  Entropy production is a key concept of thermodynamics and allows one to
analyze the operation of engines. For the Scovil-Schulz-DuBois heat engine, the
archetypal three-level thermal maser coupled to thermal baths, it was argued
that the common definition of heat flow may provide negative entropy production
for certain parameters [E. Boukobza and D. J. Tannor, Phys. Rev. Lett. 98,
240601 (2007)]. Here, we show that this can be cured, if corrections for
detuning are properly applied to the energies used for the bath transitions.
This method can be used more generally for the thermodynamical analysis of
optical transitions where the modes of the light field are detuned from the
transition energy.
