  Using a samples of 61506 spiral galaxies selected from the SDSS DR2, we
examine the luminosity function (LF) of spiral galaxies with different
inclination angles. We find that the characteristic luminosity of the LF,
$L^*$, decreases with increasing inclination, while the faint-end slope,
$\alpha$, depends only weakly on it. The inclination-dependence of the LF is
consistent with that expected from a simple model where the optical depth is
proportional to the cosine of the inclination angle, and we use a likelihood
method to recover both the coefficient in front of the cosine, $\gamma$, and
the LF for galaxies viewed face-on. The value of $\gamma$ is quite independent
of galaxy luminosity in a given band, and the values of $\gamma$ obtained in
this way for the 5 SDSS bands give an extinction curve which is a power law of
wavelength ($\tau\propto\lambda^{-n}$), with a power index $n=0.96\pm0.04$.
Using the dust extinction for galaxies obtained by Kauffmann et al. (2003), we
derive an `extinction-corrected' luminosity function for spiral galaxies. Dust
extinction makes $M^*$ dimmer by about 0.5 magnitudes in the $z$-band, and
about 1.2 magnitudes in the $u$- band. Since our analysis is based on a sample
where selection effects are well under control, the dimming of edge-on galaxies
relative to face-on galaxies is best explained by assuming that galaxy disks
are optically thick in dust absorptions.
