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Changes in the Role of Leds in Automotive Lighting Applications (2)

Jul. 10, 2020

4. LED human factors

China LED auto headlights factory believes that: LED sources are very different from most incandescent lamps used in 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 with lower energy requirements Output.

• The narrow-band spectral output of color LEDs can produce a highly saturated color appearance, while incandescent lamps produce broadband light sources that require filters to produce color illumination (Figure 3).

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

• LED has 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 photometric, chromatic and temporal characteristics of LED light sources may also affect the driver's ability 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 (Figure 4). This difference is related to the visual function when driving, because when driving at night, the brightness of the asphalt road is between 0.1 cd/m2 and 1 cd/m2, and the visual detection hazard is carried out by the cones in the eyes and the rod visual receptor of.

However, photometrics, such as illuminance (lx), brightness (cd/m2), luminous intensity (cd), and luminous flux (lm), are based entirely on the spectral response of pyramidal receptors in the eye. Cone receptors are specifically designed for viewing in daylight outdoors and indoor light levels, usually between 10 and 1000 cd/m2. The way in which light is measured is significantly different from the way we look at it, because rod-shaped receptors are generally more sensitive to short visible wavelengths (such as blue light and green light) than pyramidal receptors. Therefore, the usual light metrics (lx, cd/m2, cd, and lm) underestimate the driver's ability to see things under the LED source at night.

China Automobile Led Headlights

China Automobile Led Headlights 

The International Commission on Lighting (CIE) recently released a unified photometric system to quantify the relative effects of rod and cone cells at night. Therefore, an LED light source that is 20% to 30% lower than the light level generated by an incandescent lamp can be used to obtain an equivalent night vision effect.

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

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

Like sensing street brightness, discomfort glare also increases sensitivity to short-wavelength light. It is not yet clear whether or to what extent the increase in discomfort glare affects driving safety. There is evidence that when drivers experience uncomfortable glare from the headlights, they are more likely to experience driving behaviors, such as increased head movements, which is believed to increase the risk of collision.

Regarding visual detection of vehicle signal lights, LEDs have some advantages, especially for vehicle brake lights, because the startup time of LEDs is much shorter than incandescent lights. Bullough proved that it is possible to use the driver's eyes to sense a critical amount of light energy to predict the visual response time of colored light signals (such as brake lights or turn signals). When the tungsten filament lamp is turned on first, the illumination of the filament increases gradually, and it takes up to 250 ms to reach full brightness. LEDs have almost instantaneous start-up time and can produce a critical amount of light energy faster. Therefore, compared to the filament source of the same nominal color and peak luminous intensity, the visual reaction time caused by the LED is shorter.

Importantly, because the deceleration rate of the braked vehicle is related to the action of turning on the brake light (pressing the brake pedal), the shorter light source rise time can provide a parking distance of nearly 7 meters for the driver behind, Although this distance is small, sometimes it is actually very effective.

5 Energy and environmental issues

Compared with filament light sources, automotive lighting systems that use LED sources have higher luminous efficiency and can significantly reduce power requirements. In a separate study, Hamm and Schoettle estimated the typical power of conventional filament source vehicle lighting systems and LED lighting systems. Their estimated average values for different lighting and signal functions are summarized in Table 4. Table 4 also lists the estimated total usage time of each type of lighting system based on the driving mode in the United States. At the same time, Table 4 lists the total annual lighting energy use of filament source vehicle lighting systems and LED lighting systems.

Assuming that the lighting energy per kilowatt-hour of a gasoline-powered car is equivalent to 1.29 kilograms of carbon dioxide emissions, the conversion of incandescent filament lamps to LEDs is expected to reduce the total annual energy consumption by 27.4 kilowatt-hours, which is equivalent to the annual reduction of each car. About 35 kg of carbon dioxide emissions.

6 Future trends

China automobile LED headlights are already common in signal lighting applications, and have been introduced for headlamp systems. The rapid improvement of luminous efficiency will become more and more attractive in automobile use.

The solid-state structure, modular configuration of the LED system and relatively easy intensity control through current modulation or pulse width modulation provide important prospects for energy-efficient vehicle lighting systems. Energy-efficient vehicle lighting systems can use adaptive lighting systems (AFS) to make real-time changes based on changing road, traffic, and weather patterns.

These advantages of LEDs can also make dynamic rear signal lighting systems possible.

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