RESTORATION FOR SAMPLED IMAGING SYSTEMS.

Abstract

Digital image restoration requires some knowledge of the degradation phenomena in order to attempt an inversion of that degradation. Typically, degradations which are included in the restoration process are those resulting from the optics and electronics of the imaging device. Occasionally, blurring caused by an intervening atmosphere, uniform motion or defocused optics is also included. Recently it has been shown that sampling, the conversion of the continuous output of an imaging system to a discrete array, further degrades or blurs the image. Thus, incorporating sampling effects into the restoration should improve the quality of the restored image. The system transfer function (the Fourier transform of the point spread function), was derived for the Landset Multi-Spectral Scanner and Thematic Mapper systems. Sampling effects were included, along with the relevant optical, instantaneous field of view and electronic filter data, in the system analysis. Using the system transfer function, a least squares (Wiener) filter was then derived. A Wiener filter requires the ratio of the power spectra of the scene and noise, which is often, for simplicity, assumed to be a constant over frequency. The restoration method used here includes models for the power spectra which are based on the study of several different types of Landsat scenes. The Wiener filter is then inverse Fourier transformed to find a restoration filter which is spatially windowed to suppress ringing. Qualitative and quantitative evaluations are made of the restored imagery. Comparisons are made to the approaches taken by other investigators, in particular, to one who has had success restoring the same type of imagery. It is found that the restoration method used here compares favorably with this previous work.