Are Spiral Disks Really Opaque ?

We compare the ultra-violet, optical, and far-infrared emission for a sample of 135 spiral galaxies in order to address the widely debated problem concerning the opacity of spiral disks. We find that the re-radiation of the dust, estimated from the far-infrared emission, is on average only 31$\pm 1$ percent of the bolometric luminosity of a spiral galaxy, indicating that less than one third of the stellar radiation is absorbed and then re-radiated by dust in a spiral disk. Applying a radiation transfer model which assumes a `Sandwich’ configuration for the spiral disk, and fully takes into account the effect of scattering, we find for our sample a median of the face-on blue (4400{\AA}) optical depth ${\tau }_{\mathrm{B}}=0.49$ and the mean $\rm <\tau_B> =0.60\pm 0.04$, indicating that most spiral galaxies in our sample are {\it not} opaque for blue light (${\tau }_{\mathrm{B}}<1$).