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5 CHAPTER 3 REVIEW OF HEADLAMP DESIGN As SD is one of the limiting factors in the design of vertical curves it is the critical component which must be considered in the assessment of the sag curve design criteria. For sag vertical curves, SD is limited to that which is allowed by the vehicleâs headlamps. As mentioned, equations for determining the length of sag vertical curves assume a headlight that is 1 degree above horizontal (AASHTO, 2004). This value was likely determined using older sealed-beam headlamps which put out more light above horizontal than do modern replaceable-bulb headlamps. Since the development of these design equations, changing technology and efforts for the worldwide harmonization of headlamp standards has caused beam patterns to change drastically. Because of these changes, equations for sag vertical curves may be overestimating the SD allowed by modern headlamps. EVOLUTION OF HEADLAMP DESIGN In the United States, the first electric headlamps to be installed as standard equipment occurred in 1911. Electric headlamps installed as original equipment were not far behind in Europe, becoming standard on some vehicles in 1913 (as cited in Moore, 1998). These headlamps used a tungsten filament as the light source. Tungsten had two drawbacks, however. As the lights were operated, tungsten would boil off of the filament and would condense on the glass of the bulb, blackening it. Tungsten headlamps also produced relatively low light output for the power that they consumed. A tungsten filament bulb filled with nitrogen gas was first used on automobiles in 1915. The gas reduced the evaporation of the tungsten, slowing the blackening of the bulb, which allowed the filament to last longer (as cited in Moore, 1998). During the mid-1960s, the first halogen headlamps were used in Europe. Halogen headlamps use a tungsten filament in a bulb filled with halogen gases. The gases create a chemical reaction with the evaporating tungsten which redeposits the tungsten back onto the filament (Moore, 1998). This chemical reaction, known as the halogen cycle, increases the lifetime of the bulb, reduces blackening, and increases the light output relative to the power consumed. U.S. automobile manufacturers began installing halogen bulbs in sealed-beam headlamps in the 1970s (Moore, 1998). In Europe, regulators and manufacturers chose to use the extra efficacy of halogen bulbs to provide more light with the same power consumption. In the United States, most halogen bulbs produced the same amount of light as non-halogen bulbs, but with reduced power consumption. In the 1990s, another light source was introduced. High-intensity discharge (HID) headlamps were first offered as an option in Europe in 1991. HID headlamps, also known as âxenon headlamps,â produce light with an electric arc. Metallic salts are vaporized within the arc, producing the lampâs high intensity. The xenon gas used in automotive HID allows the lamp to provide adequate light immediately upon powering on, and increases the speed at which the lamp reaches full brightness. HID lamps provide a light with a higher color temperature, which appears bluish-white as opposed to the yellowish light from tungsten filaments. They also provide longer life, increased light-source lumens, and higher intensity beam patterns than their halogen-tungsten counterparts (Moore, 1998).
6 Headlamps with light-emitting diode (LED) light sources were first installed in 2008 (Whitaker, 2007). Benefits of LED headlamps include a longer lifespan with slightly decreased power consumption. However, with multiple high-powered LEDs, temperature management becomes key. The heat from LEDs is produced at the rear of the emitters, and requires additional heat management measures such as heatsinks and cooling fans. In addition, because there is little heat from the front of the lights, ice and snow on the lens are not effectively thawed by the LEDs. Headlamps have evolved due to two major factors: changing headlamp standards, and new technology. There are two major standards which regulate headlamps: the Society of Automotive Engineers (SAE) standards used in North America, and the Economic Commission for Europe (ECE) regulations used in much of the rest of the world. Historically, a major difference between these standards was the amount of light allowed above the horizontal axis of the headlamps; the prevailing ideas being increased visibility in SAE standards, and reduced glare in ECE regulations. Figure 4 shows the amount of light allowed above horizontal according to several SAE regulations between 1933 and 1997. Included is the Federal Motor Vehicle Safety Standard (FMVSS) 108 , which incorporates SAE standards. A trend can be seen in which the light above horizontal has been generally decreasing (particularly left of the vertical) and also that the amount of light is becoming more regulated (i.e., more limits are imposed). The figures below only show the regulations pertaining to light above horizontal for the sake of simplicity.
7 Figure 4. Light allowed above horizontal by SAE standards. From this figure it can be seen that the regulations have changed. In 1940, the uplight was increased in both magnitude and angle, but was reduced again in 1950. An interesting addition was made in 1997 where a lower limit was placed on the uplight in order to ensure that the headlamp provided some uplight component. Efforts to harmonize the disparate standards began in the mid-1900s, leading to the creation of an international group of lighting experts and vehicle manufacturers called the Groupe de Travail-Bruxelles 1952 (GTB). However, in spite of the research done at the time, the diametrically opposed philosophies of the SAE and ECE standards prevented a compromise on a common beam pattern (Moore, 1998). The greatest progress towards harmonization would come much later. In 1990, the GTB was asked by the Group Rapporteurs Eclairage (GRE) to recommend one worldwide headlamp beam pattern (Moore, 1998). A study conducted by Sivak and Flannagan (1993) recommended four test points that should be common worldwide. The GTB took these four points, made slight modifications, and established the rest of the beam pattern in an attempt to make one unified beam pattern with improved illumination in the typical area of driver vision, sufficient illumination of road signs, and reduced glare for oncoming vehicles. However, the different priorities exhibited by North American and European standards prevented a compromise from being reached at the time (Moore, 1998).
8 In 1993, Sivak, Flannagan, and Sato conducted a study which measured the light output of 150 headlamps from the United States, Europe, and Japan. By taking the median output for each group, they were able to create isocandela diagrams of the âtypicalâ headlamp for each region for the time period. As seen in Figure 5, the amount of light above horizontal is much higher for U.S. and Japanese SAE-J headlamps than for European and Japanese ECE-J headlamps. According to the study, the U.S. headlamps are representative of those manufactured in the late 1980s to early 1990s, and the European and Japanese headlamps are representative of those manufactured in the early- to mid-1980s.
9 Figure 5. Isocandela diagrams of the median luminous intensities for U.S. headlamps, European headlamps, the Japanese SAE-J headlamps, and the Japanese ECE-J headlamps (Source: Sivak, Flannagan, and Sato, 1993). Despite the apparent differences of the two standards in the diagrams, compromises have been made by both sides to bring headlamp beam patterns closer together. In 1997, a large step towards harmonization was taken when FMVSS 108 was updated to include the option of labeling headlamps as visually/optically aimable (VOA). In order to be labeled as VOA, headlamps are required to have a steeper vertical gradient than conventional U.S. headlamps. There are two types of VOA headlamps: those aimed using the vertical gradient to the left of vertical (VOL) which are conceptually similar to European headlamps, and those aimed using the gradient to the right of vertical (VOR) which are conceptually similar to
10 conventional U.S. headlamps (Sivak, Flannagan, & Miyokawa, 2000). Figure 6 shows the beam pattern and aiming points for VOR and VOL headlamps (HAP). Figure 6. Beam patterns and aiming positions for VOR and VOL headlamps (Source: Headlamp Aiming Procedure, 2006). Another major step toward complete harmonization was made in 1999, when the GTB proposed a fully harmonized beam pattern to the GRE based on the four common points recommended by Sivak and Flannagan (1993). The proposed beam pattern was a compromise between the North American and European philosophies (Sivak et al., 2000). Figure 7 shows the test points (top) and zones (bottom) above horizontal (note: while the proposed beam pattern includes points up to 90 degrees above horizontal, the figure stops at 12 degrees for simplicity) (GTB Coordinating Committee, 2002).
11 Figure 7. Test points (top) and zones (bottom) of the harmonized beam pattern proposed by the GTB. In 2004, Schoettle, Sivak, Flannagan, and Kosmatka photometered 20 headlamps representing 39% of the headlamps on passenger vehicles being sold in the United States at that time, and determined the median luminous intensities. Of the median luminous intensity values at 1 degree up and from 0 to 5 degrees left, the average value was less than 500 cd. That is 28% less than the maximum allowed (700 cd) in FMVSS 108. Figure 8 shows the isocandela diagrams of the median luminous intensity for the sales-weighted sample.
12 Figure 8. Isocandela diagrams of the median luminous intensities for sales-weighted sample representing the low-beam headlamps on current passenger vehicles in the U.S. The two panels represent the same information in two different formats. Maximum intensity: 22740 cd at 1.0°R, 1.0°D. (Test voltage:12.8V) (Source: Schoettle et al., 2004). The efforts for a worldwide harmonized beam pattern with a focus on controlling glare for oncoming drivers has created a trend in U.S. headlamp standards for reduced light above the horizontal. The resulting beam patterns of today are much different than those when AASHTO was developing their design guidelines, and may not be well represented by them. In addition to changes made to standards and regulations, the technology of headlamps has also evolved. Newer, brighter light sources, replaceable bulbs, and better reflectors and lenses have all contributed to the constant evolution of headlamps. Not only is the technology of modern headlamps different from the sealed-beam technology likely used in determining SD for sag vertical curves, but there are also many more varieties of headlamps, each with its own beam pattern. COMPARISON TO STOPPING SIGHT DISTANCE With the introduction of new light sources, the variety of headlamps has increased. Each light source has unique characteristics which have an impact on a driverâs ability to see objects ahead. This is the critical detail for the sag vertical curves.
13 In 2005, the Federal Highway Administration (FHWA) published the Enhanced Night Visibility (ENV) study that investigated the performance of several types of headlamp configurations by determining the distance at which a driver could see an object in the road (Blanco, Hankey, and Dingus, 2005). Among the configurations tested were a standard halogen low beam (HLB), an HID low beam, and a low-profile halogen low beam (HLB-LP). By comparing the mean detection distance of an object (i.e., the distance at which a participant was able to see the object) with the calculated stopping distance, it was found that the stopping distance was compromised in several situations by the HLB and HID headlamps. Table 1, Table 2, and Table 3 below show the situations in which the stopping distance might be compromised for the HLB, HID, and HLB-LP, respectively. Table 1. Detection Distance by Type of Object and Potential Detection Inadequacy when compared to Stopping Distance at Various Speeds: HLB Type of Object Det. (ft) 126 ft at 25 mi/h 197 ft at 35 mi/h 278 ft at 45 mi/h 370 ft at 55 mi/h 474 ft at 65 mi/h 529 ft at 70 mi/h Tire Tread 240 X X X X Parallel Pedestrian, Black Clothing 386 X X Perpendicular Pedestrian, Black Clothing 409 X X Child's Bicycle 464 * * Cyclist, Black Clothing 566 Perpendicular Pedestrian, White Clothing 828 Parallel Pedestrian, White Clothing 839 Static Pedestrian, White Clothing 858 Cyclist, White Clothing 862 X = stopping distance might be compromised; * = exceeds distance, but the scenario is not likely; 1 ft = 0.305 m; 1 mi = 1.6 km Source: Blanco, Hankey, and Dingus, 2005
14 Table 2. Detection Distance by Type of Object and Potential Detection Inadequacy when compared to Stopping Distance at Various Speeds: HID Type of Object Det. (ft) 126 ft at 25 mi/h 197 ft at 35 mi/h 278 ft at 45 mi/h 370 ft at 55 mi/h 474 ft at 65 mi/h 529 ft at 70 mi/h Tire Tread 212 X X X X Parallel Pedestrian, Black Clothing 275 X X X X Perpendicular Pedestrian, Black Clothing 282 X X X Child's Bicycle 417 * * Cyclist, Black Clothing 444 X X Perpendicular Pedestrian, White Clothing 683 Parallel Pedestrian, White Clothing 713 Static Pedestrian, White Clothing 734 Cyclist, White Clothing 796 X = stopping distance might be compromised; * = exceeds distance, but the scenario is not likely; 1 ft = 0.305 m; 1 mi = 1.6 km Source: Blanco, Hankey, and Dingus, 2005 Table 3. Detection Distance by Type of Object and Potential Detection Inadequacy when compared to Stopping Distance at Various Speeds: HLB-LP Type of Object Det. (ft) 126 ft at 25 mi/h 197 ft at 35 mi/h 278 ft at 45 mi/h 370 ft at 55 mi/h 474 ft at 65 mi/h 529 ft at 70 mi/h Tire Tread 177 X X X X X Parallel Pedestrian, Black Clothing 302 X X X Perpendicular Pedestrian, Black Clothing 326 X X X Child's Bicycle 399 * * Cyclist, Black Clothing 494 X Perpendicular Pedestrian, White Clothing 721 Parallel Pedestrian, White Clothing 744 Static Pedestrian, White Clothing 778 Cyclist, White Clothing 805 X = stopping distance might be compromised; * = exceeds distance, but the scenario is not likely; 1 ft = 0.305 m; 1 mi = 1.6 km Source: Blanco, Hankey, and Dingus, 2005 The ENV study (Blanco, Hankey, and Dingus, 2005) was conducted on the Virginia Smart Road, with objects presented on flat, straight portions of the road. The decreased SD in a sag vertical curve could cause a compromise of stopping distance in even more situations. This study highlighted how different headlight technologies can affect a driverâs ability to see objects near the road and, therefore, the SSD.
15 In addition to the light source, headlamps must incorporate methods for distributing the light in the desired pattern. Early headlamps used lens optics. The light source was located at the focal point of a metallic parabolic reflector, which collected the light. As light bounced off of the reflector, and through the glass lens, optics molded into the lens would shift the light into the desired pattern. This was typical of most early sealed-beam headlamps (Moore, 1998). In the 1980s, advancement in computer-aided drawing allowed the development of complex shape reflectors, which improved the efficiency of light collection and distribution. In the late 1980s, some U.S. vehicles used complex-reflector headlamps in conjunction with faceted optic lenses. The first multi-reflector headlamps to use a clear lens appeared on the 1990 Honda Accord, with the reflector designed for both light collection and distribution into the desired pattern. Today, modern reflectors are commonly made of plastic with a metallic coating. Another method for collecting and distributing light from a source is projector optics. For this system, the light source is located at the focal point of an ellipsoidal reflector and a condenser lens is located at the front of the lamp. A shade located between the lens and the reflector is used to block a portion of the light to achieve the low or dipped-beam pattern. In some headlamps a separate lamp is used for high beams and, in others, the shade is removed from the path of the light. Headlamps today come in many varieties. With several light sources with unique characteristics, several different methods of collecting and distributing light, and differentâmore regulatedâbeam patterns, it is easy to see why the equation for a sag vertical curve does not accurately represent modern headlamps with its assumed 1-degree uplight. SUMMARY Since the creation of sag vertical curve design guidelines, headlamps and their resulting beam patterns have changed significantly. The major driving forces behind this change are the introduction of newer technologies and an effort by industry groups to create a worldwide harmonized beam pattern. Key changes in technology include the introduction of new light sources â most importantly, halogen and HID light sources â and new methods of collecting and distributing light. Todayâs clear-lens, complex-reflector, replaceable-bulb headlamps are a far cry from the traditional sealed-beam technology likely used in determining AASHTOâs guidelines. Key concerns in the regulation of headlamp beam patterns include forward visibility, proper illumination of roadway signs, and the reduction of glare for oncoming drivers. As industry groups attempt to harmonize the two major headlamp standards (SAE and ECE), the desire to decrease glare has taken a major role in the U.S. headlamp beam patternsâ trend of decreasing and controlling the light above the horizontal. While more uplight is allowed to the right of vertical for the illumination of roadway signs and objects near the road, todayâs beam patterns are drastically different from those during the time of AASHTOâs guidelines creation. These factors underscore previous research which suggests that todayâs headlamps are not well represented in AASHTOâs guidelines for the design of sag vertical curves, and why a closer look is needed to determine if changes need to be made to the guidelines.
