The instrument used for the CPFM is a spectroradiometer based on a concave, holographic diffraction grating and a 1024-element diode array detector. It measures the intensities of the two linear polarization components of radiation propagating upward at the aircraft location from a range of elevation angles near the horizon. In addition, a measurement of the intensity of the direct solar beam is made by viewing a horizontal diffusing surface mounted under a quartz dome on board the aircraft. These measurements are used to verify atmospheric light-scattering calculations, which are essential for the accurate modeling of the chemistry of the stratosphere where POLARIS makes its measurements.