Optical Processes Of String Images And Their Retrieval :

Nabel Ibrahem Hendawy. 

After the discovery of laser and holography, the problem ofinterferometry in diffused light gained the attention of manywor~ers all-over the world. This resulted in an enormous numberof publications on various aspects of the problem. One of theimportant aspects of the problem is the study of the interferencephenomenon in storing and retrieval the images.The use of film as a linear storage medium is a relativelynew application in the history of photographic emulsion. Theoverlapping exposures are stored linearly by controlledphotographic processing which determine the storage capability ofthe film. Spatial filtering techniques are then applied toseparate the image spectra and to retrieve the individual images.The image of a laser illuminated surface has a random finestructure called a speckle pattern. This structure is due to thecoherent addition of the diffraction patterns generated in theimage plane by each point of the object. It can be said brieflythat the random properties of the image depend on the nature ofthe object. on its shape and inclination of the incident laserbeam. The mean properties of the image depend only on the imaginglens. Particularly, the mean size of speckle is given by thediameter of the Fraunhofer diffraction pattern of the imaginglens and the spectrum of image irradiance is modulate”! by thetransfer function of the imaging lens. Consequently, the Fourierspectru~ of an intensity speckle pattern is spread out.In the first chapter of this wor~,the characteristics of thedifferent types of holographic plates are discussed. Holographic-iiplatesas an optical device for image storage, their noise andsignal to noise ratio are also studied. To increase theefficiency of the hologram, the amplitude hologram must beconverted to phase hologram by bleaching processes. Differenttypes of bleach baths are investigated.In the methods of information processing described here(chapters II and III) the low-frequency optical signals aremodulated by a random intensity distribution to spread outinformation in the Fourier plane. In chapter II, the doubleexposuretechnique is studied theoretically and experimentallyfor image storage and retrieval of amplitude objects. TwoAmplitude objects, Eiffel tower and the Soviet PaLace, in theform of positive slides are stored and retrieved optically. Thetechnique is based on the fact that two laterally translated andidentical intensity speckle patterns display a system of Young’sfringes in their Fourier plane. The contrast of fringes ismaximum and their spacing is inversely proportional to thelateral shift.If the laterally displaced speckle patterns are notidentical, the contrast of the corresponding Young’s fringes isno longer maximum. Consequently, the contrast of Young’s fringesdisplayed in the Fourier plane by two laterally displaced specklepatterns represents their correlation degree.Moreover, not only single images are stored and retrieved butalso two overlapping images are stored and individuallyretrieved.-iii-In chapter III, an improvement of the technique of storageand retrieval images and to increase the capacity of storage arebrought about as a result of multiple exposures. Coherent lightsource has been employed in the storage as well as in theretrieval of the images. Two signals i.e. Eiffel tower and SovietPalace are recorded on a single frame with an equal lateraldisplacements for each signal. The noise of the retrieved imagecan be suppressed by taking the exposure time in the form ofbinomial coeffiCient ratio. Up to five exposures are studied inthis work. The factors affecting the contrast of the individualretrieved image are also studied. The maximum number of signalsthat can be multiplexed is discussed.