Reflector
Retro-reflectors reflect incident light preferentially in the direction from which it came. To distinguish it from other retroreflectors , reflectors are understood here that are made up of flat mirrors.
Basics
The aim is to reflect (throw back) the headlights of a motor vehicle in such a way that it reaches the eyes of the driver of the approaching vehicle.
With safety reflectors, it is not desirable to reflect the light exactly back into the light source. A scattering effect is necessary in order to cover the small angle between the vehicle headlight, reflector and viewer.
Areas of application
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Reflectors are most widespread as a safety device in road traffic. At night z. B. otherwise unlit objects visible to approaching drivers.
According to the StVZO , bicycles in Germany must have at least a red reflector on the back, a white reflector in the front and a yellow one in the pedals that looks forward and backward. In addition, there are either at least two evenly distributed yellow spoke reflectors per wheel or retroreflective tires. Often the front or rear retroreflectors are integrated in the rear light or headlight housing. However, one of the rear reflectors must be mounted separately. Other yellow retro-reflectors acting on the side are permitted. For Austria, the corresponding conditions are set out in Section 1 of the Bicycle Ordinance. According to Art. 217 of the VTS , bicycles in Switzerland must have a white reflector at the front and a red reflector at the rear of at least 10 cm 2 , as well as yellow reflectors on the front and rear pedals with a light area of at least 5 cm 2 . Yellow reflectors in the spokes or retroreflective marking on tires are allowed, but not compulsory.
Reflectors must also be attached to automobiles, trailers, and many other vehicles.
Although belonging to the group of pedestrians and therefore not obliged to equip them with a reflector according to the StVZO, parents are increasingly equipping their prams with reflectors. Classic hard plastic reflectors , such as those used on bicycles, or special, self-adhesive reflector foils are popular here .
Also delineators are equipped with reflectors. These reflect predominantly white (neutral in color).
Often house owners out of town provide their driveway with reflectors. Reflective elements on bicycle tires, school satchels or safety vests use reflector foil - a material with small, transparent plastic balls, the reflection principle of which is similar to that of a cat's eyes .
Many traffic signs are retroreflective and provided with foils or coatings for this purpose.
Reflex light barriers also use reflectors in order to be able to accommodate the transmitter and receiver in one housing and to facilitate adjustment.
Working principle
A retroreflective reflector as shown works with an arrangement of many small three-surface corner reflectors or totally reflective prisms . Many three-sided pyramids (no tetrahedra!) Are arranged in a flat array. Each pyramid has the shape of a corner cut off by a cuboid - the three surfaces that meet in the corner are perpendicular to each other. In the picture example, the pyramids (square corners) look like cubes from behind. The alignment of all cube or square corners is usually the same. The tips of these corners point away from the light source, so they protrude from the rear of the reflector body. The load-bearing pane is smooth at the front and is in the millimeter range.
A mirroring of the pyramids (optical prisms) is not necessary. The inclined rear boundary surfaces viewed from the light source reflect the rays entering through the smooth side of the reflector due to total reflection . Due to the arrangement of the three surfaces at 90 ° angles, the light is reflected back in the direction from which it came.
If the reflector is not illuminated vertically, it will refract when the light enters the smooth front surface . This has no effect on the retroreflection, since when the light emerges there is again refraction with the same angle of refraction and the same orientation.
However, the double refraction can only cancel each other out if the front surface is flat and smooth, because in the cube corner there is a small lateral offset of the light rays (parallel shift).
The total reflection of the non-mirrored, transparent plastic does not occur when the light falls from the side: Rays from directions that are clearly different from the vertical are reflected less well. The division of the reflector in z. B. two or four partial surfaces with different orientations of the pyramid surfaces prevents inclined light incidence that the entire reflector surface loses its function. In the case of other reflectors, neighboring pyramids are sometimes also rotated with respect to one another. In this way, at least one area or one neighbor always remains functional. Occasionally, partial surfaces are even angled slightly.
An extremely precise reflector without any scattering effect would reflect the light exactly back into the headlights, i.e. not or hardly at all to the driver's eyes. Therefore, an intended scattering effect is provided during manufacture. Incoming rays are then reflected back as a narrow cone at the expense of brightness. This scattering effect can, for. B. can be achieved by tiny deviations of the many small areas from the shape described above or by using diffraction effects. A controlled scattering effect requires a very high level of manufacturing accuracy. Injection molding tools for the production of reflectors are provided with special inserts, so-called galvanos.
Reflectors based on the triple mirror principle were made from glass in the past, but are now made from transparent plastic ( Plexiglas ). Often this is colored red or yellow so that only light of this color is reflected.
Remarks
A reflex reflector that reflects very well and precisely can dazzle drivers.
Many of the retro-reflectors described and also those shown show a maximum of brightness at individual points at different lateral beam angles or distances due to reflections on only two surfaces. The direction of the very bright reflected light is then dependent on the random position of headlight to reflector. The scattering effect is often missing. This unwanted mirror effect can also be reduced by rotating subsets of the cube-corner mirrors. It is almost never the whole reflector that reflects very brightly.
literature
- Fritz Winkler, Siegfried Rauch: Bicycle technology repair, construction, production. 10th edition, BVA Bielefelder Verlagsanstalt GmbH & Co. KG, Bielefeld 1999, ISBN 3-87073-131-1 .
- Michael Gressmann, Franz Beck, Rüdiger Bellersheim: specialist knowledge of bicycle technology. 1st edition, Verlag Europa Lehrmittel, Haan-Gruiten 2006, ISBN 3-8085-2291-7 .