焦点深度

depth of focus, focal depth, DOF

[目次：理論]

試料のある点に焦点を合わせたとき、その正しい像面の前後にボケが十分に少ない像が得られる範囲がある。この範囲を焦点深度という。焦点深度は、入射プローブの開き角が小さいほど深くなり、観察倍率が高くなるほど浅くなる。 SEMの入射プローブの開き角は光学顕微鏡の場合に比べて、約100倍小さい。そのため、SEMの焦点深度は、光学顕微鏡の焦点深度の約100倍大きい。図1に、対物レンズの絞りの径を変えたときに、焦点深度が変わる様子を模式的に示す。図2に、対物絞り径の違い（入射プローブの開き角の違い）によって、焦点深度が異なるSEM像の例を示す。

対物絞り径の大小による焦点深度の違い⇒図1

絞り径の大小による焦点深度の比較⇒図2
(試料：電球のフィラメント、加速電圧10kV)
左：絞り径100μm、右：絞り径200μm

The depth of focus (DOF) or the focal depth is the range of distances for which the specimen (object) is imaged with an acceptable sharpness on the image plane. DOF is larger as the opening (aperture) angle of the incident probe is smaller but is smaller as the magnification of the image is higher.
The opening angle of the incident electron probe of the SEM is about 100 times smaller than that of the optical microscope. Thus, the DOF of the SEM is about 100 times larger than that of the optical microscope. Fig. 1 schematically shows the change in DOF when the diameter of the objective lens aperture is changed. Fig. 2 shows a comparison of SEM images taken at the different DOFs determined by the different objective lens aperture sizes.

Fig.1 Difference in DOF depending on the diameter of the objective lens aperture.

Fig.2 Comparison of two SEM images taken at the two different DOFs determined by the objective lens aperture sizes of 100 μm diameter (Left) and 200 μm diameter (Right).
Specimen: Filament of a light bulb. Accelerating voltage: 10 kV.