Spiral galaxies exhibit a large range in properties such as
scalelength, surface brightness, gas surface density and star formation
(SF) history.  A recent study by of one of us (de Jong 1994) shows that
low surface brightness (LSB) galaxies are about as numerous as high
surface brightness galaxies.  This means that a very numerous class of
galaxies is hardly studied.  In order to reach this important we
obtained surface photometry in the B, V, R, I, H and K bands of 86
face-on spiral galaxies.  This is the only statistically complete sample
of galaxies with surface photometry in such a wide range in passbands
and easily correctable for selection effects.
    Further study of this sample shows that most galaxy parameters are
independent of scale size, but depend on surface brightness.  Especially
colors show a strong dependence on surface brightness, all galaxies
become bluer going radially outward and LSB galaxies are bluer than HSB
galaxies by the same amount.  Two explanations are the most obvious; a
change in stellar populations or reddening caused by dust extinction.
At the moment our data is insufficient to distinguish between the two
possibilities.
    To limit our evolution models it is imperative to have radial
information on both the current SF rate and the distribution of dust.
We propose to image a statistically complete subsample of ~35 galaxies
from the previous mentioned sample with ISOCAM using LW3.  This will
give us detailed information on the distribution of hot dust and the
current SF.  To measure the cold dust content we will scan all galaxies
at 180 micron using ISOPHOT.  This method gives us accurate background
subtraction and information on the spatial extent of the cold dust for
3-4 large galaxies.  As we already have the optical and near-IR data we
can ensure that our sample is complete in Hubble type (Sa-Sm), surface
brightness and color range.  These observations enable us to distinguish
whether color gradients are caused by dust or by population effects.