  In this work we determine the parameters of spiral structure for a sample of
face-on spiral galaxies. In practice, the solution of this problem is a hard
task because of the diversity of the observed characteristics of spiral
structure, such as the arm number, their shape, arm contrast etc. In this work
we study spiral structure in galaxies based on an analysis of photometric cuts
perpendicular to the arm direction. The method is based on an approximation of
these slices with an analytical function and derivation of the parameters of
spiral structure (arm width, asymmetry, pitch angle) using the fitted
parameters of this approximation. The algorithm has been applied to a sample of
155 galaxies selected from the Sloan Digital Sky Survey in different passbands.
In this paper we only consider the results on the arm width: most spirals show
an increase of their width with galactocentric distance. Only 14 per cent of
galaxies in our sample show an opposite trend or have an almost constant arm
width at all radii.
