  With the increasing precision requirements and growing spectrum of
applications of Monte Carlo simulations the evaluation of different components
of such simulations and their systematic ambiguities become of utmost interest.
In the following, we will address the question of systematic errors for Photos
Monte Carlo for simulation of bremsstrahlung corrections in final states, which
can not, in principle, be identified as a decay of resonances. It is possible,
because the program features explicit and exact parametrization of phase space
for multi-body plus multi-photon final states. The Photos emission kernel for
some processes consist of complete matrix element, in the remaining cases
appropriate approximation is used. Comparisons with results of simulations,
from generators based on exact phase space and exact fixed order matrix
elements, can be used. For the purpose of such validations Photos provides an
option to restrict emissions to single photon only. In the current work we
concentrate on final state bremsstrahlung in $q \bar{q}(e^+e^-) \to l^+l^-
l^+l^- \gamma$ and $\gamma\gamma \to l^+l^- \gamma$ processes. The reference
distributions used as a cross-check are obtained from the fixed-order MadGraph
Monte Carlo simulations. For the purpose of validation we concentrate on those
phase space regions where Photos is not expected to work on the basis of its
design alone. These phase space regions of hard, non-collinear photons, do not
contribute to large logarithmic terms. We find that in these phase space
regions the differences between Photos and MadGraph results do not surpass a
few percent and these regions, in turn, contribute about 10% to the observed
process rates. This is encouraging in view of the possible ambiguities for
precise calculation of realistic observables.
