Patent Publication Number: US-8526092-B2

Title: Programmable optical label

Description:
CROSS-REFERENCE 
     This application is a divisional application of U.S. application Ser. No. 11/932,370 filed Nov. 13, 2006. U.S. application Ser. No. 11/932,370 claims priority to U.S. Provisional Application No. 60865619 titled Programmable Optical Label filed on Nov. 13, 2006 and U.S. Provisional Application No. 60865608 titled “Optical Identification System and Method,” filed on Nov. 13, 2006 which are incorporated herein by reference in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     None 
     NAMES OF PARTIES TO JOINT RESEARCH AGREEMENT 
     None 
     BACKGROUND 
     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. 
     SUMMARY 
     The present invention is directed to a programmable optical label. In particular, the present invention combines the persistent but reprogrammable absorption, reflection and refractive index properties of photochromic materials with retro-reflecting optical structure and multi-layer optical coatings to construct a programmable optical label. 
     In one aspect, the present invention provides a retro-reflecting construct suitable for use as an optical label. The construct comprises a spherical lens with a first surface and a second surface. A multi-peak transmission filter layer is disposed adjacent to said second surface. An optional spacer layer is disposed between said second surface and said transmission filter. A programmable wavelength selective reflection layer, comprising a photochromic material, is disposed adjacent to said multi-peak transmission filter layer. Further, a black absorber layer is disposed adjacent to said selective reflection layer wherein all wavelengths of light reaching said black absorber are absorbed, whereby said retro-reflecting construct reflects specific wavelengths of light irradiating said construct. 
     In another aspect, the present invention provides another retro-reflecting construct suitable for use as an optical label. The construct comprises a spherical lens comprising a first surface and a second surface. A multi-peak transmission filter layer, comprising a birefringent material, is disposed adjacent to said first surface. A programmable wavelength selective reflection layer, comprising a photochromic material, is disposed adjacent to said second surface. An optional spacer layer is disposed between said second surface and said selective reflection layer. Further, a black absorber layer is disposed adjacent to said selective reflection layer wherein all wavelengths of light reaching said black absorber are absorbed, whereby said retro-reflecting construct reflects specific wavelengths of light irradiating said construct. 
     In another aspect, the present invention provides still another retro-reflecting construct suitable for use as an optical label. The construct comprises a corner cube reflector having a first surface and a second surface. A multi-peak transmission filter layer, comprising a birefringent material, is disposed adjacent to and preferably completely covering said second surface. An optional broadband reflector layer is disposed adjacent to said second surface not covered by said multi-peak transmission filter layer. A programmable wavelength selective reflection layer comprising a photochromic material and a birefringent material is disposed adjacent to said transmission filter layer. Further, a black absorber layer is disposed adjacent to said selective reflection layer wherein all wavelengths of light reaching said black absorber are absorbed, whereby said retro-reflecting construct reflects specific wavelengths of light irradiating said construct. 
     In another aspect, the present invention provides a further retro-reflecting construct suitable for use as an optical label. The construct comprises a corner cube reflector having a first surface and a second surface. A multi-peak transmission filter layer, comprising a birefringent material, is disposed adjacent to said first surface. A programmable wavelength selective reflection layer, comprising a photochromic material and a birefringent material, is disposed adjacent to said second surface. An optional broadband reflector layer is disposed adjacent to said second surface not covered by said selective reflection layer. Further, a black absorber layer disposed adjacent to said selective reflection layer wherein all wavelengths of light reaching said black absorber are absorbed, whereby said retro-reflecting construct reflects specific wavelengths of light irradiating said construct. 
     In another aspect, the present invention provides yet another retro-reflecting construct suitable for use as an optical label. The structure comprises an optical waveguide with an end, having a first side and a second side, said waveguide comprising a core layer sandwiched between a plurality of cladding layers, wherein said core layer comprises a plurality of spherical lens. A multi-peak transmission filter layer is disposed adjacent to said end wherein a light enters said waveguide. A programmable wavelength selective reflection layer, comprising a photochromic material, is disposed adjacent to said second side cupping the said plurality of spherical beads. A first broadband reflection filter layer is disposed adjacent to said first side cupping said plurality of spherical lens. A second broadband reflection filter layer is disposed adjacent to said selective reflection layer. A black absorber layer is disposed adjacent to said second broadband reflection filter layer wherein all wavelengths of light reaching said black absorber are absorbed, whereby said retro-reflecting structure reflects specific wavelengths of light irradiating said structure. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings described below. 
       Programmable Optical Label 
         FIG. 1  is an exemplary embodiment of a programmed optical identification system. 
         FIG. 2  illustrates an exemplary programming process of the programmable optical label. 
         FIGS. 3   a  and  3   b  are exemplary embodiments of the programmable optical label. 
         FIG. 4  is an expended view of the construction of an embodiment of a retro-reflecting construct. 
         FIG. 5  illustrates the operation of an embodiment of a reflecting construct. 
         FIGS. 6   a ,  6   b , and  6   c  show an embodiment and characteristics of a photochromic material used in the invention. 
         FIGS. 7   a  and  7   b  shows the transmission spectra of an exemplary embodiment of a multi-peak transmission filter. 
         FIG. 8  shows the reflection spectra of an exemplary embodiment of a programmable reflection filter. 
         FIG. 9  shows the absorption spectrum of an exemplary prior art inorganic photochromic material. 
         FIG. 10   a  is a sectional view of an embodiment of a programmable wavelength selective reflection layer. 
         FIG. 10   b  is a sectional view of the construction of an embodiment of a multi-peak transmission filter layer. 
         FIG. 11  illustrates an exemplary embodiment of the current invention in operation. 
         FIG. 12  illustrates a method to fabricate an embodiment of the invention. 
         FIG. 13  illustrates another method to fabricate an embodiment of the invention. 
         FIG. 14  is a sectional view of an embodiment of the invention in the form of a labeling strip with spherical lenses. 
         FIG. 15  is a sectional view of an embodiment of the invention in the form of a labeling strip with corner cube reflectors. 
         FIG. 16  is a sectional view of another embodiment of the invention in the form of a labeling strip with corner cube reflectors. 
         FIG. 17  is a sectional view of another embodiment of the invention in the form of a labeling strip with spherical lenses. 
         FIG. 18  is a sectional view of an embodiment of the invention in the form of a waveguide with associated programming and interrogation wavelengths. 
       Optical Identification System and Method 
         FIG. 1  is an exemplary embodiment of the present invention as a schematic diagram of an optical labeling/identification system. 
         FIG. 19  shows a block diagram of an exemplary embodiment of an optical programmer. 
         FIG. 20  shows a block diagram of an exemplary embodiment of an optical interrogator. 
     
    
    
     DESCRIPTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Further, the dimensions of layers and other elements shown in the accompanying drawings may be exaggerated to more clearly show details. The present invention should not be construed as being limited to the dimensional relations shown in the drawings, nor should the individual elements shown in the drawings be construed to be limited to the dimensions shown. 
     Programmable Optical Label Description 
     In general, the present invention is a programmable and latching retro-reflective construct suitable for use as an optical label in an optical labeling system. The retro-reflective construct comprises a photochromic material responsive to specific wavelengths of light. 
       FIG. 1  depicts an optical labeling system  100  comprising a programmable optical label  101  and an optical interrogator  102 . The programmable optical label  101  comprises a plurality of retro-reflecting constructs  103  disposed on a labeled surface  104 . The surface  104  may be an uneven or discontinuous surface; as such the surface could not easily be labeled with labels such as a bar code that contains spatial patterns of high reflectivity and low reflectivity regions. 
     In one embodiment, the labeled surface  104  is highly corrugated with small and discontinuous features similar to the surface of an automobile radiator or a window screen. In another example, the labeled surface  104  is like a crumpled piece of paper that has many folds therein. In yet another example, the surface  104  is like a piece of sheep skin with wool thereon. In another example, the surface  104  is in the form of a woven or knitted fabric. 
     Referring back to  FIG. 1 , in an exemplary embodiment the optical label  101  can take the form of a fabric label. The fabric label can contain a collection of narrow label strips that comprise yarns incorporated into the fabric, wherein the yarns comprise the reflecting construct  103 . This collection may comprise several different kinds of strips. Each kind of strip may be associated with a different wavelength of the interrogating light beam  105 . Each kind of strip may also be programmed by a different set of programming wavelengths. There generally would be at least as many differing kinds of strips in the fabric as there are different wavelengths in the code used for the labeling system. 
     The optical label  101  comprises a plurality of retro-reflecting constructs  103  that can be programmed to provide specific wavelength coded responses when illuminated with multi-wavelength interrogation light  105 . For example, the coded responses may identify the labeled surface  104  as “radiator”, “paper”, “sheep skin” or “garment”. The optical interrogator  102  illuminates the surface  104  with the interrogation light  105  comprising multiple wavelengths of light such as λE 1 , λE 2 , λE 3 , λE 4 . Each of the plurality of retro-reflecting constructs  103  on the labeled surface  104  can retro-reflect a specific wavelength of light according to its programmed state, whereby the constructs  103  on the surface  104  retro-reflect a light with specific combination of wavelengths  106  according to the wavelength-code programmed into each construct  103 . 
       FIG. 2  illustrates an exemplary programming and coding process  120 . The plurality of retro-reflecting constructs  103  can be programmed and reprogrammed with various wavelength codes. A programming light beam  121  from a programming transmitter  15  illuminates the retro-reflecting constructs  103 . The light beam  121  comprises a specific combination of wavelengths of light. As shown in  FIG. 2 , the programming wavelengths are λA 1  or λB 1 , λA 2  or λB 2 , λA 3  or λB 3 , λA 4  or λB 4 , and optionally λC where λC is a conditioning wavelength that enables the programming. These wavelengths can change the state of the photochromic materials in the retro-reflecting constructs  103 . After the programming light  121  is removed, the photochromic materials retain their programmed states. Those states can then be sensed by a beam of the interrogation light  105  shown in  FIG. 1 . Each retro-reflecting construct may be programmed differently by the programming light. Some retro-reflecting constructs  103  may be programmed to be reflecting and other constructs  103  may be programmed to be non-reflecting. Since each construct  103  may be associated with different wavelengths of the interrogation light  105 , a specific wavelength code can be programmed into the retro-reflection constructs  103  labeling an object. 
       FIGS. 3   a  and  3   b  present two exemplary embodiments  130 ,  131  of the invention.  FIG. 3   a  illustrates a plurality of retro-reflecting constructs  103  applied as a dry aerosol onto a surface.  FIG. 3   b  illustrates the constructs  103  immersed in a film  135 , such as an adhesive coating. 
     A preferred embodiment of the retro-reflecting constructs  103  shown in  FIG. 3   a  comprises a spherical lens  132  such as lens bead and a programmable reflecting coating  133 . The lens  132  has a refractive index that allows it to function as a lens. The typical values for the refractive index may range from 1.8 to 2.8. See U.S. Pat. No. 2,963,378, herein incorporated by reference. The programmable reflecting coating  133  coats a portion of the lens  132 . The coating  133  comprises photochromic material responsive to specific wavelengths of light. Another portion of the lens  132  is not coated with the reflecting coating  133 . 
     For the embodiment shown in  FIG. 3   b , a spacer layer  134  is disposed between the spherical lens  132  and the programmable reflecting coating  133 . The thickness of the spacer layer  134  is chosen to establish the reflecting coating  133  at the focal plane of the spherical lens  132  which is immersed in a film  135 . The spacer layer  134  is not included in the embodiment shown in  FIG. 3   a  since those label pieces will have their clear surfaces (that are not coated with the reflecting coating  133 ) exposed to air. 
       FIG. 4  presents an embodiment of the retro-reflecting construct  103  in expanded detail. One function of the reflecting construct  103  as shown in  FIG. 4  is to select those programming wavelengths that will determine the state of the photochromic material in the reflecting construct  103 . The programmable reflecting coating  133  preferably comprises a multi-peak transmission filter layer  141 , a programmable wavelength selective reflection layer  142  and a black absorber layer  143 . The transmission filter layer  141  is disposed closest to the surface of the spherical lens  132 . The multi-peak transmission filter layer  141  selects specific wavelengths of light to program the photochromic material contained in the selective reflection layer  142 . 
     In  FIG. 4 , the transmission filter  141  passes one specific programming light beam  121  wavelength (e.g., λA 1 ) in the band between 250 and 340 nm and one specific programming light beam  121  wavelength (e.g., λB 1 ) in the band between 460 and 680 nm. In an exemplary embodiment comprising the photochromic material of  FIG. 6 , the multi-peak transmission filter  141  in  FIG. 4  reflects other programming wavelengths (e.g., λA 2 , λA 3 , λB 2 , λB 3 ) in those two bands and thereby keeps the light at those other programming wavelengths from affecting the state of the photochromic material in the reflecting construct  103 . Note that the multi-peak transmission filter  141  also transmits the full range of interrogating light beam  105  wavelengths and a conditioning wavelength λC (if such a conditioning wavelength also is used). 
     The programmable wavelength selective reflection layer  142  comprises preferably multiple layers of photochromic material and non-photochromic material. Furthermore, the reflection layer  142  preferably comprises alternating layers of the photochromic material and the non-photochromic material whereby an optical multi-layer interference filter is formed. The programmable wavelength selective reflection layer  142  reflects a specific wavelength of interrogation light  105  associated with a given retro-reflecting construct  103 . The wavelength selective reflection layer  142  reflects the interrogation light  105  with an associated wavelength when the photochromic material is so programmed. However, when the photochromic material is programmed into another state, the programmable wavelength reflection layer  142  transmits that interrogation wavelength (as well as all of the other interrogation wavelengths) instead of reflecting it back toward the spherical lens  132 . The black absorber layer  143  absorbs, with minimal reflection, all the wavelengths of light reaching it. 
     To label an object, a plurality of retro-reflecting constructs  103 , preferably comprising several different kinds of the constructs  103 , are affixed to a labeled surface. Each kind of the constructs  103  is associated with a different wavelength of the interrogation light  105  (e.g., one of λE 1 , λE 2 , λE 3  or λE 4 ). Each kind of constructs  103  also is programmed with a different set of programming wavelengths (e.g., a pair comprising λA 1  and λB 1 , λA 2  and λB  2 , λA 3  and λB 3 , or λA 4  and λB  4 ). While the example given identifies four pairs of wavelengths, there preferably would be at least as many kinds of retro-reflecting constructs  103  on the surface of a labeled object as there are different wavelengths in the code used for labeling the object. 
       FIG. 5  illustrates how an embodiment of the retro-reflecting construct  103  selects its associated wavelengths for responding to interrogation and for being programmed. The wavelength selective reflection layer  142 , the second part, reflects only the interrogation wavelength associated with that strip (e.g., λE 1  but not λE 2 , λE 3 , and λE 4  shown in  FIG. 5 ), if it is programmed to be reflecting. If it is programmed to be not reflecting, that associated wavelength is transmitted into the black absorber  143  to be absorbed. The other wavelengths of the interrogation light (e.g., λE 2 , λE 3 , λE 4 ) also are transmitted through the reflection filter  142  to be absorbed by the black absorber  143 . Thus, each strip either will reflect its associated interrogation wavelength or not reflect that wavelength. The strip preferably will not reflect the other (non-associated) interrogation wavelengths. Thus, the background signal returned to the interrogator from the label can be reduced. 
     The photochromic material contained in the programmable wavelength selective reflection layer  142  likely will be responsive to a large range of wavelengths. A narrow-band response is achieved when the photochromic material is incorporated into an optical interference filter structure. Furthermore, the photochromic material likely can be programmed with a large range of wavelengths. The multi-peak transmission filter layer  141  selects the specific programming wavelengths within this range that are associated with a given retro-reflecting construct  103 . The use of different multi-peak transmission filter layers  141  allows each construct  103  to be distinguishable, so that some constructs  103  can be programmed to be reflecting and others can be programmed to be non-reflecting. 
     The interrogation light  105  preferably is in a range of wavelength for which the photochromic material is essentially transparent. In particular, the interrogation light  105  may be in the eye-safe wavelengths range of 1500-1800 nm because the intensity of the light  105  provided by an interrogator can be higher. This higher intensity may enable the interrogator to be located at a larger standoff distance from the object labeled. Many photochromic materials do not have significant optical absorption at wavelengths in the range of 1500-1800 nm. Thus, the change in the refractive index of these materials is used. The photochromic material is incorporated into the programmable wavelength selective reflection layer  142  so that a change in its refractive index will produce a change in the reflectivity of the selective reflection layer  142  at the interrogation wavelength of interest. It is noted that some photochromic material could have absorption at the 1500-1800 nm wavelengths. However, those photochromic materials preferably have only weak absorption at these wavelengths. In that case, the characteristics of the multi-layer programmable wavelength reflection filter  142  can be simpler, involving primarily a change in index of only one component of the multi-layer programmable wavelength reflection filter  142 . 
       FIG. 6   a  presents an exemplary photochromic material that may be used in the present invention. The material is a 1,2-bis(2-methyl-6-(2,4-diphenylphenyl)-1-1benzothiophene-3-yl) perfluorocyclopentene. See, M. S. Kim, T. Sakata, T. Kawai and M. Irie, “Amorphous photochromic films for near-field optical recording,” Japanese Journal of Applied Physics, vol. 42 (2003) pp. 3676-3681, herein incorporated by reference. The material can be converted between an open-ring isomer and a closed-ring isomer. The open-ring isomer has very little absorption of wavelengths greater than 350 nm. The closed-ring isomer, however, has a pair of strong absorption peaks at the wavelengths of 370-390 nm and 500-580 nm, as shown in  FIG. 6   b . These strong absorption peaks are accompanied by the refractive index of the closed-ring isomer being significantly different from the refractive index of the open-ring isomer. 
     The exemplary material shown in  FIG. 6   a  can be converted between an open-ring isomer  31  and a closed-ring isomer  33 . As shown in  FIG. 6   b , the open-ring isomer  31  has very little absorption of wavelengths greater than 350 nm. In contrast, the closed-ring isomer  33  has a pair of strong absorption peaks at the wavelengths of 370-390 nm and 500-580 nm. This substantial difference in absorption spectrum is accompanied by the refractive index of the closed-ring isomer  33  being substantially different from that of the open-ring isomer  31 . In one example of photochromic material, the refractive index of the open-ring isomer  31  at 1553 nm is 1.621 whereas the refractive index of the closed-ring isomer  33  is 1.684. See M. K. Kim, H. Maruyama, T. Kawai and M. Irie, “Refractive index changes of amorphous diarylethenes containing 2,4-diphenylphenyl substituents,” Chem. Materials, vol. 15 (2003), pp. 4539-4543, herein incorporated by reference. This exemplary photochromic material exhibits a change in index of more than 0.06 (or greater than 3.5%). This level of index change is observed over a large range of wavelengths including the wavelengths between 1500 and 1800 nm, as indicated in  FIG. 3   c . See J. Chauvin, T. Kawai and M. Irie, “Refractive index change of an amorphous bisbenzothienylethene,” Japanese Journal of Applied Physics, vol. 40 (2001), pp. 2518-2522, herein incorporated by reference. 
     The exemplary photochromic material of  FIG. 6  may be converted from its open-ring state to its closed-ring state by illuminating it with a light at a wavelength of 250-340 nm. This material may be converted from its closed-ring state back to its open-ring state by illuminating it with a programming light beam at a wavelength of 460-680 nm. Thus, the material can be programmed with a large range of programming wavelengths. The multi-peak transmission filter layer  141  that comprises the first part of the programmable reflecting coating  133  preferably may transmit one specific programming wavelength in the 280-340 nm band and one specific programming wavelengths in the 460-680 nm band. 
     A large change in refractive index may be achieved by using films that contain a large percentage of the photochromic material. Large percentage incorporation has been achieved in prior art photochromic films by forming films of amorphous photochromic materials, by forming liquid crystal films of the photochromic material, and by incorporating the photochromic material into a polymer (preferably into the backbone of the polymer). The material illustrated in  FIG. 6   a  is one embodiment of an amorphous film. A person skilled in the art will note that materials other than these described above may be used as substitute to form such films that exhibit a large change in refractive index. The present invention is not intended to be and is not to be construed as limited to the materials described herein. 
     Referring to  FIG. 10   a , an exemplary embodiment of the programmable wavelength selective reflection layer  142  is shown. The wavelength selective reflection layer  142  may be constructed from multiple layers of a photochromic material and a non-photochromic material. Each layer has a thickness that is an odd multiple of a quarter wavelength (in that material) of the desired reflection peak wavelength. This exemplary embodiment functions as a multi-layer interference filter that has 14 periods of layers having 5/4 wave thickness. The programmable wavelength selective reflection layer  142  is intended to selectively reflect interrogation light at a given interrogation wavelength but to not reflect other interrogation wavelengths. The refractive index of the non-photochromic material may be 1.35. The refractive index of the photochromic material may be 1.68 when the label piece is in it retro-reflecting state. In this case, the photochromic material is in its closed-ring state as discussed with respect to  FIG. 6   a . When the photochromic material is in its open-ring state, the reflection peak of the programmable wavelength selective reflection layer  142  is shifted such that the filter no longer reflects that specific interrogation wavelength. The total thickness of this exemplary reflection layer  142  may be approximately 40 μm. 
     Referring to  FIG. 10   b , an exemplary multi-peak transmission filter layer  141  may be constructed by cascading two Fabry Perot etalons  171 ,  172 . Each of the etalons is an optical filter that has a narrow transmission peak. The wavelengths of those transmission peaks correspond to the two associated programming wavelengths of the retro-reflecting construct  103 , λA and λB as shown in  FIG. 5 . Each mirror of these two etalons is formed by a 3-period quarter-wave reflective stack. The layers of that reflective stack may have refractive indices of 1.5 and 2.3. An etalon spacer  173  that establishes an optical cavity length of the etalons may have a refractive index of 2.3 or higher. In this exemplary embodiment, the etalon spacer  173  has a half-wave thickness. The first etalon  171  produces a narrow transmission peak at a particular programming wavelength. It may be approximately 320 nm and has sub-layers of thickness 50 nm and 40 nm. The second etalon  172  produces a narrow transmission peak at another programming wavelength. It may be a wavelength of approximately 640 nm and has sub-layers of approximate thickness 100 nm and 80 nm. The total thickness of this exemplary transmission filter layer  141  may be approximately 2 μm. 
     Referring back to  FIG. 5 , the black absorber layer  143  preferably comprises a material that preferably absorbs the light at the various interrogation wavelengths, and also the programming wavelengths, but has low reflection of that light. Non-limiting examples of material that may be used as the black absorber layer  143  may be gold blacks, silver blacks and carbon blacks. See, L. Harris, The Optical Properties of Metal Blacks and Carbon Blacks, Monograph Series No. 1 Dec. 1967 (Eppley Foundation for Research, Newport, R1), herein incorporated by reference. The optical reflectance of gold blacks is typically less than 1% in the wavelength range of the interrogation light  105 . Gold black coatings that can be formed on electrically insulating materials, such as the retro-reflecting construct  103 , are described in an article by Lehman, et al. See, J. Lehman, E. Theocharous, G. Eppeldauer and C. Pannell, “Gold-black coatings for freestanding pyroelectric detectors,” Measurement Science and Technol., v. 14 (2003), pp. 916-922, herein incorporated by reference. 
     The total thickness of the programmable wavelength selective reflection layer  142  places a constraint on the minimum diameter of the spherical lens  132 . The diameter of the spherical lens  132  may be at least 3 times and preferably at least 10 times larger than the total thickness of the reflection layer  142 . In general, the larger the spherical lens  132  is compared to the total thickness of the programmable wavelength selective reflection layer  142 , the better the retro-reflection characteristics of their composite structure. 
     The effectiveness with which the retro-reflecting construct  103  retro-reflects the interrogation light  105  can be degraded as a result of spherical aberration from the spherical lens  132 . This spherical aberration occurs when the spherical lens  132  has a uniform refractive index. However, the spherical aberration can be reduced substantially by using a spherical lens that has a refractive index gradient in the radial direction. See, Y. Koike, Y. Sumi and Y. Ohtsuka, “Spherical gradient-index sphere lens,” Applied Optics, vol. 25 (1986), pp. 3356-3363, herein incorporated by reference. Even better performance is anticipated with a spherical lens comprising a graded index core and a cladding of uniform index. See, K. Kikuchi, T. Morikawa, J. Shimada and K. Sakurai, “Cladded radially inhomogeneous sphere lenses,” Applied Optics, vol. 20 (1981), pp. 388-394, herein incorporated by reference. When the spherical lens  132  has a graded-index, the retro-reflecting construct  103  preferably should have the spacer layer  134  of the appropriate thickness, as discussed above (paragraph 50). 
     Referring to  FIG. 11 , generally less than half of the surface of the spherical lens  132  is covered with the programmable reflecting coating  133 . The interrogation light  105  that illuminates the un-covered portion of a spherical lens bead will be focused onto the programmable reflecting coating  133 . The selected wavelength component of the interrogation light  105  then reflects off the reflecting coating  133  according to the programmed state and is directed via the spherical lens  132  back toward the source of the interrogation light  105 . When a collection of retro-reflecting constructs  103  has been dispersed onto a labeled surface as shown in  FIG. 3   b , probably only some of the spherical lens  132  will have their un-covered portions facing the incoming interrogation light  105  and be able to retro-reflect the interrogation light  105  back toward the interrogator. Those retro-reflecting constructs  103  that have their programmable reflecting coating facing the interrogator will not act as retro-reflectors. The amount of retro-reflection also will depend on the exact orientation of the construct  103  with respect to the interrogation light  105 . The interrogation light  105  can illuminate both the un-covered portion of the spherical lens  132  and the portion that is covered by the programmable reflecting coating  133 . The variation in the effective illumination of the collection of retro-reflecting construct  103  can result in a variation in the retro-reflection response such as no retro-reflection  180 , weak-retro-reflection  181  or strong-retro-reflection  182 . 
     In another embodiment, the retro-reflecting constructs  103  can be applied to the labeled surface at different times. This can be done to increase the number of interrogation and programming wavelengths available. This also can be done to re-supply those retro-reflecting constructs  103  types whose number may have been reduced through wear such as when some constructs  103  that were applied long ago have become detached from the labeled surface. 
     A simple binary wavelength code has been presented as an example wherein each wavelength represents a bit of the code. Each bit can have a logical 1 value (reflecting) or a logical 0 value (non-reflecting). Other wavelength-based codes also are possible. In other exemplary embodiments, combinations of wavelength and multiple intensity levels could be used to form a coded response. For improved code detection, the code words containing all 0s or all 1s may be excluded. Thus, a 3 wavelength code can have 6 different code words; a 5 wavelength code can have 30 different code words and a 9 wavelength code can have 510 different code words. 
     If the material shown in  FIGS. 6   a ,  6   b  and  6   c  is used for the photochromic component of the programmable wavelength selective reflecting filter  142  of all the different constructs  103  of the label  101 , the N sets of programming wavelengths (where N is the number of code wavelengths) must be defined within the 250-340 nm range and within the 460-680 nm range (the absorption peaks for that material). If N is 4, a possible choice of the programming wavelengths are 280, 300, 320 and 340 nm for selecting the closed-ring state of the photochromic material and 560, 600, 640 and 680 nm for selecting the open-ring state. If N is 4, one may want to choose 4 interrogation wavelengths that lie in the range between 1550 and 1800 nm. For example, one may choose wavelengths that are spaced by 80 nm (e.g., 1550 nm, 1630 nm, 1710 nm and 1790 nm) to cover that entire range. 
     As an example of the design and construction of the multi-peak transmission filter  141  and the programmable wavelength selective reflection filter  142 , the wavelengths of 320 nm and 640 nm may be selected as the programming wavelengths and 1710 as the interrogation wavelength associated with an exemplary reflecting construct  103  of an optical label  101 . The programmable wavelength selective reflecting filter  142  has a reflection peak at 1710 nm. The multi-peak transmission filter  141  has a pair of transmission peaks located at 320 nm and 640 nm. The multi-peak transmission filter  141  also has fairly high transmission for 1710 nm. The desired spectral widths of the filter peaks depend on the number of wavelengths in the code (which is 4 for this example). 
     The multi-peak transmission filter  141  as shown in  FIG. 5  may be constructed by cascading two Fabry Perot filters. Each Fabry Perot filter is an optical etalon comprising two reflectors separated by a spacing distance. Preferably, the reflectors have the desired reflection level for the etalon over the range of programming wavelengths (e.g., 280-340 nm) addressed by that etalon but have substantially lower reflection for the other range of programming wavelengths (e.g., 560-680 nm) as well as the range of interrogation wavelengths (e.g., 1550-1790 nm). In this case, the transmission through that etalon will be fairly high at those other wavelength ranges. This permits multiple etalons that operate at different wavelength ranges to be cascaded together to obtain a multi-peak transmission filter. Each etalon produces a narrow transmission window within the wavelength range over which its two reflectors reflect and a broad transmission window over those wavelengths for which its two reflectors do not reflect substantially. 
       FIG. 7   a  shows the transmission spectrum of an exemplary multi-peak transmission filter  141  for the first part of the programmable reflecting structure  133 . There are transmission peaks centered at 320 nm and 640 nm to pass the two desired programming wavelengths. Note that the filter has low transmission for the other 6 programming wavelengths of this example. Referring to  FIG. 7   b , furthermore, there also is substantial transmission for wavelengths greater than 1400 nm. The two Fabry-Perot etalons comprising this filter each have mirrors comprising interference stacks with 2 or more periods of alternating high-index and low-index layers of quarter-wave thicknesses at the wavelength of the transmission peak. The spacer in each Fabry-Perot etalon has a half wave thickness at the transmission peak wavelength. Only a small gap separates the two cascaded Fabry-Perot etalons. The total thickness of this composite multi-peak transmission filter  141  structure can be less than 2 micrometers. Note that the transmission spectrum of this multi-peak transmission filter  141  has substantial features at those wavelengths outside of the ranges of programming and interrogation wavelengths. However, this often is acceptable for the label  101  and does not degrade the performance of the label  101 . 
       FIG. 8  shows the reflection spectra of an exemplary multi-layer reflection filter  142  for the second part of the programmable reflecting structure  133 . This interference filter  142  is constructed from multiple layers of a photochromic material and a non-photochromic material. In this case, the multi-layer reflection filter  142  has a peak at 1710 nm when the photochromic material is in its closed-ring state (refractive index=1.68). The width of this reflection peak is selected such that the reflection is low for the adjacent interrogation wavelengths of 1630 nm and 1790 nm. The reflection peak is shifted to shorter wavelengths when the photochromic material is in its open-ring state (refractive index=1.62). The reflection at a wavelength of 1710 nm is reduced from nearly 1.0 for the unshifted reflection filter to below 0.1 for the shifted reflection filter. Thus, the extinction ratio or signal contrast obtained with this change in refractive index is better than 10 dB. Note that the reflection at the adjacent interrogation wavelengths still is low even for the shifted filter. A total of 14 periods of 5/4 wave thick layers are used to construct this exemplary interference reflection filter  27 , whose spectrum is shown in  FIG. 5 . The total thickness of this exemplary filter is approximately 40 μm. 
     A potential weakness of the exemplary photochromic material shown in  FIG. 6  is that its programming wavelengths lie in the wavelength range where there is substantial irradiation outdoors (e.g., from sunlight). Thus, ambient irradiation may gradually cause the reflecting constructs  103  to depart from their programmed state. Therefore, it may be preferable to select other photochromic materials that have a gated reactivity. See M. Irie, “Diarylethenes for memories and switches,” Chemical Review, v. 100 (2000), pp. 1685-1716, herein incorporated by reference. As an example, a gated photochromic material may require some other input besides the programming light beam  121  as shown in  FIG. 2 , to cause it to convert efficiently from one state to the other. One possible gating mechanism is temperature. 
     Some of the photochromic materials that have a gated reactivity can convert between their open-ring and closed-ring isomers when the temperature is increased. When these materials are kept at room temperatures, the conversion process occurs very slowly. In one exemplary embodiment of the invention, a conditioning light illuminating the constructs  103  may be absorbed by the black absorber  143  material that is underneath and in close contact with the photochromic material, whereby the absorbed conditioning light heats the black absorber and thereby also heats the photochromic material. If an eye-safe wavelength is used for the conditioning light, substantial conditioning energy can be supplied to the label  101  by the programmer  15 . 
     Another embodiment may comprise photochromic materials that can be programmed with wavelengths at which the ambient radiation is weak. This may be done by selecting deep UV wavelengths (below 300 nm) and a set of IR wavelengths (e.g., between 1380 and 1420 nm) where there is substantial atmospheric absorption of the sunlight. Those wavelengths (e.g. between 300 nm and 1380 nm) where there is substantial solar irradiance could be rejected by an optical filter that is placed above the programmable reflecting structure  133 . 
     Referring to  FIG. 9 , an example of an inorganic photochromic material that may be used in the programmable wavelength selective reflection filter  142  (shown in  FIGS. 4 and 5 ) is tungsten oxide. Tungsten oxide can be optically converted from a more oxidized state (e.g., WO3) to a less oxidized state. See R. Bussjager, J. M. Osman, E. Voss and J. Chaiken, “Tungsten oxide based media for optical data storage and switching applications,” Proceedings of 1999 IEEE Aerospace Conference, pp. 343-349, herein incorporated by reference. In one exemplary embodiment, the oxidized (yellow) state has an absorption peak located at 200-320 nm with little absorption at longer wavelengths. The oxygen deficient (blue) state has a broad absorption peak located at wavelengths longer than 1000 nm. It is possible to have these absorption peaks shifted to other wavelengths by using other versions of tungsten oxide. 
     The tungsten oxide can be used with a multi-peak transmission filter  141  that passes programming light at those wavelengths (below 300 nm and between 1380 and 1420 nm) for which there is little solar irradiance. The substantial change in the absorption spectra of the two states of a given tungsten oxide film is accompanied by a corresponding change in the refractive index. This change in the refractive index is used to shift the peak wavelength of the programmable wavelength selective reflection filter  142 , as discussed above. Note, however, that there is substantial absorption by the oxygen deficient state at the range of interrogation wavelengths. Thus, the programmable wavelength selective reflection filter  142  incorporating this photochromic material should be designed to reflect the desired interrogation wavelength when the photochromic material is in its oxygen rich state. The tungsten oxide has very little absorption at the interrogation wavelengths when in this state. The degraded height of the shifted reflection peak, a result of the absorption of interrogation light by the oxygen deficient state, is then of much less consequence. 
     In another embodiment, the tungsten oxide can be used as a gated photochromic material. For example, tungsten oxide can be converted between its two states by illuminating it with conditioning light (λC) at an infra-red (IR) wavelength in addition to the programming light (λA 1 , λB 1 ) at shorter wavelengths. See R. Bussjager, et al., “Using tungsten oxide based thin films for optical memory and the effects of using IR combined with blue/green wavelengths,” Japanese Journal of Applied Physics, vol. 39 (2000), pp. 789-796, herein incorporated by reference. 
     Furthermore, tungsten oxide can have substantial absorption even at wavelengths longer than 2000 nm. If the optical intensity levels at the programming wavelengths are set low enough, it is necessary to illuminate the tungsten oxide film with both the IR conditioning light and the programming light beam in order to obtain substantial conversion of the state. Given the long-wavelength sensitivity of the blue tungsten oxide, IR conditioning wavelengths can be used for which high ambient levels do not occur naturally. In this case, the deep UV light could be used to convert the tungsten oxide to the blue state and the combination of the conditioning light and a shorter wavelength light (e.g., at 950 nm) could be used to convert the tungsten oxide to the yellow state. The IR conditioning light could be at wavelengths of 1380-1420 nm or 1800-2000 nm, for example. 
       FIG. 12   a - h  illustrates an exemplary process  190  to fabricate the retro-reflecting constructs  103  described in the previous sections. The process  190  is derived from a process for manufacturing prior art retro-reflective beads. See, for example, U.S. Pat. No. 2,963,378, herein incorporated by reference. First, a carrier  191  is constructed that contains a soft layer into which spherical lens  132  can be embedded. Multiple spherical lens  132  are then embedded into the carrier  191 , with a portion of the lens exposed ( FIG. 12   a ). An optional spacer layer  134  of spacer material is applied over the exposed surfaces of the embedded spherical lens  132  ( FIG. 12   b ). A layer of multi-peak transmission filter layer  141  is then applied over the exposed portions of the spherical lens  132  ( FIG. 12   c ). A layer of programmable wavelength selective reflection layer  142  is applied over the transmission filter layer  141  ( FIG. 12   d ). Then, a black absorber layer  143  is applied over the selective reflection layer  142  ( FIG. 12   e ). The three coatings  141 ,  142  and  143  collectively form the programmable reflective coating  133  as shown in  FIGS. 4 and 5 . A final protective coating  192  may, optionally, be applied over the black absorber layer  143  ( FIG. 12   f ). Next, the carrier  191  is removed (or the coated spherical lens  132  are detached from the carrier  191 ) to make available the individual retro-reflective spherical lens ( FIG. 12   h ). 
     Some of the coated spherical lenses  132  may still remain attached to each other, being connected together by the thin portions of the programmable reflecting coating  133 , the optional spacer layer  134  and the optional protective coating  192  deposited in the regions of exposed carrier  191  between the spherical lens  132 . This thin material may be removed by some non-limiting means such as brushing to separate the individual retro-reflecting construct  103  ( FIG. 12   h ). 
     Referring to  FIG. 13 , it illustrates another exemplary process  200  to separate the individual retro-reflecting construct  103 , instead of breaking through the thickness of the spacer layer  134 , the programmable reflecting coating  133  and the protective coating  192 . The three coatings  141 ,  142  and  143  collectively form the programmable reflective coating  133  as shown in  FIGS. 4 and 5 . A removable backing layer  201  is attached to the coated lens after the protective coating  192  has been applied, and before the carrier  191  is removed ( FIG. 13   g ). The carrier  191  is then removed ( FIG. 13   h ) in a manner similar to that described above with regard to  FIG. 12 . The front surfaces of the spherical lens  132  are thereby exposed. The spherical lens  132  protects a part of the coating that is on its back surface. However, the spacer layer  134 , programmable reflecting coating  133 , and protecting coating  192  that lies in the regions between the lenses  132  are exposed. These layer  134  and coatings  133 ,  192  may be removed by some non-limiting means such as wet or dry etching ( FIG. 13   j ). For example, the spacer layer  134 , the multi-peak transmission filter layer  141  and the programmable wavelength selective reflection layer  142  may comprise organic materials. These organic materials may be etched by oxygen plasma or by reactive ion etching with an oxygen-containing gas. The etchant preferably should be a type of which the lens  132  is not responsive thereto. The lens  132  preferably is a silicate glass material. Other selective etchants could be used to remove the black absorber layer  143  and the protective coating  192 . The etching process leaves the retro-reflecting constructs  103  mostly detached from each other, being attached primarily by the backing layer  201 . The backing layer  201  is then removed to release the individual constructs  103  ( FIG. 13   k ). 
       FIG. 14  presents another preferred embodiment of the present invention in the form of a labeling strip  210 .  FIG. 14  illustrates a cross sectional view of the strip  210  that provides enhanced retro-reflection. The strip  210  comprises one or more spherical lens  132  that serves as an optical lens. The programmable reflecting coating  133  coats a first portion  132   a  of each spherical lens  132 . A second portion  132   b  of the spherical lens  132  is not coated with the reflecting coating  133 . When the uncovered second portion of the spherical lens  132  is exposed to air, the focal plane may be located at the surface of the first portion  132   a  of the lens  132 . When an additional coating layer or film covers the surface of the second portion  132   b  of the lenses, the optional spacer layer  134  may be interposed between the lens  132  and the programmable reflecting coating  133 . The spacer layer  134  preferably has a thickness to establish the programmable wavelength selective reflection layer  142  at the focal plane of the spherical lens  132  when the lens  134  is embedded entirely within a film. The total thickness of the programmable reflecting coating  133  constrains the preferred minimum diameter of the spherical lens  132 . 
     The combination of the size of the spherical lens  132 , the total thickness of the programmable reflecting coating  133  and the thickness of the optional spacer layer  134  determines the thickness of the strip  210  embodiment. When the labeling strip  210  is to be used as a yarn, the width of a strip  210  is preferably two to five times larger than the thickness of that strip  210 . A strip having such an aspect ratio in its dimensions is more likely to lie flat when it is woven or knitted into a label fabric, with the wider side of the strip in the plane of the fabric. In some embodiments (not shown) a strip  210  may contain spherical lens on both of its sides. In these embodiments, the yarn will be retro-reflecting even though it is flipped, which may occur when that yarn is woven into a fabric. Constraints on the weight and stiffness of a labeled fabric may limit the maximum width and thickness of the label strip  210  in it. 
       FIG. 15  shows another exemplary embodiment of the present invention in the form of a labeling strip  220  comprising corner cube reflectors  221  or modified versions of such reflectors disposed on a substrate  223 . In this embodiment, the programmable reflecting coating  133  is formed on the multiple back reflecting surfaces  222  of the corner cube reflector  221 . A corner cube reflector  221  may have two or more back reflecting surfaces  222 . At least one or preferably all of these back reflecting surfaces  222  contains a programmable reflection coating  133 . Any back reflecting surface  222  that is not coated with the programmable reflecting coating  133  are preferably coated with broadband reflectors, such as a metal film. In this way, the retro-reflectance from the corner cube reflector  221  can be programmed. Retro-reflecting sheets comprising corner cube reflecting structures are described in U.S. Pat. Nos. 2,310,790; 3,712,706 and 4,895,428, herein incorporated by reference. 
     A characteristic of a corner cube reflector  221  is that a light may be incident onto a back reflecting surface  222  at a large angle, relative to the surface normal. Thus, a conventional multi-layer optical interference filter design would not be appropriate for use in the programmable reflecting coating  133 , since its reflection characteristics are appropriate only over a small range of incident angles relative to the surface normal. These conventional multi-layer filters make use of non-birefringent materials in their multiple layers. The range of incident angles is limited partly because the optical path is longer for light incident at larger angles relative to the surface normal. As a result, the effective thickness of the multiple layers becomes larger than what would be optimal for the desired filter response. 
     A multi-layer optical interference filter that retains its desired reflection spectrum over a much larger range of incident angles may be achieved by using appropriate combinations of suitably engineered optically birefringent materials. See, for example, U.S. Pat. Nos. 5,783,120 and 5,882,774, herein incorporated by reference. With the desired birefringent material, the optical refractive index for the component of the incident light that is normal to the surface is different from the optical refractive index for the component of the incident light that is parallel to the surface normal. The desired birefringent material has an appropriately smaller refractive index for the light component that is parallel to the surface than for the light component that is normal to the surface. The difference in refractive indices is selected such that the birefringent layer has approximately the same optical thickness (which is the arithmetic product of the physical distance traversed by the incident light and the refractive index) for light incident at a large angle and light incident at the surface normal. This selection of the refractive index components also is constrained by the need to establish the necessary refractive index contrast at the interface between two adjacent layers of the interference filter. Such index contrast will need to be different for the two light components if the reflection of light by that interface is to not become suppressed at Brewster&#39;s angle. 
     Similar to other embodiments of the present invention, the programmable reflecting coating  133  for a labeling strip based on corner cube reflectors has three parts. The first part is a multi-peak transmission filter layer  141 . This multi-layer transmission filter may be constructed from one or more layers of birefringent materials, as described above. The second part is a multiple-layered programmable wavelength selective reflection layer  142 . One layer component of this programmable wavelength selective reflection layer  142  may be a non-photochromic birefringent material. The other layer component may be a photochromic material. Further, the photochromic material is preferably birefringent. Such a photochromic material may be obtained by doping a birefringent polymer such as polyethylene naphthalate or polyethylene terephthalate with an appropriate photochromic molecule such as one of the diarylethenes discussed in the descriptions of the other embodiments herein of the Programmable Optical Label invention. The third part is the black absorber layer  143 . 
     Referring to another exemplary embodiment  230  shown in  FIG. 16 , the multiple parts of the programmable reflecting coating  133  need not be directly adjacent to each other. The multi-peak transmission filter layer  141  could be located separately from the other parts of the reflecting coating  133 . In particular, the multi-peak transmission filter layer  141  is located at the front surface of a corner cube reflector (instead of at the back surface as in the previous embodiment). The other parts of the programmable reflecting coating  133  are located at the back reflecting surfaces  222  of the corner cube reflectors  221 . The incident light first encounters the multi-peak transmission filter layer  141  and then passes through the corner cube reflectors  221 , being selectively reflected from the back reflecting surfaces  222  of the corner cube structure. The programmable wavelength selective reflection layer  142  may be programmed to selectively reflect a particular interrogation wavelength. The black absorber layer  143  absorbs the light that reaches it and prevents those wavelengths from being retro-reflected. 
     Since the multi-peak transmission filter layer  141  may receive incident light from a large range of angles, it may preferably contain one or more layers of birefringent materials that enable the multi-peak transmission filter layer  141  to maintain its desired transmission spectrum over a larger range of incident angles, as discussed above in relation with the previous embodiments. 
       FIG. 17  illustrates another exemplary embodiment  240 , which is based on spherical lens  132 . The multi-peak transmission filter  141  of the embodiment is located at the front surface of the beaded retro-reflector structure (instead of at the back sides of the beads as in some previous embodiments). The embodiment  240  shown in  FIG. 17  is similar in many ways to the labeling strip  220  illustrated in  FIG. 12  but this embodiment  240  also contains a cover layer  241  over the front sides of the spherical lens  132 . The multi-peak transmission filter layer  141  is located at the front side of the cover layer  241 . The other parts of the programmable reflecting coating  133  are located at the back sides of the spherical lens  132 . The incident light first encounters the multi-peak transmission filter layer  141  and then passes through the cover layer  241 , spherical lens  132  and spacer layer  134 , before being selectively reflected from the programmable wavelength selective reflection layer  142 . The programmable wavelength selective reflection layer  142  may be programmed to selectively reflect a particular interrogation wavelength. The black absorber layer  143  absorbs the light that reaches it and prevents those wavelengths from being retro-reflected. The multi-peak transmission filter layer  141  may receive incident light from a large range of angles. Thus, it preferably may contain one or more layers of birefringent materials that enable the multi-peak transmission filter layer  141  to maintain its desired transmission spectrum over a larger range of incident angles, as discussed above in relation to the previous embodiments. 
       FIG. 18  illustrates another exemplary embodiment as a retro-reflecting strip  250  that contains an optical waveguide  251 . The programming light is supplied through the optical waveguide  251  instead of being supplied through the front face of the strip  250 . The waveguide  251  has a core  252  that also includes spherical lens  132 . The portions of the waveguide core  252  between adjacent spherical lenses  132  as well as at the outer ends of the waveguide  251  have the same refractive index. The waveguide core  252  and the spherical lenses  132  are sandwiched between layers of lower index material, the spacer, which acts as a cladding  253 . The combination of core  252  and cladding  253  functions as an optical waveguide for the programming light. A programming light beam  121  is supplied from one or more ends of the waveguide  251 . Optional filters  255  that select the desired set of programming wavelengths for a particular strip can be placed at one or more ends or edges of the waveguide. These filters may be similar to the multi-peak transmission filter layer  141  shown in  FIG. 10   b.    
     The spherical lenses  132  serve as engineered “scattering” elements that direct the programming light  121  out of the path of the waveguide  251  toward a programmable reflecting coating  133 , which in this embodiment, comprises the programmable wavelength selective reflection layer  142  and the black absorber layer  143 , covering the back portions of the spherical lens  132  that are not attached to the waveguide core  252 . The programmable wavelength selective reflection layer  142  is disposed closest to the spherical lenses  132  and adjacent to the cladding  253 . The black absorber layer  143  is disposed on the backside of the programmable wavelength selective reflection layer  142 . 
     Some of the programming light beam  121 , which propagates down the waveguide  251 , passes through a given spherical lens onto the next lens and some of that light is deflected (by total internal reflection from the curved lens/spacer interface) toward the programmable wavelength selective reflection layer  142  on the back side of the strip  250 . Furthermore, some of the programming light  121  is deflected toward the front side of the strip  250 , which could serve as an undesirable loss mechanism for the programming light  121 . A broadband reflection filter  257  that reflects the programming light wavelength may be added on the front side, and disposed on the outside of the spacer/cladding  253 , to selectively reflect the programming light  121  back into the lenses  132 . In another embodiment, another broadband reflection filter  257  for the programming light  121  may be added between the wavelength selective reflection layer  142  (which contains the photochromic material) and the black absorber layer  143 . Therefore, more of the programming light  121  that is captured by a given lens region (instead of propagating through that region) will be utilized to program the photochromic material. 
     Optical Identification System and Method Description 
       FIG. 1  shows a preferred embodiment of the Optical Identification System and Method invention. The Optical Identification System and Method invention comprises an optical label  101 , an optical interrogator  102  and a programmer  15 , as shown in  FIG. 2 . Referring to  FIG. 1 , the optical label comprises a collection of reflecting constructs  103  whose reflection can be programmed by the programmer  15 , as shown in  FIG. 2 . Methods to combine the reflecting constructs  103  into an optical label are known to a person with ordinary skill in the art. Embodiments of the Optical Identification System and Method invention show that the optical label may be constructed as a fabric comprising many of the constructs  103  in the form of strands of yarn, woven or knitted into the fabric. 
     Furthermore, an optical label  101  may comprise several different kinds of the reflecting constructs  103 , wherein each kind of the constructs  103  is associated with a particular wavelength of a wavelength code carried by the label  101  and capable of reflecting that particular wavelength of an interrogating light beam  105 . In particular, each reflecting construct  103  may be programmed to reflect or not reflect its associated wavelength. Therefore, the collection of constructs  103  on the label  101  can be programmed to reflect a certain pattern of wavelengths, according to a wavelength-code associated with the label. 
     In  FIG. 1 , the optical interrogator  102  illuminates the label  101  with an exemplary interrogating light beam  105  that comprises multiple wavelengths of light (λE 1 , λE 2 , λE 3 , λE 4 ). The label  101  reflects selected exemplary wavelengths  106  λE 1  and λE 4  that correspond to the code with which the label  101  has been programmed. If reflectance equals binary 1, absorbance equals binary 0 and λE 4  is the most significant bit, then the reflected code is equivalent to 1001. 
       FIG. 2  illustrates an exemplary coding process. The programmer  15  illuminates the optical label  101  when the label is being programmed to carry a particular wavelength code. The label  101  can be programmed and reprogrammed with various wavelength codes. A programming light beam  121  comprising specific programming wavelengths of light (e.g., λA 1 , λB 2 , λB 3 , λA 4  and λC) illuminates the label  101 . λC is a gating signal that enables programming by λA and λB. Each reflecting construct  103  of the label  101  comprises photochromic material that can be in either a first state or a second state. The specific combination of wavelengths of the programming light beam  121  sets and changes the states of the photochromic materials contained in the collection of reflecting constructs  103  of the label  101 . The programming light is then removed and the photochromic materials retain their states. 
     Referring back to  FIG. 1 , the states of the photochromic materials can be interrogated by an interrogating light beam  105 . Different reflecting construct  103  in the label  101  may be programmed differently by the programming light beam  121 . In this exemplary embodiment, each of the reflecting constructs  103  selectively accepts only certain programming wavelengths (e.g., λA 1 , λB 1 , and λC) and rejects the other programming wavelengths. Each construct  103  may be programmed to reflect, or not reflect, a particular λE. Since these different constructs  103  can be associated with different wavelengths of the interrogating light, a wavelength code can be programmed into the response produced by a collection of the constructs  103  of the label  101 . 
     Furthermore, both the interrogation and programming processes can be done with the interrogator  102  and programmer  15  located at a distance, without physical contact to the label  101 , since they are done with beams of light. The interrogator  102  and the programmer  15  do not have to be in contact with nor in physical proximity to the label  101  being interrogated or programmed. Instead, the standoff distance between the interrogator  102 /programmer  15  and the label  101  is determined primarily by the optical signal intensity that is needed to effectively accomplish the programming and the sensing of the wavelength code. Standoff distances of fractions of a meter to many tens or hundreds of meters may be possible, with the distances typically larger for interrogation than for programming. 
     The photochromic material in the programmable wavelength selective reflection filter  142  of the programmable reflecting structure  103  can be responsive to a large range of wavelengths. A narrow band filter response can be realized by incorporating that photochromic material into an optical interference filter structure. As shown in  FIG. 5 , the filter spectrum of that structure changes as the state of the photochromic material is changed by the programming light beam  121 . Furthermore, the reflecting construct  103  may be illuminated by a large range of wavelengths, with only some of the wavelengths associated with that particular construct  103 . The multi-peak transmission filter  141  of the first part selects the specific programming wavelengths for a particular reflecting construct  103 . The use of different transmission filters  141  in the first parts of different reflecting constructs  103  distinguishes the constructs. In this way, some constructs  103  may be programmed to be either reflecting or non-reflecting for one wavelength and other label constructs  103  may be programmed to be either reflecting or non-reflecting for a different wavelength. 
     One exemplary type of wavelength code of this invention may be a binary code wherein each wavelength represents a bit of the code. Each bit has a logical 1 value (reflecting construct  103 ) or a logical 0 value (non-reflecting construct  103 ). For improved code detection, the code words of 00 . . . 00 and 11 . . . 11 may be excluded. Thus, a 3 wavelength code can have 6 different code words; a 5 wavelength code can have 30 different code words. A 9 wavelength code can have 510 different code words. 
       FIG. 19  shows an exemplary embodiment of the optical programmer  15 . The programmer  15  generates light beam  121  comprising various programming wavelengths of the optical label  101  as shown in  FIG. 2 . The programmer  15  contains a number of light sources  35 , such as lasers, light-emitting diodes (LEDs) and flash lamps that emit the various programming wavelengths. These light sources  35  may be grouped into three sets. One set includes the sources  35  that produce the UV and blue wavelengths. A second set includes the sources  35  that produce the yellow to near IR wavelengths. A third set (e.g. 33) includes the longer IR wavelengths, generally 1300 nm or greater. A flash lamp can produce light at the wavelengths of all three sets. The flash lamp can be used in combination with optical multi-peak transmission filters  141  that select the specific programming wavelengths. One or more multi-peak transmission filters  141  would have their transmission peaks centered at each of the programming wavelengths. 
     These multi-peak transmission filters  141  preferably have passband widths that match the bandwidths of the programmable wavelength selective reflection filter  142  of the programmable reflecting structure  103  as shown in  FIGS. 4 and 5 . 
     Laser diodes and LEDs can produce light in these wavelength ranges discussed above. The laser diodes and LEDs offer a compact and potentially energy efficient means to generate the programming light beam  121 . The wavelengths of the LEDs that have been demonstrated already cover the entire range of the programming wavelengths. For example, LEDs have produced light at UV wavelengths ranging from 250 nm to 340 nm. See M. A. Khan, “Deep ultraviolet LEDs fabricated in AlInGaN using MEMOCVD,” SPIE Proceedings, vol. 5530 (2004), pp. 224-230; and J. Han and A. V. Nurmikko, “Advances in AlGaInN blue and ultraviolet light emitters, IEEE Journal on Selected Topics in Quantum Electronics, vol. 8, n. 2 (2002), pp. 289-297, herein incorporated by referernce. Other LEDs can emit at blue to blue-green wavelengths. See S. Nagahama, Y. Sugimoto, T. Kozaki and T. Mukai, “Recent progress of AlInGaN laser diodes,” SPIE Proceedings, vol. 5738 (2005), pp. 57-62, herein incorporated by reference. Furthermore, LEDs that emit at green, yellow, orange and red wavelengths also have been demonstrated. See R. S. Kern, “Progress and status of visible light emitting diode technology,” SPIE Proceedings, vol. 3621 (1999), pp. 16-27, herein incorporated by reference. In fact, LEDs that emit multi-color white light also have been demonstrated. See S. W. S. Chi, et al., “Multi-color white light emitting diodes for illumination applications,” SPIE Proceedings, vol. 5187 (2004), pp. 161-170, herein incorporated by reference. 
     The emission spectrum of some of these LEDs discussed above may be broader than the spacing between the various programming wavelengths. In that case, optical transmission filters  25  may be used to limit the spectral width of the light produced at each programming wavelength. One or more of these optical multi-peak transmission filters  25  can be placed at the output of the LED. Note that a multi-color white LED could be used in a manner similar to flash lamp, with different filters selecting different programming wavelengths from the emission spectrum of the LED. 
     Lasers have been demonstrated at many of the programming wavelengths. A laser typically has a much narrower emission spectrum than a LED. Thus, a transmission filter  25  likely would not be needed for the laser output. Also, many lasers can produce very high output powers. In particular, both high power lasers and high power LEDs have been demonstrated at the wavelength range of 1300-1800 nm. 
     In  FIG. 19 , a code selector  37 , typically an electronic circuit that provides drive power to the LEDs or lasers, can be used to select the specific combination of programming wavelengths desired. The optical outputs from the various light sources  33 ,  35  (and wavelength selection filters  25 ) are combined together with an optical beam combiner  39 . This beam combiner  39  may comprise a diffractive element (such as a grating) or some other known means for combining multiple beams of light into an output beam. The programmer  15  also may include some means, such as mirrors, to steer the output programming light beam  121 , whereby the beam  121  is directed toward particular spots on the labeled surface. 
       FIG. 20  shows an exemplary embodiment of an optical interrogator  102  as shown in  FIG. 1 . The optical interrogator  102  typically comprises a laser transmitter  41 , a receiver  43  and some telescope optics  55 . The laser transmitter  41  can comprise multiple laser sources  45 , with each laser source  45  emitting at a different interrogation wavelength. The outputs of these laser sources  45  may be combined together by a beam combiner  39  such as an optical coupler or wavelength multiplexer. An optical fiber  47  (optional) may be used to couple the laser light to the telescope optics  55 . The telescope optics  55  forms the output beam  105  that is directed toward an optical label  101  as shown in  FIG. 1 . The same or different telescope optics  55  also is used with the receiver  43 . The telescope optics  55  coupled to the receiver  43  receives a reflected light  106  from a label  101  as shown in  FIG. 1 . An optional optical fiber  47  may be used to couple the telescope optics  55  to an optical wavelength de-multiplexer  49  (or, alternatively, an optical power splitter). The de-multiplexer  49  separates the various wavelength components of the received light. The various wavelength components are then coupled into a plurality of photodetectors  51 , with one photodetector  51  associated with each wavelength (i.e., each code bit). The photodetectors  51  produce electrical signals that correspond to the intensity of the light at each of the received wavelengths. These intensities represent the wavelength coded response from the label  101 . 
     A decoding processor  53  compares these photodetector signals with each other and with set point values to determine the code that has been returned by the label  101 . In general, the photodetected signal associated with each interrogation wavelength is noisy. There can be noise associated with either a “1” received signal or a “0” received signal. The “1” received signal may have noise because of the uncertainty in the amount of reflecting material on the label that has been illuminated by the interrogating light beam  105  and in the clarity of the optical path between the interrogator  102  and the label  101 . Additional noise in a “1” signal also could be contributed by conditions such as atmospheric turbulence or scattering particles (e.g., fog or dust) in the optic path between the interrogator  102  and the label  101 . 
     Noise in a “0” signal typically is contributed by electronic components in the receiver  43 . Noise in a “0” signal also could be contributed by unwanted scattering or reflection of the interrogating light beam  105  by other regions of the labeled object (or even by the label  101  itself). The wavelength code words preferably comprise a combination of “1” and “0” values. The specific wavelength code words of “00 . . . 0” and “11 . . . 1” are excluded. This exclusion makes the decoding process less sensitive to attenuation or broadband scattering brought about by transmission through the path between the interrogator  102  and the label  101 . Decision points or values (which could be different for signals associated with different interrogation wavelengths) may be established that result in a desired probability of correct code word determination. 
     From the foregoing description, it will be apparent that the present invention has a number of advantages, some of which have been described herein, and others of which are inherent in the embodiments of the invention described or claimed herein. Also, it will be understood that modifications can be made to the device and method described herein without departing from the teachings of subject matter described herein. As such, the invention is not to be limited to the described embodiments except as required by the appended claims.