The 'double detonation' explosion mechanism has been proposed as an explanation for Type Ia supernovae. In this scenario, helium accreted onto the surface of a sub-Chandrasekhar mass white dwarf detonates, leading to a second carbon detonation in the WD core. We present radiative transfer simulations for new 3D hydrodynamical simulations of the double detonation scenario. Our models show strong asymmetries arising from the off-centre detonation. We investigate both angle averaged properties and viewing angle effects for the light curves and spectra produced. We find the absorption viewed by an observer is a strong function of orientation. Our spectra show stronger absorption features than observed for normal SNe Ia due to the He detonation ash. SN 2016jhr and SN 2018byg have been specifically suggested to be triggered by He detonations. We find our equatorial line of sight spectra match the strong absorption features found in SN 2016jhr, and our extreme viewing angle spectra match the significant line blanketing of SN 2018byg. These results illustrate the need for multi-D simulations. Future work will investigate the effects of He shell and WD mass.