Comparison of the median dust colour temperature and the ISRF strength obtained from RT models. Blue crosses correspond to T_dust values obtained from the full surface-brightness data. The red circles correspond to values obtained from background-subtracted observations. The k(ISRF) values are obtained by modelling the same background-subtracted data. The diameter of the circles is proportional to the logarithm of the median column density of a field. These illustrates the effect of cloud opacity on the relation between the derived radiation field and dust temperature estimates.