U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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Publication Number: FHWA-HRT-04-133
Date: December 2005
Enhanced Night Visibility, Volume II: Overview of Phase I and Development of Phase II
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CHAPTER 2—ACTIVITY 1. DEVELOP UV–A HEADLAMP SPECIFICATION
Research and development of the UV–A and fluorescent technology began in Sweden in the late 1980s. Saab and Volvo® formed a joint venture called Ultralux® to develop and promote the technology. Prototype UV–A headlamps were manufactured and used in field tests and demonstrations. These headlamps incorporated components from a number of European manufacturers: 50-watt (W) high intensity discharge (HID) lamps with enhanced UV–A output from Philips®, reflectors and housings from Hella® and Valeo, filters from Schott, and ballasts from Hughes Power Products®. In the end, though, this technology was not adopted in Europe, largely because of a perceived lack of market; by the late 1990s, the European UV–A effort had been largely abandoned. The situation required that a major activity area of the ENV project be the fabrication and assembly of UV–A headlamps to help develop of a UV–A headlamp specification.
Before commercial UV–A headlamps could become a reality, specifications would need to be developed by the Society of Automotive Engineers, approved by the National Highway Traffic Safety Administration (NHTSA), and incorporated in Federal Motor Vehicle Safety Standards (49 CFR 571.108), as is the case for conventional headlamps. The specification parameters need to differentiate UV–A headlamp performance from that of conventional headlamps. Because UV–A radiation is not visible, it cannot be measured by the usual photometric equipment in units of luminous flux (such as candela per square meter (cd/m²) at a specified distance). Instead, it must be measured by UV–A-sensitive radiometric equipment in terms of radiant flux (such as microwatts per square centimeter (cm²) at a specified distance). Equally critical, the spectral characteristics of UV–A headlamps must be specified.
Research was needed to determine the following minimum specifications:
Obviously, these would concern vehicle manufacturers, but some additional design and safety considerations also required resolution. Only industry could undertake the research and development necessary to address these issues because they were beyond the direct scope of the analytical and empirical research to be conducted as part of the contract. Nonetheless, it was hoped that the ENV project, through its work with stakeholders, would help foster and coordinate these proprietary efforts:
From the outset of the ENV project it was clear that obtaining appropriate UV–A headlamps would be a critical issue. Early in the process the research team contacted vehicle and headlamp manufacturers, only to discover that there was no source for readymade UV–A headlamps; the Ultralux prototype components were no longer available, and the headlamps from North American Lighting® had much lower UV–A output. Fortunately, Labino AB manufactured UV–A lighting for other applications. Thus, bulbs, ballasts, and filters were readily available to support the testing and demonstration activities.
Project requirements for UV–A headlamps were considered in the following phases:
Establishing a basis for UV–A headlamp specifications required both analytical and empirical research. A limited number of field studies with UV technology in Sweden and the United States had demonstrated the potential for large gains in nighttime visibility: however, these were not parametric studies because the levels of UV–A output, fluorescent efficiency, and resulting luminance were not measured and specified as independent variables.
The performance of the UV–A and fluorescent technology as a system depends on the combined performance of the headlamps and the fluorescent materials. Until the UV–A output is fully specified, it is not possible to say what level of fluorescent efficiency is required of the materials to yield a given increase in luminance and visibility distance. Likewise, without knowing the fluorescent performance characteristics of the materials, a headlamp designer would not know how much UV–A is enough.
To address these issues, the team planned a two-stage approach. The first stage included development and implementation of a computerized visibility model supplemented by measurements of prototype headlamps and materials to analytically test various hypothetical UV–A headlamps in concert with various hypothetical materials. The second stage included field studies to empirically verify results of the model and test proposed specifications for UV–A headlamps and fluorescent materials.
UV–A Headlamp Measurements
Samples of prototype UV–A headlamps were to be obtained and measured, including the Ultralux, Labino, and North American Lighting units and unassembled components (lamps, lenses, filters) as they became available. The FHWA’s Photometric and Visibility Laboratory at the Turner-Fairbank Highway Research Center (TFHRC) was to make measurements according to the following plan:
Measurement of Fluorescent Materials
The evaluation of fluorescent infrastructure materials is described in chapter 3, activity 2.
Model Pavement Marking Visibility
A computerized model of visibility for retroreflective pavement markings was to be developed to predict the visibility of retroreflective and fluorescent pavement markings in halogen low beams, HID, and low beams supplemented with UV–A headlamps. The model was to serve as a tool to determine target visibility over a wide spectrum of parameters with relative ease. One major advantage of the computer model would be its ability to examine headlamp and pavement-marking scenarios that were unavailable for field evaluation.
The proposed computer model was to provide the following outputs: pavement marking luminance, road surface luminance, luminance contrast, threshold contrast, and visibility distances. The user would be able to specify the following input parameters: driver age, exposure time, probability of detection, ambient sky luminance, headlamp intensity distribution (for low-beam, HID, and UV–A headlamps), and pavement marking material retroreflectivity and fluorescent efficiency. The user could also vary the spectral composition of the headlamps and the spectral response of the fluorescent markings. Headlamps, observer, and pavement markings would have been implemented in three degrees of freedom only (translation only). The model would handle any number of headlamps attached to the vehicle at any location (three translation degrees of freedom).
The model was to be implemented in MatLab® and C++ for Microsoft® Windows® (Microsoft Windows 95, Microsoft Windows 98, and Microsoft Windows NT®). The preliminary design stage was to include definition and documentation of the required mathematical equations, definition and documentation of a thesaurus for the data structures and functions, development of an overall (global) data structure and function structure, and definition of the input and output data formats. The main design stage was to accomplish refinement of the overall design of the individual functions and local data structures. Modeling of fluorescent pavement markings was to include the spectral sensitivity. A brief final report describing the Phase I model and the results of the test scenarios were to be submitted.
The researchers planned to use the model to seek preliminary answers to several issues raised by the vehicle team:
This model would be limited to straight and level roadway situations. Only the visibility of pavement markings was to be predicted. Glare and backscatter caused by fog was not to be considered. Because these are significant limitations, the research team recommended that the model eventually be enhanced to allow for the following additional analyses:
Work on the model began in January 1999. A limited working prototype of the model was to be released to the project team as soon as practicable to allow for feedback from the model users. Availability of fluorescent pavement-marking data was not critical for the development of the computer model itself; the development could use hypothetical pavement marking fluorescence efficiency matrices for the time being; however, such data would be necessary for the final validation of the model and for performing the model runs. Both the final validation and the model runs were to occur when pavement marking fluorescent-efficiency data became available.
Field testing was to be conducted on the Smart Road and developed in coordination with the driver/pedestrian and infrastructure teams. Visibility measures were to be made for various headlamp and TCD conditions. The prototype UV–A headlamp with the highest radiant output was to be used. It was desirable that visibility be tested with at least two levels of UV–A intensity. Because the UV–A output of the preprototype headlamps might be less than optimal, a high-UV–A-output condition was to be achieved by using up to six of these headlamps per vehicle. The low-UV–A-output condition was to be implemented using fewer of these headlamps.
Equally important was the issue of what standard headlamps to use for the field tests. Until the time of the project, it had been appropriate to use a common tungsten-halogen headlamp. In view of the increased visibility claimed for the newly-developed metal halide, HID headlamps, it seemed important to include these as a basis for comparison. The use of a high-beam condition for comparison against the UV–A condition was also considered. While it would have been of interest to know how well the UV–A headlamps compared to a visible high beam, this comparison did not translate to a real-world option. That is, even if a visible high beam could outperform a UV–A system, drivers would simply be unable to make use of these high beams in the real world in most driving situations because ofthe presence of other traffic. The research team proposed that the field testing include the following six headlamp conditions:
Comparisons of visibility distances, subjective ratings, and measurement of glare in these six conditions (as part of activity 4) were planned to help answer the following questions: