aperture diaphragm
/ˌantɪfɹˈɪkʃən ˈpad/
n
English · aperture diaphragm中文 · 孔径光阑Español · abertura del diafragmaFrançais · diaphragme d’ouverture
意味：Device with an opening whose diameter can be changed to narrow or widen the diameter of the electron beam.