Retroreflector

A retro-reflector is a reflective material which is substantially independent of the orientation of the reflector largely reflects the incident radiation in the direction back to the radiation source ( retroreflection ).

Background

Diffusely scattering surfaces radiate very little light back towards the light source. Nevertheless, you usually appear brighter than a mirror that on a plane mirror, the reflection depends on its orientation from that is aligned only in exceptional cases perpendicular to the viewer. For this reason, a rain-slicked road that is lit only by the own vehicle headlights at night darker than the diffuse backscattering surface of a dry road surface appears.

Application

In the wireless technology radar reflectors help locating the illuminated objects, such as a balloon probe or a bridge pier on a waterway.

Precise beam recirculation in the optical wavelength range: triple mirror behind cut glass used in the measurement of the laser as a turning point. At a retro-reflective sensor, retroreflective sheeting or reflex reflectors is used to redirect the beam back to the sensor immediately adjacent to the light source.

In road traffic, increase retroreflectors on people, obstacles, traffic signs, guide devices and vehicles whose visibility at night in the spotlight. Vehicle drivers look closer ( about 1 m) whose light exit point on the scene in the light cone. Retroreflective throws light back against the direction of incidence. If the link now looks a retro-reflector at 60 m distance, then strain illumination beam and the direction of observation at an angle of about 1 °. Therefore illuminated objects retro -reflective agents act at their best when they have a certain small -angle scattering, but also obliquely illuminated - such as a traffic panel half right so 45 ° in front of us - still work well. These two parameters are used for their characterization

In white or colored, there are reflectors made ​​of transparent plastic injection molding (rear cube square rasterized), reflector films based embossed aluminum foil or back calendered plastic foil, cats eye from wider glass bodies ( biconvex or back with reflective facets, typical silver coated with protective coating ). Poet Tautropfenbelag or frost can dispel the retroreflectivity, highway signs can then show large dark spots.

Some bicycle headlights use the edge of the lens incorporates a retro reflexology, whose facets are designed so that light is obliquely back Arriving penetrates well from the bulbs.

Reflectors on roads as protection against wild have geometrically a different task. They are the incoming beam headlights angled laterally deflect by 90 ° into the area, so act like angle mirror.

Retro-reflective materials should be combined in a security application with diffusely reflecting surfaces so that they are recognizable even when they are irradiated with ambient light from other directions. For this reason wear delineators not just reflectors, but are also colored white. Also, aluminum, often extruded and anodized, as Verkehrstafel poles or railings is bright as long as it is not corroded by salt.

When pavement markings finest glass beads ( d < 0.3 mm) teileingebettet in the applied marking paint still wet (paint or liquid plastic ) by sprinkling, which retroreflection is achieved in order to increase the visibility when illuminated. The effect of the glass beads is similar to that of the Luneburg lens.

In nature occurs retroreflection at dewy plants, such as grass and corn stalks on, if they are very hairy. Visible is the effect as a " halo " while biking along the roadside around the shadow of one's head in the open, when the sun is medium high left behind. The phenomenon also occurs on hairless plants when their leaves are so highly hydrophobic by a wax layer, the contact angle of the drops of dew rises to 140 °, was what Alistair B. Fraser observed on conifers and therefore sylva shine (English Waldschein ) called.

Conversely, try designers of military vehicles, ships and aircraft used in particular to be avoided by strict avoidance of inner corners to the outer contour of unwanted radar reflections ( stealth technology ).

Retroreflectors made ​​of quartz glass ( to reflect UV- light components ) are used for long- path measurements based on the DOAS technique to atmospheric trace gases in the air on a defined path to prove.

Other effects

• Glory (English )
• Halo
• Opposition Effect: the full moon appears much brighter than the crescent per surface, dusty road appears around the point opposite the light source behind a lightest, also (dry ) meadows, fields and forests appear around much brighter to one's own (sun) shadows. For rough structures cast shadows on themselves only from the perspective of the lighting direction -is, these shadows are, however, obscured by the illuminated surfaces.

Retroreflective elements

In addition to flat optical corner reflectors ( corner cubes and triple prisms ) and reflectors, there are rotationally symmetrical lens reflectors (cats eyes, Luneburg lenses) and also fundamentally different types of retroreflective body, such as biconical structures.

Versions with plane mirrors

The retroreflectors from plane mirrors and flat surfaces, a distinction designs with two or three mutually perpendicular reflecting planes.

The picture above illustrates the retro-reflection in a plane ( two levels). If the third spatial direction may change, three levels are required, it creates a triple mirror, the principle see also corner reflector.

A triple prism is a glass body, which is the front plane and the rear side three mutually having a 90 ° related non- reflecting plane surfaces. It reflects in principle even less loss than a triple mirror, even if the front surface is non-reflective. The reason is the loss-free total reflection at the inclined back surfaces. Triple prisms have a greater range of angles within which reflection takes place, since the front surface of the vitreous body causes refraction of the symmetry axis.

Examples:

• Reflectors are usually made of plastic, rare ( earlier) also made ​​of glass. They carry rear molded triple prisms that work by total internal reflection as triple mirror. You are back covered in order to avoid reflection losses caused by dirt and ( condensation ) water, which is why the Tripelspitzen are palpable and visible from the rear only of broken, opened reflectors.
• Corner reflectors as Radar reflectors (English corner reflector ) of sheet metal as a retro reflector for microwave radiation ( RADAR ) approximately for shipping before bridge piers.
• Triple mirror and triple prisms of glass for retro-reflection of laser beams ( LIDAR) in the measurement, the measurement of the distance of the moon and for accurate positioning of satellites

Similar lens design

If there is a reflective surface in the focus of an imaging optical system, then the reflected light is directed by the optical system back towards the light source. Unlike a simple plane mirror, this property does not depend on the precise alignment of the mirror surface. For an ideal retro-reflection, however, the distance of the reflecting surface must be exactly right. Lens aberration also cause the light is not completely directed towards the light source. In some applications, a reflection is within a range close to, but not exactly even desirable at the light source. This applies for example to retroreflectors on the road. So that the reflected light of a head lamp can be seen, it may not be completely drawn back into the headlights.

Examples:

• The optically active component of reflector films and screens consists of many small transparent spheres. A transparent sphere made of glass or plastic focuses a large part of the incident light of a distant light source to a spot just behind the rear surface. By the difference in the refractive index in comparison to air, the rear surface of the ball acts specular. Since it is only slightly in front of the focus, the light is directed in a narrow cone around the direction of the light source. In this way, in the cinema as much light as the film projector reaches the eyes of the spectators. The same applies to the spotlight, which is preferably equipped with reflector film directed by signs to the driver of each vehicle.
• Cat eyes glass body whose front side is curved such that the incident parallel light is focused on the reflective back. Unlike transparent spheres can be determined within wide limits by the form in cats eyes of the solid angle is reflected back into the incident. In addition, the back can be completely reflective. Cats eyes can therefore draw more light in the desired direction. However, they are expensive to produce.
• The eyes especially nocturnal animals such as cats are retroreflective, since their retina is reflective background. See tapetum lucidum and red-eye effect
• Retroreflection by fog droplets is undesirable, which is why fog lights are placed as far away from the viewing direction.
• Luneburg lenses are back mirrored balls made ​​of a transparent material having a lower refractive index becoming inwardly. They are also used as radar reflectors. The mirror coating is then being executed as belts, so that takes place from all horizontal directions retroreflection.

Pictures

Recording a triple mirror with and without flash. 3 of the 6 edges are reflections. The eye and the camera seems always at the center.

Retroreflector element ( cat's eye ); the curved surface on the left is mirrored. The right curved surface has its focal point on the mirror surface.

In a sunken road reflectors.