The quantity of dust in a spiral disk can be estimated using the dust ’ s typical emission or the extinction of a known source .
In this paper , we compare two techniques , one based on emission and one on absorption , applied on sections of fourteen disk galaxies .
The two measurements reflect , respectively the average and apparent optical depth of a disk section .
Hence , they depend differently on the average number and optical depth of ISM structures in the disk .

The small scale geometry of the cold ISM is critical for accurate models of the overall energy budget of spiral disks .
ISM geometry , relative contributions of different stellar populations and dust emissivity are all free parameters in galaxy Spectral Energy Distribution ( SED ) models ; they are also sometimes degenerate , depending on wavelength coverage .
Our aim is to constrain typical ISM geometry .

The apparent optical depth measurement comes from the number of distant galaxies seen in HST images through the foreground disk , calibrated with the “ Synthetic Field Method ” ( SFM ) .
We discuss what can be learned from the SFM measurement alone regarding ISM geometry .

We measure the IR flux in images from the Spitzer Infrared Nearby Galaxy Survey in the same section of the disk that was covered by HST .
A physical model of the dust is fit to the SED to estimate the dust surface density , mean temperature , and brightness in these disk sections .
The surface density is subsequently converted into the average optical depth estimate .

The two measurements generally agree and the SED in order model finds a mostly cold dust ( T < 25 K. ) .
The ratios between the measured average and apparent optical depths of the disk sections imply optically thin ( \tau _ { c } = 0.4 ) clouds in these disks .
Optically thick disks , are likely to have more than a single cloud along the line-of-sight .