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The changing role of LEDs in car lighting apps (Part 2)

15 Dec 2014  | J. D. Bullough

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Another visual response that may favour LEDs over filament sources is the perception of roadway scene brightness, according to a brightness model developed by Rea et al. (2011). This response appears to have increased short- wavelength sensitivity. Figure 5 shows the predicted roadway scene brightness under headlamps using filament, HID and LED sources.

The relatively high amount of short- wavelength spectral power in white LED illumination might also have some possible negative impacts for vehicle lighting, however. When headlamps of different colours produce equivalent conventional photometric quantities, disability glare (a reduction in visual performance that is caused by scatter in the eyes from a bright light) is not influenced by the spectral content of the headlamp illumination (Schreuder, 1976). This is not the case for discomfort glare, which is defined as an annoying or painful sensation that is experienced when viewing a bright light in the visual scene of interest.

Like the perception of roadway scene brightness, discomfort glare also exhibits increased sensitivity to short- wavelength light (Bullough, 2009). It is not fully understood whether, or to what extent, increased discomfort glare affects driving safety. There is some evidence that shows that when drivers experience discomfort glare from oncoming headlamps, they are more likely to exhibit driving behaviours such as increases in head movements and increased throttle variability, which in turn have been found to be correlated with an increased crash risk (Bullough et al., 2008).

Regarding the visual detection of vehicle signal lights, because LEDs have substantially shorter onset times than filament lamps, they can have some advantages, especially for vehicle brake lamps. Bullough (2005) demonstrated that visual reaction times to the onset of a coloured light signal, such as a brake light or turn signal, could be predicted using a threshold quantity of light energy (in candela) received at drivers' eyes. When a tungsten filament lamp is first switched on there is a relatively gradual increase in illumination from the filament and it can take up to 250 ms to reach full brightness. LEDs have practically instantaneous rise times and can produce the threshold quantity of light energy more quickly. As a result, LEDs elicit shorter visual reaction times than filament sources of the same nominal colour and peak luminous intensity (Bullough et al., 2002).

Importantly, because the rate of deceleration of a braking vehicle is linked to the same action that turns on the brake light itself (pressing the brake pedal), shorter light source rise times can provide a stopping distance benefit of nearly 7 m for a driver following a braking vehicle (Sivak et al. 1994), a small but sometimes practically significant increase.


 Relative brightness of roadway pavement surfaces

Figure 5: Relative brightness of roadway pavement surfaces illuminated by photometrically equated light sources (halogen, HID and LED).


Energy and environmental issues
Because they have higher luminous efficacies compared to filament sources, automotive lighting systems using LED sources can have substantially reduced power requirements. In separate studies, Hamm (2009) and Schoettle et al. (2009) estimated the typical wattages for conventional filament source- based vehicle lighting systems and for LED lighting systems. The average of their estimates for different lighting and signalling functions are summarised in table 4. Also listed in table 4 are estimated values for the total annual hours of use for each type of lighting system, based on driving patterns in the United States (Buonarosa et al., 2008). Table 4 also includes the resulting total annual lighting energy use for filament- and LED-based automotive lighting systems.


 Relative brightness of roadway pavement surfaces

Table 4: Estimated power and energy use of filament lamp and LED automotive lighting systems

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Under the assumption that each kWh of lighting energy use on a vehicle powered by gasoline corresponds to CO 2 emissions of 1.29kg (Schoettle et al., 2009), the total reduction in annual energy use that would be expected to accompany a shift from filament lamps to LEDs for automotive lighting would be 27.4 kWh/year, and would correspond to an annual reduction of CO 2 emissions of about 35kg/ year for each automobile.


Future trends
LED automotive lighting systems are already common for signal lighting applications, and have been introduced for forward headlamp systems. The rapid advances in luminous efficacy will continue to make them increasingly attractive for automotive use.

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