Optical labels are used for a variety of purposes such as identification of labeled objects and display of information. One example of an optical label is an optical bar code in which the information carried in the label is represented as a spatial pattern of dark and bright areas. The dark areas do not reflect light and the bright areas do reflect light. Another example of an optical label is a colored decal in which information is represented as a spatial pattern of areas that reflect certain colors of light. The information carried in an optical label has some permanence although that information (such as the pattern of dark and bright areas or the colored pattern) can fade with time or from exposure of the label to the surrounding environment. An optical label generally is not programmable.
The concept of enhancing retro-reflection by placing a reflecting surface at the focal plane of a lens bead is well known. See, for example, A. B. Fraser, “The sylvanshine: retroreflection from dew-covered trees,” Applied Optics, vol. 33, n. 21 (1994), pp. 4539-4547.
Photochromic materials were investigated in the past for optical memory and display applications, however, they have not generally been considered for wavelength coded optical labels. Prior art optical structures with photochromic materials have been used for fixed-response optical labels; however, they have not been used for programmable and rewritable optical memory. Furthermore, the prior art optical structures with photochromic materials contained therein have not been combined with retro-reflective structures. Lens beads have been incorporated into prior retro-reflective sheets that have been used for highway safety signs, wearable safety products and displays; however, they have not been used for programmable and rewriteable optical memory.
Spherical retro-reflectors have been combined with a layer of coating that comprises conventional reflective metals such as aluminum, tin and chromium. The coating is situated on the focal plane of the spherical beads to produce retro-reflection result. See, U.S. Pat. No. 2,963,378.
Some retro-reflective sheets are fabricated to comprise a plurality of lens beads, a spacer layer and a reflective layer located at the focal plane of each plurality of the lens beads. See, U.S. Pat. No. 4,367,920. Only conventional reflective materials such as aluminum, silver and chromium are used for the reflective layer.
Another patent describes using a multi-layer dielectric mirror instead of a specular reflecting material in a beaded retro-reflective structure. The dielectric mirror comprises a multi-layer quarter-wave construction, which is known to form a wavelength selective reflection peak. See U.S. Pat. No. 3,700,305. Multi-layer wavelength selective structures have been used for fixed-response optical labels but they have not been associated with rewriteable optical memory. The programmable reflective structure of the present invention, like this prior patent, includes a multi-layer quarter-wave reflection filter. However, the present invention also comprises Fabry Perot transmission filters. Furthermore, any light transmitted through such a multi-layer dielectric reflector will be diffusely reflected by the underlying material.
A paper describes a combination of optical spheres covered with multiple layers of material for scattering incident light in a specific wavelength range. See, Y. Liu, K. Chen, Y. L. Kim, G. Ameer and V. Backman, “Multilayer resonant light scattering nanoshells as a novel class of nonbleaching labels for multi-marker molecular imaging,” SPIE Proceedings, v. 5326 (2004), pp. 73-81. However, the spheres do not contain materials that can be programmed or altered in real-time such that the specific wavelength scattered is modified.
Another paper describes a plurality of lens beads combined with a photographic emulsion used for permanently recording an optical image in the photographic emulsion. See, C. B. Burckhardt and E. T. Doherty, “Beaded plate recording of integral photographs,” Applied Optics, v. 8, n. 11, (1959), pp. 2329-2331. However, this combination does not have the capability to erase and rerecord a different image. Instead, the photographic emulsion is a permanent recording medium.
For the foregoing reasons, there is a need for optical labels that are programmable and rewriteable so that the information they carry can be changed. There also is a need for programmable optical labels to retain their programmed state until the next time those labels are re-programmed to the same state or to another state.
There also is a need for retro-reflective optical labels. A retro-reflective label reflects incident illumination that it receives from a light source back toward the area of that light source. Retro-reflective labels typically can be viewed from larger distances than labels whose reflection are specular or disperse, if a viewer also illuminates the label. A retro-reflective label also may provide for some degree of privacy since only viewers who also illuminate the label can see the information represented in the label.
Further, there is also a need for a method to label objects with rough, uneven, or discontinuous surfaces.