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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).
Page from 'The Autocar' magazine discussing the optical principles of vehicle headlamps, including lenses and reflectors.

Identifier  ExFiles\Box 61a\1\  scan0004
Date  23th November 1912
  
THE AUTOCAR. November 23rd, 1912.

These form a diverging beam, and since, as we proved before, the intensity of illumination falls off as the square of the distance, the arrangement would be of little use for long-distance illumination.
This diverging beam is, however, required for hedge, or side of road illumination, so as to enable

Fig. 4.

the driver clearly to distinguish the borders between which he is travelling.
This beam then requires supplementing by another that will show up obstructions at a sufficient distance to allow safety in driving.
If the rays of light remain parallel to each other after leaving the lamp, a concentrated or parallel beam is produced.
The intensity of illumination on a surface at right angles to such a beam would, neglecting absorption and dispersion due to the atmosphere, be everywhere the same no matter what the distance from the source of light. Such a beam, then, would possess admirable qualities for long-distance illumination.
To obtain such a beam use may be made of a lens. If a beam of light, consisting of a number of parallel rays, falls on a lens, the rays in passing through it are bent or refracted. The rays nearest the edge of the lens are bent more than those nearer its centre. The result of this bending is that the rays on leaving the lens all meet at one point—the focus of the lens.
Conversely, if a source of light is placed at the

Fig. 5.

focus of a lens the rays falling on it will leave as a parallel beam.
If, then, a lens be placed in the glass door of the lamp, formerly considered, at such a distance from the source of light that the light is at the focus of

Autocar Head

the lens, a parallel beam of light will leave the through the lens, as well as the divergent beams which do not pass through the lens but through the remainder of the glass front as in fig. 3, so that the diverging hedge lighting beam is still retained.
It will be quite obvious that only a small proportion of the energy available is being used in illuminating the road. Most of the rays are absorbed by the inside of the lamp. These can be utilised, however, by making use of a reflector. The choice of a reflector depends upon the nature and the source of light. If this latter be a flame source as in the acetylene lamp, the use of a metal reflector close to the flame is not practicable on account of the fumes emitted. The reflector would be quickly tarnished and corroded, so that the light, instead of being reflected, would be simply absorbed. A glass reflector of special construction to withstand heat must be used at the back of the flame, the span directly above the flame being occupied by a chimney to allow of the fumes being easily dispersed.
If the source of light be an electric bulb, then, since very little heat is given out and no corrosive influences are at work, the bulb may be surrounded by

Fig. 6.

the reflector, so that very little light need be wasted.
Considering first the glass reflectors. The ordinary spherical mirror is simply part of a spherical reflecting surface. The centre of the sphere of which it is a part is the centre of curvature of the mirror, and half this distance is the focal length or distance of the focus from the mirror.
For reasons that will be given later, this mirror has really no true focus, so that it cannot reflect a true parallel beam. The beam will diverge on leaving the mirror and prove of little use for long distance illumination. Further, if the lens be retained, the beam is broken up, so that several sets of rays are obtained, the result being patchy illumination. (Fig. 4.)
The true parallel beam may be produced by reflection from a lens-mirror. This is a form of concave mirror, but possesses a true focus. In this it differs from the spherical concave mirror. This result is obtained by making the mirror thin at the centre and thick near the edge, in a manner that will be indicated in a later article.
Having then obtained the necessary apparatus for the production of the parallel beam by reflection, the lens on the front glass can be omitted. (Fig. 5.)
  
  


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