 The two input beams have to be able to access the surface, and the output beam needs to be able to leave the surface to be picked up by a detector.There are two main types of sum frequency spectrometer, "scanning" systems and "broadband" systems.For the former type of spectrometer, one of the beams is a visible wavelength laser held at a constant frequency and the other is a tunable infrared laser — by tuning the IR laser, the system can scan over resonances and obtain the vibrational spectrum of the interfacial region in a piecewise fashion.