Real-space filtering of electron images from optical autocorrelation patterns.

The capability for filtering electron images, in real space, is demonstrated to be inherent to a two plate, incoherent optical procedure (originally described by Meyer-Eppler and Darius) for recording autocorrelation functions from transparencies. If an image contains features that occur with more than random frequency, these features may give rise to a resolvable peak in the optically recorded autocorrelation function. The optical requirements for obtaining an image of the transparency that is filtered to observe only features giving rise to some set of such peaks, or to exclude them, are described. The principle is to form an image of the transparency, with a properly placed plano-convex lens, from the incoherent light transmitted through apertures positioned over the peaks in the autocorrelation plane. The application of the method in defining the position and orientation of specific projections of protein molecules, as observed in negative stain by bright-field, or unstained by dark-field electron microscopy, is also described.