Coding and quantization of computer generated Fresnel holograms

Computer generated holograms are thin diffractive structures with specified diffraction characteristics. In the Fourier case such a hologram possesses a desired Fraunhofer diffraction pattern. In order to obtain this pattern in a finite distance from the hologram plane it is illuminated by a convergent or divergent spherical wave. The application of a digital hologram as a diffraction optical element usually requires the incorporation of the convergence of the illuminating wave in the hologram structure. The hologram has to diffract as well as to focus the illuminating wave. Holography allows this combination. The resulting structure, a Fresnel hologram, is outlined.