Motivations leading to lowering the primary beam energy in the SEM are summ
arized and arguments are presented in favour of extending the operation ran
ge down to tens and units of eV. Possibilities to produce beams at low ener
gies below 1 keV and very low energies below 50-100 eV are reviewed, partic
ularly the principle to keep the primary beam travelling fast within the co
lumn and to decelerate it near the specimen. Both possible retarding field
elements, an electrostatic immersion lens and a cathode lens, are compared
in detail as regards important parameters of microscopes containing them. T
hese include imaging parameters and aberrations, electric field on the spec
imen surface, possible electrical and mechanical configurations :including
the detection principles, and sensitivity to stray fields.