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. regarding the technical specifications and disadvantages of Bifocal Bulbs for headlamps.
| Identifier | ExFiles\Box 61a\1\ scan0386 | |
| Date | 30th October 1928 | |
| X4533 JOSEPH LUCAS LIMITED GREAT KING STREET BIRMINGHAM. RLN/FEA. 30th October, 1928. Messrs. Rolls-Royce Ltd., DERBY. X4477. Dear Sirs, Re Bifocal Bulbs. EFC8/T. We thank you for your letter of the 24th inst., with reference to the above, and we are glad to be able to give you our experienced with this type of bulb. As you probably know, the majority of American headlamps employ some sort of dipped beam arrangement, and this effect is achieved by using a bifocal type of bulb, and a special form of headlamp glass. The bulb is provided with two filaments, one at the focus of the reflector and the other slightly above it. To achieve any marked displacement of the beam it is necessary that the source of light, i.e. the filament should be as small as possible, and also that the bulb should be correctly focussed. This naturally results in a pencil of light quite unsuitable as a driving light, and it is customary to spread this beam in a lateral direction by a moulded front glass comprising in its most elementary form a series of vertical curved faces. You will have noticed the wonderful 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 two 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 | ||