IR luminosity surface density vs. dust temperature for dusty galaxies in the literature and this work. Left and right panels show dust temperatures from optically thin and thick models, respectively. Blue squares mark data points from Simpson et al. (2017); red diamonds show the ALESS sample in Hodge et al. (2019). The AS2UDS sources (Stach et al. 2019) are shown in open circles, which are calculated using results from Gullberg et al. (2019) and Dudzevičiūtė (2020). The Stefan-Boltzmann (S-B) law is shown as a solid curve, representing the expected limit for a black body. The dot-dashed curve marks the 10% systematic bias of T_d, thin, where above the curve T_d would be underestimated more than 10% by thin models. Dashed curves show the expected relations for general models with different λ_eff as labeled. This figure provides a diagnostic method identifying optically thick dust.