minimisation for IC63 and the Orion Bar. Top: FWHM of the observed dust emission profiles in different photometric bands as a function of zo (left for IC63 and right for the Orion Bar). Bottom: as a function of z₀ (left for IC63 and right for the Orion Bar). The vertical black dashed lines corresponds to the minimum value of . Radiative transfer has been done using diffuse ISM-like dust with an incident radiation field corresponding to a blackbody at 25 000 K (38 000 K) with G₀ = 1100 (G₀ = 2.6 × 10⁴) for IC63 (Orion Bar).