End-to-end spectral response characterization of the Clouds and the Earth´s Radiant Energy System sensors from 0.3 to 200 microns

The Clouds and the Earth´s Radiant Energy System (CERES) instruments each contain three spectrally broadband scanning sensors operating in the bands from 0.3 to 5 μm, 8 to 12 μm and 0.3 to >150 μm. The broad spectral response of the third band is limited only by the reflectance of two protected silver coated mirrors and the response of a thermistor bolometer detector. The end-to-end relative spectral response of this sensor over the entire spectral range must be accurately measured to permit longwave radiometric calibration with a blackbody radiance standard. This spectral response is measured with a Fourier transform spectrometer (FTS) as a source for a vacuum chamber facility which operates over the entire spectral range from 0.3 to 200 μm. The FTS is capable of operating in a step scan mode so that the dwell at each point in the interferogram is long enough for the CERES sensor to respond at a time constant of 8 milliseconds. The sensor´s relative spectral response is referenced to a windowless, uncoated pyroelectric trap detector consisting of two specular detectors in a wedge configuration. This wedge produces a total of 10 reflections of input radiant power from the two detector surfaces. The resulting spectral absorptance of the reference detector is flat and or well characterized over most of the spectral range from 0.3 to 200 μm. The facility and reference detector designs and spectral measurement techniques are described. Measurements made to validate the pyroelectric reference detector performance are presented