Abstract
Image quality is improved definitely by a local variation of the transmission factor which provides in any optical system a non uniform illumination of the exit pupil. One of the most favorable distributions of transmission factor T(X) is the EQUILIBRIUM DISTRIBUTION which corresponds to the greatest concentration of radiant energy in a diffraction pattern around its focus. The criterion used to determine T(X) is the FACTOR OF ENCIRCLED ENERGY E(W) which is the ratio of the energy distributed within a circle of radius W centered on the focus, to the total energy of the diffraction pattern. When an amplitude filter which provides T(W) is fitted to an aberration-free optical system, the diffraction pattern appears, in practice, as a luminous disk without rings of diffracted light. Consequences are in most cases a higher resolution, a more correct contrast relationship and sharper images of extended objects. Moreover geometrical aberrations are partly reduced; For example depth of focus is increased and chromatism partly corrected.
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