distortion

An aberration where an image does not exhibit rectilinear projection of an object. This aberration, "distortion" produces pin-cushion and barrel distortion for a square object. It should be noted that this aberration distorts the image but does not produce a blurred image as other aberrations do. In a TEM, the distortion becomes a matter of concern for the projector lens. In TEMs of previous generations, a distortion correction method was adopted, in which the barrel distortion of the intermediate lens was compensated by the pin-cushion distortion of the projector lens by setting the excitations of the intermediate and projector lenses to a similar value. In modern TEMs, however, the technique is not used to fix the projector lens to a strong excitation and to vary the excitation of the intermediate lens. In a real TEM, the image distortion is not serious because the distance from the projector lens to the viewing plane is large.

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