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We have, thus far, assumed the image-forming wavefronts to be spherical. As we understand from Chapter 2, the emerging wavefronts from the pupil are in general aspherical, even if the lens surfaces are spherical. Deviation of wavefronts from sphericity is called aberration. Within the domain of geometrical optics, an aberrated image can be examined by considering the intersection of aberrated rays with the image plane. Such treatment, adequate for instruments in which the aberrations are on the order of wavelengths, is known as the Seidel theory of aberrations. But the precision required of imaging systems in optical lithography means that the wave aberration is a small fraction of the wavelength. In this situation, the geometrical theory loses its validity; the Seidel treatment is insufficient. We need to study the effects of aberrations based on diffraction theory.
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