  We suggest and verify a new photometric method enabling derivation of
relative thickness of a galactic disk from two-dimensional surface-brightness
distribution of the galaxy in the plane of the sky. The method is applied to
images of 45 early-type (S0-Sb) galaxies with known radial exponential or
double-exponential (with a flatter outer profile) surface-brightness
distributions. The data in the r-band have been retrieved from the SDSS
archive. Statistics of the estimated relative thicknesses of the stellar disks
of early-type disk galaxies shows the following features. The disks of
lenticular and early-type spiral galaxies have similar thicknesses. The
presence of a bar results in only a slight marginal increase of the thickness.
However, we have found a substantial difference between the thicknesses of the
disks with a single-scaled exponential brightness profile and the disks that
represent the inner segments of the Type III (antitruncated) profiles. The
disks are significantly thicker in the former subsample than in the latter one.
This may provide evidence for a surface-brightness distribution of a
single-scaled exponential disk to be formed due to viscosity effects acting
over the entire period of star formation in the disk.
