Dipped Beam Dipped Beam
Dipped beam is designed for continuous use and it should not dazzle traffic ahead. It should provide light for up to 40 m ahead of the driver. Another 150 m (see picture three) is allowed, yet the light has to spread asymmetrically away from the middle, towards the passenger's side.
Next to the
bulb, it is necessary to have a reflector, making sure the outgoing light is parallel. In earlier days, the reflector was manufactured from sheet steel, nowadays plastic is used. A thin chromium layer is attached to the reflector. It has a parabolic shape. The reflector is used as a mirror, thus the angle of incidence equals the angle of reflection. The extraordinary feature of the parabolic shape is that all light initiated in one point, the so called focalpoint, is reflected in a parallel manner.
For the Bilux-bulb (2 headlights), just the front coil is used.
The lower part of the reflector is covered by a mirror. Its rays just hit the upper part of the reflector. Since the coil is arranged slightly before the focalpoint, the angles are a bit larger, hitting the road earlier as compared to main beam.
For normal passenger cars, the ray going to the road sinks roughly one cm per meter.
The method for further illumination of just one side of the road is by using a lamp lens. The lamp lens cuts after a certain point the rays to the left, towards the middle of the road. And it refractions fewer rays the nearer they are towards the edge of the road.
Lamp lenses are necessary for systems with 4 headlights, too. Naturally, there is no mirror, and the rays are fully used.
The diagram in picture 4 outlines the dipped beam (red). To its left the parking lights and the license plate illumination, to the right the fog lamps and one rear fog light.
Introducing the alternator in 1913, electrical headlights became possible for vehicles. At that time, a large range and homogeneous illumination was important, comparable to the main beam nowadays. The asymmetrical illumination was introduced only in 1957.