2009-01-0110 Simulating Headlamp Illumination Using Photometric Light Clusters William T.C. Neale, David R. Hessel Kineticorp, LLC Copyright © 2009 SAE International ABSTRACT which and the degree to which something is visible. Current methods exist fore evaluating the limits of Assessing the ability of a driver to see objects, visibility which rely on replicating as closely as possible pedestrians, or other vehicles at night is a necessary the conditions present at the time of the accident and precursor to determining if that driver could have performing an in situ evaluation, through observation avoided a nighttime crash. The visibility of an object at and light measurement (Adrian, 1998, pp. 181-88; Klein, night is largely due to the luminance contrast between 1992; Owens, 1989). the object and its background. This difference depends on many factors, one of which is the amount of However, replicating the lighting conditions under which illumination produced by a vehicle’s headlamps. This an accident occurred can be difficult and expensive and paper focuses on a method for digitally modeling a may be impossible if the accident site no longer exists or vehicle headlamp, such that the illumination produced by has changed significantly. If one were able to digitally the headlamps can be evaluated. The paper introduces simulate the accident environment these constraints the underlying concepts and a methodology for could, in many cases, be eliminated. However, creating simulating, in a computer environment, a high-beam a simulated environment would have its own obstacles, headlamp using a computer generated light cluster. In including the need to accurately model the various light addition, the results of using this methodology are sources in that environment. This paper does not evaluated by comparing light values measured for a real attempt a comprehensive methodology for modeling the headlamp to a simulated headlamp. lighting conditions for an entire accident environment. Instead, it focuses on one of the building blocks that will INTRODUCTION ultimately form the basis for such a methodology, namely simulation of the illumination from a headlamp. Nighttime crashes often involve complex lighting This particular light source is often a critical one in conditions. The visibility of animals, pedestrians, cars, or determining a driver’s visibility for an accident scenario. other objects both on and off the road are affected by conditions such as the illumination by headlamps, street With the recent development of sophisticated computer lamps, ambient lighting, oncoming traffic, and weather. lighting modules and ability to create and control both Primarily, it is the luminance contrast between an object light and material behavior in a three dimensional and its background that makes an object visible to an computer environment, a headlamp can be modeled and observer at night (Olson, 2003, p. 158). Luminance is assembled in a virtual environment to reasonably mimic the amount of light that is reflected off of a surface, and the behavior of that headlamp in the real world. The the luminance contrast, or difference in luminance methodology for simulating headlights as discussed in between two objects, is dependent on surface properties this paper involves modeling the illuminance levels such as color and reflectivity, as well as the amount of produced by a physical headlight source. While low and illumination arriving at that surface by light sources. In high beams are both of interest in modeling the light reconstructing a nighttime crash, evaluating the limits of simulation, only high beams are analyzed in this paper. visibility is important for determining both the distance at The Engineering Meetings Board has approved this paper for publication. It has successfully completed SAE’s peer review process under the supervision of the session organizer. This process requires a minimum of three (3) reviews by industry experts. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. ISSN 0148-7191 Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solely responsible for the content of the paper. SAE Customer Service: Tel: 877-606-7323 (inside USA and Canada) Tel: 724-776-4970 (outside USA) Fax: 724-776-0790 Email: [email protected] SAE Web Address: http://www.sae.org Printed in USA Headlight illumination depends on design factors such Headlamps range from light types such as incandescent, as the beam pattern of the projected lamp, its orientation HID, or LED and include sophisticated reflectors and relative to the object it is illuminating, and the intensity of lenses that shape and distribute light in distinctive ways. the light. Environmental factors such as the atmosphere Each headlamp model will have its own unique beam between the headlight and the object, or even dirt or pattern, and hence, each headlamp model will have its debris that covers the headlamp lens can also affect the own unique characteristics in terms of its changes in illumination. This study examines a new and clean, high- intensity, hot spot location and shape along the area it is beam headlamp, but because the methodology in this illuminating. In accident reconstruction, because there paper utilizes the performance of a real world headlamp, are conditions unique to each accident, it is important to environmental effects that are relevant, such as dirt or look at the specific headlamp model and analyze its debris, can be added to the actual headlamp and unique patterns and distribution when evaluating incorporated in the results. visibility. Equally, it is important to be able to simulate the light distribution properties of a specific headlamp. To measure the accuracy of the computer model, photometry values are calculated in the simulated SIMULATED LIGHT PHOTOMETRICS lighting environment and compared to the actual photometric values measured in a real life environment. Vehicle headlamps have sophisticated light distribution The results demonstrate that despite the complexity of patterns because of the performance standards they are the light patterns and distributions of a headlamp, an required to meet and because of the use of parabolic accurate headlamp simulation can be produced. As this reflectors and lenses to achieve the necessary efficacy. process develops and is validated for larger more The image in Figure 1 is taken from SAE J2595 complex lighting environments, it is the intention that “Standard Performance Requirements for Sealed Beam computer simulated light values could be used in Motor Vehicle Headlamps”. The grid of points denotes a conjunction with visibility models developed by Blackwell pattern of angles the light projects from the headlamp (1981) and Adrian (1987, pp. 3-12) to assess the limits and associated candela values. The primary goal of of a driver’s visibility within a specific accident scenario. these standards is to provide sufficient light power to the headlamps to illuminate objects ahead of the vehicle, BACKGROUND ON LIGHT SIMULATION while reducing the amount of glare that may be discomforting or distracting to oncoming traffic.10 The One’s ability to visually perceive an object is due to the standard has many data points, specifying both the difference in contrast between an object and its angle and candela power. This standard represents the background (Alexander, 2002, p. 158; Olson, 2003, p. minimum performance criteria and many manufacturers 183). This contrast results from differences in each have chosen additional criteria to augment their designs. object’s color, texture, spatial frequency, and illuminance The various points along the grid represent both contrast. However, in nighttime driving conditions, the minimum luminance values and maximum luminance effect of illuminance contrast tends to be the greatest values and as indicated in the top of Figure 1, the influence on the ability to perceive an object (Olson, standard shown here applies to high beams only. 2003, p. 121). For this reason, the ability to visually perceive an object at night on the roadway is primarily determined by the relative illumination of these objects from the headlamps of the vehicles, the ambient lighting from the atmosphere or moon, or man-made lights on or off the street. Techniques for digitally simulating light have existed for decades, though applications of light simulation have often focused on interior lighting or lighting design. In the Figure 1 context of accident reconstruction, Phillips (1990) has discussed the use of programs like Lumenpoint to To visually understand the application of this standard, determine light values given a particular scene Figure 2 shows (top portion) a canvas mounted on a wall environment. These programs work from basic light in a lit room, upon which a headlamp beam can be parameters and deal with the general behavior of light. projected and then photographed. The bottom portion What makes nighttime accidents a special problem for shows the photograph taken of the projected headlamp lighting simulation is the complexity of the light sources on that canvas, when the ambient lighting has been and the accuracy needed to evaluate the behavior of turned off. each of the light sources. Light simulation programs do not necessarily deal with the unique attributes that surround light being emitted from a complex light source such as a headlamp. 2 other words, if the photo in Figure 2 were a projection map, the areas in dark would prevent light from shining through, while the areas in white would allow it. The values that fall between white or black would allow a percentage of light through, equal to the percent of whiteness or blackness of that area of the projection map. This concept is illustrated in Figure 3. In this illustration, a light source sits behind a projection map through which the light passes to project an image further away.