Although infrared observations are ideal to study a molecular cloud, an infrared spectrum alone is not enough to determine the physical parameters in the source (Rowan-Robinson 1980). An intensity gradient in sub-millimeter wavelengths was considered by some authors as a measure of the density fall off. But the derived distribution laws are still controversial (Scoville and Kwan 1976, Westbrook et al. 1976, Keene et al. 1982). Other parameters entering the interpretation of the infrared observations are the emissivity and the temperature distribution of the dust. In most models of molecular clouds, major efforts have been devoted to accurately determine the dust temperature for the assumed density gradient and emissivity (Jones and Merrill 1976, Leung 1976, Unno and Kondo 1977, Rowan-Robinson 1980).