We study the correlations with inclination of H -band disk and bulge structural parameters and I - H colour profiles for a sample of 154 spiral galaxies , in order to detect possible effects due to internal extinction by dust .
The selection of the sample assures that galaxies at different inclinations are not intrinsically different , so that the observed correlations represent the real behaviour of the parameter considered .
All the parameters are derived from a bi-dimensional fitting of the galaxy image .
We find that extinction , though small at near infrared wavelengths , is sufficient to produce observable effects .
In particular the observed increase of the average disk scalelength and the reddening of the disk I - H colour at high inclinations are clear signatures of the presence of dust .
The total H -band disk luminosity depends little on inclination ; on the other hand significant corrections to the face-on aspect are derived for the H -band central disk brightness and the disk scalelength .
The bulge parameters exhibit little or no dependence on inclination .
Simulations carried out with a simple model for an internally-extincted galaxy show that these results imply a central H -band optical depth between 0.3 and 0.5 .