The variable source sets the radiation source type source type where type is either solar (source=1 default value) or thermal (source=2). In ALG, source is automatically set to solar for waveleghts ≤2.5 μm and to thermal for wavelenghts ≥8 μm regardless the user-selected value. However, notice in the intermediate range 2.5-8 μm, users have the flexibility to select the source type, and that the thermal and solar cannot be calculated together. Thus it will be necessary to run twice libRadtran in these cases (i.e. for calculations around 3 μm).
Regarding the units of the solar radiation, in ALG units are always converted to mW/(m2·nm) when saving the data into the LUT file. This is different than the units used in libRadtran, which depend on the value of the mol_abs_param parameter.
Additionally, values of source≤0 (not yet implemented) allow selecting user-defined extraterrestrial irradiance spectrum. This is ignored if mol_abs_param other than lowtran or reptran is specified.