Images of periodic phase objects are calculated for oblique illumination, the scanning method of Dekkers and de Lang, and differential interference contrast. The close relation between the scanning method and oblique illumination is shown. All the methods are capable of resolving grating periods larger than λ/2a, where a is the numerical aperture of both the objective and the condenser lenses. Differential interference gives higher contrast for object periods below λ/a; contrast is slightly higher above λ/a with oblique illumination.
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