  The optical properties of ice in the far infrared are important for models of
protoplanetary and debris disks. In this report we derive a new set of data for
the absorption (represented by the imaginary part of the refractive index
$\kappa$) of crystalline water ice in this spectral range, including a detailed
inspection of the temperature dependence, which had not been done in such
detail before. We measured the transmission of three ice layers with different
thicknesses at temperatures $\vartheta = 10...250$K and present data at
wavelengths $\lambda=80...625$ microns. We found a change in the spectral
dependence of $\kappa$ at a wavelength of $175 \pm 6$ microns. At shorter
wavelengths, $\kappa$ exhibits a constant flat slope and no significant
temperature dependence. Long-ward of that wavelength, the slope gets steeper
and has a clear, approximately linear temperature dependence. This change in
the behaviour is probably caused by a characteristic absorption band of water
ice. The measured data were fitted by a power-law model that analytically
describes the absorption behaviour at an arbitrary temperature. This model can
readily be applied to any object of interest, for instance a protoplanetary or
a debris disk. To illustrate how the model works, we simulated the spectral
energy distribution (SED) of the resolved, large debris disk around the nearby
solar-type star HD 207129. Replacing our ice model by another, commonly used
data set for water ice results in a different SED slope at longer wavelengths.
This leads to changes in the characteristic model parameters of the disk, such
as the inferred particle size distribution, and affects the interpretation of
the underlying collisional physics of the disk.
