nitride semiconductor light-emitting diodes (leds) || leds in automotive lighting

© Woodhead Publishing Limited, 2014

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20 LEDs in automotive lighting

J . D. BULLOUGH, Rensselaer Polytechnic Institute, USA

DOI: 10.1533/9780857099303.3.595

Abstract: This chapter discusses the use of light- emitting diode (LED) sources in vehicular lighting for illumination and signaling. Photometric requirements for automotive lighting are discussed, along with the key differences between LED and fi lament sources in terms of visual responses. Finally, the energy and environmental implications for LEDs in automobiles are discussed.

Key words: automotive headlamps, vehicular signals, forward illumination, vehicle emissions.

20.1 Introduction

The majority of roads in North America and much of the rest of the world are not illuminated by fi xed pole- mounted roadway lighting systems (NHTSA, 2007). Because of this, automotive lighting is a key component for driving safely at night. The performance requirements for vehicle headlamps (such as those published in the United States as Federal Motor Vehicle Safety Standard 108) are based on standards and recommendations published by the Society of Automotive Engineers (SAE) and similar industry organizations. These requirements stipulate certain minimum or maximum luminous intensities toward different directions from the center of the vehicle lighting system. A similar set of photometric performance requirements exists for countries outside North America; these differ in the particulars but have the same objectives of specifying luminous intensities to ensure vehicle lighting systems provide suffi cient light for drivers to see at night while minimizing glare to other drivers, and to ensure that vehicle signal lights can be detected promptly and without ambiguity.

The present chapter summarizes some of the performance requirements for vehicle lighting systems and includes a discussion of the impact of light- emitting diode (LED) sources on driver visual responses, compared to fi lament sources (such as incandescent and tungsten- halogen lamps), the traditional light source used in most automotive lighting applications.

20.2 Forward lighting

For automotive headlamps that provide illumination ahead of the vehicle, two headlamps are required, mounted as far apart as practical. Each headlamp must meet the same performance requirements. There are two primary types of beam

596 Nitride semiconductor light-emitting diodes (LEDs)


patterns (beam patterns are the resulting distributions of luminous intensity produced by vehicle headlamps): the high (or driving) beam and the low (or passing) beam. North American requirements for several angular locations for high- and low- beam headlamps are given in Table 20.1 and Table 20.2 .

Table 20.1 Selected photometric requirements for high- beam headlamps in the United States

Angular location (degrees left/right, up/down) Maximum luminous intensity (cd)

Minimum luminous intensity (cd)

(0° right, 0° up) 75 000 40 000 (3° left, 1° up) and (3° right, 1° up) – 5 000 (0° right, 2° up) – 1 500 (3° left, 0° up) and (3° right, 0° up) – 15 000 (6° left, 0° up) and (6° right, 0° up) – 5 000 (9° left, 0° up) and (9° right, 0° up) – 3 000 (12° left, 0° up) and (12° right, 0° up) – 1 500 (0° right, 1.5° down) – 5 000 (0° right, 2.5° down) – 2 500 (9° left, 1.5° down) and (9° right, 1.5° down) – 2 000 (12° left, 2.5° down) and (12° right, 2.5° down) – 1 000 (0° right, 4° down) 12 000 –

Table 20.2 Selected photometric requirements for low- beam headlamps in the United States

Angular location (degrees left/right, up/down) Maximum luminous intensity (cd)

Minimum luminous intensity (cd)

(8° left, 0° up) and (8° right, 0° up) – 64 (8° left, 4° up) and (8° right, 4° up) – 64 (4° left, 0° up) and (4° right, 0° up) – 125 (4° left, 2° up) and (4° right, 2° up) – 125 (1.5° right, 0.5° down) 20 000 8 000 (6° left, 1° down) – 750 (2° right, 1.5° down) – 15 000 (9° left, 1.5° down) and (9° right, 1.5° down) – 750 (15° left, 2° down) and (15° right, 2° down) – 700 (1.5° left, 1° up) 700 – (1.5° left, 0.5° up) 1 000 – (1.5° left, 0.5° down) 3 000 – (1° right, 1.5° up) 1 400 – (1° right, 0.5° up), (2° right, 0.5° up) and (3° right, 0.5° up)

2 700 –

(4° right, 4° down) 8 000 –

LEDs in automotive lighting 597


As expected, the requirements for high beams have higher intensities and fewer maximum intensity values than low beams. Additionally, the high beam has a symmetrical beam pattern. In contrast, the low beam has an asymmetrical beam pattern, with more stringent maxima toward the left side (where oncoming traffi c in North America is more likely to be found; beam patterns in countries with left- side traffi c are reversed left- to-right). Figure 20.1 shows the intensity requirements for a low- beam headlamp pattern overlaid onto the angular locations of a straight, two- lane road. Using the inverse- square law, it is possible to convert the angular luminous intensity values to illuminances on the roadway and on objects located ahead of the vehicle; illuminances from each headlamp should be added together to obtain the total.

Figure 20.2 is a photograph of a low- beam headlamp pattern projected onto a wall in front of the headlamp. To reduce glare on oncoming and preceding drivers,

20.1 Photometric requirements for low- beam headlamp patterns in the United States, superimposed onto an image of a two- lane roadway.

20.2 Photograph of a low- beam headlamp beam pattern projected onto a wall.



low- beam patterns usually have the sharp vertical gradients shown in Fig. 20.2 . Above the so- called cutoff boundary between the light and dark portions of Fig. 20.2 , the intensities are low and there is little light. The cutoff boundary makes it possible to check and adjust the vertical aim of the headlamp. Most North American headlamps require the location of the right- side cutoff boundary to be at the same height as the headlamp (Schoettle et al. , 2008). The left- side cutoff boundary is usually lower than the right- side boundary to reduce the amount of light entering oncoming drivers’ eyes. The sharp cutoff boundary of low- beam headlamp patterns restricts the forward visibility of drivers. When driving speeds exceed 60–65 km/h, it can be diffi cult for a driver to detect some potential hazards and stop in time (Andre and Owens, 2001) when using low- beam headlamps. High beams are warranted for such conditions, except if approaching traffi c is within 100 m or so. However, most drivers underutilize their high- beam headlamps (Sullivan et al. , 2004).

As a result of the sharp cutoff boundaries of low- beam headlamp patterns, the vertical aim is a very important factor for optimum performance. In the United States, for example, most states do not require headlamps to be properly aimed as part of a safety inspection (NHTSA, 2007). A recent study of vertical aim among vehicles (Skinner et al. , 2010) found that most vehicles had at least one poorly aimed headlamp. When the aim is too high, the headlamps can contribute to disability glare and discomfort glare (Perel, 1996; Sivak et al. , 1998). When the aim is too low, the driver’s forward visibility can be compromised because of the sharp vertical cutoff.

Vehicle headlamp systems that can change or adapt in response to different driving conditions, called adaptive forward- lighting systems (AFSs), are starting to cause a re- evaluation of the fi xed high- and low- beam headlamp patterns that have been used for many decades (Wördenweber et al. , 2007). Some vehicles have cornering and bending lights; bending lights sometimes use mechanical elements to swivel one or both headlamps toward roadway curves. Some European vehicles are equipped with a ‘town’ headlamp beam pattern that has lower maximum luminous intensities and a broader distribution than most low beams to help detect pedestrians while driving at low speeds in urban locations. AFS requirements for most nations are promulgated by the UN Economic Commission for Europe (ECE) in Vehicle Regulation No. 123. The Federal Motor Vehicle Safety Standard 108 in the United States is presently silent with respect to AFSs.

Presently, most automotive headlamps use fi lament sources (tungsten- halogen or more simply, halogen) with refl ector or projector optical systems to produce the necessary beam pattern. A relatively small proportion of headlamps use high- intensity discharge (HID) lamps, which use a metal halide and xenon to allow the lamps to be switched on immediately. Headlamps using LEDs are just beginning to be used on a few vehicle models. Regardless of the light source used, all headlamps are required to meet the same photometric requirements.



20.3 Signal lighting

Vehicles need to have signal lights to allow drivers to communicate their actions with respect to braking and turning during both the day and night. An increasing proportion of vehicles use LEDs for signal lighting. Different signal lights have different requirements for both color and luminous intensity. The Federal requirements for vehicle signals in the United States are based on SAE standards and recommendations. Table 20.3 lists the color and permissible luminous intensity values for several vehicle signal light types.

Performance requirements for vehicle signal lights in Europe do not differ much from those in North America regarding color and luminous intensity (Bullough et al. , 2007), with an important exception. Turn signal lights at the back of a vehicle can be red or yellow in the United States with different intensity requirements depending upon which color they are. In most of the rest of the world, rear turn signals must be yellow. Allen (2009) reports that yellow rear turn signals tend to result in fewer crashes, possibly because of their higher luminous intensities than red rear turn signals, or because the yellow color makes them easier to tell apart from brake or tail lights. The National Highway Traffi c Safety Administration (NHTSA) is considering whether yellow should be required for all rear- mounted automotive turn signals.

Table 20.3 Photometric and color requirements for vehicle signal lights in the United States

Signal function Required color Minimum–maximum luminous intensity (cd)

Tail (presence) light Red 2–18 Stop light Red 80–300 Center high- mounted stop light Red 25–130 Rear turn light Red or yellow 80–300 (if red), 130–750

(if yellow) Front turn light Yellow 130–750 Backup or reversing light White 80–300 (if two), 80–500

(if one)

20.4 Human factor issues with LEDs

LED sources are substantially different from fi lament lamps used in most present- day automotive lighting applications in a number of important ways:

• LEDs have higher luminous effi cacies (in lm/W) than fi lament sources, meaning they can produce higher intensities or broader beam patterns for the same amount of energy, or have a similar light output with lower energy requirements.



• The narrowband spectral output of colored LEDs produces a highly saturated color appearance, in contrast to broadband sources such as fi lament lamps, which require fi lters in order to produce colored illumination ( Fig. 20.3 ).

• White phosphor- converted LEDs can be produced with a higher correlated color temperature (CCT) than fi lament lamps, which results in a more bluish color appearance.

• LEDs have very rapid onset and offset times: 10–20 ns, including the decay time of yttrium aluminum garnet (YAG) phosphors, compared to about 80–250 ms for fi lament lamps.

The photometric, colorimetric and temporal properties of LED sources can also infl uence drivers’ ability to see and respond to potential hazards in and along the roadway. For vehicle headlamp systems, the spectral distribution of typical phosphor- converted white LEDs, based on blue InGaN devices in combination with YAG phosphors, has a larger proportion of short- wavelength (blue) light than the spectral distribution of fi lament sources like incandescent and halogen lamps ( Fig. 20.4 ). This difference is relevant to visual performance while driving, because at light levels commonly experienced while driving at night, which result in asphalt pavement luminances between 0.1 cd/m 2 and 1 cd/m 2 (He et al. , 1997), the visual detection of hazards is supported by a combination of cone and rod visual receptors in the eye.

However, photometric quantities such as illuminance (in lx), luminance (in cd/m 2 ), luminous intensity (in cd) and luminous fl ux (in lm) are entirely based on the spectral response of the cone receptors in the eye. Cone receptors are used

20.3 Spectral distributions of yellow and red LED and (fi ltered) fi lament sources.



exclusively for seeing at light levels typically experienced outdoors and indoors during the daytime, which is usually between 10 and 1000 cd/m 2 . This apparent discrepancy between the way light is measured and how we see matters because, collectively, rod receptors are more sensitive to short visible wavelengths (such as blue and green light) than cone receptors (Rea et al. , 2004). Thus, the usual photometric quantities (lx, cd/m 2 , cd and lm) can underestimate a driver’s ability to see under LED sources at night, relative to his or her ability to see under fi lament lamps.

A unifi ed photometric system has recently been published by the Commission internationale de L’Éclairage (CIE) to quantify the relative role of rods and cones (CIE, 2010) in seeing at night. As a consequence, it could be possible to obtain equivalent nighttime visual performance using LED sources that produce light levels that are 20% to 30% lower than those produced by fi lament lamps (Van Derlofske and Bullough, 2006). Another visual response that may favor LEDs over fi lament 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 20.5 shows the predicted roadway scene brightness under headlamps using fi lament, 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 colors 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 infl uenced by the spectral content of the headlamp illumination (Schreuder, 1976). This is not the case for discomfort glare, which is defi ned as an annoying or painful sensation that is

20.4 Spectral distributions of white LED and (unfi ltered) fi lament sources.



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 behaviors 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 fi lament lamps, they can have some advantages, especially for vehicle brake lamps. Bullough (2005) demonstrated that visual reaction times to the onset of a colored light signal, such as a brake light or turn signal, could be predicted using a threshold quantity of light energy (in cd·s) received at drivers’ eyes. When a tungsten fi lament lamp is fi rst switched on there is a relatively gradual increase in illumination from the fi lament 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 fi lament sources of the same nominal color 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 benefi t of nearly 7 m for a driver following a braking vehicle (Sivak et al. 1994), a small but sometimes practically signifi cant increase.

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



20.5 Energy and environmental issues

Because they have higher luminous effi cacies compared to fi lament 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 fi lament source- based vehicle lighting systems and for LED lighting systems. The average of their estimates for different lighting and signaling functions are summarized in Table 20.4 .

Also listed in Table 20.4 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 20.4 also includes the resulting total annual lighting energy use for fi lament- and LED-based automotive lighting systems. Under the assumption that each kWh of lighting energy use on a vehicle powered by gasoline corresponds to CO 2 emissions of 1.29 kg (Schoettle et al. , 2009), the total reduction in annual energy use that would be expected to accompany a shift from fi lament 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 35 kg/year for each automobile.

Table 20.4 Estimated power and energy use of fi lament lamp and LED automotive lighting systems

Power per vehicle (W/vehicle) Annual energy use (kWh/year)

Function Filament source

LED source

Annual use (h/year)

Filament source

LED source

Low- beam headlamp 124 87 97 12.08 8.47 High- beam headlamp 132 64 10 1.29 0.63 Daytime running lamp 48 18 382 18.30 7.03 Position lamp 14 3 107 1.54 0.29 Front turn signal 52 14 22 1.15 0.31 Rear turn signal 52 10 22 1.15 0.22 License plate lamp 17 2 107 1.80 0.16 Reverse lamp 43 7 4 0.16 0.03 Center high- mounted stop lamp

34 4 81 2.73 0.28

Brake signal 52 11 81 4.16 0.86 Tail lamp 14 2 107 1.52 0.26 Total annual energy use (kWh/year) 45.9 18.5

20.6 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 effi cacy will continue to make them increasingly attractive for automotive use. The solid- state construction of LED systems, modular confi gurations and relative ease of intensity control through current modulation or pulse width modulation provide signifi cant promise for energy- saving vehicle lighting systems that can adapt in real time to changing roadway, traffi c and weather patterns using AFS technologies. These advantages of LEDs will likely make dynamic rear signal lighting systems practical as well.

20.7 Sources of further information and advice

For additional information about automotive lighting in general including the growing use of LED sources, consult Wördenweber et al. (2007). An overview of the components of the roadway transportation lighting system, including automotive lighting, roadway lighting and traffi c signals, is provided by Bullough (2011). For an extensive discussion of the human factor aspects of lighting for transportation, Boyce (2009) is an excellent resource. Research from the National Highway Traffi c Safety Administration on vehicle lighting systems can be found online at http://www.nhtsa.gov/Research/Human+Factors, and reports from the Lighting Research Center at Rensselaer Polytechnic Institute are available online at http://www.lrc.rpi.edu/programs/transportation/TLA/PublicInformation.asp.

20.8 Acknowledgments

The preparation of this chapter was supported by the members of the Transportation Lighting Alliance (www.lrc.rpi.edu/programs/transportation/tla): Audi, Automotive Lighting, Hella, OSRAM Sylvania, Philips Lighting and Varroc Lighting Sytems.

20.9 References

Allen , K. ( 2009 ) The Effectiveness of Amber Rear Turn Signals for Reducing Rear Impacts , Washington , US Department of Transportation .

Andre , J. and Owens , D. A. ( 2001 ) ‘ The twilight envelope: a user- centered approach to describing roadway illumination at night ,’ Hum Factors , 43 , 620 – 630 .

Boyce , P. R. ( 2009 ) Lighting for Driving , New York , CRC Press . Bullough , J. D. ( 2005 ) ‘ Onset times and the detection of colored signal lights ,’ Transp Res

Rec , 1918 , 123 – 127 . Bullough , J. D. ( 2009 ) ‘ Spectral sensitivity for extrafoveal discomfort glare ,’ J Mod Optics ,

56 , 1518 – 1522 . Bullough , J. D. ( 2011 ) ‘ Roadway transportation lighting ,’ in Kutz , M. , Handbook of

Transportation Engineering: Volume II , New York , McGraw-Hill , pp. 8.1 – 8.24 . Bullough , J. D. , Yan , H. and Van Derlofske , J. ( 2002 ) ‘ Effects of sweeping, color and

luminance distribution on response to automotive stop lamps ,’ in Advanced Lighting Technology for Vehicles , Warrendale , Society of Automotive Engineers , pp. 179 – 183 .



Bullough , J. D. , Van Derlofske , J. and Kleinkes , M. ( 2007 ) ‘ Rear signal lighting: from research to standards, now and in the future ,’ in Automotive Lighting Technology and Human Factors in Driver Vision and Lighting , Warrendale , Society of Automotive Engineers , pp. 157 – 166 .

Bullough , J. D. , Skinner , N. P. , Pysar , R. P. , et al. ( 2008 ) Nighttime Glare and Driving Performance: Research Findings , Washington , National Highway Traffi c Safety Administration .

Buonarosa , M. L. , Sayer , J. R. and Flannagan , M. J. ( 2008 ) Real-World Frequency of Use of Lighting Equipment , Ann Arbor , University of Michigan .

CIE ( 2010 ) Recommended System for Mesopic Photometry Based on Visual Performance , Vienna , Commission Internationale de L’Éclairage .

Hamm , M. ( 2009 ) ‘ Green lighting: analysing the potential for reduction of CO 2 emissions in full-LED headlamps ,’ in Automotive Lighting Technology , Warrendale , Society of Automotive Engineers , pp. 9 – 14 .

He , Y. , Rea , M. S. , Bierman , A. and Bullough , J. D. ( 1997 ) ‘ Evaluating light source effi cacy under mesopic conditions using reaction times ,’ J Illum Eng Soc , 26 , 125 – 138 .

NHTSA ( 2007 ) Nighttime Glare and Driving Performance: Report to Congress , Washington , US Department of Transportation .

Perel , M. ( 1996 ) ‘ Evaluation of headlamp beam patterns using the Ford CHESS program ,’ in Gaudaen , G. , Motor Vehicle Lighting , Warrendale , Society of Automotive Engineers , pp. 153 – 157 .

Rea , M. S. , Bullough , J. D. , Freyssinier , J. P. and Bierman , A. ( 2004 ) ‘ A proposed unifi ed system of photometry ,’ Lighting Res Technol , 36 , 85 – 111 .

Rea , M. S. , Radetsky , L. C. and Bullough , J. D. ( 2011 ) ‘ Toward a model of outdoor lighting scene brightness ,’ Lighting Res Technol , 43 , 7 – 30 .

Schoettle , B. , Sivak , M. and Takenobu , N. ( 2008 ) ‘ Market- weighted trends in the design attributes of headlamps in the US ,’ in Automotive Lighting Technology , Warrendale , Society of Automotive Engineers , pp. 85 – 93 .

Schoettle , B. , Sivak , M. and Fujiyama , Y. ( 2009 ) ‘ LEDs and power consumption of exterior automotive lighting: implications for gasoline and electric vehicles ,’ 8th International Symposium on Automotive Lighting , Munich , Herbert Utz Verlag , pp. 11 – 20 .

Schreuder , D. A. ( 1976 ) White or Yellow Lights for Vehicle Head- lamps? Voorburg , Institute for Road Safety Research .

Sivak , M. , Flannagan , M. , Sato , T. , et al. ( 1994 ) ‘ Reaction times to neon, LED, and fast incandescent brake lamps ,’ Ergonomics , 37 , 989 – 994 .

Sivak , M. , Flannagan , M. J. and Miyokawa , T. ( 1998 ) Quantitative Comparisons of Factors Infl uencing the Performance of Low-Beam Headlamps , Ann Arbor , University of Michigan .

Skinner , N. P. , Bullough , J. D. and Smith , A. M. ( 2010 ) ‘ Survey of the present state of headlamp aim ,’ Transportation Research Board 89th Annual Meeting , Washington , Transportation Research Board .

Sullivan , J. M. , Adachi , G. , Mefford , M. L. and Flannagan , M. J. ( 2004 ) ‘ High- beam headlamp usage on unlighted rural roadways ,’ Lighting Res Technol , 36 , 59 – 65 .

Van Derlofske , J. and Bullough , J. D. ( 2006 ) ‘ Spectral effects of LED forward lighting: visibility and glare ,’ in Automotive Lighting Technology and Human Factors in Driver Vision and Lighting , Warrendale , Society of Automotive Engineers , pp. 11 – 18 .

Wördenweber , B. , Wallaschek , J. , Boyce , P. and Hoffman , D. D. ( 2007 ) Automotive Lighting and Human Vision , New York , Springer .

nitride semiconductor light-emitting diodes (leds) || leds in automotive lighting

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