LED application in automotive lighting systems

1 Introduction

In the mid-1980s, LEDs began to be used in automotive central high-position brake lights (CHMSL). In the 1990s, LED backlights for automotive instrument panels generally used LED as a solid new light source. After entering the new century, with the improvement of LED power and brightness, in addition to automotive headlamps, LEDs have fully entered the application of automotive lighting and signal systems. In 2005, global LEDs accounted for 52% of the market share of mobile phones. Since 2005, as the mobile phone market has become saturated, it is no longer growing. Instead, it is the automobile, traffic signal, landscape decorative lighting and general lighting market. The automotive LED market has great temptation.

2 Development history and classification of traditional car lights

2.1 The development of traditional cars

In 1879, Edison invented a vacuum incandescent bulb that uses electricity as an energy source. At the beginning of the car in 1886, the classic car was swaying at night when it was swaying. Since there were few vehicles at that time, the speed was very low (less than 30km/h), and although the road was narrow and rugged, there was no risk of injury and no risk of collision.

With the increasing number of cars and the increase in speed, there are more and more pedestrians on the road, and the car is no longer allowed to sway by a single lamp. So the light source engineer designed a pair of bright incandescent eyes for the car based on the emerging incandescent lamps. The electrical engineers of the car at that time had equipped the car with a small vehicle-mounted generator, which laid the foundation for the development of automotive electrical lighting.

In the situation of increasing number of vehicles and increasing speed, lighting is not enough. It is necessary to look around and avoid worries. From now on, not only the headlights, but also various taillights such as running lights, brake lights, turn signals, rear fog. A series of lights, such as lights, have come out one after another, in order to avoid the collision of night driving or the collision when turning. Then the headlights developed into a variety of lights such as low beam, high beam and front fog.

In order to improve filament temperature and luminous efficiency without reducing the life of incandescent lamps, light source engineers invented inflatable incandescent bulbs in the 1950s, and in the 1960s invented halogen halogen lamps in gas-filled incandescent lamps. In the 1970s, tungsten halogen lamps with a luminous efficiency of 201m/W replaced large-scale self-contained lamps for automotive lighting, and are still the dominant lamp in automotive headlamps. In order to meet the lighting requirements of high-speed vehicles, more advanced high-intensity discharge (HID) lamps have emerged, and the representative light source is Xenon metal halide lamps. A 35W xenon metal halide lamp radiates light (3200lm) three times that of a 55W tungsten halogen lamp (1101lm), which is 4.5 times lighter than a metal halide lamp and has a lifetime of more than 10 times.

2.2 Classification of car lights

Car lights are divided into two categories according to the purpose of lighting and signal lights. The lights are like the "eyes" of the car, illuminating the road in the direction of the car in the dark, extending and widening the driver's field of vision; the signal light is like the "mouth" of the car, and the vehicle can communicate with other vehicles through the light signal. Express the presence of the car and the state of its driving to other car drivers so that the car can travel safely and quickly on the road.

The lights are further divided into exterior lights and interior lights. Exterior lighting includes headlights, front fog lights and license plate lights; interior lights mainly refer to instrument lights and reading lights.

The signal light also separates the external signal lights and the interior signal lights. The signal lights outside the vehicle refer to the turn indicator light, the brake light, the tail light, the width light, the reverse light, etc.; the internal signal light refers to the indicator light of the instrument panel, mainly including steering, oil pressure, charging, door closing prompt, sound, air conditioner, etc. Indicator light. Among these lights, there are exterior lights and off-board lights that are subject to control inspections because they are a key component of driving safety.

The front lights of the car are shown in Figure 1. The headlights, like the eyes of a person, play a finishing touch, adding a lot to the car, as shown in Figure 2.

In automotive lighting, people usually refer to rear turn signals, brake lights, tail lights, rear fog lights, reverse lights and trunk lights as auxiliary lamps, as shown in Figure 3.

2.3 Repeated action of car lights

Car lights are an essential safety component for driving a car. According to statistics, 25% of the total mileage of the vehicle is driven at night or under natural light, and traffic accidents here account for 33% of the total accidents, and 50% of casualties occur at night. Among the multiple ECE regulations, there are nearly 40 regulations related to headlights. Among the 74 mandatory inspections of China's current automotive products, 23 projects related to vehicle lights.

3 A new generation of solid-state light source LEDs has become the new favorite of car lights

3.1 Advantages of LED application in automotive lighting systems

A new generation of solid-state cold light source LEDs was introduced in the 1960s. Until the 1980s, because the brightness of the LED was not high at that time, just use it as an indicator light, etc., no one would think that it would be used as a car light. After entering the 1980s, the brightness of LEDs has been greatly improved. In 1985, it was used on a passenger car. In 1986, Nissan used 72 φ5mm LEDs on the 300ZX car as the central high-position brake light, marking the beginning of LED applications in the automotive field.

Compared with traditional lamp light sources, the advantages of LEDs are as follows:

(1) Long life, maintenance-free, it is possible to replace the lamp during the entire vehicle life.

(2) It is very energy efficient and saves at least half of the energy of incandescent lamps of the same brightness. Table 1 shows the comparison of the energy consumption of car signal lights and incandescent lamps.

According to the 2003 US Department of Energy (DOE) automotive LED energy efficiency assessment report, the results were surprising. For small cars alone, if LED lights are used throughout the United States, fuel savings of 1.4 billion gallons per year (US gallons = 3.785 liters), and trucks can save twice as much fuel as cars.

(3) There is no need for hot start time, and the lighting response speed is fast. The startup time of a conventional self-igniculating light bulb is generally 100-300 ms, and the LED is usually less than 70 ms. For important brake lights, such a time difference means that the braking distance at high speed is 4 to 7 meters, which reduces the incidence of rear-end collisions by 5%.

(4) The structure is simple, and the vibration and impact resistance are strong.

(5) Small size, great design flexibility, can change the lamp mode at will, suitable for all kinds of cars.

(6) The cold light characteristics of the LED make the lamp not deformed due to long-term heating, thus improving the life of the whole set of lamps.

(7) The influence of voltage conversion is much smaller than that of ordinary light bulbs, and it is easy to control.

Table 2 gives a comparison of the LED high-position brake light and the self-ignition high-position brake light.

With its excellent safety, reliability, environmental protection and energy saving, LED has become the new darling of automotive lighting systems, bringing wedges to the upgrade and revolution of automotive lights.

3.2 LED application in automotive lighting systems

LEDs are widely used in automobiles. In addition to headlights, other outdoor and interior lights, lighting and signal lights, high-illuminance and low-illumination lights can be used with LED light sources, as shown in Figure 4.

3.2.1 Application of LED in low illumination of automobiles

LEDs in automotive low-light illumination systems are mainly instrument panel backlighting, operating switches, reading lights, width lights, license plate lights, etc. This is the first time that automakers have applied LEDs to automobiles, and currently LEDs are in cars. The most applied part.

The interior of the car, such as the instrument panel, used to use incandescent lamps or vacuum fluorescent (VF) to provide backlighting. Later, CCFLs were used as backlights. With the continuous advancement of LED technology, LEDs have replaced traditional light sources into backlighting. From dashboards to entire entertainment, navigation, trip calculations, and information center control displays, LEDs are almost everywhere. There are three main ways for LEDs to be used for backlighting:

(1) The easiest way is to mount the LED directly behind the LCD scattering film. Many packaged LEDs can be used, or unpackaged dies can be used.

(2) Another way is to introduce the light emitted by the LED into the fiber bundle, and form a flat sheet behind the scattering film of the fiber bundle, and the light can be taken out from the sheet in different ways as the backlight of the LCD.

(3) Another way is to use edge-lit LCD backlighting. A transparent or translucent rectangular plastic block is used as a light guide body, which is directly mounted behind the LCD scattering film. The back surface of the plastic block is coated with a white reflective material. LED light is incident from one side of the plastic block, with the remaining sides or white reflective material.

When multiple LEDs are used together for car dashboard lighting or entertainment system control, the consistency of color and brightness is particularly important, and the brightness of the illuminated area needs to be uniform and there should be no shadows.

LED lighting in the interior of the car mainly includes general lighting ceiling lights, reading lights, beauty makeup lights, door lock lights and so on. The goal is to provide a relaxed and friendly atmosphere to reduce driver fatigue, improve and improve comfort, and help drivers get a lot of readable and clear information.

3.2.2 Application of LED in high illumination of automobiles

(1) Application of LED in car signal lights

Car signal lights mainly refer to brake lights, direction lights and tail lights, and their illumination and color are clearly defined. In the mid-1980s, LEDs began to enter the automotive industry. China's first LED car light is the high-position brake light of the Santana 2000 sedan. It was jointly developed by Shanghai Automotive Electronics Engineering Center and Shanghai Xiaoshi Auto Lamp Co., Ltd., and was put into production in 2000 by Shanghai Volkswagen. So far, LED applications in automobiles, in addition to instrument LCD panel backlighting, the most popular is the central high-position brake lights, more than 80% of European and Japanese cars have installed LED central high-position brake lights.

At the 2004 Beijing Auto Show, the French company Fuao showed a high-position brake light made of 4 LEDs, and an LED light to realize all the tail signal functions, including stop lights, rear small lights, turn lights, fog lights And reversing lights, etc. By 2010, most automotive taillights will use LEDs, including reversing lights and license plate lights. On the 2000 Cadillac and S-Class Mercedes, the rear lights (taillights, brake lights and turn signals) all use LEDs. The new Audi A8 sedan side turn signals, running lights, brake lights and turn signals, etc., also use LEDs. At present, new high-end cars in the world, such as Cadillac, BMW, Toyota, Mercedes-Benz and Ford, are equipped with colorful LED lamps.

(2) The application of LED in automobile headlights is beginning to take shape

The headlights are the lamps that illuminate the road ahead when the car is driving at night. It is one of the important parts to ensure the safe operation of the car. The farther the illumination distance of the headlights is, the better the light distribution is, and the higher the safety performance of the car. The headlights of the car emit two beams of high beam and low beam, of which the high beam travels at a higher speed on the road where no other party is coming. The high beam should ensure bright and even illumination on the road 100m or more in front of the car.

Among all the LED lamps, the most difficult and last to use is the headlights. At present, LED headlamps are used in concept cars on some auto shows, as shown in Figure 5.

Since 2003, more than 15 automakers have exhibited concept cars using LED headlights at relevant auto shows. One of the Ford concept cars that debuted at an international auto show in North America used LED headlights. Japanese small companies and other companies also have sample lamps on display. The 2007 Lincoln Aviator and other models also use a new lighting system, including LED headlamps. In 2007, Lexus launched a 2008 LS GoohL hybrid luxury car production car with LED headlights. Toyota Japan began installing LED headlights on the Lexus 600H in 2007, and Volkswagen plans to install LED headlamps on the Audi R8 in 2008. From the current situation, LED headlamps are still in the research and development stage, and it is expected to be more commonly used in ordinary cars within 10 years.

The new light source of LED has greater flexibility in the shape and arrangement of automotive headlamps. By taking advantage of the small size of the LED, the volume of the entire set of headlights can be greatly reduced, giving up some valuable space to other related devices. The existing tungsten halogen lamps or HID lamps have a total length of about 30 cm, while the LED lamps on many concept cars are only 12.5 cm, and can break through the circular design of the traditional lamps in the shape. Due to the modular design of the LEDs, the headlamp design gives people a new visual impact. The use of an LED as a headlight not only has higher color recognition than an old-fashioned lamp such as a tungsten halogen lamp, but since the old-style lamp has only one light source, it is difficult to enlarge the illumination area, and the LED lamp has a plurality of light sources, thereby making the illumination area wider.

(3) LEDs face many challenges in automotive headlamps

One of the biggest obstacles in the full entry of LED into the automotive application market is the headlamps. The traditional light source of the headlamps is very mature, like the representative light source tungsten halogen lamp, although the light effect is not very high (about 201m / W), but the price is low. Although the high-pressure xenon lamp is relatively expensive, it is safe, efficient, energy-saving, and small in size, and the lamp design is simple, so it is favored. It is not an easy task for LEDs to replace traditional light sources in automotive headlights. The following issues need to be addressed.

One is the brightness output. The brightness requirements of the headlamps are: near-light 900lm, high-light 1100lm, overall 2000lm, about the total light output of Lumileds' 40 1W LuxeonLED at 25 °C. When the ambient temperature rises to 50 ° C, the efficiency of this type of LED will drop to over 80%. In order to increase the brightness, a larger number of LEDs are required, which not only causes an increase in cost, but also increases the failure rate of the LED.

The second is the issue of heat dissipation. Although the heat generated by a conventional light source is much higher than that of an LED, it does not impair heat and lower its light output. However, the LED light output will decrease due to the junction temperature rise. Therefore, heat dissipation issues are critical in the design of LED headlamps. To prevent the LED from overheating and burning it, you need to add a heat sink to it. Since the headlamps require a large number of LEDs, the volume after the heat sink is added is too large to be incorporated into the luminaire.

The third is optical design. Using LED as the light source to design the luminaire needs to turn the traditional column light source into a surface light source. In order to obtain the required lumen output, the LED requires a large package area, making the optical design more difficult. In some current concept cars, the existing single lamp room design is replaced by a modular design, and multiple sets of lamp light sources are used to achieve the illumination level of the traditional lamps, thereby reducing the difficulty of optical design and increasing the sense of design of the car body. .

3.3 LED market prospects in automotive applications

Prior to 2006, LED's biggest application highlight was mobile phones, which accounted for 52% of the global market. After 2005, mobile phones became saturated, replaced by cars, traffic lights, landscape lighting and general lighting.

The automotive industry remains a pillar industry in the global economy. At present, China's annual production capacity of automobiles is more than 800, which has surpassed Germany, and its output ranks third in the world. The automobile industry of such a large scale has promoted the development of automotive lamps and provided a broad market space for the application of LEDs in automobiles. According to the current LED technology level, a car needs more than 300 LEDs, and its demand is very large. In the next few years, China's headlights will have a large development, forming an annual output value of more than 1 billion yuan.

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