  The exact resolution function in transfer vector for the reflectometer
HERM\`ES at Laboratoire L\'eon Brillouin is calculated as an example of neutron
time-of-fligh spectrometer with double-disk chopper. Calculation accounts for
wavelength distribution of the incident beam, tilt of the chopper axis,
collimation and gravity, without approximation of gaussian distributions or
independence of these different contributions. Numerical implementation is
provided that matches the sections of the paper. We show that data fitting
using this exact resolution function allows us to reach much better results
than its usual approximation by a gaussian profile.
