Optical: systems and elements – Signal reflector – Including a curved refracting surface
Patent
1996-10-04
1998-07-07
Phan, James
Optical: systems and elements
Signal reflector
Including a curved refracting surface
359538, 359539, 359540, 359541, G02B 5128
Patent
active
057777900
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The present invention relates to microsphere-based retroreflective articles, particularly microsphere-based retroreflective articles wherein the microspheres are air-incident, i.e., exposed-lens or encapsulated-lens microsphere-based retroreflective constructions, and wherein the articles exhibit wide angularity of incidence and observation.
BACKGROUND
Microsphere-based retroreflective articles are well known and have been widely used for safety purposes, such as warning emblems on vehicle or hazard warnings on roadways, and for information purposes, such as traffic control and nagvigational signs. Retroreflective articles reflect incident light rays substantially back toward the light source as a cone of light; thus light emitted by headlights of a motor vehicle toward a sign with a retroreflective face will be reflected back toward the vehicle so as to be visible to the occupants of same.
Perhaps the most common form of such articles is retroreflective sheeting, often in flexible form, which is adhered to a substrate, such as an aluminum sign panel or the side of a vehicle. Some other illustrative uses of microsphere-based retroreflective products include pavement markings and coating compositions which can be used to form retroreflective coatings on desired surfaces.
Early microsphere-based retroreflective products were typically of the exposed-lens variety wherein microspheres, having air-incident front surfaces and hemispheric reflective layers disposed behind the rear surfaces thereof, were provided on a surface, e.g., a sign on the side of a road. U.S. Pat. No. 2,326,634 (Gebhard et al.) discloses such retroreflective articles. One drawback of exposed-lens constructions is that if the surface of the article, e.g., retroreflective sheeting on a road sign, becomes wet, such as by rainfall, the water which is deposited on the front surfaces of the microspheres interferes with desired retroreflection by the sheeting, thereby "blacking out" the affected sheeting. One solution is to provide a cover film disposed in front of the microspheres, typically in substantially no more than tangential contact therewith, so as to prevent deposition of water on the front of the microspheres while retaining their air-incident interface so as to maintaining the necessary optical relationships for retroreflection. Such retroreflective articles are sometimes referred to as "encapsulated-lens" type retroreflective articles. U.S. Pat. Nos. 3,190,178 (McKenzie) and 4,025,159 (McGrath) disclose such retroreflective articles.
U.S. Pat. No. 4,957,335 (Kuney) discloses that microsphere-based retroreflective articles having high retroreflective brightness at narrow divergence or observation angles, i.e., up to 0.5.degree. , may be made by selection of microspheres having defined combinations of average diameter and average refractive index. This patent teaches at column 4, lines 18-23, that variation in the size of the microspheres will increase the observation angle or divergence angle of the resultant retroreflective article.
U.S. Pat. No. 3,946,130 (Tung et al.) discloses that maximum retroreflectivity is obtained in a retroreflective product when all the microspheres in the product have the same predetermined index of refraction such that precise focusing of incident light rays on the reflective surface at the back of the microspheres is obtained.
Japan Unexamined Patent Application (Kokai) No. 4-263202 (to Toshiba Glass Co., Ltd.), filed Feb. 18, 1991, discloses a microsphere-based embedded-lens retroreflective sheet comprising a mixture of microspheres having a first refractive index and microspheres having a second refractive index. Such retroreflective sheeting would be expected to have relatively low retroreflective brightness as compared to exposed-lens and encapsulated-lens constructions and to exhibit poor, i.e., relatively narrow, entrance angle angularity because of its inherent configuration with a spacing layer between the microspheres and specularly reflective layer. For example, such shee
REFERENCES:
patent: 2326634 (1943-08-01), Gebhard et al.
patent: 3190178 (1965-06-01), McKenzie
patent: 3204537 (1965-09-01), Searight
patent: 3946130 (1976-03-01), Tung et al.
patent: 4025159 (1977-05-01), McGrath
patent: 4726134 (1988-02-01), Woltman
patent: 4896943 (1990-01-01), Tolliver et al.
patent: 4957335 (1990-09-01), Kuney, Jr.
patent: 5066098 (1991-11-01), Kult et al.
Patent Abstracts of Japan--Publication No. JP4263202, published Sep. 18, 1992.
Minnesota Mining and Manufacturing Company
Olson Peter L.
Phan James
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