The fine detail in a photographic image is limited by the effective point spread of the optical system. Additional sources of image blur, such as uniform image motion, may further limit the detail. In an attempt to enhance the image and recover the detail masked by these degrading processes, we must consider how to design the restoring filter. One approach which we discuss is the inverse filter" where the noise in the image (granularity) and realizability are important factors which force us to modify the initial concept. An example of the restoration of detail blurred by uniform image motion is presented and the results discussed. Another approach to the design of the restoring filter is to require that the net point spread of the enhanced image be non-negative, a characteristic common to all real imaging optical systems for incoherent illumination. The requirement is translated into the conditions on the shape of the restoring filter. It is shown, for example, that the filter should not allow the modulation at any spatial frequency to increase above 1.0. This requires that the optical transfer function for the net degrading process be known. Other conditions on filter roll-off and curvature are also described.
R. E. Kinzly,
R. C. Haas,
P. G. Roetling,
"Designing Filters For Image Processing To Recover Detail", Proc. SPIE 0016, Image Information Recovery, (1 April 1969); doi: 10.1117/12.946801; https://doi.org/10.1117/12.946801