We examined the Fourier power spectrum characteristics of cirrus structures in 13 sky fields with faint to bright cirrus emission observed with ISOPHOT in the 90–200 \mu m wavelength range in order to study variations of the spectral index \alpha .
We found that \alpha varies from field to field with –5.3 \leq \alpha \leq –2.1 .
It depends on the absolute surface brightness and on the hydrogen column density .
We also found different spectral indices for the same sky region at different wavelengths .
Longer wavelength measurements show steeper power spectra .
This can be explained by the presence of dust at various temperatures , in particular of a cold extended component .
For the faintest areas of the far-infrared sky we derive a wavelength independent spectral index of \alpha = –2.3 \pm 0.6 for the cirrus power spectrum .
The application of the correct spectral index is a precondition for the proper disentanglement of the cirrus foreground component of the Cosmic Far-Infrared Background and its fluctuations .