  We present a numerical code for continuum radiative transfer that is based on
the idea of a `library' describing the relation between the intensity of the
local radiation field and the resulting dust emission. With this information
and local intensities at a few reference wavelengths, the radiative transfer
equation can be integrated through the source and an approximation of the
emission spectrum is obtained. Tests with small models for which the radiative
transfer problem can be solved directly show that with our method one can
easily obtain an accuracy of a few per cent. We show spectra computed from
three-dimensional MHD simulations containing up to 128^3 cells. The models
represent starless, inhomogeneous interstellar clouds embedded in the normal
interstellar radiation field. The intensity ratios between IRAS bands show
large variations that follow the filamentary density distribution. The power
law index of the spatial power spectrum of the column density map is -2.8. In
infrared maps temperature variations increase the power at high spatial
frequencies, and the index varies between -2.5 and -2.7. Assuming constant dust
properties throughout the cloud, the IRAS ratio <I_60/I_100> decreases in
densest cores only by a factor of ~4 compared with the value in diffuse medium.
In observations the ratio can decrease twice as much even in optically thinner
clouds. This requires that most of the small grains are removed in these
regions, and possibly a modification of the properties of large grains.
