Optical: systems and elements – Signal reflector – Including a curved refracting surface
Patent
1996-02-05
1998-06-30
Phan, James
Optical: systems and elements
Signal reflector
Including a curved refracting surface
359547, 359551, 404 12, 404 14, G02B 5128
Patent
active
057742650
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to retroreflective elements such as can be placed in pavement markings to guide and direct motorists traveling on a roadway.
BACKGROUND OF THE INVENTION
The use of pavement markings (e.g., paints, tapes, and individually mounted articles) to guide and direct motorists traveling along a roadway is well known. During the daytime the markings may be sufficiently visible under ambient light to effectively signal and guide a motorist. At night, however, especially when the primary source of illumination is the motorist's vehicle headlights, the markings are generally insufficient to adequately guide a motorist because the light from the headlight hits the pavement and marking at a very low angle of incidence and is largely reflected away from the motorist. For this reason, improved pavement markings with retroreflective properties have been employed.
Retroreflection describes the mechanism where light incident on a surface is reflected so that much of the incident beam is directed back towards its source. The most common retroreflective pavement markings, such as lane lines on roadways, are made by dropping transparent glass or ceramic optical elements onto a freshly painted line such that the optical elements become partially embedded therein. The transparent optical elements each act as a spherical lens and thus, the incident light passes through the optical elements to the base paint or sheet striking pigment particles therein. The pigment particles scatter the light redirecting a portion of the light back into the optical element such that a portion is then redirected back towards the light source.
In addition to providing the desired optical effects, pavement markings must withstand road traffic and weathering, adverse weather conditions, and cost constraints.
Vertical surfaces provide better orientation for retroreflection; therefore, numerous attempts have been made to incorporate vertical surfaces in pavement markings, typically by providing protrusions in the marking surface. In addition, vertical surfaces may prevent the build-up of a layer of water over the retroreflective surface during rainy weather which otherwise interferes with the retroreflection mechanism.
One means of providing vertical surfaces is to place raised pavement markers at intervals along a pavement marking line (e.g., U.S. Pat. Nos. 3,292,507; 4,875,798). These markers are relatively large, generally several centimeters in width and 5 to 20 millimeters in height. Typically, the markers require assembling together different components, some of which were previously individually molded or casted. Therefore, the markers are relatively expensive to manufacture. The size of the markers subjects them to substantial impact forces from passing vehicles. As a result, the markers must be substantially secured to the pavement, increasing the installation costs and removal costs when they wear out. Moreover, because the markers are applied at intervals, the bright spots of light are discontinuous, rather than the desired continuous bright line.
Embossed pavement marking tapes are a second means of providing vertical surfaces (e.g., U.S. Pat. Nos. 4,388,359, 4,069,281, and 5,417,515). Selective placement of transparent optical elements on the vertical sides of the embossed protrusions results in a highly effective marking material. However, such tapes are relatively expensive compared to conventional painted markings, and thus their use is often limited to critical areas such as unlighted intersections and railway crossings. Also, these embossed tapes are constructed with polymeric materials which are susceptible to wear.
A third means of providing vertical surfaces for retroreflection is a composite retroreflective element or aggregate (e.g., U.S. Pat. Nos. 3,254,563, 4,983,458). Many variations are known, but the retroreflective elements essentially have a core with optical elements embedded in the core surface. Some known embodiments also contain optical elements dispersed throughout
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Forester Thomas E.
Goodbrake Chris J.
Hachey Kathleen A.
Hedblom Thomas P.
Humpal Kathleen M.
Fagan Lisa M.
Minnesota Mining and Manufacturing Company
Phan James
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