The role of the coherence function in holographic imaging.

Abstract

A holographic system illuminated with a temporally or spatially incoherent source can produce significant imaging characteristics. In this situation, the image from the hologram become localized with the consequence that different parts of the image can be seen only through different parts of the hologram. This distribution of the localized image on the hologram is determined by the coherence of the light source. The concept of coherence cell is introduced to model the holographic image formation with an incoherent source. Each coherence cell represents a localized area on the hologram for one object point where its image can be reconstructed and the integration of all coherence cells produces a motion picture of the optical field interacting with the object. The interaction can be a time or space function depending on the type of the incoherence of the source. A light-in-flight image is produced by coherence cells if a temporally incoherent but spatially coherent source is used in holography. The source for producing such an image can be either a pulse laser or a CW broad spectrum laser, and in our case, a CW swept frequency dye laser. The first arriving light principle for image formation through highly diffusing media is demonstrated by light-in-flight holography. Conversely, if the source is spatially incoherent but temporally coherent, the holographic system can become a lensless image system in which the image is formed by the mutual intensity function. In this case, the coherence cells function like a bridge connecting the object plane with the hologram plane and by which the image can be transferred directly. A holographic counterpart of the confocal scanning microscope is found to be incoherent image plane holography. This holographic system has some interesting properties, such as depth discrimination and suppressing of noise from out of plane scatterers, that are also possessed by confocal scanning imaging; however, the holographic system requires no scanning. The property of depth discrimination enables us to build a non-scanning confocal ranging system based on incoherent image plane holography. The system can measure the 3-D contour of an object by slicing one cross-section image at each depth and scanning through the entire depth of the object.