  Achieving a comprehensive understanding of the star and planet formation
process is one of the fundamental tasks of astrophysics, requiring detailed
knowledge of the physical conditions during the different phases of this
process. During the earliest stages, i.e., concerning physical processes in
molecular clouds and filaments, the column density N(H2), dust temperature T
and dust emissivity index \b{eta} of these objects can be derived by adopting a
modified blackbody fit of the far-infrared to (sub-)millimeter spectral energy
distributions. However, this often applied method is based on various
assumptions. In addition, the observational basis and required, but only
assumed cloud properties, such as a limited wavelength-coverage of the spectral
energy distribution and dust properties, respectively, may differ between
different studies. We review the basic limitations of this method and evaluate
their impact on the derived physical properties of the objects of interest,
i.e., molecular clouds and filaments. We find that the highest uncertainty when
applying this method is introduced by the often poorly constrained dust
properties. Therefore, we propose to first derive the optical depth and
subsequently the column density with the help of a suitable dust model as the
optical depth can be obtained with high accuracy, especially at longer
wavelengths. The method provides reliable results up to the high densities and
corresponding optical depths observed in molecular clouds. Considering
typically used observational data, i.e., measurements obtained with
far-infrared instruments like Herschel/PACS, JCMT/SCUBA-2 and SOFIA/HAWC+, data
at four wavelengths are sufficient to obtain accurate results. Furthermore, we
find that the dust emissivity index \b{eta} derived with this method is not
suitable as an indicator of dust grain size.
