DIAGRAMS FOR SHOWING HEADLAMP PERFORMANCE
Instead of representing light intensity by lines to indicate photometric values the author recommends an arrangement for denoting the intensity by varying degrees of tint on the surface of a chart that is supposed to represent the roadway. In the opening paragraph the thought is brought out that present-day automobile lighting-equipment is not designed in such a way as to make its performance a selling feature and the several reasons why the efficient distribution of light on the road has been overlooked are pointed out, emphasis being laid on the fact that the average car-designer is not an illuminating engineer, and that even if he did wish to use the best light available on the car he would have to make personal tests of the devices under all conditions of night driving before being in a position to recommend the most efficient head-lighting device.
The method of comparing devices by illumination charts that illustrate the performance of head-lighting devices is described, and charts from four typical specimens chosen from the many that are on the market are reproduced to show that while the light from each of the devices does not exceed the glare limit above the level of the head-lamp and is at least sufficient to meet the minimum driving-light that the law demands below the level of the head-lamp, a marked difference in the efficiency of these equally legal devices will be found. The construction of the devices for which the road illumination charts are reproduced is described.
Tabular data show the report of a test of the candlepower distribution from the combined beams of a pair of standard head-lamps as directed toward various points on a vertical screen located 100 ft. ahead of the test lamps. The outlining of areas of equal illumination is described at some length, the text being supplemented by diagrams and tables.
The lighting equipment of present-day automobiles is not designed in a way that makes its performance a selling feature. True, various attempts have been made at the modification of the cone-shaped beam of light that comes from an electric bulb within a parabolic reflector by glass fronts or lenses, and there have been attempts to use specially shaped reflectors that deviate from the true parabola. These modifications, however, have been largely made with a view to eliminating glare, and if the thought of improving the distribution of the light on the road has been present, it has, judging from the results obtained, been a secondary consideration.
Efficient distribution of light on the road has been overlooked for several reasons. The average car designer is not an illuminating engineer. He does not design his own head-lamps. If he has a hobby, it is more likely to run to some item of mechanical equipment or to body design. Head-lamps are to him a necessary evil, like a kit of tools or a horn or spark-plugs, something to be bought as cheaply as possible and consequently within the jurisdiction of the purchasing, rather than the engineering, department. What if the light is not so good? Cars do not often have their driving lights compared. If the lamps look good on the outside and if they look sufficiently legal so that dealers will not enter a protest, nothing more is necessary.
Furthermore, even if the car designer realized that exceptionally good driving-light might enhance the value of his car, and if he had made up his mind that he wanted the best light available, he would have a hard time making a choice without personally testing dozens of devices under all conditions of night driving. He can easily eliminate those that are illegal by referring to the lists published by the various States. From these lists he could even determine which device was the most efficient from the standpoint of stopping glare. But nothing has been published to show the comparative lighting efficiency of head-lighting devices.
If it were a case of lighting a factory or an office, no trouble would be experienced in choosing proper equipment by comparing the illumination charts that are available for industrial lighting fixtures. But despite the importance of good driving-light to the comfort and safety of the driver, no data more useful than the claims of manufacturers have been published.