The Sciencetech Polarizing variable frequency optical chopper is used with the detectors during Martin-Puplett mode. The chopper is positioned in front of the output port, and captures the output at 45 degrees from the two planes of polarization. This facilitates the retrieval of the Fourier Transform Interferogram during M-P mode. The chopper itself is designed for high stability and a broad range of frequency control (from 0.5Hz to 100Hz). The price includes the servo motor and software controller.