Miniature micro prism retroreflector

DETAILED DESCRIPTION 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.
degree.
, 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.
degree.
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.
degree.
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