Optical: systems and elements – Signal reflector – 3-corner retroreflective
Reexamination Certificate
1997-12-01
2001-03-27
Phan, James (Department: 2872)
Optical: systems and elements
Signal reflector
3-corner retroreflective
C359S529000
Reexamination Certificate
active
06206525
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention pertains to retroreflective materials and most particularly retroreflective material using micro cube corner prisms as the retroreflective elements.
Retroreflective materials are employed for various safety and decorative purposes. Particularly, these materials are useful at night time when visibility is important under low light conditions. With perfect retroreflective materials, light rays are reflected towards a light source in a substantially parallel path along an axis of retroreflectively. For many applications, perfect retroreflectivity is not required. Rather, a compromise is required in which a cone of divergent light is retroreflected which permits enough light to strike the viewer's eye, yet not so much that the intensity of the reflective light at the viewer's eye is unduly diminished. Under circumstances where the only source of illumination is the headlights of an automobile on an unlit road, the ability to retroreflect such a cone of divergence to the eye of the driver is important for safety reasons.
Many types of retroreflective material exist for various purposes. These retroreflective materials can be used as reflective tapes and patches for clothing, such as vests and belts. Also, retroreflective bands can be used on posts, barrels, traffic cone collars, highway signs, warning reflectors, etc. Retroreflective material may be comprised of arrays of randomly oriented micron diameter spheres or close packed cube-corner (prismatic) arrays.
Cube-corner or prismatic retroreflectors are described in U.S. Pat. No. 3,712,706, issued to Stamm (Jan. 23, 1973). Generally, the prisms are made by forming a master negative die on a flat surface of a metal plate or other suitable material. To form the cube-corners, three series of parallel equidistance intersecting V-shaped grooves 60 degrees apart are inscribed in the flat plate. The die is then used as a mold to form a transparent cube-corner array which is then processed into sheets of retroreflective material.
When the groove angle is 70 degrees, 31 minutes, 43.6 seconds, the angle formed by the intersection of two cube faces (the dihedral angle) is 90 degrees and the incident light is reflected back to the source. For automobile headlight reflectors, the dihedral angle is changed slightly so that the incidental light is reflected non-orthogonally towards the driver instead of the source.
Preferably, the retroreflected light from the vehicle headlights should be returned in a cone wide enough to encompass the eye of the vehicle's driver (this angle is referred to as the angle of observation).
At long distances the cone of light need only encompass two-tenths of a degree, but as the distance is decreased and/or as the distance from the head lamps to the eyes of the driver increase (as in the case of the driver of a large truck verses that of a sports car) then the cone of light should be increased to five-tenths or even one degree.
Many attempts have been made to keep the intensity of the retroreflected light uniform over this larger cone. Changing the dihedral angle of the cube corner prism will spread this cone of light, but in a star shaped pattern that is not uniform.
Diffraction of the light (see Stamm U.S. Pat. No. 3,712,706) by the small effective aperture of the cube corner prisms spreads the light, but again in a non-uniform manner with hot spots and nulls in decreasing intensity as the angle of the cone increases.
Mild diffusers have been tried such as texturing the front surface of the material or incorporating light scattering pigments or light refracting particles in a top coating on the front surface or in a top film. This technique scatters or redirects the light over much larger angles beyond the viewing cone so that much of the light is lost.
SUMMARY OF THE INVENTION
Diffraction scattering is most useful but has several drawbacks. Relatively small prisms in the size of 0.006″ to 0.12″ on centers which are air backed will diffract the light out into a cone of 0.5°, but the light pattern is not uniform. Furthermore, air backed prisms are troublesome and expensive. The reflecting faces of the prisms must be protected from contact with all other materials by constructing air cells in the backing materials. However, when the same size prisms are metalized, the diffraction is much reduced and will not sufficiently encompass the 0.5° angle.
We have found that if the cube corner prism pattern is ruled with prisms spaced in the range of 0.001″ to 0.003″ on center, and most preferably 0.002″ on center, and the resulting prisms are metalized, the retroreflected cone of light is spread out to include a 0.5° observation angle, and the intensity throughout the area is very uniform despite substantial change of the dihedral angle. This result is believed to occur because for very small prisms (0.001″ to 0.003″), diffraction effects spread or diverge the light over wide observation angles, and therefore a change in the dihedral angles of the prisms, such as may occur during master generation or product manufacturing, will have less impact on the change in the light distribution. The six overlapping return beams caused by diffraction (see
FIG. 4
of U.S. Pat. No. 5,171,624 issued Dec. 15, 1992, and incorporated in its entirety herein by reference) are diverging much more in the very small metallized prisms, so that as the dihedral angles change and the six beams move apart the central portion of the entire light distribution will retain light longer (at a greater dihedral angle) than with larger prisms, i.e., in excess of 0.003″.
The extreme cases are very large prisms that return six well collimated beams that do not overlap each other versus metalized very small prisms that return very divergent beams that overlap each other. A substantial dihedral angle change will cause the beams retroreflected from the large prism to completely separate from each other leaving a dark area in the center of the return beam. The same dihedral angle change in the small prisms will cause the beam spread to be the same, but because of the divergence caused by diffraction, the edges of the beams will still be overlapping, and a dark area will not occur.
The result will be a much safer product, because a dihedral angle change will not leave dark areas in the retroreflected light distribution.
REFERENCES:
patent: 3712706 (1973-01-01), Stamm
patent: 3810804 (1974-05-01), Rowland
patent: 4202600 (1980-05-01), Burke et al.
patent: 4243618 (1981-01-01), Van Arnam
patent: 5171624 (1992-12-01), Walter
patent: 5491586 (1996-02-01), Phillips
patent: 5508084 (1996-04-01), Reeves et al.
patent: 5558740 (1996-09-01), Bernard et al.
patent: 5565151 (1996-10-01), Nilsen
patent: 5780140 (1998-07-01), Nilsen
patent: 0 588 504 A1 (1994-03-01), None
patent: WO 94/09974 (1994-05-01), None
patent: WO 95/11466 (1995-04-01), None
patent: WO96/10148 (1996-04-01), None
patent: WO 97/21121 (1997-06-01), None
patent: WO 98/12581 (1998-03-01), None
Park, B.C., et al., “Polarization Properties of Cube-Corner Retroreflectors and Their Effects on Signal Strength and Nonlinearity in Heterodyne Interferometers,”Applied Optics, 35 (22): 4372-4380 (Aug. 1, 1996).
Nilsen Robert B.
Rowland William P.
Hamilton Brook Smith & Reynolds P.C.
Phan James
Reflexite Corporation
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