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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-04-145
Date: December 2005
Enhanced Night Visibility Series, Volume XIV: Phase III—Study 2: Comparison of Near Infrared, Far Infrared, and Halogen Headlamps on Object Detection in Nighttime Rain
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Most of the information necessary for the task of driving a vehicle is acquired through the driver’s visual system. Some estimate that up to 90 percent of the necessary information for operating a motor vehicle is visual.(1) Before the visual system can detect and recognize various objects in the field of view, there must first be sufficient lighting.(2) Headlamps are the most common lighting systems used for nighttime visibility of the roadway. Over the years, improvements in headlamp design, including parameters such as beam pattern, aiming, luminous output, and intensity have helped increase visibility in night driving; however, headlamp design requires a tradeoff in illumination directed onto the roadway and the illumination directed farther down the road. Some portion of this illumination is directed into the eyes of oncoming drivers, resulting in glare. Glare can be described as the blinding experience that results from bright light sources in the visual field of view.(3) More specifically, glare can be further described in terms of disability glare and discomfort glare. Disability glare is the glare that results in reduced visual performance, while discomfort glare is glare that results in physical discomfort, but which does not necessarily result in reduced visual performance.
Disability glare occurs when the introduction of stray light into the eye reduces the ability to resolve spatial detail.(4) It is an objective impairment in visual performance.(5) Many of the classic models of this type of glare attribute these deleterious effects to intraocular light scatter in the eye.(6) This scattering produces a veiling luminance over the retina, which effectively reduces the contrast of stimulus images formed on the retina. The disabling effect of the veiling luminance may have serious implications for nighttime driving visibility. Researchers investigating disability glare caused by conventional halogen headlamps have found that glare from an oncoming vehicle can significantly reduce detection distances on the roadway at night. Theeuwes, Alferdinck, and Perel found that a glare source of 1,380 candela (cd) reduced detection distances for a given scenario from around 35.4 m (116 ft) down to 27.4 m (90 ft) and also resulted in many missed targets.(3) Age was also found to have significant effects on detection performance under a glare situation.
Discomfort glare has been defined as the level of illumination bright enough to result in a measurable level of subjective discomfort or annoyance.(2) It is known to be related to the degree of homogeneity between the glare source and its background.(7) Light source characteristics affecting discomfort levels include the intensity, or luminance, as well as the size of the light source. Discomfort glare can vary among different individuals because of many factors including personality, preference, and experience. It has also been shown that the degree of discomfort that a driver feels when exposed to glare may depend partly on the difficulty of the driver’s visual task;(8) as the difficulty of the driver’s visual task increases, the subjective rating of discomfort may increase. The most common method for evaluating discomfort glare is the use of the deBoer scale rating system.(9) The deBoer scale has endpoints at 1 and 9 and verbal anchors for each of the odd numbers as follows: (1) “Unbearable,” (3) “Disturbing,” (5) “Just acceptable,” (7) “Satisfactory,” and (9) “Just noticeable.” Although only odd numbers 1 through 9 have descriptors, the responses can be any number, odd or even. The Enhanced Night Visibility (ENV) discomfort glare study discussed previously used this scale to measure the discomfort glare of the 11 vision enhancement systems (VESs) (ENV Volume VII).
At present, the two most common types of headlamp systems are those based on tungsten-halogen incandescent lamps (halogen headlamps) and those based on metal-halide high intensity discharge lamps (HID headlamps).
Halogen headlamps use technology similar to most basic electrically powered light sources in which passing an electric current through a high-resistance tungsten filament generates light. The use of a halogen gas in the bulb allows the lamp to operate at a higher temperature, and it produces light in the visible spectrum that is better suited for driving than light from standard incandescent filament lamps. With the use of complex reflectors and lenses, halogen lighting systems can provide an output of just over 1,000 lumens (lm) while operating at 12.8 volts (V). The average luminance of halogen bulbs can be around 1,400 candela per square meter (cd/m²) (408 footlamberts (fL)).(10) One of the disadvantages of halogen lamps (indeed, of all incandescent lamps) is that roughly 80 percent of the output is lost as heat in the infrared spectrum.(11) In addition, the filaments in halogen systems can be damaged from road vibrations and other effects associated with long-term use. As a result, halogen headlamps may not last as long as other components in the vehicle.
High Intensity Discharge Headlamps
High intensity discharge (HID) lamps have been in use for many years in roadway luminaires and other outdoor lighting systems. Bosch developed the first automobile HID headlamp in the fall of 1991. HID headlamp systems are attractive to automotive manufacturers because of their longer lifespan, better durability, greater performance and power efficiency, and new stylistic freedom.
One of the main reasons that HID headlamps are more durable is the lack of a current-carrying filament, such as is found in halogen lighting systems. Instead of a filament, an arc is created between two electrodes, which excites a gas (commonly xenon) inside the lamp that vaporizes metallic salts. These metallic salts help sustain the arc and provide a consistent light source.(12) HID headlamps are estimated to be able to last the life of an automobile (10 years, 160,394.4 km (100,000 mi)) under normal operating conditions.(10) This is a significant improvement in overall durability compared to halogen headlamps.
HID headlamps also provide better performance and efficiency compared to conventional halogen headlamps. The luminous efficacy, which is the ratio of luminous output to electrical power consumption, is much greater in discharge lamps compared to halogen designs. Various lighting manufacturers claim that discharge lamps have at least twice the lumen output as halogen lamps with comparable wattage.(10)
Another characteristic of HID lighting systems that certain automobile designers find appealing is their flexibility in styling. Different shapes and sizes of headlamps may be advantageous for designers, but they also may have effects on glare. The size of headlamp fixtures has been linked to the subjective rating of discomfort glare.(13) A smaller light source (e.g., projector type) with the same light output as a larger source headlamp (e.g., reflector type) may be rated differently on its perceived discomfort;(14) therefore, it is important to consider headlamp size when making direct comparisons of discomfort glare.
COMPARISON OF HID AND HALOGEN GLARE
Public concern continues about the glare produced by an increasing number of new HID headlamps. In 2001, the National Highway Traffic Safety Administration (NHTSA) asked drivers to submit their opinions on the issue of glare and its many sources. Although drivers complained about the use of fog lamps in normal weather conditions and indirect glare in the rearview mirror from sport utility vehicles (SUVs) and trucks, the single most complained-about source of glare was the HID headlamp. Many drivers stated that these headlamps are too blinding and dangerous. Other drivers said the blue color is distracting and uncomfortable. On the other hand, drivers who own the new headlamps praise the increased visibility they provide. The owners commented on how safe they feel at night driving with these new headlamps and how well the headlamps light their forward view.(12) However, Phase II of the ENV project showed that drivers’ preference for HID lamps was not associated with increased visibility. Although drivers rated the HID lamps as helping them better detect and recognize objects, the objective data indicated that the HID lamps chosen for this study often performed worse than other VESs (ENV Volume XII). Other research has found a subjective preference (i.e., less discomfort glare) for halogen headlamps over the HIDs.(15) Some researchers believe this subjective difference in the perception of brightness or visual discomfort may result in part from the differences in spectral power distribution of the two headlamp designs. Halogen headlamps tend to have a warmer appearance because their spectral distribution is predominantly comprised of longer wavelengths. In contrast, HID designs have distinct peaks of spectral power throughout the visible spectrum, with more output in the short wavelength portion of the spectrum. This results in HID lamps having a slightly bluish appearance. Spectral power distributions for typical HID and halogen headlamps are illustrated in figure 1.
This variation in appearance may be one reason some drivers perceive HID lamps to be more discomforting. A recent study of both discomfort and disability glare from halogen and HID light sources showed that two headlamps of the same intensity (measured at the eye) had different discomfort ratings.(16) Drivers rated the HID headlamps (using the deBoer scale) as being more discomforting than the halogen beams. Disability glare, however, was not affected by the spectral power distribution.
On the other hand, another part of the ENV project indicated that the HID headlamp used in this research was rated as acceptable using the deBoer scale and as causing less glare than the halogen headlamp tested (ENV Volume VII). However, these were only two specific headlamps and should not be considered representative of the headlamp types.
The main goal of this study was to evaluate the disability and discomfort glare of headlamps with varying beam distributions and intensities. A total of five different sets of VESs were evaluated in this study. Each VES was specifically chosen for its beam distribution and output characteristics. Four of these systems were different designs of HID headlamps that recently have become available for use on public roadways. The fifth system was a standard halogen headlamp that has been used in previous ENV studies involving detection tasks and discomfort glare evaluation. In addition, the effect of three different driver ages, two driver light adaptation levels, and two pedestrian locations were also assessed. Disability glare was measured as detection of pedestrians against oncoming glare. Discomfort glare was measured using the deBoer scale ratings. All the experimental variables are discussed in more detail in chapter 2. The following are specific questions this study was designed to answer: