Traffic safety reflectors how they work

Retroreflector

Principle of a retroreflector: The light beam is always reflected back to the source regardless of the angle of incidence.

A Retroreflector (from lat. retro backward, reflexio Back diffraction) is a reflective material which largely reflects the incident radiation, largely independent of the orientation of the reflector, in the direction back to the radiation source (retroreflection).

background

Diffuse surfaces reflect only a small amount of light back to the light source. Nevertheless, they usually appear brighter than a mirror, because the reflection of a plane mirror depends on its orientation, which is perpendicular to the viewer only in exceptional cases. For this reason, a wet road surface that is only illuminated by the vehicle's own headlights appears darker at night than the diffuse backscattering surface of a dry road surface.

application

Retroreflectors for bicycles for visibility and safety
Outdated traffic sign, which no longer reflects at night

In radio technology, radar reflectors support the location of irradiated objects, such as a balloon probe or a bridge pillar on a waterway.

Precise beam return in the optical wavelength range: Cube-corner behind ground glass, a laser serves as a turning point when measuring. A retroreflective sheeting or a reflector is used on a reflex light barrier to redirect the beam back to the sensor immediately next to the light source.

In road traffic, retroreflectors on people, obstacles, traffic signs, guidance systems and vehicles increase their visibility at night in the headlights. Vehicle drivers look close (around 1 m) to the light exit point at the scene in its light cone. Retroreflective material throws light back against the direction of incidence. If the driver sees a retroreflector at a distance of 60 m, the illuminating beam and the direction of observation span an angle of about 1 °. Illuminated retroreflective media therefore work best when they have a certain small amount Angular scattering have, but also illuminated at an angle - like a traffic sign half right 45 ° in front of us - still work well. These two parameters are used to characterize them.

In white or colored versions, there are reflectors made of transparent plastic injection molding or glass, which have a structure made up of many cube corners at the back. They do not have to be mirrored, since total reflection takes place and thus triple mirrors are created. These elements are also often referred to as cat eyes.

Retroreflective foils are made of embossed aluminum foil, plastic foil embossed on the back or contain transparent, retroreflective beads.

Cat eyes are made of glass bodies (biconvex, silver mirrored and coated with protective coating).[1]

Dense dewdrop coating or hoar frost can dissipate or prevent retroreflection; Motorway signposts, for example, show large dark spots.

Some bicycle headlights have a retroreflective zone integrated on the edge of the lens, the facets of which are designed in such a way that light coming at an angle from the back of the lamp penetrates well.

In security applications, retroreflective materials should be combined with diffusely reflective surfaces so that they are also recognizable when they are irradiated with extraneous light from other directions. For this reason, delineators not only have reflectors, but are also colored white. Even aluminum, often extruded and anodized, as a traffic signpost or railing appears light as long as it is not severely corroded by salt.

The finest glass beads (d <0.3 mm) partially embedded in the applied marking paint (lacquer or liquid plastic) by sprinkling it on, whereby retroreflection is achieved in order to increase the visibility when illuminated. The effect of the glass beads is similar to that of the Lüneburg lens.

In nature, retroreflection occurs on dewy plants, such as grass and stalks of grain, if they are very hairy. The effect becomes visible as a so-called “halo” when cycling on the side of the road around the shadow of your own head in the green, when the sun is medium-high to the left behind you. The phenomenon also occurs on hairless plants if their leaves are so strongly hydrophobic due to a wax layer that the contact angle on the dewdrop increases to 140 °, which Alistair B. Fraser observed on conifers and therefore sylvanshine (English forest shine) was called.[2]

Conversely, designers of military vehicles, ships and especially aircraft try to avoid unwanted radar reflections by strictly avoiding inside corners on their outer contour (stealth technology).

Retroreflectors made of quartz glass (to also reflect UV light components) are used in long-path measurements based on DOAS technology in order to detect atmospheric trace gases in the air on a defined light path.

Other effects

  • glory (engl.)[3][4]
  • gloriole
  • Opposition effect: the full moon appears much brighter than the crescent moon per area, a dusty road appears brightest around the opposite point of the light source behind you, and (dry) meadows, fields and forests appear significantly lighter around your own (sun) shadow. Rough structures cast shadows on themselves. Viewed only from the perspective of the direction of illumination, however, these shadows are covered by the illuminated surfaces.[5]

Retroreflective elements

Comparison of corner reflector (1) and lens reflector (2) at three different angles of incidence. The reflective surface is shown in dark blue.

In addition to plan-optic corner reflectors (triple mirrors and triple prisms) and retro reflectors, there are rotationally symmetrical lens reflectors (cat's eyes, Lüneburg lenses) and, in principle, other types of retroreflective bodies, for example biconic constructions.

Versions with plane mirrors

Illustration of the retroreflection on the 90 ° double mirror

In the case of retroreflectors made of flat mirrors and flat surfaces, a distinction is made between designs with two or three mutually perpendicular reflective levels.

The picture above illustrates the retroreflection in one plane (two mirrors). If the third spatial direction can also change, three mirrors are required, a triple mirror is created, for the principle see also corner reflector.

A Triple prism is a glass body that is flat at the front and has three non-mirrored flat surfaces at an angle of 90 ° to each other on the back. In principle, it even reflects with less loss than a triple mirror, even if the front surface is not anti-reflective. The cause is the loss-free total reflection on the sloping rear surfaces. Triple prisms have a larger angular range within which the reflection occurs, since the front surface of the glass body causes a refraction towards the axis of symmetry.

Examples:

  • Reflectors are usually made of plastic, more rarely (earlier) also made of glass. They have triple prisms molded on the back, which function like triple mirrors through total reflection. They are covered at the back to avoid reflection losses due to dirt and (condensation) water, which is why the triple tips can only be felt on broken, open reflectors and visible from behind.
  • Corner reflectors as
    • Radar reflectors corner reflector) made of sheet metal as a retroreflector for microwave radiation (RADAR), e.g. for shipping in front of bridge piers.
    • Triple mirrors and triple prisms made of glass for retroreflecting laser beams (LIDAR) when surveying, measuring the distance to the moon and for precisely determining the position of satellites
  • Glass reflective spheres (poured as a fine powder) are pressed into freshly applied road marking paint, preferably about half of the diameter, but not too dense so as not to shade (Retroreflective paint).

Lens-like design

Retroreflector element based on the principle of a cat's eye; the curved surfaces on the left are mirrored. The right curved surface has its focal point on the mirror surface. The drawings illustrate the function with incident light from different angles

If there is a reflective surface in the focus of an imaging optics, the reflected light is directed back towards the light source by the optics. Unlike a simple flat mirror, this property does not depend on the exact orientation of the reflecting surface. For an ideal retroreflection, however, the distance between the reflective surface must be exactly right. In addition, lens defects mean that the light is not completely directed towards the light source. In some applications, reflection in an area close to, but not exactly at, the light source is even desirable. This applies, for example, to retroreflectors in road traffic. In order for the reflected light from a headlamp to be seen, it must not be completely directed back into the headlamp.

Examples:

  • The optically effective component of reflector foils and canvases consists of many small transparent spheres. A clear ball made of glass or plastic focuses much of the incident light from a distant light source on a spot just behind the rear surface. Due to the difference in the refractive index compared to air, the rear surface of the sphere has a reflective effect. 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, a particularly large amount of light from the film projector reaches the eyes of the audience in the cinema. The same applies to headlights, which are preferably directed in the direction of the driver of the respective vehicle by signs equipped with reflector foil.
  • Cat eyes are glass bodies, the front of which is curved in such a way that incident parallel light is focused on the mirrored back. In contrast to transparent spheres, with cat's eyes the solid angle into which the incident is reflected back can be determined within wide limits by the shape. In addition, the back can be fully reflective. Cat eyes can therefore direct more light in the desired direction. However, they are more complex to manufacture.
  • The eyes of nocturnal animals such as cats in particular are retroreflective because their retina has a reflective background. See tapetum lucidum and red-eye effect
  • Retroreflection due to fog droplets is undesirable, which is why fog lights are arranged as far away from the line of vision as possible.
  • Lüneburg lenses are spheres mirrored on the back and made of a transparent material with a refractive index that decreases towards the inside. They are also used as radar reflectors. The mirroring is then designed as a belt, so that retroreflection takes place from all horizontal directions.

Illustrations

  • Triple prism at a measurement target (measurement), recording with camera flash

  • Photo of a triple mirror with and without flash; three of the six edges are reflections; the eye or the camera appears always in the centre

  • Retroreflector mounted on a delineator post on the side of the road

  • Reflectors embedded in a street

  • One of five retroreflectors located on the moon for measuring distances

Individual evidence

  1. Crystal Glass Reflectors. Swareflex.com, accessed August 28, 2013
  2. ^ Alistair B. Fraser: The sylvanshine: retroreflection from dew-covered trees. In: Applied Optics. Volume 33, No. 21, July 20, 1994, pp. 4539-4547, doi: 10.1364 / AO.33.004539.
  3. ↑ H. Moysés Nussenzveig: The Science of the Glory. Scientific American, December 30, 2011, accessed August 29, 2013
  4. ↑ Lawrence J. Mayes: Glories - an Atmospheric Phenomenon. January 9, 2003, accessed August 29, 2013
  5. Opposition Effect, Atmospheric Optics. atoptics.co.uk, accessed August 29, 2013