Ratio between the solid angle of a blackbody emitting half of the PACS fluxes at the blackbody temperature fitting those fluxes, and the measured half light solid angle, for 260 galaxies. Values approaching one indicate emission that is on average optically thick in the far infrared over the measured source size. Low values indicate optically thin emission and/or a scattered distribution of small sources which might still have considerable far-infrared optical depth. The inset shows the relation between this ratio of solid angles and the far-infrared surface brightness. The tightness is somewhat trivial, because both axes involve the measured solid angle and galaxies span a limited range of dust temperatures.