The Engineering Resource For Advancing Mobility 400 COMMONWEALTH DRIVE WARRENDALE, PA 15f ica

830567

Motor Vehicle Forward Lighting

Michael Perei, Paul L. Olson, Michael Sivak and Jere W. Medlin, Jr.

International Congress & Exposition Detroit, Michigan February 28 - March 4, 1983 830567

Motor Vehicle Forward Lighting

Michael Perel, Paul L. Olson, Michael Sivak and Jere W. Medlin, Jr.

MOTOR VEHICLE FORWARD LIGHTING hazards. Adverse weather can reduce visibility distances in many circumstances. In fog, ABSTRACT backscatter from headlamps increases glare and reduces the ability of headlights to illuminate This paper surveys the literature on motor road delineation and other critical objects. vehicle headlighting and its influence on the In rain, the specular reflectance of light from ability of drivers to avoid accidents. The roadways makes it virtually impossible for review identifies the key relationships between drivers to see lane markings. headlamp design characteristics and driver and Not all drivers are affected to the same environmental factors. The major safety degree by the nighttime visual environment. problems associated with headlighting are For example, drivers with various visual discussed, and issues needing the attention of deficiencies can be more sensitive to nighttime the research community are identified. driving problems. Some of the visual characteristics which can reduce drivers' visual performance at night include glare resistance, contrast sensitivity, peripheral THE NIGHTTIME ACCIDENT RATE IS OVER THREE acuity, and night myopia. Drivers who are TIMES THE DAY RATE. Many factors can be blamed fatigued are more adversely affected by the for this difference including alcohol, young nighttime visual environment because of changes reckless drivers, and fatigue. It is also in their visual search behavior and even lower reasonable to assume that the poor visual sensitivity to peripheral cues. The use of environment at night can affect safety since so alcohol and other drugs can impair driver much of the information required by drivers is visual information processing and make night obtained visually. driving more dangerous. There are a number of major differences Vehicle characteristics can also play a between the visual environment at night and day role in influencing the nighttime visual that can make night driving a difficult task. environment. Headlamps are the primary source At night, the broad fields of view of daylight of illumination for drivers on unlighted narrow to the region illuminated by artificial roads. The design goal of headlamps is to lighting—headlights or fixed road lighting. illuminate the roadway and potential hazards At night, objects are seen under lower without subjecting oncoming drivers to contrasts and non-uniform illumination, which excessive glare. There are a number of makes detection difficult. As ambient light headlamp design characteristics which can decreases, colors fade, shadows disappear, influence driver vision and, hence, capability resolution is lowered, and object detail to avoid accidents, including beam pattern, vanishes. Peripheral vision is reduced, intensity, and lamp construction. The causing the driver to modify search and scan performance that headlamps are designed to behavior. Increased glare causes eye strain achieve can be degraded by misaim, dirt, lamp and fatigue and further increases the outage, and incorrect voltage. difficulty of detecting hazardous objects. In order to identify the major problems in Various aspects of the highway environment headlighting, the above driver, vehicle, and can further degrade driver vision at night. environmental factors were examined in a review The variation in pavement reflectivity from of the literature to determine their frequency asphalt to concrete can affect the contrast of occurrence, their relation to crashes, and and, hence, detectability of potential their relation to driver performance. The

0148-7191/83/0228-0567$02.50 Copyright 1983 Society of Automotive Engineers, Inc. review also describes the historical were frequently confounded with changes in development of the design of headlighting and traffic volume, upgraded enforcement tries to identify the safety-related rationale activities, or other roadway modifications. In for the various engineering developments and some cases accident rates actually increased standard requirements. with illumination levels. But, the general Most of the review focuses on data from trend is toward fewer accidents as illumination laboratory and field tests and computer levels increase. Two examples will be offered models. Very little information exists for purposes of illustration. describing the exposure characteristics of One of the better recent studies is by drivers, the relation of the headlamp Scott (2). He looked at the relationship of characteristics to accidents, and the rationale eight variables to accidents at 41 sites in for headlamp development over the years. Where Great Britain. Average road surface luminance possible, the relation of headlamp design to gave the best correlation and it could not be safety was determined through the use of the improved by the addition of other variables. proxy measures that have been used in lab, Scott's data are summarized in Figure 1, and field; and computer modeling studies. These show that the proportion of accidents during relationships show the sensitivity of hours of darkness dropoed in a relatively safety-related performance to variations in linear fashion as the level of illumination headlamp characteristics. The factors, such as increased. driver and environmental characteristics that interact with headlamp design to affect performance were also examined. The goal of examining all the above types of data was to determine the nature and extent of the hazards associated with headlight factors and the extent to which modifications to headlamp characteristics or their interacting factors could lead to improved driver performance. One problem area that became clear at the outset of the review was the need to improve performance of the low beam rather than high beam. This need arises from two basic reasons. First, most night driving is on low beam because of the high traffic densities that are encountered. Second, high beam intensities 0.5 1.0 1.5 2.0 have reached the point of diminishing returns Average road surface luminance (cd/m2) beyond which no significant increases in seeing distance could be gained without extremely high Fig. 1 - Best fitted.relationship between levels of intensity. Thus, the majority of the Dark/Day accidents and average review will focus on low beam. road surface luminance (2) REVIEW OF PERTINENT ACCIDENT EXPERIENCE VISIBILITY - If the great bulk of Another recent and very interesting information necessary for the safe and analysis has been reported by Merritt et al. effective operation of a motor vehicle is (3). Using data from the 1977 Fatal Accident acquired visually, and if darkness impairs Reporting System (FARS) developed by the vision, then lighting systems which improve National Highway Traffic Safety Administration vision ought to have an effect on safety. (NHTSA), they compared the ratio of Researchers have been more successful in single-vehicle fatal accidents on straight and correlating accident rates with fixed curved road sections under three lighting illumination systems characteristics than conditions: daylight, night but lighted and headlighting. The reason is that controlled night not lighted. An update of this analysis studies are easier to run with fixed using 1980 FARS data is reproduced in Table 1. illumination systems than headlights. There The percentages in the first two columns [i.e., are thousands of miles of roads illuminated at daylight and night (but lighted)] are different levels. Lighting systems are being strikingly similar. However, the proportion installed where none existed, and old systems occurring on curves is higher on unlighted are being upgraded somewhere at almost any roads at night, as shown in the third column. time. Sometimes it is even possible to alter These data suggest that improved lighting helps the lighting levels on a particular street and to reduce accidents on curves. study the effects. There are several good summaries of this research. One of the better is provided by Gallagher and Janoff (1)*. The results of these studies are quite variable. Among the *Numbers in parentheses designate References at reasons are that changes in lighting levels end of paper TABLE 1 - Summary Of 1980 Accident Statistics range for bicycle accidents is 23-32%. The Relating To Lighting Conditions, Road large variability in these estimates is due in Geometry, and Run-Off Road Accidents part to the difficulty of judging the role of visibility in accidents. GLARE - If glare reduces the ability to SINGLE VEHICLE FATAL ACCIDENTS AS A FUNCTION OF LIGHTING AND ROAD GEOMETRY see, then glare should cause accidents. Some evidence that it does is provided by Burger et DAYLIGHT NIGHT (BUT LIGHTED) NIGHT (DARK) al. (6), who used a critical incident approach

Straight to identify vehicle driver mismatch problems and Level 5454 (67*) 3009 (641) 6758 (581!) contributing to accidents or near accidents. Curved Based on 3,500 survey returns, citing nearly and Leve-l 2693 (331) 1663 (361) 4876 (421) 1,700 accidents or near accidents, the authors Totals 8147 (100) 4671 (100X) 11634 (IOCS) conclude that glare from the headlamps of oncoming vehicles is the single most important Straight driver-vehicle mismatch problem to which NHTSA and Grade 2137 (461) 714 (451) 2329 (391) should direct its attention. Curved What does not become clear from these data and Grade 2539 (5SX) B73 (551) 3575 (611) is whether the glare problem is caused by Totals 4676 (100X) 1587 (lOOt) 5904 (1001) failure to dim, misaim, or inadequate low beam design. Clearly, the solution recommended All Straight depends very much on the source of the problem. (Level + Grade) 7591 (591) 3723 (591) 9087 (521) In sum, the two attributes most important All Curved in headlamp design, illumination and glare, (Level + Grade) 5232 (411) 2535 (411) 8451 (481) both seem to relate to the probability of Totals 12823 (1001) 6258 (1001) 17538 (1001) accidents. What we do not have is the relationship of various levels of headlight illumination and glare to accidents. Further, It seems clear that better roadway the difficulties of acquiring such information lighting reduces accidents at night. But, does are such that it is unlikely to be available in this hold true for headlighting as well? the foreseeable future. In lieu of accident Unfortunately, there is no direct evidence that data, researchers must rely on surrogate it does. The problem is that the various criteria such as visibility distance to conditions mentioned earlier that made it evaluate the problem of nighttime visibility. relatively easy to run controlled studies of WEATHER - Adverse weather can make the fixed lighting have no parallels in relatively poor visual conditions at night even headlighting. worse. However, there is only limited The best evidence for headlighting as a information on the relation between factor in accidents comes from in-depth headlighting, adverse weather, and accidents. studies. For example, the Tri-Level study Koth et al. (7) reviewed data from a carried out at Indiana University (4) found number of studies of fog and accidents. The that improved forward lighting would have been conclusions of this review were that nighttime of at least possible benefit in about 3% of the increases the incidence of fog accidents. The accidents studied. However, there are problems scope of fog accidents is minor, representing with the in-depth approach that may tend to about 2 or 3% of all accidents, but the work against variables such as lighting. For severity of fog accidents can be great. example, a single-vehicle, run-off-the-road Rain can increase glare and affect the crash may be ascribed to driving too fast or contrast of critical objects illuminated by poor delineation. However, better limiting headlamps. Accidents in rain were investigated might have provided the operator with by Haghighi-Talab (8) and reviewed in an OECD sufficient information to properly adjust his report (9). The results of these studies or her speed or see where the road was going indicate that the degrading effects of rainfall despite poor delineation. In short, unless the on vision contribute to accident causation. operator ran into something because of lack of None of the studies of adverse weather visibility, inadequate headlighting could be a were able to associate headlamp design with contributory factor that is easily overlooked. accident rates. However, the influence of Another assessment of the potential role headlamp design on visibility in adverse of headlighting in accidents was presented by weather is an important factor that should be Hale and Blomberg (5). They estimated the considered in headlamp evaluations. percent of pedestrian and bicycle accidents where poor visibility could have been a causal REVIEW OF EXPOSURE AND OTHER EXPERIENCES OF THE factor. Since improved headlighting is one DRIVING PUBLIC approach to increase visibility, their estimates provide upper bounds on the The seriousness of the night driving headlighting problem. Using data from in-depth safety problems is demonstrated in part by the accident investigation studies, they calculated poor performance of drivers in being able to that from 3-23% of all pedestrian accidents stop safely, read signs, and follow the road were attributable to poor visibility. The path. It is also a function of the exposure of drivers to the more hazardous lamp design benefits must be balanced against certain characteristics and environmental conditions. disadvantages, such as a slight loss in seeing Another component of the night driving hazard distance, and change in apparent color of is the extent to which driver limitations can roadside objects. reduce the seeing distance provided by Lamp mounting height and driver eye height headlamps. Unfortunately, there is only in cars have been decreasing since the 1950's limited data on the exposure characteristics of as car manufacturers have sought to change the the night driving environment and the range of looks of their cars. In the early 1950's lamp capabilities of the driving population. height was about 32 inches. Currently the VEHICLE AND LAMP CHARACTERISTICS - A minimum mounting height is 24 inches measured considerable number of vehicle and lamp to the center of the lamp. For reasons of characteristics affecting night driving international harmonization and improved performance vary from car to car. These aerodynamics some proposals have suggested that include beam intensity, beam pattern, beam the height be slightly lowered even further. color, aim, lamp mounting height, driver eye Driver eye heights in cars have also dropped height, lens and reflector deterioration, over the last 30 years, from about 3.95 feet to windshield tint, dirt, inoperative lamps, and 3.5 feet (for the 15th percentile) (14). lamp voltage. Although there are many drivers in vans, Beam intensity and pattern, although pickups, and trucks with higher eye heights, regulated by FMVSS 108, can vary significantly the exposure of drivers with low eye heights is from manufacturer to manufacturer. FMVSS 108 the critical factor determining the degree of references SAE photometric requirements, which glare hazard. set maximum and/or minimum intensity values for Misaim and dirt on lamps are encountered a limited number of points* in the beam rather frequently by drivers. They are pattern. The permissible range of values for discussed more fully in a later section of the some of the test points can be relatively report. large. Thus, a variety of beam patterns and Lens and reflector deterioration of glass intensities can be designed to meet the sealed beam lamps is practically non-existent. standard. The quality control employed in the A very limited number of plastic headlamps have manufacturing process can also affect variation been introduced and drivers may encounter some from lamp to lamp. lens abrasion depending on the quality of the Most U.S. vehicles use sealed beam lamps protective coating and the harshness of the which are basically similar in terms of environment. intensity pattern. Intensity levels do vary Defects, whether in-operative lamps or from lamp to lamp. However, no systematic lamps operated at less than design voltage are survey of this variability has been published. a problem of unknown size. The Ford Motor A small selection of tungsten sealed beam lamps Company in their comments to the NHTSA was examined by Bhise el al. (10), who found estimated that 51.9 million headlamps were variations in maximum output as great as two to replaced on passenger cars in the U.S. in 1979, one. of which 63% were due to burnout. It is not Motorcycles can use European non-sealed known how many nighttime miles are travelled headlamps with a "sharp cut-off" beam pattern. with one lamp out before it is replaced. With It is not known how many "sharp cut-off" lamps halogen lamps, sealed or unsealed, it is are currently in use in the U.S. possible for a driver to use them even if the In recent years the frequency of use of lens has cracked. Under this condition halogen sealed beams has increased. One reason moisture and dirt can enter the lamp and reduce for the advent of halogen sealed beams is to the road illumination. take advantage of the raised maximum intensity Drivers have also been exposed to now permitted on high beam by Federal visibility degradation at night because of the standards. No published data has been found increased use of tinting on windshields. Not which reports the actual intensities of halogen only do more cars use tinted glass, but the high or low beams. Also no information on the transmissivity has decreased over the last 30 percent of cars equipped with halogen lamps was years. The estimated percent of cars with found. tinted windshields is about 80%. Light The halogen lamps operate at higher transmittance through a' tinted windshield can filament temperatures and hence emit a whiter be as low as 65% depending on installation light than tungsten units. The effect of such angle, compared to 82-84% transmittance for a color differences on safety are nil although non-tint windscreen. some drivers have the impression that they can ENVIRONMENTAL CHARACTERISTICS - The nature see better with the whiter light. In France, of the roadway enviroment to which drivers are headlamps are yellow, a policy based on the exposed also affects night driving safety and assumption that it reduces glare. Yellow the performance of headlamps. Environmental headlamps (other than fog lights) are illegal factors include weather, roadway design, and in the U.S. In Europe the lamp color issue has critical object visual characteristics. been extensively researched (11, 12, 13). The The weather under which drivers must findings indicate that if there is any benefit operate varies from region to region. A to yellow headlamps, it is slight. These summary of weather patterns has been prepared by Thomas and»reported by Olson et al. (15). 100 These data show that the incidence of fog varies from practically never in much of the country to 10 to 20% of the days in coastal areas. The incidence of precipitation of 0.01 inch or more varies from 20 to 50 days in the Southwest, to 100 to 150 days in the Central and Eastern regions, to 150 to 170 days in the far Northwest and around the Great Lakes. Bhise et al. (10) estimated that the typical U.S. driver will encounter 91 days of driving annually with precipitation greater than 0.01 inch. Thus, adverse weather cannot be ignored Summer Clothing (Beiow Knee) in evaluations of headlamps performance. Summer Clothing (Above Belt Line) The distribution of artificial Winter Coats illumination has been reported by Bhise et al. (10) and is reproduced in Table 2. These data show that artificial illumination is rare on N=80 Subjects rural roads. This suggests that most high-speed driving is done using only headlamp illumination.

TABLE 2 - Frequency Of Fixed Roadway Liahting on U.S. Roads (10) 10 20 30 40 50 60 70 Luminous Reflectivity (Percent) ENVIRONMENT Fig. 2 - Cumulative distribution of Rural Urban pedestrian clothing reflectance XIllu- X Not X illu- X Not minated Illuminated minated Illuminated (10)

Interstate 6.0 94.0 91.1 8.9 System

Al 1 Other 5.3 94.7 85.5 la. 5 Freeways JU Divided Roads 7.9 92. 1 87.8 12.2 0) (Nonfreeways) 0 TO Undivided Roads 0.9 99.1 87.8 12.2 O (Over 2 Lanes) 0) • Undivided Roads 2.5 97.5 55.2 44.8 (2 lanes) *

t -* • r v •' • • * I " Obstacles on the road must be brighter or •. • — Percen t Retrore f darker than their background in order to be ''-%C• • -V f •

detected. The reflectivity of pavements, that 0 C often form the background for objects of 200 400 600 800 1000 concern to drivers, has seldom been measured Distance trom Driver (feet) accurately. Some data have been provided by Measured pavement retro- Bhise et al. (10) and are reproduced in Figure reflectivity characteristics (10) 2. These data should be compared with those in Figure 3, also from Bhise et al. (10). Clearly, much clothing has reflective characteristics similar to road pavements that might constitute a background in a potential collision situation. Thus, low contrasts In summary, the above types of factors between targets and background are common when form the environment under which headlamps must the target is a pedestrian. provide visibility to drivers. Driver safety The exposure of drivers to hills and can be reduced if headlamp design does not take curves can also influence headlamp these environmental factors into account. performance. A beam pattern that provides good DRIVER CHARACTERISTICS - A number of visibility distance under straight, flat road driver related factors can affect the conditions, may produce too much glare to visibility provided by headlamps. These oncoming drivers on curves or hills. A survey characteristics include vision, fatigue, of road topography was performed by Bhise et alertness, and alcohol. The distribution of al. (10) and incorporated in the Ford headlamp these characteristics in the driving population evaluation model, which is discussed later in and their effect on headlamp performance is the paper. only partially known. A number of visual deficiencies can affect how well drivers can see at night. These include glare sensitivity, lens yellowing, contrast sensitivity, and acuity. The degree to which these factors can vary from individual to individual can be significant. They have also been shown to vary with age. One source of information on the variation on drivers' response to glare is by Pulling et al. (16). They measured the headlight glare resistance of 30 subjects in a driving simulator. They increased the intensity of simulated oncoming headlights until the subject slowed down or otherwise gave evidence of being impaired. The headlight glare resistance was measured as the logarithm of the ratio, at the threshold for glare acceptability, between the illuminance from oncoming headlights and the •0.5 0-0 0.5 1.0 15 ambient illumination. The results are shown in LOG BACKGROUND LUMINANCE (CO/SQ M) Figure 4, A small scale survey was also conducted to assess the log glare ratios found -ig. 5 - Effect of background luminance in typical driving environments. These- on visual acuity of three age measurements showed glare ratios as much, as groups (17) 50:1 (log*!.7) on residents! streets and over 500:1 (log=2.7) on an unlighted interstate Considerable information does exist about the highway. Comparing these values to the alcohol levels of the driving population. headlight glare resistance suggests that Blood alcohol concentration levels vary increasing age can have a significant impact on depending on time and day of the week, reaching the ability of drivers to drive under many a peak in the early morning hours of weekends. typical night conditions. At such times, one driver in eight may have a blood alcohol level of .10% or more (18). ENGINEERING DEVELOPMENT AND TESTING BACKGROUND - Headlamps, like many vehicle in 3.0 components, have evolved over a long period of u time. The history of this process has been z2 5 reviewed by a number of authors (19, 20, 21, - 22, 23, 24). A brief summary is presented here. ua HEADLIGHTING IN THE UNITED STATES - K 2.0 Headlights first appeared about 1906. The first headlights used acetylene and, like the first electric lamps which became available later, had a "beam pattern" much like a search light. About the time of World War I, an effort was made to more adequately illuminate the road by molding prisms into the lenses. 0.5 Further developments continued in the 1920's and early 1930's, resulting in substantial 20 40 60 80 improvements in light distribution intensity. AGE Unfortunately, this era was also characterized by a proliferation of beam Fig 4 - Headlight glare resistance vs. patterns, lamp sizes and shapes that not only age (16) made headlamp components expensive, but also made them difficult to replace when necessary. In the middle 1930's work began toward the development of the sealed beam, which was Another indication of the variability of introduced on 1940 model cars. The sealed beam driver visual capabilities with age is shown in is probably the most significant single a study by Sturgis and Osgood (17). Figure 5 development ever to occur in headlighting. It from that study shews how visual acuity solved some serious problems associated with decreases as luminance is reduced and as age aging of the lamp unit, virtually guaranteeing increases. consistent, good quality headlighting Of the many other factors that can throughout the life of the vehicle. Headlamps influence driver performance, such as fatigue were also standardized dimensionally as well, and alertness, little is known about their making them readily available at relatively low frequency of occurrence in the population. cost. Since the introduction of the sealed beam, low beam was asymmetrical (i.e., it directed some rather significant changes have been the most intense portion of the beam to the introduced to solve certain problems and right), it produced greater visibility improve performance. For example, it was felt distances on the right side of the road. As a that performance of the early lamps in rain, result, it was recommended that changes be made snow and fog was not as good as it might have to the Graves shield to allow a greater amount been, due to illumination scattered above of light to be projected along the right edge horizontal. In 1955 this scatter was reduced of the road. This was accomplished by removing by the addition of a "fog cap" (a metal shield a portion of the shield on one side. The sharp positioned near the filament). cut-off characteristic was retained. However, Visual aiming of sealed beams was found to instead of presenting a symmetrical appearance be very difficult, due to the lack of when projected against a wall of screen, it now well-defined edges in the pattern. This appeared flat on the left with a 15 degree problem was aided greatly by the introduction upward slant on the right. This revised of mechanically aimable units in 1956. concept became the European standard. In 1957 the first four-lamp system was More recently a further modification has introduced. By partially separating high and taken place, with the high intensity portions low-beam functions, the need for compromise in above horizontal being cut off at +1 degree. lens design was reduced. Although in theory This produces a shape approximating the letter the four-lamp system was developed to provide "Z," instead of a shallow "V." This change drivers with better illumination, no published reduces problems with glare on curves and into research studies predating 1957 have been found the rear view mirrors of vehicles ahead. that evaluated the visibility benefits of this Another advance in European headlighting system. came with the introduction of iodine (halogen) Further refinements in design resulted in sources. The first mention of these in the an improved two-lamp system in 1959. Again, no literature occurs in the early 1960's (28, 29), controlled experimental studies have been found although they did not appear on cars until that evaluated the visibility performance of sometime later. this development. The use of iodine vapor inside a light The 1970's saw the introduction of bulb makes possible a chemical reaction that rectangular two-and four-lamp systems. The causes vaporized tungsten to redeposit on the primary benefit was in styling and the fact filament itself rather than on the glass that they would permit better utilization of envelope (30). Thus, the problem of bulb the space on the front of the vehicle. blackening is eliminated. It also makes it Halogen technology was not applied to possible to generate substantially more sealed beams until the late 1970's. The major luminous flux/watt and to use a smaller reason for this development was that FMVSS 108 filament. The smaller filament simplifies the had been changed to allow a more powerful high problem of focusing the beam. Because the beam (from 75,000 cd to 150,000 cd), with the filament must be run at a much higher limitation that power consumption had to stay temperature in order to operate the chemical the same. The low beam of these units have to reaction just described, it was necessary to meet the same photometric requirements as their use a quartz envelope on the bulb. It is for tungsten counterparts. this reason that such sources are termed HEADLIGHTING IN EUROPE - European quartz-iodine or quartz-halogen. headlighting systems are based on the Substantial development has taken place in development of the Graves "anti-dazzle" bulb, the last several years since halogen lamps were which was patented in 1920. The Graves bulb first introduced. Earlier versions could use provides a simple and inexpensive way of only a single filament in the bulb, making it greatly reducing the amount of light projected applicable only for four-lamp systems. above horizontal. A metal shield surrounds the Present-day versions incorporate two filaments, front, sides and bottom of the low beam so that both high and low beams can be filament, preventing any light from being generated from a single source. projected directly forward or to the lower One of the major differences between portion of the reflector. While this reduces European and U.S. lamps is in construction. the efficiency of the lamp in terms of luminous European lamps are not hermetically sealed and flux/watt compared to the sealed beam system, are typically of composite construction, i.e., it does produce significantly less glare. a glass lens glued to a metal reflector. In 1953-54 a number of lighting tests were Unsealed and composite lamps can be designed in carried out under the auspices of the CIE shapes and sizes that may give them full (International Commission on Illumination). economy and styling benefits. However, These tests have been described by de Boer (25, European construction can lead to two potential 26) and Jones (27). As part of this program, problems that the sealed beam does not have: comparisons were made between American and (1) accumulation of dirt and moisture on European lighting systems. The results interior surfaces due to the "breathing" action suggested that visibility distances on the left of such units, and (2) oxidation of the side of the road were comparable under most reflector. One of the best surveys of this conditions tested. However, since the American problem (31) found significant distortion of beam patterns in as little as six months in a drivers may balk at paying the cost of lamp sample of European lamps they tested. replacement which may be relatively high for European safety inspection data (as quoted by limited production lamp shapes. Because, the Ehrhardt, 30) have shown serious deterioration replaceable bulb will still put light on the as a common reason for failures, but such road, some drivers may not realize that the surveys are unsystematic and do not cracked lens allows dirt, moisture, and quantitatively measure lamp performance. The possibly corrosion to reduce lamp performance. Europeans have worked hard to solve these lamp Another potential problem with some replaceable durability problems, but data on newer headlamp bulb systems is that photometries can become designs are scarce. Although some European degraded if the filament is not aligned lamps may provide good long term performance, precisely when a new bulb is installed. This the current standards do not set requirements problem can be solved by having precise that prevent poorer quality lamps from being tolerances for filament position and bulb' sold. For example, they do not specify mounting. corrosion performance tests. Maintaining correct aim may become more Another major difference between European difficult if new lamp designs do not have and U.S. headlamps is the method of aim. aiming pads compatible with current mechanical European lamps are aimed visually by projecting aimers. The quality of aim in the vehicle the beam on a screen directly or on a screen fleet will thus be dependent on the extent to through a condensing lens. Because the which service shops purchase and can properly European low beam pattern has a relatively use adapters for these lamps. sharp intensity gradient, aim can be adjusted The use of headlamp covers is another by positioning the lamp until the gradient approach with the potential to improve falls in line with the reference markings on aerodynamics. Testing of glass covers was the screen. Aiming accuracy depends in part on conducted by the California Highway Patrol and how precisely the beam pattern can be aligned reported in comments to NHTSA Docket 81-11. with the reference markings. Because the U.S. Their testing found that the covers reduced beam pattern does not have intensity gradients maximum low beam intensity from 10-23 percent as distinctive as those in European lamps, and increased glare intensity above visual aiming is more difficult. Visual aiming horizontal. Their conclusion was that the is used in the U.S. although it has been shown types of covers tested degrade headlamp to be less accurate than mechanical aiming performance and should be prohibited. (32). Mechanical aiming checks alignment by Another new development with potential reference to the aiming pads built into the effects on lamp performance is the lowering of headlamp lens. A vertical plane through the passenger car lamp mounting heights. The surface of these pads forms the aiming plane. impetus comes from the desire for international Mechanical aimers are used to align the aiming harmonization, since Europe allows a slightly plane vertically and horizontally using spirit lower mounting height, and the desire to lower levels and a split image/mirror system. This hood lines for reduced drag. Over the years aiming method is not affected by beam pattern minimum headlamp heights have been lowered from intensity gradients. 30-32 inches to 24 inches from the ground to Questions of aim and durability are the center of the lens. In practice the range becoming increasingly important in the U.S. as of current passenger car headlamp heights is the impetus grows to improve aerodynamics and from 24-28 inches. The effect of lowering increase styling options. These motivations mounting heights on seeing distance was studied have been the major force behind the in a test conducted by Roper and Meese (33). engineering developments that seek to develop Figure 6 shows the results, which apply to new lamp shapes, sizes, aiming devices, lamps meeting SAE specifications for low beam construction, and materials. The safety and a target 16 inches square with 7 percent implications of these possible changes are only reflectance. These data show that about 10 beginning to be understood. feet of seeing distance are lost for each inch Durability may become a problem because the lamp height is lowered. This figure can plastic lamps are being proposed. This type of vary, however, depending on such factors as lamp needs a protective coating on the lens to driver eye height, beam aim, and beam pattern. protect it from abrasion. A scratched lens can Changes are also being suggested that have alter the beam pattern and increase glare to the potential for upgrading low beam oncoming drivers. The degree to which photometries. For example, four lamp systems performance Suffers depends on the coating used are being proposed which use filaments parallel Durability can also be affected if the to the longitudinal axis and single function reflector materials corrode or the inside of low beam units. This development is said to the lamp becomes dirty, which may be more allow higher light output in the lower right likely if replaceable bulb systems are quadrant for visibility with the same or less introduced. Problems with European type glare intensity in the upper left quadrant as systems have been noted above. New materials current U.S. low beams. Also, composite lamps and techniques used in lamp design can improve with replaceable bulbs are being proposed which this problem but may have other durability could permit lamp designers to develop separate problems. In particular, if a lens cracks, left and right-side beam patterns for improved visibility wtth less glare. Not all the procedures which are standardized on a company, engineering changes claim they will increase not an industry basis. If modifications to seeing distance. Some of the new lamp designs industry standards are sought, the problem being proposed may be limited in terms of the moves to an appropriate committee of the extent to which their photometries can be Society of Automotive Engineers (SAE). In the improved although all claim to meet current case of beam development, hardware is usually standards. fabricated and appropriate demonstrations are staged for committee members. Ultimately, when it appears as though consensus is likely, the NO ATTENTION FACTOR APPLIED issue is balloted to the committee. Some lamp modifications may require changes to Federal Motor Vehicle Safety Standard 108 before they can be implemented. While this method of headlamp development is not "scientific" in the usual meaning of the term, it cannot be said to have produced a poor lighting system. However, there is a problem in that the process is not documented in the open literature. This make it difficult to review and critique the rationale for proposed changes and to quantify the improvements in safety that these changes may bring. 2*00 2000 !600 1200 900 400 0 400 800 1200 1600 DISTANCE BETWEEN APPROACHING CARS DISTANCE BEYOND • •FEET- MEETING POINT LABORATORY AND FIELD TESTING AND COMPUTER Fig. 6 - Effect of lamp mounting height MODELING on visibility distance (33) In view of the difficulty of relating headlamp design factors to accidents and the limited information on the safety related MOTORCYCLE HEADLAMPS - The main focus of aspects of engineering developments over the this report is automobile headlighting. years, the most descriptive and detailed However, a great variety of special lighting information on headlamp safety problems comes systems have been developed for certain from experimental studies and computer applications. Motorcycles are such a case, and modeling. The dependent variables used in are of interest here because they operate in these tests and analytic analyses serve as the same environment as automobiles. The proxy measures of the degree to which headlamp special problems to lighting design posed by characteristics can affect night driving safety. motorcycle use are: VISIBILITY - The most basic function of a. The vehicle rolls and the lamp is headlamps is to provide drivers with the steered in cornering, distorting the visibility for vehicle control, for detection patterns and changing its illumination and assimilation of road sign information, as distance. well as detection and identification of roadway b. There is usually only one lamp. hazards (e.g., pedestrians, stalled vehicles, c. Many bikes (especially smaller ones) and pot holes). Most headlight visibility have limited electrical power research has been concerned with the roadway available. hazard problem. The standards controlling motorcycle One way to evaluate the seriousness of the lighting are less restrictive than those road hazard problem is to compare vehicle controlling automobile lighting. As a result stopping distance to driver seeing distance. there are many varieties of motorcycle If the seeing distance is greater than the headlamps, making them potentially more stopping distance, then the driver would have a expensive and difficult to find when a positive safety margin and should be able to replacement is required. Many of these lamps, avoid hitting the obstacle. especially thoje designed for mid-size and Many studies have been run to determine smaller motorcycles, have considerably less seeing distances and most have used a somewhat output than a car headlamp. similar methodology. For example, the targets This problem has been explored in detail are either small (0.5 meter square is typical) in two recent NHTSA-sponsored studies (34, or man-size. They are painted a uniform dark 35). Among their findings was that gray (e.g., 7% reflective), or covered with considerable improvement has resulted from the dark-colored fabric. They are typically flat, introduction of halogen sealed beams. but are occasionally three-dimensional. The ENGINEERING DEVELOPMENT - Lighting subjects ride in an automobile that is driven development work in the United States has been at a set speed on a straight, flat road. generally described by Kilgour (23). It is a Subjects are typically under 30 years old, complex process, often initiated by a single alert, and unfatigued. One or more targets are person or company. This is typically followed placed at known positions along the right edge by laboratory and field testing, using of the road, and a glare source, representing an oncoming car, is placed to the left. The U.S. and European headlamps. Most used a subjects are instructed to indicate when each methodology similar to that just described target is detected, usually by pressing a (called "standard" in the table). Those that button. The results are reported in terms of used a different method are briefly described. mean detection distances for the various The differences in visibility distance conditions of interest. Occasionally, among studies are impressive. For example, distributional data are provided as well. with no opposing glare car, mean visibility Table 3 is a summary listing of distances to a large target range from 14 twenty-five headlighting studies that have in meters [46.2 feet] (Johansson and Rumar, 45) to common the detection and/or identification of 120 meters [396 feet] (Hemion, 43), a large and/or small, low-contrast targets placed difference of about 8.5:1. on the right side of the road using standard

TABLE 3 - Means (X) And Standard Deviations (sd) of Visibility Distance to Low-Contrast Targets Reported In Twenty-Five Field Studies T i LOW BEAM VISIBILITY DISTANCE (Meters)

NO GLARE MAXIMUM GLARE AUTHOR(S) YEAR METHOD Large Tgt Small Tgt Large Tgt Small Tgt

X sd X sd X sd X sd

Adler and 1973 Standard 61 20 65 15 Lunenfeld (36)

Bhise, Farber 1976 Standard 107 58 and McMahan (37)

DeBoer (25) 1955 Standard 65

DeBoer and 1956 Standard 80 53 Morasz (38)

Graf and 1976 Non-alerted subjecta, normal 35 36 Krebs (39) driving environment* Eye

Halstead- 1979 Subjects drove car on public 30 Nusaloch et al- roads, encountering (40) "natural" targets at unexpected locations

Helmers and 1973 Standard 75 6 48 7 Rumar (Al)

Helmers and 1974 Standard 70 60 Rumar (42)

Hemion (A3) 1968 Standard 120 10 76 10

Hemion (44) 1973 Standard 122 107

Johansson and 1963 Subjects drove toward man- 14 2.5 Rumar (45) sized obstacle - braked as soon as they cov'd see it. „ , Johansson et at* 1963 Subject Car Stationary. 25 23 (46) Glare car moved toward subjects, measured distance _ when target disappeared.

Kazenmaier (47) 1956 Standard no 50

Rumar (53) ' 1970 Standard 45 40

Rumar, Helmers 1973 Standard 125 100 and Thorell (54)

Schmidt- 1982 Standard (European Lamps) 51 6 65 6 Clausen (55)

1968 Standard 69 38 Yeatman (56)

Zechnall (57) 1963 Standard 95 50

10 TABLE 3 - (-Continued) 1 LOU BEAM VISIBILITY DISTANCE (Meters)

NO GLARE MAXIMUM GLARE AUTHOR(S) YEAR METHOD Large Tgt Small Tgt Large Tgt Small Tgt

' X sd X sd X sd X sd 1 Lindae (48) 1962 Standard 1OO 60 L L t Moore (20) 1958 Standard 61 50 1 f Mortimer and 1974 Standard method, except 88 70 Olson (49) subjects had to identify orientation of target. r T Roper and 1938 Driver approached unex- 20 53 Howard (SO) pected man-sized target in[mph center of the lane. Measured — distance from target when! 50 30 subject lifted foot from Imph _ accelerator. _[ Roper and 1963 Standard 145 114 Meeae (33) .

Roper and 1939 Standard 69 55 Scott (51) .] Rumar (52) 1965 Standard 45 8.6 38 7.3 __ — _-J

Some of the reasons for these differences By comparing the range of seeing distances in mean visibility distances are known. For to the estimated stopping distances, the example, the studies which report the shortest maximum safe speed can be determined. Because distances used methods which introduced of the variability in both seeing distance and uncertainty about target position (e.g., stopping distance, the maximum safe speed can Johansson and Rumar. 45), the nature of the vary from about 20 mph to about 55 mph, under target (e.g., Halstead-Nussloch et al., 40), or optimum conditions without an oncoming glare deceived the subject about the nature of the car. With an opposing glare source seeing test in an effort to determine the visibility distances are less, and maximum safe speeds distance of unalerted subjects (e.g., Roper and would be correspondingly lower. Thus, since Howard, 50; and Graf and Krebs, 39). many of the seeing distances and stopping In some cases mean visibility distances distances were conservative estimates and did vary from study to study because the test not account for the limit conditions associated conditions vary. For example, targets may have with many accidents, one can conclude that different shapes, reflectances and locations. under real world conditions many drivers cannot Beam intensities and patterns may vary, and see far enough on low beams to stop and prevent subjects may have different visual a collision with a roadway hazard. capabilities. The ambient lighting and target In order to determine ways to reduce this background may not be the same from study to negative safety margin, researchers have study. All of these factors can affect the conducted many tests to determine the extent to visibility distances measured. which various factors increase detection Sometimes the reason for the differences distance. One of the key factors in lamp among studies cannot be determined because design that can be changed to increase important information is often omitted. For visibility distance is beam intensity example, many investigators do not provide BEAM INTENSITY - Data provided by Roper photometric data on the headlamps, which is and Howard (50) serve as an illustration of the critical because of the manufacturing and relationship between beam intensity and design variability from lamp to lamp. visibility distance for three levels of target Background reflectivity, which is as important reflectivity. (See Figure 8.) These data are in detection as target reflectivity, is rarely for a pedestrian target, no opposing glare car, measured. unalerted subjects and car speed of 50 mph. As Campbell (58) has calculated automobile a reference, current U.S. low beams would stopping distances for level roads, and braking direct about 18-30,000 candlepower toward a in the skid mode. Figure 7 shows these right side target 200 feet down the road; high calculated stopping distances as a function of beams, about 60,000 to 120,000. initial velocity, two road frictions, and two The data indicates that gains in driver reaction times. These curves can be visibility distance associated with increasing considered to be conservative, minimum lamp output are relatively rapid up to 25,000 estimates of stopping distance. candlepower, and flatten out considerably after 11 200 300 STOPPING DISTANCE IN FEET Fig. 7 - Automobile stopping distances (58)

First, illumination reaching a surface varies as the square of the distance from the source. That is, the illumination reaching a surface is equal to the luminous intensity of the source directed toward that surface divided by the square of the distance. This means, for example, that to double the distance between a source and surface yet maintain' the latter at the same level of illumination, the source intensity will have to be increased fourfold. Second, the key to target detection is not simply target luminance, but the contrast provide by the target and its background. 100 125 150 Typically, the headlamps are illuminating both BEAM CANOLEPOWER surfaces. If the source is moved further away, but its output is increased to provide the same Fig. 8 - Visibility distance as a target luminance, contrast will actually function of beam candlepower and decrease because the background is now target reflectivity (50) relatively closer to the target. Thus, if the (Candlepower values are X 1,000) target was at threshold before, it becomes (Visibility distances are X 10 sub-threshold under the revised conditions. feet) Required threshold contrast varies inversely with level of illumination, so the only that. Obviously, it is not easy to obtain solution is to increase source intensity great improvements in seeing distance over what further to a point where the reduced contrast is provided today simply by using more powerful will be at threshold again. lamps, even ignoring the problem of glare. For Third, the atmosphere has a significant example, the recent increase in output allowed scattering effect. Except for extreme for high beams (from 75,000 cd to 150,000 cd conditions such as fog, this problem has max.) resulted in about a 20% improvement in largely been ignored until recently. However, visibility distance, according to Figure 8. measurements reported by Huculak (59) show that Some of the reasons for the relatively poor atmospheric scattering can be quite gain in visibility per unit increase in significant. Huculak offers one example in intensity are as follows: which the retroreflectance of the road surface 12 at about 200 meters appeared to be 50% higher in making such comparisons is that the seeing than it actually was, due to atmospheric distances were usually measured with the glare scatter. And this was reported to be a clear lamps the same as the visibility lamps. night. What this means is that if we use more Different results would be obtained if unlike powerful lamps to try to see further, beams were used. Thus, the driver of a car atmospheric scattering works against us by with European lamps meeting a car with U.S. low reducing the contrast of more distant targets. beams might not have seeing distances as good In sum, a great deal of evidence about as the opposing driver using a U.S. beam. visibility leads to the following conclusions: Because of the difficulty of substantially a. When considering low-contrast targets improving seeing distance with conventional such as dark-clad pedestrians, animals systems, polarized headlighting was and unlighted vehicles, present-day investigated in a number of studies. In this low beams headlights do not offer system polarized filters are placed in front of sufficient visibility for safe the headlamps and in front of the driver's operation at the national limit (55 eyes. The axes of polarization are oriented so mph or about 90 km/hr). There is some that when the driver looks through the filter, debate about the degree of shortfall, light from the opposing lamps is blocked out. but under real world conditions the Because of the significant glare reduction maximum safe speed is probably no possible with the system, it permits extreme higher than 70 km/hr (45 mph). lamp intensities and reduces the need for high b. The output of present-day low beam standards of aim and maintenance. However, headlamps is at the point that large because of concerns over cost and proper use of increases in visibility distances the system by drivers, it was never implemented. require very great increases in Another approach to lengthen seeing intensity. The main restriction to distance through beam pattern changes, was the further increases is glare. three-beam system. The third beam was designed GLARE CONTROL - In the presence of to increase seeing distance in certain meeting oncoming vehicle glare, visibility distance can situations. One study found that target be reduced significantly compared to open road detection distance was extended 29 percent driving. Table 3 previously shown, lists the further in two-lane meeting situations than seeing distances with oncoming vehicles under standard low beams. (60). However, the mid- maximum glare conditions and shows the beam was too bright to be used in meetings on reduction in distance compared to the no glare curves and following closely or overtaking and condition. The negative safety margin for passing preceeding cars. Because of concern unopposed driving is even lower in the presence over proper driver use of the beams and the of oncoming traffic. Glare that reduces criticality of proper aim for the mid beam visibility distance is called disability lamp, this system was never implemented. glare. Glare can also have the effect of Ongoing research at the Unversity of causing discomfort to. the driver without Michigan is investigating whether more reducing visibility. This is called discomfort conventional approaches to low beam pattern or psychological glare. If low beam intensity design can result in increases in seeing could be increased without increasing glare, distance in meeting situations. The extent of then seeing distance could be increased, at the increase possible depends on the least a little. Efforts to improve seeing limitations imposed by discomfort and distance by controlling glare have concentrated disability glare. on beam pattern shaping and polarized DISABILITY GLARE - Light entering the eye headlighting. is to some extent scattered by the optic media The major beam patterns that have been so that random illumination is superimposed studied and compared have been the U.S. over the image of both target and background. tungsten sealed beam pattern and the European As a result, background luminance is sharp cut-off pattern. There is no clear effectively increased and contrast is reduced evidence that one pattern is any better than (61). The effect has been shown to be the same the other. The results depend on the as that produced by supplying veiling luminance circumstances under which the beams were directly over the target and background (62, evaluated, the evaluation criteria selected, 63). and the particular lamps that were chosen to be A systematic evaluation of the representative of the beam pattern. For relationship between illuminating and glaring example, U.S. lamps typically show higher intensities and seeing distance has been seeing distances for right side targets; provided by Grime (64). His data are European beams because of their reduced glare reproduced in Figure 9. To provide a frame of show longer seeing distances for left side reference, present U.S. low beams, in the targets. Furthermore, a beam pattern with a situation described for Figure Q, would produce sharp intensity gradient like the European beam about 18-30,000 cd of illumination towards A that may provide good performance on flat right side target 200 feet down the road and straight roads, may not provide as qood a about 1500 cd or less of glare towards an visibility distance on curves, hills, and with opposing car about 200 foot away. less than perfect aim. Another caution to note increase intensity in the direction of hazards on the'right side of the road. Glare intensities directed at oncoming drivers may Beam Intensity need to be raised slightly if conventional on Object— sealed beam technology is used. However, new Candle-Power technology (e.g., single-function, axial-filament low beams) may allow glare intensities to be lowered. The key to determining how much intensity can be selectively increased is the discomfort glare acceptable to drivers. Discomfort Glare. It is common knowledge that glaring lights can produce a sensation of discomfort. While the sensation is real enough, the mechanism(s) involved are not 3,000 4,000 known. Lacking an understanding of what physical responses are involved, it has been Glare Intensity—Candle-Power necessary to approach discomfort glare as a Fig. 9 - The relation between seeing problem in subjective scaling. distance, beam intensity directed at an object and Most headlight discomfort evaluations have glaring intensity (64) [Object employed the following 9-point scale described position 10 feet to side of by DeBoer (1965): opposing lamp and 10 feet behind 1. unbearable it; object reflectance, 7 2. percent; height of object, 18 3. disturbing inches; car speed, 30 mph] 4. 5. just acceptable 6. The main point from Figure 9 is that the 7. satisfactory disabling effects of glare are ndn-linear. Low 8. levels of glare produce more degradation per 9. just noticeable unit increase than higher levels. The DeBoer scale has been used by a number Extrapolating from the relationships shown in of different investigators. Of special this figure indicate that doubling current low interest is the meticulous laboratory work beam intensity towards the target increases reported by Schmidt-Clausen and Bindels (66). seeing distance by about the same percentage as Their results are reproduced in Figure 10. reducing glare by 50%. The advantage of Based on these data, the peak glaring reducing glare is that discomfort is reduced. intensities of U.S. low beams in a meet on a However, this approach requires a very sharp straight-flat road would be rated about "4," intensity gradient that may be difficult to suggesting that a further increase in glare is achieve in practice. The European beam undesirable. Some verification of this was patterns ire designed to produce lower glare obtained by Bhise et al. (10), based on a but in doing so they sacrifice intensity dimming request study. Their results are towards roadside objects and also lamp reproduced in Figure 11, and show that requests efficiency. The U.S. sealed beams rely on lens for dimming began increasing rapidly when the prescription changes to control the beam glare experienced by the opposing driver was patterns but this approach cannot produce sharp such as to produce a rating of "4" or less. intensity gradients. Hence, glare levels Mortimer and Olson (67) also used the cannot be easily reduced without lowering DeBoer scale to obtain glare ratings for overall output. several lighting systems in a full-scale There are also problems with the approach meeting situation. Their data are reproduced of increasing intensity to increase seeing in Figure 12. The considerable effect of glare distance. The effects of glare as represented angle, as determined by where the subjects were in Figure 9 assume straight, flat road meeting searching for targets, should be noted. The situations. Because of misaim and various road data also suggest that ratings obtained in a geometries, the eyes of oncoming drivers are realistic meeting situation may be different sometimes exposed to portions of the beam other from those derived from laboratory studies. than the "design" glare zone. Being hit Assuming the driver chooses to look" ahead or to suddenly with the high intensity portion of the the right rather than at the approaching low beam pattern when cresting a hill or headlamps, it appears that U.S. low beams could rounding a curve is an unpleasant experience at be rated "5" or better, rather than the "4" best. It can lead to severe, if brief, losses predicted by the DeBoer equation. in visibility. More intense lights would A problem with ratings of the type exacerbate this problem. described has been pointed out by Lulla and The solution to these seemingly Bennett (68). They note that the scale values irreconcilable tradeoffs may be to selectively assigned a given glare level depend to some

14 10 Not Noticeable 9 • •

Satisfactory

5 Just C3 5 • • Acceptable O) B S 5 a \ Left cc. Target

tiso

Disturbing 3 • •

2 - •

Intolerable -f- U.S. Mid-A Mid-B ECE ECE-U.S \ Low Low Mid Beam

—3—2—10 12 Fig. 12 - Mean ratings of maximum glare discomfort in meetings with five Log glare illuminance log Eg (lux) beams when searching for targets Fig. 10 - DeBoer ratings (W) as a function on the right and left side of of glare illuminance (66) the lane (67)

extent on the range of stimuli presented. This 1001 suggests that: (1) the ratings cannot be treated as absolutes, and (2) if the 105,000 CP composition of glare to which people are 80 exposed in everyday driving changes, their judgment of what is "just acceptable" will change as well. Some additional work on this 60 phenomenon is currently being carried out at the University of Michigan. This work was carried out in two stages. S 40 Stage 1 was a laboratory study very much like that of Schmidt-Clausen and Bindels. The results were identical. Stage 2 used the 20 laboratory approach and rating scale, but was carried out in on a roadway with automobiles. t The stage 2 results are shown in Figure 13. It 6 - is apparent from Figure 13 that the subjects in 1 the field setting rated the same glare levels Unbearable Disturbing jus! AcceptaDie more comfortable than did the subjects in the laboratory setting (DeBoer prediction). Discomfort Glare Index (W) Discomfort glare is one of the principal Fig. 11 - Request for dimming from limiting factors in attempting to gain improved approaching drivers as a visibility by increasing lamp output. There function of calculated glare are only a few rigorous studies of discomfort discomfort rating (10) glare in a setting applicable to headlighting.

15 FULL RANGE Some indication of .how well lighting PARTIAL RANGE systems actually function under real-world conditions has been provided by Yerrell (72). In this study measurements were taken of the glare and, illuminating intensities of a large number of passing vehicles at several sites in Great Britain. These data are reproduced in Figures 14 and 15.

& 3

-3 -Z -I 0 LOG LUX Fig. 13 - Comparison of mean ratings assigned each glare condition in the field compared with prediction based on use of DeBoer equation (Olson and 4 6 8 x 1Q3 Sivak, unpublished) Illumination intensity (cd) Fig. 14 - Frequency distribution of Unfortunately, as just noted, the results are headlamp illumination not in agreement. Given the critical nature of intensities in Great Britain(72) this work to headlamp design, further work is desirable to arrive at more definitive 100 conclusions. Complicating the glare issue, both from the point of disability and discomfort, is the fact that it is experienced not only from vehicles in the forward field, but from following vehicles, via the rearview mirrors. The problem of rear view mirror glare has been studied in some depth (69, 70, 71). Briefly summarized, the findings are that both kinds of glare are a problem with the present U.S. low-beam system. Obviously, any increases in intensity will worsen the rear view mirror problem. However, Olson and Sivak (71) argue that this is a problem for which there is a potential solution, namely lower reflectivity for interior and exterior mirrors. However, there is some uncertainty about what this reflectivity should be, especially for outside mirrors. CONDITIONS DEGRADING LIGHTING SYSTEM PERFORMANCE - Headlamps are designed to provide certain performance levels. Since they are 6 8 10 12 14 16 x 10' important safety systems, they should come as close to the design performance as possible 'Glare' intensity (cd) under normal operating conditions. Fiq. 15 - Frequency distribution of glare intensities in Great Britain(72)

16 Based on*1dealized British isocandela diagrams, and the photometric target placements LOWED SEAM used by Yerrell, the "expected" glare and VERTICAL 5" illumination values were about 1600 and 4000 iio cd, respectively. The 4000 cd illumination S value falls slightly below the median in Figure •- • S 14, indicating it was met by fewer than half of i- « the vehicles in the sample. About 10% of the wi. measured illuminating intensities were & one-quarter or less of the expected value. 2 Glare, as shown in Figure 15, was also quite -• -4 -t 0 I 4 « t 10 variable, although most measurements (about INCHES DOWN (-1 OH UP AT 29 FT 85%) were less than the expected value. Fig. 16 - Distribution of headlight aim in Thus, the available evidence indicates a vehicle inspection state that field headlamp performance varies greatly (Texas) (32) and that a large fraction of vehicles (about 25%, according to Yerrell) are running with illuminating intensities about one-half or less of levels expected based on design criteria. The reasons for the state of affairs illustrated in Figures 14 and 15 are many and LOWER BEAM probably not fully known. But, research has VERTICAL been carried out to identify some of the more I" significant ones. In the next portion of this report these factors will be reviewed, together with other variables affecting system performance not considered in the Yerrell study. Aim - The first systematic work on the aim problem was carried out at the Transportation and Road Research Laboratory in Great Britain MTh^ (73, 74, 75, 76). They identified the major -• -• -4 -2 0 2 4 • • 10 12 sources of misaim as improper setting by IMCHU DOWN I-) OR UP AT 23 FT service facilities, changes in the adjusting Fig. 17 - Distribution of headlight aim in mechanism, and changes in vehicle attitude due non inspection states (Kansas to load. and Missouri) (32) With the implementation of vehicle inspection programs in several states, headlamp aim was noted as the most common "defect," and misaims far in excess of accepted standards TABLE 4 - Headlamp Aim Variances From All were commonly found (77, 78, 79, 80). Significant Sources Excluding Load Hull et al. (32) measured the extent of (81) misaim in the motor vehicle population by SOURCE VARIANCE (Degrees) surveying cars in Texas, Missouri, and Kansas. Figure 16 shows low beam vertical aim for Horizontal Vertical Texas, a State with vehicle inspection. Figure Lamp 17 shows low beam vertical aim for Kansas and Mlsorient aiming plane .05 .05 Missouri, states without lamp inspection. As a Difference between beam .92 .92 baseline, SAE standards specify limits of ^ 4 and mounting plane inches at 25 feet. One degree equals 5.2 Change in use .77 .77 inches at 25 feet. About 68% of the lamps were Vehicle within the SAE limits for low beam vertical aim Lamp mounting .12 .38 in Texas compared to 59% in the other two Dog tracking* .19 States. Matchboxing** .11 Olson and Mortimer (81) surveyed the Aiming literature to identify the major sources of Long axis alignment misaim and estimate their magnitude. Photometric .52 Disregarding vehicle attitude changes, the data Visual 1.06 indicate that a large fraction of the cars on Aiming the road at any given time can be expected to Visual screen .29 .08 have one or more lamps significantly misaimed. Visual screen .96 .29 Photometric .04 .02 The sources of aim error are summarized in Mechanical .02 .04 Table 4. Adding to these sources the problem Human Factors .58 .58 of vehicle load effects, the misaim problem is seen to be very significant. *Condition when rear axle is not perpendicular to the long axis of the car. **Condition when headlamps are not positioned in a line perpendicular to the long axis of the vehicle.

17 Relatively minor discrepancies between Detailed measurements of the effect of where a lamp is actually aimed and where it was dirt on photometric performance have been intended to be aimed can produce significant reported by Cox (84). Data from two lamps performance changes. By way of illustration, measured by Cox are shown in Figure 19. The consider Table 5, taken from Hull et al. (32). top one was pre-judged "moderately dirty," the These data show target detection distances bottom one "very dirty." The moderately dirty under various beam and misaim conditions. condition reduced output on both beams by about Changes of as little as one degree can result 50%. According to Rumar, this is about the in very large changes in visibility, either due least reduction which the drivers can be to altered illumination or glare. expected to notice.

TABLE 5 - Visibility Distance As A Function Of Aim On Observer And Meeting Cars (32)

Test Observer's Car Glare Car Target Detection No. Lamp Aim Lamp Aim Distance, ft. V 40.000 cc 1 Low beam aimed No glare care 397 / Clean —. correctly - Dirry — 2 Low beam aimed Low beam aimed 345 \ correctly correctly.. \20.C 100 CO 3 Low beam aimed L.B.1°U-1°R 265 /. correctly 4 Low beam aimed L.B . 1 -1 /2^U-1 -1 /2°L 250 Mambea m correctly // \\ / x 5 L.B.1°D-V l.B. correct 208 Transvers e dislribulio n o f intensi l s X. 6 L.B.1°D-V No glare car 219 •^.-^ 7 L.B.1°U-V L.B.1°U-V 444 15' 10° 5' Let! 0« Right 5' ^ 8 . L.B.1°U-V No glare car 697 ^ 9 H.B. aimed No glare car 729 correctly 10 H.B.1-1/2°U-V No g8are car 642 11 H.B.1-1/2°D-V No glare car 594

•Location referenced to "correct" aim position.

For some misaim problems there are available solutions. For example, vehicle V30.000 cc attitude can be controlled with several types Clean § 20000 1 Duly — - of self leveling devices ( 82). Other sources I \ of aim error are more difficult to solve. 5 10000 / / \ Dirt - As headlamps accumulate dirt, 20.000 cc v - intensity and seeing distance become degraded. _ — ~^^M Some indication of the extent of this problem — '5- Leu 0- H.gt« 5* 10- 15- in Sweden has been provided by Rumar (83), Fig. 19 - Measurements of attenuation due whose data are reproduced in Figure 18. to dirt, taken in the horizontal plane for a lamp judged moderately dirty (top) and very dirty (bottom) (84)

Some indication of what various levels of attenuation mean in terms of loss of visibility has been provided by Rumar (83), whose data are reproduced ^n Figure 20. It will be noted that light dirt layers can actually increase visibility distance slightly. This is due to scattering that causes additional illumination to be directed upward. Rumar's data indicate 20 40 60 80 that a 50% reduction in output would reduce LIGHT REDUCTION (%) visibility distance by 10-15%. Based on .the information described, dirt Fig. 18 - Proportion of cars at gas stations having various degrees looms as a significant factor in reduced of light reduction in the headlamp performance. Several lamp central part of the high beam washing/wiping systems have been suggested as caused by dirt under three road solutions and have been marketed in other conditions (83) countries. In the U.S., the use of wiper 18 systems is limited because Federal regulations The first serious effort to develop a prohibit wiper blades from obstructing light computer seeing distance model was that of Jehu from the lamp. The aiming pads on U.S. lamps (86, 87). This model was written first to make it difficult for wiper blades to "park" simulate a meeting of two cars on a off the lens. Further study to develop the straight-flat road, and then extended to most cost-effective option is needed. consider curves. The model was used to evaluate various lighting systems in use at that time. An extensive research effort was undertaken at the University of Michigan in the early 1970's which led to the development of a computer model that computes seeing distance. The model was developed based on extensive z field tests (49) and originally applied to o meeting situations on flat-straight roads 20 (88). The model was subsequently expanded to o include hills and curves (89). sLrU The Unversity of Michigan model has been BEAM used extensively over the years for evaluation of alternative lighting systems (90), and as a OPPOSING LOW means of screening candidate lighting systems CD BEAM >' for later field evaluation (40, 34, 35). CO One problem that was addressed recently using the U of M model is lamp mounting height - 60 8O (71). Current regulations allow mounting LIGHT REDUCTION (%) heights to vary from a minimum of 24 inches to a maximum of 54 inches. There was concern that Fig. 20 - Visibility reduction caused by the greater heights, common on trucks, caused light attenuation due to dirt excessive glare for drivers. for three conditions obtained in In dealing with this question, the model controlled experiments (83) was run to simulate a number of possible meeting situations using an experimental, high intensity low beam. Some of the data which resulted are shown in Table 6. This table Weather - Headlamps must be designed to be indicates the gains in visibility distance minimally affected by adverse atmospheric associated with greater mounting heights, as conditions. Conditions suoh as fog, snow and well as disability and discomfort glare rain cause the illumination projected from a effects. Clearly, greater mounting heights headlamp to be scattered. This reduces the provide both more visibility and glare. Based illumination actually reaching a target of on this and other analysis, it was concluded interest. Some of the illumination is that reducing the upper limit on mounting scattered upward, causing the atmosphere height is unwarranted. between the driver and target to light up, with the result that target contrast is reduced (7). There have been some efforts to quantify the effects of rain and fog (7, 85, 10) with a TABLE 6 - Effects Of Lamp Mounting Height On view toward designing lighting systems to be Visibility And Glare (71) more effective in adverse weather. The results indicate that performance is improved by using VISIBILITY DISTANCE DISCOMFORT GLARE lamps which project relatively little Mounting Heights Distance at illumination above horizontal, and mounting (Inches) Maximum Minimum Maximun. 5.0 or worse them as low as possible. Performance is not (No Gl«re) (Max Glare) Rating* (feet) improved by tinting the lens a yellow color. 24 292 259 4.7 550 COMPUTER MODELS - Head lighting is a complex topic, and full-scale lighting studies 40 315 253 4.5 600 are expensive to set up and run. The results 54 338 249 4.4 650 of these experimental studies are only applicable to the limited conditions of the test. It is thus difficult to evaluate •Based on a 9-po1nt scale where 5 • just acceptable, 1 • unbearable, and lighting system parameters systematically. For 9 • just noticeable. example, experimental results measured under straight, level road meeting situations may not provide a realistic evaluation of performance While computer models of the type on curves or when misaimed. Because computer described above are a considerable help in models permit lamp performance to be readily evaluating certain aspects of beam performance, evaluated under many possible conditions, they they are limited in the number of conditions are valuable tools in lighting research. (target type, road topography) that can be

19 considered at one time. Since lamp performance to real-world accidents is unknown. Thus, to varies as a function of conditions tested, the understand the meaning of the figure of merit, investigator still has the problem of weighting it is important to understand the performance often conflicting results to arrive at a criteria built into the model. For example, meaningful assessment of performance. pedestrian visibility is determined for Investigators at Ford set out to produce a "unalerted drivers," and discomfort glare is computer model which would provide a acceptable if the DeBoer glare index is greater comprehensive evaluation. This work has been than 4 or if it is less than the discomfort summarized by Bhise et al. (10). As a first glare of a U.S. low beam. Greater figures of step the investigators developed a seeing merit and larger differences between beam distance model, something like the others which patterns could be achieved if, for example, have been described. A simulated standardized some pedestrian encounters were with alerted test route was then assembled. This route drivers and greater discomfort glare could be consists of a series of highway sections in the tolerated by drivers. form of environmental parameters which are The Ford model is the only approach thought to have an influence on visual presently available for evaluating headlights performance while driving at night. It from a systems point of view. It can be of includes such factors as pavement, lane line great help in serving as a standardized means and target reflectance, road geometry, lane of evaluating the relative consequences of configuration, ambient illumination, and glare changes in headlight parameters. What is from fixed lighting and%traffic. needed is general agreement among the lighting When headlighting systems are run in the research community on the logic and criteria model, the output is a figure of merit. This used in calculating the figure of merit. figure of merit is the percentage of the distance travelled on the standardized test SUMMARY route during which the seeing distance to pedestrians and pavement lines and the Driver recognition and detection errors discomfort glare levels experienced by opposing often occur at night because of the obvious drivers simultaneously meet certain acceptance visibility degradation that occurs in criteria. darkness. Although present-day low beams are In one analysis using the model, a number adequate for many tasks involving the detection of different headlamp systems were compared, of high contrast objects, they are less than including 5-3/4 inch round, 7-inch round, and adequate in revealing low contrast objects. 4-x 6-1/2 inch rectangular US systems; and The available illumination may not be European Bilux 2-lamp, and Halogen H4, 2-lamp sufficient to permit drivers to see obstacles, systems. The figures of merit were 74.1, 74.0, signs, and the direction of the road. The 73.9, 67.3, and 71.4, respectively. The lower extent of this insufficiency cannot.be assessed ratings for the European systems were directly but has been inferred from accident attributed to their lower intensity low beam statistics, driver reports, experimental and poorer performance in wet road driving and studies, and analytical evaluations. at speeds over 40 mph. However, the European In examining the broad accident system discomforted from about 2-6% of the statistics, nighttime accident rates are found drivers compared to about 10% for the U.S. to be three times the day rate. Although some beams. of this difference can be attributed to greater Other analyses conducted with the Ford alcohol involvement, fatigue, and young driver model have shown that the figure of merit is problems, inadequate lighting can be assumed to much more sensitive to environmental variables play a role as well. Even drunk and fatigued than headlamp factors. The largest difference drivers might benefit from improved between two headlamps had been about 10 visibility. A number of studies have shown percentage points, while different that improved roadway illumination using fixed environmental conditions (e.g., road type, road lighting reduces accidents. However, for the surface condition) have produced differences as majority of vehicle miles in this country, the large as 60 percentage points. This implies only practical means of increasing roadway that changing the beam patterns will not have illumination is to improve headlight systems. as much effect on system performance as One way to quantify the safety margin changing roadway environment factors. However, drivers have at night is to compare the to a large extent this approach is not visibility afforded by headlights with the practical, since certain key variables (e.g., stopping distance of the car. The measured road geometry) cannot really be changed. seeing distance with low beam headlamps varies Another implication of the model is that as a function of many factors, including driver the intensity of the current U.S. low beam age, target characteristics, beam intensity, appears to be close to optimum. Some beam and test methodology. For pedestrian type patterns did show slight increases in the targets, seeing distance on typical U.S. low figure of merit over U.S. low beams. However, beams has been shown to vary from about 30 it is not clear how much weight to attach to meters (99 ft) to 120 meters (396 ft). In the relatively small differences that are found meeting situations, the seeing distance ranges because the relationship of the figure of merit from about 25 to 80 meters (83-264 feet).

20 The maximum safe speeds for these stopping Headlamp performance is also affected by distances can be calculated for various the extent and nature of the limit conditions combinations of road friction and assumed imposed by various environmental and driver driver perception reaction time. For example, factors. For example, weather and road on a dry pavement the maximum speed for a 396 topography have been shown to influence foot stopping distance is about 55 mph, headlamp illumination. Also, the increasing assuming a 2.5 second driver reaction time. In proportion of older drivers could further opposing traffic the 264 foot stopping, distance reduce the level of visibility provided by corresponds to about a 42 mph safe speed. headlamps. Since these maximum visibility distances are While each of the above factors may have a likely to be considerably less in the real relatively small effect on nighttime visibility world, the margin of safety for many drivers when considered individually, the combined will be negative. For example, the 83 foot action of all of them suggest that the need to visibility distance measured in some tests maintain and improve low beam performance is translates to about 28 mph under the most ideal more critical than ever. driving conditions (i.e., dry pavement and one One of the ways to evaluate the effects of second driver perception reaction time). these conditions on headlamp performance is The problem of nighttime driving has been through the use .of a figure of merit such as recognized for many years by researchers who that developed by Bhise et al. The figure of have sought to develop improved headlighting merit provides a systematic and comprehensive systems. Progress has been slow because of the evaluation of the many interacting factors difficulty of resolving the many tradeoffs influencing night driving safety. involved with headlight design. The primary One of the factors affecting the figure of tradeoff involves illuminating intensity versus merit is discomfort glare. Discomfort glare is glare intensity. Seeing distance in open-road an issue which divides American and European driving can be increased somewhat by simply lighting engineers. There are relatively few increasing the lamp intensity; but in vehicle studies on discomfort glare. Some of these, meeting situations the number of drivers especially those conducted in laboratory complaining about glare would also increase and settings, suggest we are at the limit for seeing distance would be impaired. The glare. Others, especially those conducted in polarized headlighting system had been proposed field settings, indicate further increase in as a solution to this tradeoff. However, cost glare may be possible. And there are other and implementation problems caused researchers data that indicate that people's judgment of to seek simpler and more practical approaches. what is objectionable varies depending on the The research on improving the low beam has context in which the judgment is made. concentrated on how to change beam intensity Other components of the figure of merit are and distribution. Such an .approach has shown performance degrading factors such as misaim that only relatively small increases in and dirt. However, existing data on the extent visibility can be made because of the and nature of these factors is not up to date limitations of current lamp design and for the current U.S. vehicle fleet. manufacturing techniques and because of the AN AGENDA FOR FUTURE ATTENTION - The laws of physics and response of the human information discussed in this literature review visual system. However, even small increases suggests a number of problem areas that need in visibility are important to help counter the further attention: many environmental, driver, and vehicle factors 1) Because headlamp performance has been that degrade low beam performance. shown to vary as a function of test Some of the vehicle design factors that conditions, a standardized approach to can reduce driver visibility have become more evaluation of headlamp performance significant in recent years because of basic needs to be developed. This approach changes in the characteristics of the vehicle should be based on the performance of fleet. Although basic low beam technology has headlamps under the hazardous not changed significantly since the 1950's, the conditions that can lead to vehicle fleet has changed in a number of ways accidents. The figure of merit that can reduce headlamp performance including technique used in the Ford model can 1) increased use of tinted windshields, 2) be employed as a basis for such lower lamp mounting height in passenger cars, evaluations. A figure of merit for and 3) lower driver eye height. In addition, headlamp system performance can be the increased percentage of smaller cars raises used to evaluate the safety the question of whether there is increased lamp implications of changes to the vehicle susceptibility to misaim due to vehicle fleet that affect nighttime vision. loading. Currently, a number of proposals have The components of the figure of merit been advanced which would significantly change developed by Ford comprise many of the the technology of lamp construction. While driver, vehicle, environment, and these technologies may offer some styling and hazard characteristics associated with fuel economy benefits to consumers, their long accidents caused by inadequate term road illumination performance is uncertain. nighttime visibility. A review of the

21 model is needed to assess the model 2. Scott, P.P. The relationship between components and performance criteria road lighting quality and accident frequency. that affect the figure of merit The Transport and Road Research Laboratory, value. It would also be worthwhile to Digest LR929, 1980. explore the possibility of relating 3. Merritt, J.O., Helander, M., Abrams, changes in the figure of merit to C.,and Miller, J.C. The effect of wide-angle accident rates for various subfleets headlight illumination on driver performance. even though such a task may prove ,o Human Factors Research Inc. Contract No. be very difficult. DOT-HS-7-01724, June 1982 2) Because the most recent data on the 4. Treat, J.R. and Romberg, R.A. extent and nature of headlamp misaim Tri-levelstudy: Modification. Task 1: Final is over 10 years old, a survey of report on potential benefits of various current aim practices is needed. This improvements in vehicle systems preventing survey will help to evaluate the accidents or reducing their severity. Final seriousness of the aim problem and report. Indiana University, Bloomington, identify the sources of aim variance Institute for Research in Public Safety, Report that need to be remedied. The extent No. DOT-HS-805 094, 1977. of the headlamp dirt problem also 5. Hale, A. and Blomberg, R.D. needs to be surveyed for the same "Conspicuity for Pedestrians and Bicyclists - reasons. Task 2 Interim Report: Operational Definition 3) Because of the significance of of Conspicuity and Accident Analyses" NHTSA discomfort glare to the evaluation of Contract, DTNH22-80-C-07052, February, 1982. headlamps, further work is needed to 6. Burger, W.J., Smith, R.L., Queen, increase our knowledge of the J.E., and Slack, G.B. Accident and near subject. Additional testing under accident causation: The contribution of actual field conditions with a broad automobile design characteristics. Dunlap and range of subjects is required. Also, Associates, LaJolla, California, Contract No. evaluations of the exposure of drivers DOT-HS-802-714, 1977. to different glare levels is needed to 7. Koth, B.W., McCunney, W.D., Dverk, determine frequency of exposure to D.P.,Janoff, M.S., and Freedman, M. Vehicle glare and the intensity of the Fog Lighting: An Analytical Evaluation. exposure. Such information can be Franklin Institute Research Laboratory. Report used in assessing the tradeoffs DOT-HS-803-442, March 1978. between a beam pattern which might 8. Haghighi-Talab, D., "An provide intense glare in relatively Investigation into the Relationship Between few meeting situations versus a lamp Rainfall and Road Accident Frequencies on Two which provides lower glare levels in a Cities" Accident Analysis and Prevention - Vol large number of meetings. 5, No. 4, 1973 pp 345-349. 4) Because many new technologies for 9. OECD Road Research Group, "Adverse headlamp construction are being Weather, Reduced Visibility, and Road Safety" developed, a better understanding of The Organization for Economic Cooperation and their potential advantages and Development, August 1976. disadvantages is needed. 10. Bhise, V.D., Parker, E.I., Saunby, From a broader perspective, a better C.S., Troell, G.M., Walunas, O.B., and understanding of the visibility conditions Bernstein, A. Modeling Vision With Headlights associated with accidents needs to be In a Systems Context. Ford Motor Company, developed. A knowledge of the visibility Automotive Safety Office, Dearborn, Michigan. characteristics of struck obstacles or Society of Automotive Engineers, International pedestrians or road delineation could help Automotive Engineering Congress and Exposition, pinpoint the major safety problems of nighttime 28 Feb-4 March 1977, Report No. SAE 770238. driving and provide a better understanding of 11. Devaux, P. Unified European Passing appropriate countermeasures. Given what we Beam and Yellow Light. International Road know now about the limited seeing distance Safety and Traffic Review, 4, 1956, 33-39. available to drivers, it might also be 12. Christie, A.W., Ashwood, J.E., and worthwhile to try to answer the question of why Symons,R.D.H.. Visual Acuity In Yellow drivers maintain speeds so far above their Headlights. Road Research Laboratory, ability to stop or maneuver to avoid a Crowthorne, England, Report No. RRL Report LR collision. 156, 1968. 13. Schreuder, D.A. White or yellow REFERENCES light for Vehicle Headlamps? Institute for Road Safety Research SWOV, Voorburg (The 1. Gallagher, V.P. and Janoff, M.S. Netherlands). Report No. 1976-2E, 1976. Interaction between fixed and vehicular 14. Lee, R.L. and Scott, D.P. "Driver Eye illumination systems. Franklin Institute Height and Vehicle Sales Trends" Federal Research Laboratories, Philadelphia, Pa, Report Highway Administration, Office of Highway No. F-C2873/ FHWA-RD-72-51, July, 1972 Safety, January, 1981.

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The viewsexpressed in this paper are those of the authors and not necessarily those of the National Highway Traffic Safety Administration

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