歪曲収差

distortion

[目次：レンズ系]

像が物体と相似な形にならない収差。物面の正方形上の位置から出射した電子がレンズを通ったあと、糸巻き型(pin-cushion)または樽型(barrel)に結像される収差。この収差は像を歪ませるが、他の収差のように像のボケを与えるものではない。電子顕微鏡では投影レンズで問題になる。昔の電子顕微鏡では中間レンズと投影レンズの励磁を同程度にして中間レンズによる樽型歪を投影レンズの糸巻型歪で打ち消すという方法(distortion free)が取られていた。近年の電子顕微鏡では投影レンズを強励磁に固定して使用し中間レンズを可変するためにこの歪の打消しは行われていない。実際には投影レンズから観察面までの距離が大きいので像の歪曲は目立たない。

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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