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).
Monograph on the theory and practice of Acetylene and Electric headlamp design, Part I covering the production of a suitable beam.
| Identifier | ExFiles\Box 61a\1\ scan0003 | |
| Date | 23th November 1912 | |
| 1070 THE AUTOCAR, November 23rd, 1912. Autocar Head Lights. A Series of Three Elementary Monographs dealing with the Theory and Practice of Acetylene and Electric Head Lamp Design. By D.{John DeLooze - Company Secretary} H.{Arthur M. Hanbury - Head Complaints} Ogley. PART I.—The Production of a Suitable Beam. NOT the least interesting feature of the recent motor show was the brave display of lighting apparatus of all descriptions. The struggle that is taking place between acetylene and electricity for premier place in the ranks of car illuminants was distinctly evident to the most casual observer. The more or less extravagant claims, however, of the rival manufacturers, coupled with the numerous new designs, and aided by the multiplicity of terms, must have made the task of the average motorist, bent on deciding the case on its merits, almost hopeless. Fig. 1. To a certain extent the choice of a suitable lighting equipment will always depend upon individual taste and requirements, but it is as well before coming to a final decision to look into the claims of both systems. In the following articles an attempt will be made to furnish the reader with the necessary theoretical knowledge to enable him to do this. The presence of light always indicates radiation, and radiation is a process involving the expenditure of energy. The usual type of visible radiation, known as incandescence, is common to all bodies when heated above a certain temperature—the red heat. In the case of a luminous flame the heavier hydro-carbons are decomposed at a high temperature and liberate incandescent particles of carbon, which continue to glow with incandescence until consumed or carried away as smoke. This smoke, of course, is one of the inherent defects of the flame source of light. The source of light in the electric system is a filament of metal which is raised to a high temperature and so becomes incandescent by the passage of an electric current through it. A luminous body, or source of light, such as either of the above, may be supposed to emit rays in all directions—a number of such rays emanating from a source form what is known as a beam of light. The shape of the beam depends entirely on the method employed to control the rays. Thus, if the rays are allowed to issue unrestricted from the source, a diverging beam will proceed in any direction. Let O (fig. 1) be a luminous body. Rays of light are emitted equally in all directions, so that if a sphere surrounds O each unit of area of its surface will receive an equal quantity of light upon it, so the illumination over the surface will be equal. The actual illumination depends simply upon the quantity of light falling on the surface, and has nothing to do with the colour or the reflecting power of the surface. Thus rainfall is independent of the nature of the soil. If rain falls on clay it remains, but if on sand it disappears or is absorbed. In the same way a quantity of light, or illumination, falling on a dark coloured surface is absorbed and wasted, but when the same illumination falls on a light surface the latter is clearly distinguishable. Replace the sphere that surrounded O with a second one, of radius twice OA. The area of this sphere will be four times the area of the first sphere, since the area of a sphere depends upon the square of its radius. Fig. 3. The quantity of light emitted has not changed, so that the illumination or quantity of light on unit area will now only be one quarter of what it was in the first case. The illumination therefore falls off as the square of the distance. Such a light placed in the front of a car would serve little useful purpose. Since rays are emitted in all directions, only a small number would be usefully employed in illuminating the road, and even then, since the beam is diverging, the rapid decrease in intensity with the distance would render the illumination too feeble for safety in travelling. Further, some of the rays would cause inconvenience to the driver by shining in his direction, and so destroying his sense of contrast. This latter defect is, however, readily overcome by simply surrounding the source of light with a metal casing, furnished with a glass door in front as in fig. 2. The walls of the lamp will now absorb all rays but those issuing from the front of the source of light Fig. 2. | ||