LED's Evolving Role in Automotive Lighting Applications

Release date: 2018-02-28 Source: Aladdin Lighting Network Share:

01

Introduction

In many parts of the world, including North America, roads are not illuminated by fixed road lighting systems. As a result, car lighting plays a crucial role in ensuring safe driving at night.

The performance requirements for automotive headlamps are based on standards issued by organizations like the Society of Automotive Engineers (SAE) and similar industry groups. These standards specify the minimum or maximum luminous intensity that should be emitted from the center of the vehicle's lighting system in different directions.

Similar photometric performance requirements exist in countries outside North America. While these regulations may vary in detail, they all aim to ensure that the vehicle's lighting system provides enough light for the driver to see the road clearly, minimizes glare for other drivers, and ensures that the lights are easily detectable.

02

Headlight

For car headlights that provide illumination for the front of the vehicle, two are typically required and installed as far apart as possible. Each headlamp must meet the same performance requirements. There are two main types of beam patterns: high beam and low beam.

As expected, the requirement for high beam is higher intensity and smaller maximum intensity values. In addition, the high beam has a symmetrical beam pattern. In contrast, the low beam has an asymmetrical beam pattern, with the more stringent maximum intensity tending to the left (in North America; if it is a left-hand country, the beam pattern is reversed).

The cutoff boundary allows you to check and adjust the vertical target of the headlamps. Most North American headlights require the right cutoff boundary to be at the same height as the headlamps. The left cutoff boundary is usually lower than the right border to reduce the amount of light entering the oncoming driver's eyes. The "glare" cutoff boundary of the low beam headlight pattern limits the front visibility of the driver.

When driving at speeds above 60-65 km/h, it may be difficult for the driver to use low beam headlamps to detect potential hazards in time and stop. In this case, the speed of light is guaranteed unless the nearby vehicle is within 100 meters. However, most drivers do not make full use of high beam headlights.

Since the cutoff boundary of the low beam headlamp pattern is clear, the vertical target is an important factor in achieving optimum performance. For example, in the United States, most states do not require headlamps as part of a security check.

A recent study of vertical targets on vehicles found that most vehicles had at least one headlamp with poor targets. When the target is too high, the headlights may cause uncomfortable glare. When the target is too low, the visibility of the driver's front will be affected.

The adaptive lighting system automatically adjusts the steering angle side of the low beam according to the speed of the vehicle and the direction of the direction, and expands the effective illumination range when the vehicle turns.

The automatic leveling function ensures that the light always shines in front of the ground, regardless of the load condition. Some vehicles have turning and cornering lights; curved lights sometimes use mechanical components to orient one or two headlamps toward the road.

Some European vehicles are equipped with a "town" headlamp beam pattern that has a lower maximum luminous intensity and a wider distribution than most low beams, helping drivers to detect pedestrians while driving at low speeds in the city.

Adaptive lighting system requirements in most countries are based on the United Nations Economic Commission for Europe (ECE) No. 123 vehicle regulations. The US Federal Motor Vehicle Safety Standard 108 currently has reservations about adaptive lighting systems.

Currently, most automotive headlamps use a filament source (tungsten halogen or simpler halogen) of the mirror or projector optics to produce the necessary beam pattern. A relatively small proportion of headlamps use high intensity discharge (HID) lamps, using metal halides and xenon lamps to quickly turn the lights on.

The use of LED headlights has just begun to be used on several models. Regardless of the source used, all headlamps need to meet the same luminosity requirements.

03

Signal Light

Vehicles need to have signal lights so that the driver can alert others during braking and steering during the day and night.

Nowadays, more and more vehicles use LEDs for signal illumination. Different signal lights have different requirements for color and luminous intensity. The US federal requirements for vehicle signals are based on SAE standards and recommendations. Table 3 lists the color and allowable luminous intensity values for several vehicle signal types.

The performance requirements of European car lights are not much different from those in North America in terms of color and luminous intensity, with one exception.

In the United States, the lights on the back of the vehicle may be red or yellow, with different strength requirements depending on their color. In most other parts of the world, the turn signal must be yellow.

Allen previously reported that the yellow rear turn signals tend to have fewer collisions, either because their luminous intensity is higher than the red rear turn signal, or because yellow makes it easier to distinguish between the brake light or the tail light. The National Highway Traffic Safety Administration (NHTSA) is considering whether it needs to turn yellow for all rear-mounted car steering signals.

04

LED Human Factors

LED sources are very different from the incandescent lamps used in most current automotive lighting applications:

• LEDs have higher luminous efficiency (lm/W) than filament sources, which means they can produce higher intensity or wider beam patterns with the same amount of energy, or produce similar light output with lower energy requirements.

• The narrow-band spectral output of a color LED produces a highly saturated color appearance, while the incandescent lamp produces a broadband source that requires a filter to produce colored illumination (Figure 3).

• White phosphor converted LEDs can produce a higher correlated color temperature (CCT) than incandescent lamps, resulting in a more blue appearance.

• LEDs have a very fast response time: 10-20 ns, including the decay time of yttrium aluminum garnet (YAG) phosphors, while incandescent lamps are about 80-250 ms

The luminosity, chromaticity, and time characteristics of an LED source can also affect the ability of the driver to see and respond to potential hazards on the road. For vehicle headlamp systems, the spectral distribution of a typical phosphor-converted white LED based on a combination of a blue InGaN device and a YAG phosphor has a greater proportion of short-wavelength (blue) light (Fig. 4).

This difference is related to the visual function of the driving, because the asphalt pavement brightness is between 0.1 cd / m2 and 1 cd/m2 during night driving, and the visual detection risk is caused by cones and rod visual receptors in the eye.

However, optical metrics such as illuminance (lx), brightness (cd/m2), luminescence intensity (cd), and luminous flux (lm) are based entirely on the spectral response of the pyramidal receptor in the eye.

Cone receptors are specifically designed for viewing outdoors and indoors at ambient light levels, typically between 10 and 1000 cd/m2. The way light is measured is significantly different from the way we look at it, because in general, rod-shaped receptors are more sensitive to shorter visible wavelengths (such as blue and green) than cone receptors.

Therefore, the usual light metrics (lx, cd/m2, cd, and lm) underestimate the driver's ability to see things at nighttime LED sources.

The International Commission on Illumination (CIE) recently released a unified photometric system to quantify the relative role of rods and cones at night. Therefore, an equivalent nighttime visual effect can be obtained by using an LED light source that is 20% to 30% lower than the light level produced by an incandescent lamp.

According to the brightness model developed by Rea et al., another visual response of LEDs over filament sources is the sensing of street brightness. This induction seems to increase the sensitivity of short wavelengths. Figure 5 shows road brightness predicted using halogen, HID, and LED light sources.

However, the relatively high short-wavelength spectral power in white LED illumination may also have some potential negative effects on vehicle illumination. When the headlights of different colors produce the same conventional luminosity, the disability glare is not affected by the spectral content of the headlamp illumination. This is not the case for discomfort glare.

Like sensory street brightness, glare is also more sensitive to short-wavelength light. It is not yet clear whether discomfort glare increases or does not affect driving safety.

There is evidence that when drivers encounter discomfort from

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