The opacity of a spiral disk due to dust absorption influences every measurement we make of it in the UV and optical .
Two separate techniques directly measure the total absorption by dust in the disk : calibrated distant galaxy counts and overlapping galaxy pairs .
The main results from both so far are a semi-transparent disk with more opaque arms , and a relation between surface brightness and disk opacity .
In the Spitzer era , SED models of spiral disks add a new perspective on the role of dust in spiral disks .
Combined with the overall opacity from galaxy counts , they yield a typical optical depth of the dusty ISM clouds : 0.4 that implies a size of \sim 60 pc .
Work on galaxy counts is currently ongoing on the ACS fields of M51 , M101 and M81 .
Occulting galaxies offer the possibility of probing the history of disk opacity from higher redshift pairs .
Evolution in disk opacity could influence distance measurements ( SN1a , Tully-Fisher relation ) .
Here , we present first results from spectroscopically selected occulting pairs in the SDSS .
The redshift range for this sample is limited , but does offer a first insight into disk opacity evolution as well as a reference for higher redshift measurements .