Median filters are of interest because they eliminate impulse noise and preserve image edges. The output of these nonlinear filters depends upon the local statistics and therefore, no analytic expression exists to describe filter performance. Performance can only be inferred by using representative inputs. For imaging systems operating at or near the Nyquist frequency, median filters will eliminate unresolved and small targets. The effects of median filters are noticeable on periodic targets such as the four-bar resolution target used for minimum resolvable temperature measurements.