Optical: systems and elements – Signal reflector – 3-corner retroreflective
Patent
1996-06-05
1999-08-10
Phan, James
Optical: systems and elements
Signal reflector
3-corner retroreflective
359530, G02B 5122
Patent
active
059367695
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a reflecting sheet and a reflecting plate to be used for road signs, information boards, and reflectors. (In the present invention, the term "reflection" embraces "retroreflection".)
BACKGROUND ART
Road signs, information boards, and reflectors for safety and security are provided with reflecting sheets or plates (collectively referred to as "reflecting sheet" hereinafter), which employ glass beads or corner-cube prisms as the reflecting elements. Unfortunately, these reflecting elements have drawbacks in their reflecting characteristics as explained below. They do not meet industrial standards if they are used as such for reflecting sheets.
The disadvantage of glass beads as reflecting elements is that they do not accomplish complete reflection on account of spherical aberration inherent in spherical lens. FIG. 12 shows the optical paths of incident rays and reflected rays of a glass bead (3) whose focal length is equal to its curvature radius. Judging from the focal length, the luminous flux of all incident rays should converge at point P on the reflection surface of the bead (3). In actual, however, this is not the case due to spherical aberration, as shown in the diagram of light paths. The resulting reflection is diffuse reflection, with incident rays reflected in all directions. This is the reason for the low luminance of reflected rays from glass beads. Therefore, the conventional reflecting sheets do not fully meet requirements for luminance.
The disadvantage of corner-cube prisms as reflecting elements is that their reflection with high luminance is highly directional. This is due to the fact that incident rays are reflected twice in the prism and there is very little difference between the angle of incidence and the angle of reflection. In other words, the reflected rays from the prism are visible only for a person standing close to the light source, and the luminance of reflected rays is very low at the optical path deflected (by several degrees) from the light source. Moreover, the fact that the luminance of reflected rays is extremely high for a person standing near the light source makes corner-cube prisms unsuitable for use on information boards. (In other words, information boards with corner-cube prisms would be dazzling for observers near the light source.)
For this reason, reflecting sheets provided with corner-cube prisms as reflecting elements do not meet the industrial standards (such as JIS) which stipulate the luminance at different angles of observation, and hence they are seldom used for road signs and information boards.
It is an object of the present invention to provide a reflecting sheet which permits reflection with higher luminance than the conventional reflecting sheets with glass beads and which fully meets various industrial standards.
DISCLOSURE OF THE INVENTION
The first aspect of the present invention resides in a reflecting sheet which comprises a transparent sheet capable of reflection by its reverse, said transparent sheet having the face which is partly or entirely constructed of a plurality of contiguous cylindrical surfaces (1), each having the identical or different curvature radius, and also having the reverse which is constructed of a plurality of contiguous corner-cube prisms (2). Each cylindrical surface (1) corresponding to the plane of incidence of each corner-cube prism is inclined in one direction only, as shown in FIGS. 1 to 5.
The second aspect of the present invention resides in a reflecting sheet which comprises a transparent sheet (of synthetic resin) capable of reflection by its reverse, said transparent sheet having the face which is partly or entirely constructed of a plurality of contiguous composite surfaces (1), each being a combination of a cylindrical surface inclined in one direction only and a curved or flat surface inclined in the opposite direction (as shown in FIGS. 16 to 20), said composite surfaces (1) being constructed such that a large part of their surface area is accounted fo
REFERENCES:
patent: 2685231 (1954-12-01), Onksen
patent: 5237449 (1993-08-01), Nelson et al.
Japan Invention Spread Promotion Association Corporation
Nath Gary M.
Novick Harold L.
Phan James
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