A new Holographic Method of Generating a High Efficiency, Extended Range Spatial Filter with Application to Restoration of Defocussed Images

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Abstract
After a short analysis of current methods for preparation of spatial filters for restoration of defocussed images it is shown that such a filter can be made in a single step by holographic technique which uses a very weak reference beam. A high reconstruction efficiency is obtained since bleaching of the filter is necessary in order to realize the desired filter function. Measurements on a monobath developer show that the diffraction efficiency of a hologram can be made almost independent of the exposure over a range greater than 250:1, possibly of the order of 1 000:1. This effect has been utilized to increase significantly the range of linear response of the filter. It is also shown that the filter generated in this way is automatically optimum with respect to granularity noise in the input image. In order to reduce the number of optical elements in the image processing system the filter has been made self-focussing and acts as the imaging lens. A quantitative analysis of the effects of defocussing and restoration has been made possible by introducing a spoke target in the picture to be restored.