Elliptical polarizers, tags and identification systems using frequency selective surfaces

An identification system, comprising:

• at least one tag having encoded information attached to a surface of an object to be identified, said tag comprising a first elliptical polarization filter, said first elliptical polarization filter comprising a first frequency selective surface (FSS) disposed on said surface, said first FSS comprising least a first periodic pattern of spaced apart electrically conductive lines having a sub-wavelength line-to-line spacing, said lines including at least one subwavelength geometrical feature, said pattern being operable to receive incident radiation having a wavelength between 400 nm and 1 mm and output radiation having a phase differential for a first orthogonal linear polarization state relative to the other linear polarization state;
• a receiver remotely located from said tag operable for differentially attenuating said first and said other orthogonal polarization states comprising a second elliptical polarizing filter coupled to receive said output radiation, said second elliptical filter comprising a second FSS comprising least one periodic second pattern of spaced apart electrically conductive lines having a sub-wavelength line-to-line spacing, said lines including at least one subwavelength geometrical feature, and a first linear polarizer optically coupled to said second elliptical polarizing filter;
• a photodetector optically coupled to said linear polarizer for generating an intensity pattern, and
• a processor operable for determining whether said intensity pattern corresponds to said encoded information,
• wherein said first pattern comprises a plurality of first and a plurality of second sub-patterns, said lines associated with said first sub-pattern aligned along a first axis and said lines associated with said second sub-pattern being along a second axis that is rotated 70 to 90 degrees relative to said first axis, and
• wherein at least a portion of said second sub-pattern is substantially aligned and has at least a pattern portion substantially matching said first sub-pattern or said second sub-pattern is operable for passing said output radiation from said first or said second sub-pattern, wherein after processing by said linear polarizer and detection by said photodetector radiation transmitted by said pattern portion forms bright regions in said intensity pattern.