Abstracts Category : Other

Coherent optical techniques for processing stereophotographs

by Victor Paul Bennett

The detection of the regions of conjugate imagecoincidence between two transparencies finds many applications. Inparticular it forms a powerful means of extracting direct heightinformation from stereo photographs. This thesis presents threedifferent methods of achieving image coincidence detection. Thetheoretical basis for each of the methods described and theexperimental results are presented to substantiate the basicconcepts. </br>The first method detects thecorrelation between two spatially varying signals (i.e., theamplitude transmittance of two transparencies) and it is achievedusing optical heterodyne detection. The basic system consists of aMach-Zehnder interferometer. The vertical stereo transparencies arerelatively oriented, one in each arm of the interferometer, toobtain the same scale in the common image plane. Through theoscillation of one of the mirrors of the interferometer, thefrequency of the light in one arm is shifted resulting in ascanning fringe pattern in the common image plane. A smallphotodetector placed in the common image plane is used to producethree current signals. The three current signals are provided by:one, the intensity due to only one arm of the interferometer, two,the intensity due to the other arm, and, three, the intensity whenboth arms are open. Analysis shows that these current signals aresufficient to determine image matching (of the two transparencies)over the region of the detector regardless of the relative exposurelevels of the stereo pair. It is shown that this method can also beextended to non-vertical transparencies. </br>Thesecond method (image plane filter method) of image coincidencedetection makes use of the properties of a hologram. In this methodthe two vertical stereo transparencies are bleached, which convertsdensity variations to phase variations. An image plane hologram ismade of one of the bleached transparencies and the hologram recordsthe interference between the image wavefront and an off-axis planewave. When the processed hologram is illuminated the off-axis planewave is blocked and the bleached transparency used to make thehologram is replaced by the conjugate bleached transparency. Onthose regions where the conjugate transparency has the same phaserelief as the transparency used to make the hologram, the hologramreconstructs the off-axis plane wave. A suitable spatial filter isplaced behind the hologram to remove the extraneous scatteredlight. The light that is allowed to pass is used to illuminate animage plane giving a fringe of equal height. </br>The third method (pseudo-Fizeau fringe method)does not directly detect image coincidence but instead produces aninterferogram from which the height information is read. Theinterferogram is formed by placing a vertical stereo pair in aMach-Zehnder interferometer, one in each arm. The twotransparencies are imaged onto a common image plane. In the backfocal plane of the imaging lens a knife edge is placed. Theintroduction of the knife edge deforms the