Estimating the spectral response function of the CASI-2

The spectral response function (SRF) of an instrument describes its relative sensitivity to energy of different wavelengths and is normally determined in the laboratory using a tunable laser or a scanning monochromator. In the case of an imaging spectrometer, this procedure is exceptionally difficult and time-consuming as it is necessary to determine the SRF for each of the many hundreds of detectors which make up the sensing array. This paper describes a simplified method to approximate the SRF of the CASI-2 (Compact Airborne Spectrograph Imager), using the same low pressure gas lamps that are used to determine the wavelength calibration of the instrument. The method has been applied to the NERC (Natural Environment Research Council, UK) CASI and results suggest that the SRF is variable across the range of wavelengths sensed, from a full-width half maximum (FWHM) of 2.7 nm at approximately 668 nm, to a FWHM of over 5 nm in the near IR. The results have implications for sensor simulation studies and for applications such as red edge detection