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We have obtained surface photometry for a sample of 86 face-on spiral galaxies in the near-infrared ($H$ and/or $K$-passbands) and the optical ($B,V,R$ and $I$-passbands). This statistically complete diameter limited sample can be used to represent a volume limited sample. Therefore we can examine the number frequency per volume of these galaxies with respect to their parameters.
An important parameter that has been studied is the central surface brightness of the disk. As the radial light profile of spiral galaxies can be described by an exponential disk, they are characterized by two parameters, the central surface brightness ($\mu_0$) and the scalelength ($h$). Since the classical paper of Freeman (1970) it is assumed that the value for $\mu_0$ is constant among the galaxies, at least in the $B$-passband. We will show that $\mu_0$ has no constant value but depends on galaxy type and that this dependence is even stronger going to longer wavelengths.
Furthermore we find that almost all the disks of the galaxies in our sample are becoming bluer with increasing radius. These color gradients do not seem to have any correlation with any other global parameter of the galaxies. We will present a dust model and a stellar population model in order to constrain the explanations for the color gradients.
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