  We present the results of a Fourier photometric decomposition of a
representative sample of ~100 isolated Sb-Sc CIG/AMIGA galaxies. It complements
the analysis presented in Durbala et al. 2008 for the same sample by allowing a
description of the spiral structure morphology. We estimate dynamical measures
like torque strength for bar and spiral, and also the total nonaxisymmetric
torque. We explore the interplay between the spiral and bar components of
galaxies. Both the length and the contrast of the Fourier bars decrease along
the morphological sequence Sb-Sbc-Sc, with bars in earlier types being longer
and showing higher contrast. Bars of Sb galaxies are ~3x longer than bars in Sc
types. We find that longer bars are not necessarily stronger (as quantified by
the torque Q_{b} measure), but longer bars show a higher contrast, in very good
agreement with theoretical predictions. Our data suggests that bar and spiral
components are rather independent in the sense that the torque strengths of the
two components are not correlated. The total strength Q_{g} is a very reliable
tracer of the bar strength Q_{b}, the two quantities showing a very tight
linear correlation. Comparison with a similar sample of disk galaxies extracted
from the OSUBGS indicates that the isolated CIG/AMIGA galaxies host
significantly longer Fourier bars and possibly show a different distribution of
spiral torque Q_{s}. The Fourier analysis also revealed a potential case of
counterwinding spiral structure (NGC 5768), which deserves further kinematic
study. We find that m = 2 (i.e., dominating two-armed pattern) is the most
common spiral arm multiplicity among the sample of Sb-Sc CIG/AMIGA galaxies
(~40%), m = 2&3 and m = 1&2 are found in ~28% and ~13% of isolated galaxies,
respectively.
