Expected fluxes of secondary photons for a pure proton composition, a spectral index β = 2.0, a maximum energy E_max = 10²¹ eV, compared to estimates by Kalashev et al. (2009) and Ahlers et al. (2010). The units are arbitrary but the different fluxes were consistently normalized with respect to their corresponding cosmic ray flux at 10¹⁹ eV (these UHECR fluxes are omitted in this figure, but agree very well). The IR/Opt/UV backgrounds estimated by Kneiske et al. (2004) and Stecker et al. (2006) are used for these comparisons (see text).