<title>Immersion lenses for low-voltage SEM and LEEM</title>

Spherical and chromatic aberration coefficients C_s and C_c of various immersion lenses for low voltage SEM and LEEM are calculated. The minimum values of magnetic immersion lens are C_s equals C_c equals 1 mm. For the combined electrostatic and magnetic lenses, those values are at most C_s equals 1 mm and C_c equals 0.7 mm, when the specimen is free from the electrostatic field. When the specimen is immersed in the electrostatic field, those values reduce to C_s equals 0.2 mm and C_c equals 0.1 mm at 1 kV.