  Dust infrared emission possesses scaling properties. Overall luminosity is
never an input parameter of the radiative transfer problem, spectral shape is
the only relevant property of the heating radiation when the inner boundary of
the dusty region is controlled by dust sublimation. Similarly, the absolute
scales of densities and distances are irrelevant; the geometry enters only
through angles, relative thicknesses and aspect ratios, and the actual
magnitudes of densities and distances enter only through one independent
parameter, the overall optical depth. Dust properties enter only through
dimensionless, normalized distributions that describe the spatial variation of
density and the wavelength dependence of scattering and absorption
efficiencies. Scaling enables a systematic approach to modeling and
classification of IR spectra. We develop a new, fully scale-free method for
solving radiative transfer, present exact numerical results, and derive
approximate analytical solutions for spherical geometry, covering the entire
range of parameter space relevant to observations. Scaling implies tight
correlations among the SEDs of various members of the same class of sources
such as young stellar objects, late-type stars, etc. In particular, all members
of the same class occupy common, well defined regions in color-color diagrams.
The observational data corroborate the existence of these correlations.
