1977-02-01

Modeling Vision with Headlights in a Systems Context 770238

A Headlight Evaluation Model has been developed which provides a broader and more comprehensive method for characterizing the performance of headlamps than is possible in traditional headlight seeing distance field tests.
The Headlamp Evaluation Model accepts as input the candlepower patterns of the headlamp system being evaluated and provides a measure of driver visual performance based on a large number of simulated seeing distance tests and glare discomfort checks on a standardized test route. The output of the Model, termed the Figure of Merit, is the percentage of the distance traveled by the simulated driver on the standardized test route in which the seeing distance to pedestrians and pavement lines, and the discomfort glare levels experienced by opposing drivers, simultaneously meet certain acceptance criteria.
The standardized test route is a computer representation of a series of highway sections in the form of a file of environmental parameters which have an influence on visual performance in night driving. The simulation includes such parameters as pavement, lane line and pedestrian reflectance, road geometry, lane configuration, ambient illumination and glare from fixed lighting, and traffic and pedestrian density. The standardized test route is a representation of a U.S. night driving environment as measured in a series of field surveys covering thousands of miles of highway and as reported in the literature.
The seeing distance calculations are performed by an integral seeing distance model which is based on the human visual performance literature and validated by field studies. Response to glare is based on published discomfort glare formulations, modified and validated on the basis of highway tests.
Because driver visual performance, as expressed in the Figure of Merit, is functionally dependent on environmental factors as well as on headlamp characteristics, the Figure of Merit is a systems measure. The Model can be used not only to evaluate headlamps but to measure the sensitivity of driver visual performance to environmental factors and thus to improvements in certain aspects of the highway itself.
Applications of the Model have shown that driver visual performance at night is more sensitive to environmental conditions and to the driver's visual capabilities than to the range of characteristics exhibited by existing and proposed headlight systems. This is partly because large increases in candlepower are required to provide useful increases in visibility and because such increases in candlepower produce a concomitant increase in glare discomfort. Other applications include a comparison of several European and mid-beam systems with current U.S. systems, evaluation of headlamp misaim effects and a determination of the effectiveness of improving the brightness of pavement lines.

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