 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.In the case of broadband spectrometers, one laser (typically the visible laser) is kept at a fixed narrow wavelength, and the other laser produces a spectrally broad beam.