If specified, the calculated spectrum is multiplied with a filter function
defined in file.
filter_function_file file
The file must contain two columns.
Column 1 is the wavelength, in nm. Column 2 is the corresponding filter
function value. Comments start with #. Empty lines are ignored.
In combination with output sum or output integrate this option
is useful e.g. to calculate weighted irradiances or actinic fluxes or
to simulate broadband or satellite observations.