Heat radiation at the nanoscale is a relatively young but flourishing research field, that has attracted much attention in the last decade. This is on the one hand due to the fact that this effect is nowadays experimentally accessible (Hu et al. (2008); Kittel et al (2005);Narayanaswamy et al. (2008); Ottens et al. (2011); Rousseau et al. (2009); Shen et al. (2009)), and on the other hand due t…