Comets formed in the early phases of the solar system and may still contain
unprocessed material. Approching the sun, the cometary surface is heated up
and a lot of material is released from the cometary surface into the coma.
The cometary coma is composed of solid and semi-refractory particles which
are, after their ejection from the surface, driven by the forces of  radiation
pressure and solar wind. The extend of the coma and the coma structure
strongly depends on the processes on the cometary surface and its properties.
The shape of the coma shows a lot of variations caused by fans and jets. Since
grains radiate most efficient close to their own size, particles of about the
same properties  can be traced by observing the coma at different wavelength.
Therefore, coma structures observed at wavelengths shortwards of the wavelength
of maximum thermal emission will be caused mainly by 'small'(hot) particles,
because they are 'overheated' in comparison to the black body equilibrium
radiation. Whereas coma structures visible at wavelengths longwards the
wavelength of maximum thermal emission result from emission of 'larger'
particles. This would be the direct proof of the presence of large grains in
the cometray coma. The observations of the temperature distribution in the
coma could  be compared directly with isochron-isodyn-models for cometary
comae. Due to the opaqueness of the earth's atmosphere longwards of 20microns,
no observations have been possible to test the grain distribution hypothesis
raised by several models. The shape of the coma at different wavelengths would
give a slight hint on the strength of radiation pressure and other forces and
their influence on selceted kinds of particles. Mapping the coma at different
wavelengths would allow to determine precise absolute flux values and spacial
resolved flux distributions. This would be the basis for modelling grain size
distributions and multi-temperture evaluations within the cometary coma.