  Double detonations of sub-Chandrasekhar mass white dwarfs are a promising
explosion scenario for Type Ia supernovae, whereby a detonation in a surface
helium shell triggers a secondary detonation in a carbon-oxygen core. Recent
work has shown that low mass helium shell models reproduce observations of
normal SNe Ia. We present 3D radiative transfer simulations for a suite of 3D
simulations of the double detonation explosion scenario for a range of shell
and core masses. We find light curves broadly able to reproduce the faint end
of the width-luminosity relation shown by SNe Ia, however, we find that all of
our models show extremely red colours, not observed in normal SNe Ia. This
includes our lowest mass helium shell model. We find clear Ti II absorption
features in the model spectra, which would lead to classification as peculiar
SNe Ia, as well as line blanketing in some lines of sight by singly ionised Cr
and Fe-peak elements. Our radiative transfer simulations show that these
explosion models remain promising to explain peculiar SNe Ia. Future full
non-LTE simulations may improve the agreement of these explosion models with
observations of normal SNe Ia.
