Polarised Headlight Filters

One way of reducing the glare from headlamps is to use a polarised headlight system. In this paper the relative merits, for this purpose, of commercially available filters are discussed. A second paper will discuss the widely differing proposals that have been put forward in America and in Germany for the use of polarised headlights, and gives the results of tests made with an alternative system. If an object which is neither very light nor very dark is to appear as bright in polarised light, when seen through the visor, as it normally appears in unpolarised light, about five times as much light is needed. When the transmission and extinction coefficients of the polarising material are known, it is possible to compare the performance of a polarised headlight system with that of systems using ordinary light, in terms of the seeing distances of a standard object. A high intensity polarised beam can only be used if the beam is almost completely polarised. At lower beam intensities less stringent demands are made on the polariser. The extinction of oncoming polarised lights by the visor depends upon the accuracy of its setting, and may also be influenced by the properties of the windscreen. For a particular polariser the light leakage was increased on the average 150 times for a toughened-glass windscreen and four times for a laminated-glass screen. It is evident that existing types of toughened-glass windscreen could not be used with polarised headlights, unless the visor were mounted outside the windscreen