Rainbow holography using external spatial filters for chromatic corrections

Abstract

A promising method using external spatial filters is described for the chromatic correction of images from transmission holograms produced in white light. The spatial corrective filter or screen represents a 1-D random diffuser and is characterized by the sinc function describing the Fourier transform of its amplitude transmittance. Holograms of nondiffuse or specularly reflecting objects, for which the far field diffraction pattern can be approximated to the delta function, have been considered for chromatic corrections by these spatial filters. However, the technique is also applicable to opaque 3-D objects coated with ret- roreflective materials. By using suitable optics the image of the object illuminated with a parallel beam of laser light is formed downstream with respect to the Fourier plane. The hologram is recorded in a plane suitably positioned with respect to the image plane. When reconstructed with a conjugate wave a real image of the Fourier transform plane is obtained downstream with respect to the real image of the object. In white light a highly blurred image is obtained on a ground glass screen placed in the plane of the image. Now let the corrective spatial filter screen be placed in the plane of the real image. It would produce a slitlike distribution containing the object information in the Fourier plane. A chromatic corrected image is then obtained in the same manner as in the rainbow holographic reconstruction process.