  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 $\rm \tau_B=0.49$ 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 ($\rm \tau_B <1$).
