  Future far-infrared space missions require highly-sensitive spectroscopy as a
primary diagnostics tool. However, these systems are sensitive to straylight,
due to the ultra-sensitive few-mode detectors used, which affects the
measurement and calibration of the spectrum, as revealed by the Herschel
mission. To ensure that the science goals of future missions are met, the
complex modal behaviour has to be understood, and appropriate verification and
calibration strategies must be developed. We propose a modal framework to
addresses these issues, using Herschel-SPIRE as a case study, and demonstrate
how the technique can be used for the design and verification of spectrometers
in future far-infrared missions.
