Improved Lighting for Fog Driving
Page 36
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FFOG is one of the weather conditions which, whilst being infrequent, temporarily presents one of the greatest obstacles to the operation of commercial vehicles, and anything which will help the driver to overcome this difficulty and lessen the danger deserves attention. Recently we have had opportunities of trying out some special glasses for headlamps, which are known as Nebulite. They have been developed by Dr. A. H. Stuart, and are being marketed by the London Motor Accessories Co., Gloucester House, 19, Charing Cross Road, London. W.C.2. They are available in all standard sizes.
These glasses are lightly tinted in such a manner as to filter out of the beam of light those pariticular rays which cause back glare, and thus permit the driver to benefit by being able to see objects reached by those rays which can penetrate the fog, but which would, where ordinary white light is utilized, be screened off from him by the glare.
The effect is almost startling, even when no mist is present. At first sight, the light appears to be feeble, because there are no clearly defined 'edges tg the beam, but one soon finds that objects at a distance become clearly visible, and if one's headlamps be turned on to the unlighted headlamps of another vehicle a long distance away. these become brilliantly illuminated by reflection. Another advantageous point is that there is considerably less dazzling of approaching drivers and far less frequent demands upon one to dip.
The exact tinting of the glass is the result of experiments extending over several years. Two years ago it was applied to ordinary glass in the form of a dye, but there were difficulties in utilizing this method, and now the light-filtering media are embodied in the glass.
White light is a blend of many colours, and experiments show that those colours with a range from red to yellowish-green with a longer wavelength have better fog penetration, whilst those of shorter length, corresponding to green, blue and violet, fail to get through, whilst the human eye is most sensitive to a wave length approximating to yellowish-green.
The aim has been to provide a light filter giving a sharp cut-off within the narrow range of wave lengths between the maximum sensitivity of the eye and the point at which fog definitely ceases to transmit the light to any appreciable extent. It was found possible to obtain such a filter, the effects of which, apart from actual experience, can best be judged from the spectrographs which we repmduce, where A is the spectrum of a tungsten are unfiltered to indicate the full wave-length band ; X shows the position giving the best resolving power to the eye, Y that at which the eye is most sensitive, and 2 the point at which the cut-off should be made for fog penetration. That part of the spectrum to the right is short-wave light, which will cause back glare.
The spectrograph of Nebulite is shown by B, and is close to the ideal prescribed. The other spectrographs are those of various yellow glasses which have been tested under precisely similar conditions.