  The investigation of light-induced structural changes by diffractive methods
has improved much in the last two decades. We present here the case of neutron
photocrystallography for which we have built a special experimental setup at
the single crystal neutron diffractometer TriCS at the Swiss Spallation Neutron
Source SINQ. We illustrate the progress of the method on the example of the
structural determination of photoinduced nitrosyl linkage-isomers in
Na2[Fe(CN)5NO] 2H2O. The in-situ determination of the population of the
light-induced linkage isomers by optical transmission measurements enhances the
reliability of such structural investigations considerably. Additionally we
present a new simulation tool within the program package JANA2006 which allows
to plan a photocrystallographic experiment thoroughly since the required
q-range, the minimally needed population of the photoinduced species, as well
as the necessary counting statistics for a successful single crystal
diffraction experiment can be evaluated in advance.
