Bias level reduction of incoherent holograms.

The known techniques for making incoherent holograms succeed for an object containing a small number of points; however, they fail in practice for an object with a great many discrete points or for an object with a continuous intensity distribution. This results because the intensity pattern generated in incoherent holography in a superposition of random intensity patterns generated by the individual object points. Thus, the intensity pattern contains a large bias level which masks the relatively small spatial variations of the pattern in the recording film grain noise. This paper describes a technique for eliminating the bias. By introducing a narrow band time modulation of the light in one path of an interferometer used to generate the hologram intensity pattern, the spatially varying part of the hologram intensity pattern is modulated at the time frequency, while the bias part is unmodulated. The bias part can then be eliminated by performing a point-by-point time correlation over the hologram plane. Experimental results are presented.