An expression is derived for the energy resolution of a planar trochoi
dal monochromator. The expression is generally valid for entrance and
exit apertures (beams) of circular cross section, and obviates the pre
vious practice of assigning to the resolution an ad hoc term represent
ing the full potential drop across the entrance aperture. Good agreeme
nt is demonstrated with results of practical monochromators over a wid
e range of energies.