Basic principles of LED car lights – Definition, structure, and function
Here you will find useful information and important tips relating to LED headlamps 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.
The light emitting diode is also called a luminescence diode or, in short, LED. LED stands for "light emitting diode", as it turns electrical energy into light. Physically, it is a cold-light source and an electronic semiconductor component part in optoelectronics. Its conductivity lies between that of conductors (e.g. metals, water, graphite) and non-conducting materials (e.g. non-metals, glass, wood).
LEDs are available in the widest range of sizes, designs, and colours, depending on requirements. The classic variant (standard LED) has a cylindrical shape and is closed by a hemisphere at the spot where the light is emitted.
Simple LEDs consist of the following components
High-performance diodes possess a large metal blank that allows for a better heat regulation. As the heat is dissipated more easily, more current can flow through the diode, the light emission covers more area, and the light output is higher. Compared to a simple 5 mm LED, the heat resistance is reduced tenfold. In practical terms, this means that a high-performance diode, such as the Luxeon Rebel, has a square emission area of about 1 mm and an efficiency of approx. 40-100 Lumen. The power of a normal 5 mm standard LED pales in comparison to this. With a size of 0.25 mm and a power of less than 0.1 W and 20-30 mA, it only reaches an efficiency of 1-2 Lumen.
The small, flat design of LEDs offers considerable leeway for ground-breaking product designs: For example the "LEDayFlex" daytime running light module for passenger cars, trucks, and caravans.
There are different types and designs of LEDs. According to their area of application, they differ in structure, power, and service life. Among the most important LEDs are:
Leaded LEDs are the forerunners of all LEDs, and they are mainly used for control purposes. Nowadays they are used as a combination of several LEDs in LED spotlights, fluorescent tubes or modules. They are available in 3, 5 and 10 mm sizes. You recognise the cathode, the negative pole of a leaded LED, by the fact that it is shorter than the anode (positive pole) and that the plastic coating is flattened. The exit angle of the light is determined by the lens shape of the housing.
SuperFlux LEDs are more powerful than regular leaded LEDs, and they have up to four chips (semiconductor crystals). Among the commonly used models are "Piranha" and "Spider". They offer a broad beam angle and are particularly used for area lighting, as the light is emitted over an area. A good heat dissipation is achieved via four contacts, which can be individually controlled. The structure of the High Flux ensures a long service life and makes them an efficient light source that can be universally used.
SMD stands for "Surface Mounted Device", which means that this diode is used surface-mounted. SMD LEDs usually consist of three to four chips and have solder contacts, which are soldered to the printed circuit board or connection surface. Regarding the current density, they are relatively insensitive and therefore can shine intensively. There are numerous versions of SMD LEDs. Size, shape of housing, and luminous flux strength can be chosen variably. They are used in combination with other SMD LEDs in LED fluorescent tubes or modules. In the automotive industry, they are primarily used for direction indicators, stop lamps, or daytime running lights.
High-power LEDs are powerful and robust LEDs, which can be operated at currents of 1000 mA under ideal operating conditions. They are often used on metal-core PCBs. Their unusual design places increased demands on thermal management.
The "Chip On Board" LED (COB) is the most advanced LED. It has this name because it is directly attached to the circuit board. This is achieved by so-called "bundling" which attaches chips though a fully automatic process on the gold-plated PCB. The contact to the opposite pole is achieved via a gold or aluminium wire. As COB LEDs do not use reflectors or lens optics, the beam angle of the emitted light is very wide. The greatest advantages of COB technology are the high illuminating power, the homogenous illumination and the numerous areas of application.
Basically, an LED consists of several layers of semiconductor compounds. Semiconductors, such as silicon, are materials whose electrical conductivity lies between that of conductors, such as the metals silver and copper, and non-conducting materials (insulators) such as PTFE or quartz glass. The conductivity of semiconductors can be greatly influenced by the specific introduction of electrically effective external substances (by means of a process known as doping). The different semiconductor layers together form the LED chip. The type of structure of these layers (various semiconductors) has a crucial bearing on the luminous yield (efficiency) and light colour of the LED.
If a current flows through the LED in the flow direction (from anode + to cathode –), light is created (emitted).
The n-doped layer is prepared by the incorporation of foreign atoms so that there is a surplus of electrons. In the p-doped layer, there are only a small number of these charge-carriers. This produces so-called electron holes (band gaps). When a voltage (+) is applied across the p-doped layer and n-doped layer (-), the charge-carriers move towards each other. At the pn junction, recombination takes place (where oppositely-charged particles combine to form a neutral entity). This process releases energy in the form of light.
If voltage is applied to an LED, the resistance falls to zero. LEDs are extremely sensitive components, and if the permissible current is exceeded even by a small quantity, they may be destroyed. Therefore it is important never to connect LEDs directly to a voltage source. They may only be connected if a current limiter or series resistor is built into the circuit. High-power LEDs are controlled via an electronic ballast that provides a constant current.
The adjacent graphic shows the circuit required for an optional functioning of the LED. In this case, a series resistor is used as a limiter which controls the forward current IF that flows through the LED. In order to choose the proper resistor, the forward voltage UF must be determined beforehand.
In order to calculate the series resistor RV, you need to know the total voltage, the forward voltage and the forward current.
As LEDs require only a little current, they already illuminate when they receive only a fraction (a few mA) of the permitted forward current. This is often enough to provide sufficient light. As already mentioned, there are different ways of operating LEDs, depending on the area of application.
Currently, due to its high cost, the LED is used by the automotive industry only in the premium segment, but in the long run it will become standard. This is because in addition to economic reasons, there are mainly technical arguments for using LEDs in standard production.
LEDs offer great functionality, technical performance, and optimal lighting results. They support saving energy resources and provide more safety in traffic. Furthermore, the daylight-like colour of the light allows for a pleasant and subjectively increased perception of the light.
The LED market for lights and headlamps will permanently develop in two directions: For one thing, the premium segment will gain in importance; it requires high functionality combined with excellent light output. On the other hand, the economically and ecologically motivated segment will be promoted more; it requires both low energy consumption and inexpensive solutions. Highly developed, functional, economical – LEDs have much to offer.
There are different methods for directing light into a particular direction. The most important methods of directing light in automotive lighting are reflection, refraction, and hybrid (combination of reflection and refraction).
As LEDs require only a little current, they already illuminate when they receive only a fraction (a few mA) of the permitted forward current. This is often enough to provide sufficient light. As already mentioned, there are different ways of operating LEDs, depending on the area of application. Here are three of these ways.
The thermal management plays a decisive role in the use of LEDs, as these component parts react very sensitively to heat.
LEDs are cold-light sources as they emit light, but practically no UV or IR radiation. The emitted light is cold and does not heat up illuminated objects. The LED does, however, heat up through the light creation process. Up to 85% of the energy is converted into heat. The lower the temperature, the brighter and longer the LED shines. Appropriate cooling should therefore always be ensured. In addition to the heat produced by the LED, for headlamps or lamps, other sources of heat, such as heat from the engine or insolation, must also be taken into account. Therefore, depending on the LED and its intended purpose, today different techniques are used to increase the heat transfer or dissipation.
a) Finned heat sink
b) Pin heat sink
c) Heat sink with heat pipe
Furthermore, it is usually possible to regulate the current for the LEDs. Under extreme conditions the power of the LED can be reduced to a certain level in order to lower heat production. In order to improve cooling further, the air circulation is raised by axial or radial blowers between the cooling elements.
LEDs are superior in several aspects. They might be more expensive to purchase than normal light bulbs or halogen bulbs, but their use pays for itself in a short time. The automotive industry in particular uses the positive features of the LED and employs it increasingly in new vehicles due to the following advantages:
|Light Source||Luminous flux|
|Conventional bulb W5W||~ 50||~ 8||~ 2700||~ 5|
|Halogen bulb H7||~ 1100||~ 25||~ 3200||~ 30|
|Gas discharge D2S||~ 3200||~ 90||~ 4000||~ 90|
|LED 2.5 Watts||~ 120 (2010)|
~ 175 (2013)
|~ 50 (2010)|
~ 70 (2013)
|~ 6500||~ 45 (2010)|
~ 70 (2013)
Environmental protection and increasing fuel prices are both the most convincing arguments showing that saving energy is more important than ever these days. When buying a new vehicle, consumers now focus clearly on the fuel consumption. Though often they ignore the potential savings related to the energy requirements of the vehicle lighting system.
|Vehicle configuration (headlamp/rear light)||Fuel consumption [l/100 km]||CO2 emission [kg/100 km]||Decrease|
|Halogen/conventional||~ 0.126||~ 0.297||-|
|Xenon/LED||~ 0.077||~ 0.182||39%|
|LED/LED (Potential for 2015)||~ 0.051||~ 0.120||60%|
Fuel consumption and CO2 emission for an average operating time of the lighting system
|Daytime running light system||Fuel consumption [l/100 km]||CO2 emission [kg/100 km]||Decrease|
|Halogen headlamp||~ 0.138||~ 0.326||-|
|LED (separate daytime running light function)||~ 0.013||~ 0.031||91%|
Additional fuel consumption and CO2 emission for daytime running lights
|Comparison of light sources||Fuel consumption|
|Halogen/incandescent bulb configuration||0.10 – 0.25 l /100 km|
|Xenon/LED configuration||0.05 – 0.15 l /100 km|
|Full LED configuration (Potential 2015)||0.03 – 0.09 l /100 km|
Fuel consumption according to lighting configuration (OE car)
The number of vehicles on the road is increasing worldwide. The increased traffic density on the roads leads to more frequent rear-end collisions. To avoid these, the driver must perceive light signals quickly. While a conventional incandescent bulb needs up to 0.2 seconds to light up, an LED reacts immediately. It does not require a warm-up phase and lights up immediately as soon as the brake pedal is depressed. The rear vehicle can thus react more quickly to the braking action of the vehicle in front.
Two vehicles are driving in the same direction at a speed of 100 km/h (safety distance 50 m). The vehicle in front brakes, and the driver of the second vehicle reacts to the LEDs lighting up immediately and brakes at almost the same moment. This reduces the stopping distance by almost 5 m and represents an enormous increase in safety.
Generally, all LED headlamps can be adjusted with a normal beamsetter. LED headlamps with only one optical lens (low beam) are treated the same during testing and adjusting of the light distribution as all other headlamps with only one light source. In the case of some headlamps with multiple light sources, a special fact has to be taken into account. Due to the design of some headlamps, the collecting lens of the beamsetter is simply too small to capture the emitted light (low beam) of all of the LEDs. In such cases, it is important to know which LED is responsible for which lighting function.
During preparation of the vehicle the manufacturer's data must be taken into account! This will be shown for the low beam of the Audi A8. As mentioned before, three vertically placed LEDs generate both a symmetrical and an asymmetrical part of the low beam.
Therefore, the beamsetter must be aimed at these lenses. If the beamsetter is directed according to the specifications, the light distribution can be set as usual.