The light coming from the supernovæ which we observe comes from the diffusion from the photons 
Co.
It is the study of the transport and the diffusion of these photons in
the successive layers of the supernova which is called radiative
transport. The diffusion is determined by the opacity of the mediums
show again which depends on their nature.
Contrary to the other astrophysical objects, the supernovæ of the Ia type contain neither hydrogen, nor helium, opacity is thus dominated by the diffusion of the free electrons and a very great number of atomic lines (in particular in UV). Moreover, the external layers acquire relativistic speeds.
Initially, the supernova is an opaque matter sphere expanding into which energy is injected in the form of photons resulting from the radioactive decreases. Progressively with the expansion of the matter, times of diffusion lengthen and become longer than times of expansion; the supernova becomes transparent.
The Doppler shifts of the atomic lines have significant effects because of the fast expansion of star: a photon emitted inside photosphere of the supernova will find matter increasingly transparent until meeting a Doppler line which will diffuse it several times. The spectrum can thus resemble a spectrum of black body whereas the temperature of the photon has nothing to do with the temperature of the matter.
Modelings still require to be refined in particular by the taking into account of the à.trois.dimensions effects. All in all, there is not for the moment of consensus on the methods and the results even if the starting ingredients are the same ones.