From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Letter from Joseph Lucas Ltd. discussing the technical limitations of bifocal headlamp bulbs and alternative P.100 headlamps.
| Identifier | ExFiles\Box 61a\1\ scan0393 | |
| Date | 30th October 1928 | |
| P/126 JOSEPH LUCAS LTD., To Messrs. Rolls Royce Ltd., D E R B Y. PAGE 2 DATE 30th October, 1928. patterns on many American front glasses, and they are all used to try and eliminate the disadvantage of a too narrow driving beam. We have found that even the best of these glasses absorb or disperse an appreciable amount of light when compared with a flat glass, and this can only be obviated by using plate glass which has been optically ground to the correct curvatures and then polished, and the cost of such work is naturally prohibitive. The angle through which the beam can be dipped by the bifocal bulb arrangement is strictly limited, and in our opinion insufficient, nor is the anti-dazzle effect in the dipped position very much pronounced. You may ask the question "Could not the offset filament in the bulb be moved further from the main filament, and so increase the angle of displacement?", but in practice there is a limit beyond which one must not go owing to the distortion of the beam as the filament moves away from the axis of the reflector. We have made many experiments with special bulbs made to our specification to see if the standard arrangement could not be improved upon, but the bifocal system is distinctly one of compromise, and it is not practicable to have the offset filament very much further offset without entirely destroying the beam. You will gather from these remarks that we consider that the bifocal bulb to be a rather poor palliative to the dazzle problem, and not worth consideration in conjunction with our P.100 headlamp. In addition to this, the reflector of the P.100, which as you know combines a glass mirror and a parabolic reflector, does not lend itself to the system under discussion quite so well as an ordinary parabolic reflector, which is another reason for our doubt of the advisability of your pursuing the matter further. You may have noticed at our Stand at the Motor Show, an improved model P.100 headlamp | ||