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84-491: The surface of this type of vibrating coating line is distributed and scattered with raised bumps. Some bumps are coated with high-refractive-index glass beads . When a speeding vehicle runs over the raised road lines, it produces a strong warning vibration to remind the car driver of deviation from the lane. Perpendicular to driving directions, these marking lines are used for settled mainline toll plaza, ramp entrances, mountainous areas, continuous sharp turns, downhill sections and

168-512: A "time-reversed" wave is generated by the PCM with temporal encoding of the phase-conjugate wave (see, e.g., SciAm, Oct. 1990, "The Photorefractive Effect," David M. Pepper, et al. ). Inexpensive corner-aiming retroreflectors are used in user-controlled technology as optical datalink devices. Aiming is done at night, and the necessary retroreflector area depends on aiming distance and ambient lighting from street lamps. The optical receiver itself behaves as

252-450: A bubble in the center of the strand to serve as the hole in the bead. In Arekamedu this was accomplished by inserting a hollow metal tube into the ball of hot glass and pulling the glass strand out around it, to form a continuous glass tube. In the Venetian bead industry, molten glass was gathered on the end of a tool called a puntile ("puntying up"), a bubble was incorporated into the center of

336-420: A form of raised pavement marker but unlike most such markers, delineators are not supposed to be hit except by out-of-control or drifting vehicles. Unlike their smaller cousins, delineators are tall enough to impact not only a vehicle's tires but the vehicle body itself. They usually contain one or more retroreflective strips. They can be round and open in the center or curved (45-degree sections) of plastic with

420-414: A gather of molten glass, and a second puntile was attached before stretching the gather with its internal bubble into a long cane. The pulling was a skilled process, and canes were reportedly drawn to lengths up to 200 feet (61 m) long. The drawn tube was then chopped, producing individual drawn beads from its slices. The resulting beads were cooked or rolled in hot sand to round the edges without melting

504-434: A hole. The beads again are rolled in hot sand to remove flashing and soften seam lines. By making canes (the glass rods fed into the machine) striped or otherwise patterned, the resulting beads can be more elaborately colored than seed beads. One "feed" of a hot rod might result in 10–20 beads, and a single operator can make thousands in a day. Glass beads are also manufactured or moulded using a rotary machine where molten glass

588-403: A passenger automobile. As the observation angle increases, the reflector performance decreases. For example, a truck has a large separation between the headlight and the driver's eye compared to a passenger vehicle. A bicycle reflector appears brighter to the passenger car driver than to the truck driver at the same distance from the vehicle to the reflector. The light beam and the normal axis of

672-543: A patent pending, and then submitted the white epoxy-resin reflective raised pavement markers to the California Bureau of Highways, the predecessor to the California Department of Transportation , now known as Caltrans. In areas where snowplowing is frequent, conventional markers are placed in a shallow groove cut in the pavement, or specially designed markers are used which include a protective metal casting that

756-511: A point. Two prisms may also serve as targets for angle measurements , using total stations or simpler theodolites ; this usage, reminiscent of the heliotrope , does not involve retroreflection per se, it only requires visibility by means of any source of illumination (such as the sun) for direct sighting to the center of the target prism as seen from the optical instrument. Astronauts on the Apollo 11 , 14 , and 15 missions left retroreflectors on

840-417: A quieter tool and less dirty flame. Also unlike metalworking, the torch is fixed, and the bead and glass move in the flame. American torches are usually mounted at about a 45-degree angle, a result of scientific glassblowing heritage; Japanese torches are recessed, and have flames coming straight up, like a large bunsen burner; Czech production torches tend to be positioned nearly horizontally. Dichroic glass

924-442: A reflective strip. They are also used in low reflective markers in a "T" shape. They can also be used to indicate lane closures as in cases where the number of lanes is reduced. Delineators are often used in snow-prone areas in lieu of raised pavement markings as the latter can be easily dislodged from the road surface by snowplows. In addition to marking the road edge, delineators are used to mark ramp gores, medians, and objects near

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1008-461: A retroreflector—usually referred to as a prism —is normally attached on a surveying pole and is used as a target for distance measurement , for example, a total station . The instrument operator or robot aims a laser beam at the retroreflector. The instrument measures the propagation time of the light and converts it to a distance. Prisms are used with survey and 3D point monitoring systems to measure changes in horizontal and vertical position of

1092-519: A series of scientific research satellites designed to provide an orbiting laser ranging benchmark for geodynamical studies of the Earth. There are two LAGEOS spacecraft: LAGEOS-1 (launched in 1976), and LAGEOS-2 (launched in 1992). They use cube-corner retroreflectors made of fused silica glass. As of 2020, both LAGEOS spacecraft are still in service. Three STARSHINE satellites equipped with retroreflectors were launched beginning in 1999. The LARES satellite

1176-536: A thin sheet or in paint. In the case of paint containing glass beads, the paint adheres the beads to the surface where retroreflection is required and the beads protrude, their diameter being about twice the thickness of the paint. A third, much less common way of producing a retroreflector is to use the nonlinear optical phenomenon of phase conjugation . This technique is used in advanced optical systems such as high-power lasers and optical transmission lines . Phase-conjugate mirrors reflect an incoming wave so that

1260-525: A type of Luneburg lens , was developed and manufactured by the Institute for Precision Instrument Engineering (IPIE) in Moscow. The mission was interrupted in 2013 after a collision with space debris . Modulated retroreflectors, in which the reflectance is changed over time by some means, are the subject of research and development for free-space optical communications networks. The basic concept of such systems

1344-533: A variety of colors. They have enjoyed widespread usage in the British Isles and elsewhere around the world. Nonreflective raised pavement markers (also known as Botts' dots) are usually round, are white or yellow, and are frequently used on highways and interstates in lieu of painted lines. They are glued to the road surface with epoxy and as such are not suitable in areas where snow plowing is conducted. They are usually made out of plastic or ceramic materials. In

1428-481: A very special shape: the upper half is a dome shape, and the lower half is a base covered with a reflective layer. They come in a variety of colors and sizes to meet the requirements of different applications. Currently they are widely adopted in Taiwan. Cat's eyes, in their original form, consist of two pairs of retroreflective glass spheres set into a white rubber dome, mounted in a cast-iron housing. They generally come in

1512-470: A weak retroreflector because it contains a large, precisely focused lens that detects illuminated objects in its focal plane. This allows aiming without a retroreflector for short ranges. Retroreflectors are used in the following example applications: Many prey and predator animals have naturally retroreflective eyes by having a reflective layer called the Tapetum lucidum behind the retina, since this doubles

1596-448: Is a device or surface that reflects radiation (usually light ) back to its source with minimum scattering . This works at a wide range of angle of incidence , unlike a planar mirror , which does this only if the mirror is exactly perpendicular to the wave front, having a zero angle of incidence. Being directed, the retroflector's reflection is brighter than that of a diffuse reflector . Corner reflectors and cat's eye reflectors are

1680-399: Is a vehicle's headlights, where the light is sent to the traffic sign face and then returned to the vehicle operator. Retroreflective traffic sign faces are manufactured with glass beads or prismatic reflectors embedded in a base sheeting layer so that the face reflects light, therefore making the sign appear more bright and visible to the vehicle operator under darkened conditions. According to

1764-557: Is called marvering, a term derived from the French marbrer , 'to marble'. It can also be pressed into a mold in its molten state. While still hot, or after re-heating, the surface of the bead may be decorated with fine rods of colored glass called stringers creating a type of lampwork bead. The drawing of glass is also ancient. Evidence of large-scale drawn-glass bead making has been found by archeologists in India, at sites like Arekamedu dating to

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1848-555: Is embedded in recesses in the pavement, allowing the marker to protrude slightly above the pavement surface for increased visibility, much like a cat's eye . Lit LED in-pavement Raised Pavement Markings (RPMs), help improve road safety, as they are more generally visible than reflective RPM markings as they are internally lit and don't require headlights to show up. And, whilst the RPMs are usually raised, being so visible, in snowplow areas, these types of RPMs can be installed flush-mounted within

1932-435: Is fed in to the centre of a rotary mould and solid or hollow glass beads are formed. The Bohemian glass industry was known for its ability to copy more expensive beads, and produced molded glass "lion's teeth", "coral", and "shells", which were popular in the 19th and early 20th century Africa trade. A variant of the wound glass bead making technique, and a labor-intensive one, is what is traditionally called lampworking . In

2016-515: Is generally held in place using butyl pads , epoxy glue, or bitumen . In areas with little snowfall, reflective raised pavement markers are applied directly on top of the road surface. The device's retroreflective surface enables the device to be clearly visible at long distances at night and in rainy weather. The devices come in multiple colors which vary in usage depending on local traffic marking standards. In 1965 San Diego Police Motorcycle Officer Kenneth Grant Maine, improved upon, applied for

2100-616: Is more helpful as in a wider variety of weather conditions and light conditions. Some lit linear visual aids / RPMs, melt snow, without needing any additional heating elements, as such, these types or self-cleaning RPMs are more energy-efficient compared to those that need additional heating elements to melt snow so are NOT self-cleaning. Linear visual aids are also more useful since they can be used for creating in-pavement signs and messages such as lit merge arrows. Whilst point source RPMs could be used for such signage, in practice, because they are installed so close together and much deeper within

2184-878: Is recognized and recommended by the International Convention for the Safety of Life at Sea ( SOLAS ) because of its high reflectivity of both light and radar signals. Application to life rafts , personal flotation devices, and other safety gear makes it easy to locate people and objects in the water at night. When applied to boat surfaces it creates a larger radar signature —particularly for fiberglass boats, which produce very little radar reflection on their own. It conforms to International Maritime Organization regulation, IMO Res. A.658 (16) and meets U.S. Coast Guard specification 46 CFR Part 164, Subpart 164.018/5/0. Examples of commercially available products are 3M part numbers 3150A and 6750I, and Orafol Oralite FD1403. In surveying ,

2268-412: Is sometimes preferable to have an imperfect, slightly divergent retroreflection, as in the case of road signs, where the illumination and observation angles are different. Due to spherical aberration , there also exists a radius from the centerline at which incident rays are focused at the center of the rear surface of the sphere. Finally, high index materials have higher Fresnel reflection coefficients, so

2352-413: Is that a low-power remote system, such as a sensor mote, can receive an optical signal from a base station and reflect the modulated signal back to the base station. Since the base station supplies the optical power, this allows the remote system to communicate without excessive power consumption. Modulated retroreflectors also exist in the form of modulated phase-conjugate mirrors (PCMs). In the latter case,

2436-402: Is used on road surfaces, road signs , vehicles , and clothing (large parts of the surface of special safety clothing , less on regular coats). When the headlights of a car illuminate a retroreflective surface, the reflected light is directed towards the car and its driver (rather than in all directions as with diffuse reflection ). However, a pedestrian can see retroreflective surfaces in

2520-628: Is used to produce high-end art beads. Dichroic glass has a thin film of metal fused to the surface of the glass, resulting in a surface that has a metallic sheen that changes between two colors when viewed at different angles. Beads can be pressed, or made with traditional lampworking techniques. If the glass is kept in the flame too long, the metallic coating will turn silver and burn off. Italian glass blowing techniques, such as latticinio and zanfirico , have been adapted make beads. Furnace glass uses large decorated canes built up out of smaller canes, encased in clear glass and then extruded to form

2604-638: The Moon as part of the Lunar Laser Ranging Experiment . The Soviet Lunokhod 1 and Lunokhod 2 rovers also carried smaller arrays. Reflected signals were initially received from Lunokhod 1 , but no return signals were detected from 1971 until 2010, at least in part due to some uncertainty in its location on the Moon. In 2010, it was found in Lunar Reconnaissance Orbiter photographs and

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2688-646: The United Kingdom and other parts of the world. They were invented in the United Kingdom in 1933 by Percy Shaw and patented in 1934 (UK patents 436,290 and 457,536), and the United States in 1939 (US patent 2,146,359). On March 15, 1935, Shaw founded Reflecting Roadstuds Ltd, which became the first manufacturer of raised pavement markers. The plastic markers now used widely throughout the United States and elsewhere appeared three decades later. They were originally invented by American engineer Sidney A. Heenan in

2772-405: The refractive index of the material is exactly one plus the refractive index n i of the medium from which the radiation is incident (n i is around 1 for air). In that case, the sphere surface behaves as a concave spherical mirror with the required curvature for retroreflection. In practice, the optimal index of refraction may be lower than n i + 1 ≅ 2 due to several factors. For one, it

2856-449: The 2nd century CE. The small drawn beads made by that industry have been called Indo-Pacific beads , because they may have been the single most widely traded item in history—found from the islands of the Pacific to Great Zimbabwe in southern Africa. There are several methods for making drawn beads, but they all involve pulling a strand out of a gather of glass in such a way as to incorporate

2940-766: The American design. Countries formerly part of the British Empire are likely to retain a modified version of the British laws including the basic principles regarding road safety. Hong Kong's raised marker colors are identical to those used in the United Kingdom. While Australian designs generally follow those in the US, the colors generally follow European usage. Differences from European usage include: For countries in Latin America,

3024-462: The U.S. are made with retroreflective sheeting materials, they degrade over time. Until now, there has been little information available to determine how long the retroreflectivity lasts. The MUTCD now requires that agencies maintain traffic signs to a set of minimum levels but provide a variety of maintenance methods that agencies can use for compliance. The minimum retroreflectivity requirements do not imply that an agency must measure every sign. Rather,

3108-491: The UK, the area in which pedestrians should cross at pelican crossings is marked out by a series of markers. Usually, these are painted as squares on the road but occasionally a metal stud is used. These are usually square and made from unpainted steel or aluminum. Delineators are tall pylons (similar to traffic cones or bollards ) mounted on the road surface, or along the edge of a road, and are used to channelize traffic. These are

3192-547: The United States Federal Highway Administration (FHWA): Approximately 24% of all vehicle accidents occur during adverse weather (rain, sleet, snow and fog). Rain conditions account for 47% of weather-related accidents. These statistics are based on 14-year averages from 1995 to 2008. The FHWA's Manual on Uniform Traffic Control Devices requires that signs be either illuminated or made with retroreflective sheeting materials, and though most signs in

3276-635: The United States National Highway Traffic Safety Administration (NHTSA), the Traffic Safety Facts 2000 publication states the fatal crash rate is 3-4 times more likely during nighttime crashes than daytime incidents. A misconception many people have is that retroreflectivity is only important during night-time travel. However, in recent years, more states and agencies require that headlights be turned on in inclement weather such as rain and snow. According to

3360-411: The United States, as it is not affected by the passage of snowplows and does not affect the interior of the roadway. Where weather permits, embedded or raised retroreflectors are preferred as they last much longer than road paint, which is weathered by the elements, can be obscured by sediment or rain, and is ground away by the passage of vehicles. For traffic signs and vehicle operators, the light source

3444-453: The Venetian industry, where very large quantities of beads were produced in the 19th century for the African trade, the core of a decorated bead was produced from molten glass at furnace temperatures, a large-scale industrial process dominated by men. The delicate multicolored decoration was then added by people, mostly women, working at home using an oil lamp or spirit lamp to re-heat the cores and

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3528-403: The above rules are: Colors can also be combined, with a different color facing each direction: The current trend for lane markings is to intersperse retroreflective paint lines with reflectors as seen on the majority of American highways. This scheme only applies to the US and Canada. Mexico, on the other hand, generally follows European usage. If marked on the road, the color should match

3612-422: The bead maker usually grinds from commercially available glass seed beads and recycled glass. Molded ground glass, if painted into the mold, is called pate de verre, and the technique can be used to make beads, though pendants and cabochons are more typical. Lampwork (and other) beads can be painted with glass paints. Retroreflector A retroreflector (sometimes called a retroflector or cataphote )

3696-413: The bead making process, the bead can be further fired in a kiln to make it more durable. Modern bead makers use single or dual fuel torches, hence the more modern term flameworked . Unlike a metalworking torch, or burner, a flameworking torch is usually "surface mix"; that is, the oxygen and fuel (typically propane, though natural gas is also common) is mixed after it comes out of the torch, resulting in

3780-483: The beads with linear and twisting stripe patterns. No air is blown into the glass. These beads require a large scale glass furnace and annealing kiln for manufacture. Lead crystal beads are machine-cut and polished. Their high lead content makes them sparkle more than other glass, but also makes them inherently fragile. Lead glass (for neon signs) and, especially borosilicate is available in tubing, allowing for glass blown beads. ( Soda-lime glass can be blown at

3864-495: The cat's eye retroreflector uses a normal lens focused onto a curved mirror rather than a transparent sphere, though this type is much more limited in the range of incident angles that it retroreflects. The term cat's eye derives from the resemblance of the cat's eye retroreflector to the optical system that produces the well-known phenomenon of "glowing eyes" or eyeshine in cats and other vertebrates (which are only reflecting light, rather than actually glowing). The combination of

3948-559: The colors of raised markers vary by country to country. For most countries, they tend to generally follow the European counterparts rather than North American counterparts, but in several countries of Latin America, such as Mexico, the usage of blue follows the North American counterpart. Cat's eyes made out of metal were the earliest form of retroreflective pavement markers, and are in use in

4032-471: The composition of the glass beads could be analyzed and help archaeologists understand the sources of the beads. Glass beads are usually categorized by the method used to manipulate the glass – wound beads, drawn beads, and molded beads. There are composites, such as millefiori beads, where cross-sections of a drawn glass cane are applied to a wound glass core. A very minor industry in blown glass beads also existed in 19th-century Venice and France. Probably

4116-657: The course of his employment with the Stimsonite Corporation in Niles, Illinois . Heenan filed an application for a patent on October 23, 1964. Patent No. 3,332,327 was subsequently granted on July 25, 1967. Stimsonite went on to become the leading manufacturer of retroreflective raised pavement markers in the United States and was acquired in 1999 by Avery Dennison Corporation . For six years, Avery sold Stimsonite's line under its Sun Country brand. In 2006, Avery sold its raised pavement marker division to Ennis Paint, one of

4200-514: The dark only if there is a light source directly between them and the reflector (e.g., via a flashlight they carry) or directly behind them (e.g., via a car approaching from behind). " Cat's eyes " are a particular type of retroreflector embedded in the road surface and are used mostly in the UK and parts of the United States . Corner reflectors are better at sending the light back to the source over long distances, while spheres are better at sending

4284-400: The earliest beads of true glass were made by the winding method. Glass at a temperature high enough to make it workable, or "ductile", is laid down or wound around a steel wire or mandrel coated in a clay slip called "bead release". The wound bead, while still hot, may be further shaped by manipulating with graphite, wood, stainless steel, brass, tungsten or marble tools and paddles. This process

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4368-409: The efficiency of coupling of the light from the ambient into the sphere decreases as the index becomes higher. Commercial retroreflective beads thus vary in index from around 1.5 (common forms of glass) up to around 1.9 (commonly barium titanate glass). The spherical aberration problem with the spherical cat's eye can be solved in various ways, one being a spherically symmetrical index gradient within

4452-509: The end of a metal tube, or, more commonly wound on the mandrel to make a hollow bead, but the former is unusual and the latter not a true mouth-blown technique.) In addition, beads can be fused from sheet glass or using ground glass. Modern Ghana has an industry in beads molded from powdered glass. Also in Africa, Kiffa beads are made in Mauritania, historically by women, using powdered glass that

4536-542: The end of the highway (intersection of highway exit and the plane of the common roadway), gates and entrances of enterprises, institutions, and school. In the same direction of traffic driving direction, they are mainly settled in the median strip , edge lines, and dangerous sections of the road. In the United States , Canada , Mexico , some countries of South America , Thailand and Australia , these plastic devices commonly have two angled edges facing drivers and containing one or more corner reflector strips. The marker

4620-515: The eye's lens and the cornea form the refractive converging system, while the tapetum lucidum behind the retina forms the spherical concave mirror. Because the function of the eye is to form an image on the retina, an eye focused on a distant object has a focal surface that approximately follows the reflective tapetum lucidum structure, which is the condition required to form a good retroreflection. This type of retroreflector can consist of many small versions of these structures incorporated in

4704-424: The fine wisps of colored glass used to decorate them. These workers were paid on a piecework basis for the resulting lampwork beads. Modern lampwork beads are made by using a gas torch to heat a rod of glass and spinning the resulting thread around a metal rod covered in bead release. When the base bead has been formed, other colors of glass can be added to the surface to create many designs. After this initial stage of

4788-455: The first side, say x, the ray's x -component, a , is reversed to − a , while the y - and z -components are unchanged. Therefore, as the ray reflects first from side x then side y and finally from side z the ray direction goes from [ a , b , c ] to [− a , b , c ] to [− a , − b , c ] to [− a , − b , − c ] and it leaves the corner with all three components of its direction exactly reversed. Corner reflectors occur in two varieties. In

4872-456: The higher angle required to properly reflect the light. Raised reflectors are generally not used in areas that regularly experience snow during winter, as passing snowplows can tear them off the roadways. Stress on roadways caused by cars running over embedded objects also contributes to accelerated wear and pothole formation. Retroreflective road paint is thus very popular in Canada and parts of

4956-415: The holes closed; were sieved into sizes; and, usually, strung onto hanks for sale. The most common type of modern glass bead is the seed bead , a small type of bead typically less than 6 mm (0.24 in), traditionally monochrome, and manufactured in very large quantities. They are a modern example of mechanically drawn glass beads. Seed beads, so called due to their tiny, regular size, are produced in

5040-444: The key differences between these types is that lit linear RPMs, being linear, each lit unit provides both positional and directional guidance to viewers. For single-point source viewer directional guidance, a number of these units must all be viewed at the same time. In severe weather, such as fog or snow whiteouts, this may not be possible. Therefore, as each linear visual aid provides both positional and directional guidance, this type

5124-565: The largest manufacturers worldwide of paint for pavement markings (particularly lane markings). The company (based in Ennis, Texas ) changed its name to Ennis Traffic Safety Solutions and now markets the Stimsonite product line and descendants under the Stimsonite brand. Other manufacturers of retroreflective raised pavement markers sold in the United States under various designs include 3M, Apex Universal, Vialume, and Ray-O-Lite. Glass road studs have

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5208-427: The light that their retina receives. Inspired by the natural world, the inventor of road 'cat's eyes' was Percy Shaw of Boothtown , Halifax, West Yorkshire , England. When the tram-lines were removed in the nearby suburb of Ambler Thorn, he realised that he had been using the polished steel rails to navigate at night. The name "cat's eye" comes from Shaw's inspiration for the device: the eyeshine reflecting from

5292-420: The light to a receiver somewhat off-axis from the source, as when the light from headlights is reflected into the driver's eyes . Retroreflectors can be embedded in the road (level with the road surface), or they can be raised above the road surface. Raised reflectors are visible for very long distances (typically 0.5–1 kilometer or more), while sunken reflectors are visible only at very close ranges due to

5376-503: The mechanical accuracy of the construction. Retroreflectors are devices that operate by returning light back to the light source along the same light direction. The coefficient of luminous intensity, R I , is the measure of a reflector performance, which is defined as the ratio of the strength of the reflected light (luminous intensity) to the amount of light that falls on the reflector (normal illuminance). A reflector appears brighter as its R I value increases. The R I value of

5460-472: The modern day from machine-extruded glass. Seed beads vary in shape; though most are round, some, such as Miyuki delicas, resemble small tubes. Pressed or molded beads are associated with lower labour costs. These were commonly produced in the Czech Republic in the early 20th century. Thick glass rods are heated to molten and fed into a complex apparatus that stamps the glass, including a needle that pierces

5544-496: The more common form, the corner is literally the truncated corner of a cube of transparent material such as conventional optical glass. In this structure, the reflection is achieved either by total internal reflection or silvering of the outer cube surfaces. The second form uses mutually perpendicular flat mirrors bracketing an air space. These two types have similar optical properties. A large relatively thin retroreflector can be formed by combining many small corner reflectors, using

5628-410: The most used kinds. There are several ways to obtain retroreflection: A set of three mutually perpendicular reflective surfaces, placed to form the internal corner of a cube, work as a retroreflector. The three corresponding normal vectors of the corner's sides form a basis ( x , y , z ) in which to represent the direction of an arbitrary incoming ray, [ a , b , c ] . When the ray reflects from

5712-431: The new MUTCD language describes methods that agencies can use to maintain traffic sign retroreflectivity at or above the minimum levels. In Canada , aerodrome lighting can be replaced by appropriately colored retroreflectors, the most important of which are the white retroreflectors that delineate the runway edges, and must be seen by aircraft equipped with landing lights up to 2 nautical miles away. Retroflective tape

5796-434: The observation angle and entrance angle between the automobile's headlights, bicycle, and driver. The observation angle is the angle formed by the light beam and the driver's line of sight. Observation angle is a function of the distance between the headlights and the driver's eye, and the distance to the reflector. Traffic engineers use an observation angle of 0.2 degrees to simulate a reflector target about 800 feet in front of

5880-472: The observer when it is directly in line with the light source. The brightness of a reflector is also a function of the distance between the light source and the reflector. At a given observation angle, as the distance between the light source and the reflector decreases, the light that falls on the reflector increases. This increases the amount of light returned to the observer and the reflector appears brighter. Retroreflection (sometimes called retroflection)

5964-422: The oldest known beads dating over 3,000 years. Glass beads have been dated back to at least Roman times. Perhaps the earliest glass-like beads were Egyptian faience beads, a form of clay bead with a self-forming vitreous coating. Glass beads are significant in archaeology because the presence of glass beads often indicate that there was trade and that the bead making technology was being spread. In addition,

6048-493: The pavement compared to linear RPMs, such usage would tend to damage the road, especially within the asphalt roads. In almost all European countries, such markers will include reflective lenses of some kind. Most appear white or gray during daylight; the colors discussed here are the color of light they reflect. Because of their inconspicuousness during the day, they are always used in conjunction with painted retro-reflective lines; they are never seen on their own. The exception to

6132-454: The pavement so that they completely avoid the snowplows. There are a number of types, ranging from a single LED point source RPMs, with limited daytime visibilities, (road studs), to multi-LED linear type RPMs, visible in all full sunlight and nighttime applications. Some lit RPMs can be visible and useful in both bright sunlight as well as nighttime applications, whilst others are only visible at nighttime or in low light applications. One of

6216-610: The purpose according to European counterparts. Also, fluorescent yellow markers are used to indicate temporary lanes during roadworks on major roads in Japan. Two other markers are adopted for use in Japan, taken from North American usage: The design of the markers is diagonal. In Thailand, raised marker colors generally follow European usage but road markings follow the MUTCD. However, Thailand also adopted three uses of marker colors, one different from European usage: Thailand's raised markers use

6300-404: The reflected wave exactly follows the path it has previously taken, and require a comparatively expensive and complex apparatus, as well as large quantities of power (as nonlinear optical processes can be efficient only at high enough intensities). However, phase-conjugate mirrors have an inherently much greater accuracy in the direction of the retroreflection, which in passive elements is limited by

6384-448: The reflector as shown in Figure 2 form the entrance angle. The entrance angle is a function of the orientation of the reflector to the light source. For example, the entrance angle between an automobile approaching a bicycle at an intersection 90 degrees apart is larger than the entrance angle for a bicycle directly in front of an automobile on a straight road. The reflector appears brightest to

6468-427: The reflector is a function of the color, size, and condition of the reflector. Clear or white reflectors are the most efficient, and appear brighter than other colors. The surface area of the reflector is proportional to the R I value, which increases as the reflective surface increases. The R I value is also a function of the spatial geometry between the observer, light source, and reflector. Figures 1 and 2 show

6552-542: The retroreflectors have been used again. Lunokhod 2's array continues to return signals to Earth. Even under good viewing conditions, only a single reflected photon is received every few seconds. This makes the job of filtering laser-generated photons from naturally occurring photons challenging. Vikram lander of Chandrayaan-3 left Laser Retroreflector Array (LRA) instrument supplied by NASA 's Goddard Space Flight Center as part of international collaboration with ISRO . On 12 December 2023, Lunar Reconnaissance Orbiter

6636-522: The roadway such as fire hydrants. In such areas, the height of the delineators is established based on the typical amount of snowfall the area receives, ranging from 4 feet to more than 8 feet in height in areas with extreme snow depths. The name "delineator" is also used for reflective devices attached to other objects which are technically not pavement markers. [REDACTED] Media related to Raised pavement markers at Wikimedia Commons Glass beads Glass bead making has long traditions, with

6720-494: The sphere, such as in the Luneburg lens design. Practically, this can be approximated by a concentric sphere system. Because the back-side reflection for an uncoated sphere is imperfect, it is fairly common to add a metallic coating to the back half of retroreflective spheres to increase the reflectance, but this implies that the retroreflection only works when the sphere is oriented in a particular direction. An alternative form of

6804-433: The standard hexagonal tiling . Another common type of retroreflector consists of refracting optical elements with a reflective surface, arranged so that the focal surface of the refractive element coincides with the reflective surface, typically a transparent sphere and (optionally) a spherical mirror. In the paraxial approximation , this effect can be achieved with lowest divergence with a single transparent sphere when

6888-636: Was launched on February 13, 2012. (See also: List of laser ranging satellites .) Other satellites include retroreflectors for orbit calibration and orbit determination, such as in satellite navigation (e.g., all Galileo satellites , most GLONASS satellites , IRNSS satellites , BeiDou , QZSS , and two GPS satellites ) as well as in satellite gravimetry ( GOCE ) satellite altimetry (e.g., TOPEX/Poseidon , Sentinel-3 ). Retroreflectors can also be used for inter-satellite laser ranging instead of ground-tracking (e.g., GRACE-FO ). The BLITS (Ball Lens In The Space) spherical retroreflector satellite

6972-711: Was placed into orbit as part of a September 2009 Soyuz launch by the Federal Space Agency of Russia with the assistance of the International Laser Ranging Service , an independent body originally organized by the International Association of Geodesy , the International Astronomical Union , and international committees. The ILRS central bureau is located at the United States' Goddard Space Flight Center . The reflector,

7056-804: Was successfully able to detect transmitted laser pulses from Vikram lander. A similar device, the Laser Retroreflector Array (LaRA), has been incorporated in the Mars Perseverance rover . The retroreflector was designed by the National Institute for Nuclear Physics of Italy, which built the instrument on behalf of the Italian Space Agency . Many artificial satellites carry retroreflectors so they can be tracked from ground stations . Some satellites were built solely for laser ranging. LAGEOS , or Laser Geodynamics Satellites, are

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