WHAT HAPPENS WHEN A HEADLIGHT IS OUT OF FOCUS
Since the layman and not the engineer buys and drives most of the automobiles produced and because the literature on automobile headlighting presents to[ILLEGIBLE] technical a picture of what happens when the light source of an automobile head-lamp is out of focus, the authors planned and executed an extensive study of the subject in an endeavor to clarify the technicalities by presenting them in the forms of photographs and simple charts, the chief object being to obtain data that emphasize the necessity of accurate control of the size and location of the light source with respect to the focal point of parabolic headlight-reflectors. A great difference in the resultant beam of light is produced by a very small displacement of the light source, either through poorly constructed lamps or due to lack of proper adjustment, and the tests made evaluate how small these displacements and how great these differences are.
After considering the difficulties of locating the light source or filament of the 21-cp. headlight-bulb at the exact focal point of the reflector and of taking account of the practice of certain manufacturers who measure tolerances in sixty-fourths of an inch and of trouble due to wabbly sockets, distorted reflectors and the like, the authors constructed a device that enabled the lamp socket to be moved in any direction by micrometer screws having 32 threads per in., thus causing a one-half revolution of the screws to move the light source exactly 1/64 in., backlash being compensated for by springs. A test reflector made as perfectly as possible was used in connection with the device. Variations due to filament size, shape or relative position in the bulb were eliminated so far as possible by selecting lamps exactly correct as to light-center length, axial alignment and bulb image, and the same lamp was used for all the light-center length and axial-alignment tests, these being made by moving the one lamp with the aforesaid accurate focusing-device. Three types of lamp were chosen to give beams of wide, medium and narrow spread, and, with this equipment, the authors set out to ascertain the effects of various specified changes in equipment arrangement and to record them by photographs, as well as by photometric and linear measurements.
In making the measurements, the test reflector with the universal focusing-device was mounted on a rotatable table, graduated in degrees, located 25 ft. from a screen having the Illuminating Engineering Society's headlight-specification test-points plotted upon it. Photometric readings were taken with a Macbeth illuminometer. The headlight-lamps were operated at exactly 21 mean spherical cp. A single reflector was used. The tests brought out clearly the need for accurately made lamps and equipment and for accurate focusing if the headlight situation is ever to be brought under control.