Image Reconstruction with Fraunhofer Holograms*

The holograms considered are formed from opaque or transparent diffracting objects which are contained in an aperture illuminated with a coherent, collimated, quasimonochromatic beam of light. It has been shown that the intensity distribution in the near-field of the aperture, but in the far-field of the individual objects which are contained in the aperture, is given by a function whose essential term represents the interference between the Fraunhofer diffraction pattern from the object and the coherent background. The hologram thus formed is referred to as a Fraunhofer or far-field hologram because of the imposed condition. The reconstruction, which is accomplished by placing the recorded hologram in another coherent collimated quasimonochromatic beam and again going to the far-field of the individual objects, yields an intensity which is essentially the original object distribution. In the far-field region of the individual objects for which this result is valid, the reconstruction is seen to be devoid of the evidence of a virtual image. One advantage of this particular method is that the virtual image which appears in the conventional (Fresnel) hologram method creates no problem here since it reduces to a constant for the far-field approximation.