Lighting technology variables: Luminous flux

Luminous flux Φ

Unit: lumen [lm]

Luminous flux F is the term used to describe the complete light output radiated from a light source.

Luminous intensity I

Unit: candela [cd]

The luminous intensity is the portion of the luminous flux radiating in a specific direction.

Lighting technology variables: Luminous intensity

Illuminance E

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.

Lighting technology variables: Illuminance

Luminance L

Unit: candela per square meter [cd/m2]

Luminance L is the impression of brightness the eye has from a luminous or illuminated surface.

Luminous efficiency ŋ

Unit: lumen per watt [lm/W]

Luminous efficiency h specifies the rate of efficiency with which the consumed electrical power is transformed into light.

Colour temperature K

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

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.

Bulb

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.

Halogen bulb

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.

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.

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.

Advantages/differences of halogen bulbs compared with standard bulbs

Filament thinner
Can be operated at higher temperatures
Higher luminance, up to 30/50/90% more between
50 and 100 meters in front of the vehicle and a range of illumination increased by up to 20 meters
More driving safety at night and in adverse weather conditions

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

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.

Direction indicators

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

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.

Comparison between filament (halogen) / light arc gas discharge lamp (xenon)

Comparison between halogen filament and xenon light arc

	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)
Electronic control	no	yes

Vehicle bulbs have to be standardized in compliance with ECE-R37 or R99. This makes bulb replacement possible while at the same time avoiding mix-ups with other bulbs.

The following lettering can be found on bulbs

Name of the manufacturer
6 or 6 V, 12 or 12 V, 24 or 24 V stands for the rated voltage in compliance with ECE regulation 37.
H1, H4, H7, P21 W stands for the international category description of ECE-standardised bulbs e.g. 55 W.
E1 indicates which country the light source has been tested and approved in. "1" stands for Germany.
"DOT" means that it is also approved for use on the American market.
"U" stands for UV-reduced bulbs, according to ECE. The bulbs are used in headlamps with plastic cover lenses, for example.
The approval mark granted by the appropriate authorities, e.g. E1 (Federal Department of Motor Vehicles in Flensburg), is inscribed on the bulb and is either 37 R (E1) + a five-digit number or only (E1) + a three-digit number (sometimes alphanumerical codes, see illustration).
Most bulbs have a coded manufacturer's mark. This makes traceability to the manufacturer possible.
Since not all bulbs have enough room for the markings, the legislator prescribes only the following obligatory information: Manufacturer, output, inspection tag, and approval mark.

Factors that influence a vehicle interior light source

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

Negative factors of influence

Mechanical loads through impact and vibrations
High temperatures
Switching-on process
Voltage spikes
Excessive on-board voltage
High luminance due to extreme
Filament density

Positive factors of influence

Filling pressure
Filling gas

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.

Lighting technology tips - light sources

Lighting technology tips for light sources

Xenon headlights require a high voltage for ignition, which is why the ballast voltage supply connector should always be removed before any work is carried out on the headlamps.
When using a new bulb , never touch the new glass bulb since fingerprints will be burnt on and make the bulb opaque.
If a xenon bulb breaks in a closed room (workshop), the room should be ventilated to prevent a health hazard due to toxic gases. D3 and D4 xenon lamps no longer contain mercury and are therefore more environmentally compatible.
Standard filament and halogen bulbs do not contain any materials which are problematical from an environmental point of view, and can be disposed of with normal household waste.
Xenon bulbs are special waste. If the bulb is faulty but the interior glass bulb is still intact, it has to be disposed of as special waste since the gas/metal vapor mixture contains mercury and is thus extremely toxic when inhaled. If the glass bulb has been destroyed e.g. in an accident, the xenon bulb can be disposed of with normal household waste since the mercury will have evaporated.
In D3 and D4 xenon lamps, the mercury was replaced with non-toxic zinc iodide. These bulbs can be disposed of with normal household waste.
The waste code for disposal is: 060404.
There are no separate tips for LEDs, as these usually cannot be replaced.