Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Automotive Lighting

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_116



Light is needed for road safety at night, supporting the driver’s orientation and early recognition of obstacles. This light is produced by the headlamp. However, the same light useful for the driver will also glare oncoming traffic. Thus, the light of headlamps increases and reduces road safety at the same time. The main task of headlamp designers is to solve the optimization problem between a good view for the driver and as little glare as possible for oncoming traffic. But not only the own headlights are necessary for a good orientation of the driver by night. Signal lamps of other cars give information about the change of direction, reducing of speed, drive back, and the contour of cars. So automotive lighting deals with the construction of headlamps and signal lamps based on perception conditions by night.


The following article will give a short overview about automotive lighting. It is focused on car lighting and does not lit the interaction between car and street lighting. It starts with the perception conditions by night. The knowledge of this field helps to understand the special construction of current and future headlamps.

Perception at Night Time

Figure 1 shows the same street during the day and at nighttime. The gray values represent luminances. The day-to-night luminance ratio is 1:1,000. Additionally, the quantity of information is much higher during the day than it is at night. The daylight viewing distance of 200 m and more opposes to no more than 65 m in darkness – very often even lower. However, the driving speed is nearly the same. The density of traffic is lower in the middle of the night – still in winter between 17 and 20 h, traffic density is nearly the same as in summer. It is clear that perception conditions at nighttime are completely different from daylight conditions.
Automotive Lighting, Fig. 1

Compare the luminances of night and day of the same street [1]

What does that mean for the perception? The visual power functions that followed give an answer. Figure 2 left shows the contrast sensitivity and Fig. 2 right the visual acuity as a function of adaptation luminance.
Automotive Lighting, Fig. 2

Contrast sensitivity (up) and visual acuity (down) as function of adaptation luminance (~road luminance)

Because the luminance of the road is not constant, the right adaptation level has to be defined. Figure 3 gives an impression in which area the driver is looking for objects. According to this data, the accumulation point of view is in 30–60 m ahead of the car where luminance levels are measured to 0.01–0.1 cd/m2 depending on the road surface and the luminance intensity of the headlamp. Consequently, the contrast sensitivity and the visual acuity (Fig. 2) are much lower compared to daytime or road lighting running conditions.
Automotive Lighting, Fig. 3

Accumulation point of view on a straight road (left) and a bending road (right) [2]

Another very important point is glare for oncoming traffic caused by car headlamps. Figure 4 shows increasing of the vertical illuminance of an oncoming car. Figure 5 presents the threshold contrast depending on the vertical illuminance at the eye. The threshold contrast is doubled if the legally required illuminance value of 0.5 lx is reached. Because light above the horizontal line is needed for recognition of traffic signs, bridges, tunnels, and so on, a typical optimization task has to be solved.
Automotive Lighting, Fig. 4

Vertical illuminance at the eye produced on oncoming car is situated in different distances [3]

Automotive Lighting, Fig. 5

Threshold contrast as function of different illuminance level on the eye [4]

Construction of Headlamps

The aim of a good headlamp is to bring as much light as possible to the road surface beyond a distance of 30 m. The headlamp should not glare oncoming traffic additionally. So a limitation of light above the horizontal line is necessary. That leads to the typical light distribution of a low beam headlamp shown in Fig. 6 (up). The illuminance distribution is demonstrated in false color on a measuring screen in 25 m distance. Because of the dynamic swiveling of a car and production tolerances in the headlamp, the cutoff line has to be tilted down by a minimum of 1°. Only if there is no oncoming traffic can the driver use high beam headlamp; see Fig. 6 (down). High beam is used in Europe approximately between 5 % and 15 % at night. This very rough estimate shows the limitation of good driving conditions.
Automotive Lighting, Fig. 6

Illuminance distribution of a low beam (up) and a high beam (down) on a 10-m-wall measured in 25 m presented in false color [Hella]

What is needed to get such a light distribution? The headlamp has to produce:
  1. 1.

    A hot spot with more than 10 lx in 25 m

  2. 2.

    More than 1 lx in an area between ±30° (left/right) and −10° to 0° (up/down)

  3. 3.

    A cutoff line with a gradient of 20:1 on the vertical lines by ±5°

  4. 4.

    Less stray light above the cut-off-line (<0.4 lx for halogen lamps and <0.5 lx for discharge lamps)

Light sources with high luminances are needed to create a bright hot spot. So halogen lamps, gas discharge lamps, or LED is used. For the shape of the cutoff line, the light source should have an aspect ratio of 5:1. Figure 7 shows the luminance pictures of a halogen lamp and a LED module fulfilling this requirement.
Automotive Lighting, Fig. 7

Luminance picture of a halogen bulb (up) and an LED module (down)

Generally, in addition to the lamp, a reflector and/or a lens is necessary to distribute the light in the desired direction. The following standard reflectors are used:
  1. 1.

    Parabolic (lenses in cover glass) – Fig. 8a

  2. 2.

    Freeform reflector (transparency of cover glass) – Fig. 8b

  3. 3.
    Ellipsoid (projection system with projection lens and shutter) – Fig. 8c.
    Automotive Lighting, Fig. 8

    Different reflector systems (a): parabolic [Hella]; (b): freeform [Hella]; (c): ellipsoid [5]

Figure 8c presents the simulation of the light distribution in a projection system (down) and demonstrates the beam of a real system (up). The comparison of a reflection and a projection system is illustrated in Table 1.
Automotive Lighting, Table 1

Comparison of reflection and projection system


Reflection system

Projection system

Cover glass

With lenses/transparent


Luminous surface



Optical system

Large reflector + cover glass with lenses

Small reflector + shutter + projection lens + transparent cover glass




Reflector type









Every headlamp has to fulfill the demands and regulations of ECE (for Europe, Asia, Australia, South America) or SAE (for North America). These standards describe the photometric and geometric requirements of headlamps and are adapted to the state of the technological development.

Construction of Signal Lights

A headlamp supports the driver’s vision and provides information about oncoming traffic and obstacles. Further lighting equipment on a car is necessary to signalize position, speed, and change of direction. All lighting equipment not supporting vision but to be seen is called signal lights or signal lamps. Stop lamps, rear lamps, day time running lamps, marking lamps, and position lamps belong to this group. Figure 9 shows some lamps.
Automotive Lighting, Fig. 9

Different signal lamps (incandescent lamps, LED, Neon tubes) [Hella]

What are the requirements for signal lamps? The luminance of the light source should not be as high as for a headlamp. The light distribution has to be very wide. Figure 10 shows the relative luminous intensity.
Automotive Lighting, Fig. 10

Relative luminous intensity as a function of angle (maximum luminous intensity = 100)

The usage of white light for signal functions is prohibited. Signals have to be well defined. Different colors, different luminance, and blinking are used to indicate different information (Fig. 11). For example, a higher luminance is used for a stop lamp than for a rear lamp. Blinking yellow light (for ECE) or red light (for SAE) shows that the driver will change his/her direction.
Automotive Lighting, Fig. 11

Luminous intensity and light color of different signal function [Hella]

Vision for Automotive Lighting

Headlamps will change significantly in the next decade. The headlamps presented above cannot completely solve the task to produce enough light for the driver with no glare for oncoming traffic. Considering the maximum viewing distance of 65 m, the maximum speed should not be more than 65 km/h without high beam. At a higher speed, these headlamps do not provide enough light on the left side to see objects in time to react. Development of headlamps should look into providing high beam all the time without causing glare for the oncoming driver. Some prototypes of such headlamps have been available lately.

A glare-free headlamp needs a sensor system and actuating element. The aim of such a system is to detect oncoming traffic and to blank out the light in the direction of the oncoming cars immediately (Fig. 12). Radar, lidar, infrared scanners, or cameras are used as sensors. A LED array (Fig. 13) or a dynamic AFS system (actor) is responsible for the adaptive light control. This way, it is possible to bring enough light right and left of an oncoming car without glaring the driver.
Automotive Lighting, Fig. 12

The function of a glare-free headlamp [Hella]

Automotive Lighting, Fig. 13

LED array – up low beam; down high beam [5]

With this “glare-free” technology in a large number of cars, severe accidents with pedestrians and bicycles at night could be reduced dramatically. To achieve this, further technological improvements are necessary as well as cost reduction and the employment of these systems in a large number of cars. “Intelligent light for safety!” should be a political, economic, and human objective for automotive lighting in the future.



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© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.University of Technology BerlinBerlinGermany