Retroreflective optical system and methods

A retroreflective optical system for creating a passive optical tag in an absence of electrical power, involving: a retroreflector having a surface and a retroreflective element disposed in relation to the surface, the retroreflective element configured to: passively impart a unique signature in relation to incoming light by using at least one of spectral filtration and color filtration, whereby a plasmonic response is effectible; and reflect outgoing light having the unique signature; and an optical device having an input aperture, the optical device disposed at a distance from the retroreflector and configured to transmit the incoming light and the outgoing light.