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Different spiral generation mechanisms are expected to produce differentmorphological and kinematic features. In this first paper in a series wecarefully study the parameters of spiral structure in 155 face-on spiralgalaxies, selected from the Sloan Digital Sky Survey, in the three $gri$ bands.We use a method for deriving a set of parameters of spiral structure, such asthe width of the spiral arms, their fraction to the total galaxy luminosity andtheir colour, which have not been properly studied before. Our method is basedon an analysis of a set of photometric cuts perpendicular to the direction of aspiral arm. Based on the results of our study, we compare the main threeclasses of spirals: grand design, multi-armed and flocculent. We conclude that:i) for the vast majority of galaxies (86\%) we observe an increase of their armwidth with galactocentric distance; ii) more luminous spirals in grand designgalaxies exhibit smaller variations of the pitch angle with radius than lessluminous grand design spirals; iii) grand design galaxies show less differencebetween the pitch angles of individual arms than multi-armed galaxies. Apartfrom these distinctive features, all three spiral classes do not differsignificantly by their pitch angle, arm width, width asymmetry, andenvironment. Wavelength dependence is found only for the arm fraction.Therefore, observationally we find no strong difference (except for the viewand number of arms) between grand design, multi-armed and flocculent spirals inthe sample galaxies.

A multi-wavelength study of spiral structure in galaxies. I. General characteristics in the optical