  The paper is devoted to the understanding of the infrared emission of nuclear
regions in galaxies. a) ISO data of NGC1808 are presented: spectro--photometry
from 5.1 to 16.4 mic., a 25"x25" map at 6 mic. and 170mic. photometry. b) The
data are complemented by a polarization measurement at 170 mic. (2.5+/-0.4% at
position angle 94+/-5deg) and a map at 6 mic. In the map, the degree of
polarisation goes up to 20% in the outer regions. We argue that the
polarisation is produced by emission of big grains and exclude very small
grains and PAHs or scattering and extinction. c) The mid infrared spectrum
shows, beside the main emission bands, a so far unknown plateau of PAH features
in the >13 mic.region. d) The total spectrum can be fit under the assumption of
optically thin emission. However, such a model fails to reproduce the 25mic.
point and implies that the mid infrared is due to very small grains and PAHs.
These particles would then also have to be responsible for the 6 mic.
polarisation, which is unlikely. e) To avoid these difficulties, we
successfully turn to a radiative transfer model whose major feature is the
existence of (hot spots) produced by the dust clouds around OB stars. We
demonstrate the decisive influence on the mid infrared spectrum of both the
PAHs and the hot spots.
