  We apply the radiative transfer (RT) code SKIRT on a sample of ~14000
low-redshift (z<= 0.1) galaxies extracted from the TNG50 simulation to enable
an apples-to-apples comparison with observations. The RT procedure is
calibrated via comparison of a subsample of TNG50 galaxies with the DustPedia
observational sample: we compare several luminosity and colour scaling
relations and spectral energy distributions in different specific SFR bins. We
consistently derive galaxy luminosity functions for the TNG50 simulation in 14
broadband filters from UV to submillimetre wavelengths and investigate the
effects of the aperture, orientation, radiative transfer recipe, and numerical
resolution. We find that, while our TNG50+RT fiducial model agrees well with
the observed luminosity functions at the knee (+/- 0.04 dex typical agreement),
the TNG50+RT luminosity functions evaluated within 5R_1/2 are generally higher
than observed at both the faint and bright ends, by 0.004 (total IR)-0.27
(UKIDSS H) dex and 0.12 (SPIRE250)-0.8 (GALEX FUV) dex, respectively. A change
in the aperture does affect the bright end of the luminosity function, easily
by up to 1 dex depending on the choice. However, we also find that the galaxy
luminosity functions of a worse-resolution run of TNG50 (TNG50-2, with 8 times
worse mass resolution than TNG50, similar to TNG100) are in better quantitative
agreement with observational constraints. Finally, we publicly release the
photometry for the TNG50 sample in 53 broadbands from FUV to submillimetre, in
three orientations and four apertures, as well as galaxy spectral energy
distributions.
