ABSTRACT
In order to transform and filter the images within an optical processing system, a useful tool is the diffractive optical element (DOE). The DOE is a beam shaping optical element for a coherent light beam, where beam shaping is achieved by significant modulation of the refractive index of the element on the microscale. In contrast to refractive optical elements such as a lens, or a prism, the modulation of refractive index occurs over shorter spatial distances. Due to this high spatial frequency modulation, individual elements of the image can be processed differently to other elements, so that geometrical transformation of the image can be accomplished [34]. This is known as space variant interconnection in contrast to the space invariant interconnection provided by a lens or a bulk prism. DOEs are widely used in optical systems, which require space variant connections, such as optical interconnects and multichannel Fourier optical systems. Feature extraction and image transforms with DOEs have been described in [222,224–226].
