The existence of a photon subsection in the output section of the configuration file enables the photon output. If photons are enabled, the output file named Photons (followed by the appropriate suffix) is generated when SMASH is executed. It's format is identical to the collision output (see OSCAR collisions format), it does however only contain information about all particles participating in the photon producing interactions at each timestep.
Further, the block headers differ from the usual collision output:

# interaction in nin out nout rho density weight photon_weight partial part_weight type proc_type

where

density: Density at the interaction point
photon_weight: Weight of the photon process relative to the underlying hadronic interaction. Make sure to weigh each photon in your analysis with this value. Otherwise the photon production is highly overestimated.
part_weight: Always 0.0 for photon processes, as they are hardcoded.
proc_type: The type of the underlying process. See Process types for possible types.

Note, that "interaction", "in", "out", "rho", "weight", "partial" and "type" are no variables, but words that are printed.
The photon output is available in binary, OSCAR1999, OSCAR2013 and OSCAR2013 extended format.