Patent US 6,563,582

In addition to having color, light waves have the attribute of polarization. An apparatus and method to convert circular polarized light into linearly polarized light over a wide range of wavelengths is provided by utilizing a first surface-relief grating functioning as a quarter-wave waveplate and a second surface-relief grating functioning as a half-wave waveplate. A plurality of such devices are arranged in a two-dimensional array and combined with an array of linear polarizers and an array of photodetectors to form a polarization imaging sensor. Such a sensor could have applications in automobiles to alert drivers of the presence of other vehicles, especially at night, in fog, or in rain. Military applications include the detection of vehicles placed among trees and shrubs. Another advantage of using circular polarization images is that the sign and magnitude of the circular polarization can potentially be used to reveal the spatial orientation, material, and surface roughness of the object's surface.