Effect of Spectral Shape of Calibration Source on Retrieved Ocean Radiance and Reflectance

Radiometric calibration of ocean colour sensor establishes a quantitative relationship between the sensor output and the incoming spectral radiance or reflectance. Calibration coefficients, generated using precise sources in laboratory, are validated after launch, using Sun/Moon. Vicarious calibration uses ocean targets. The calibration coefficients are used to retrieve ocean spectral radiance/reflectance. Sensor provides the convolved output of input spectrum and its own spectral response function. The shapes of input spectra are different during calibration and ocean measurement. Also, sensor response function varies with sensor. The effect of above variations, on the accuracy of calibration coefficients and retrieved radiance/reflectance, are presented in this paper for two ocean colour sensors. Calibration coefficient and retrieved spectral radiance and reflectance are found varying upto 2.8% and 3% respectively depending on the shapes of spectra of calibration source, sensor response and ocean radiance. This study shows that spectral shape effects need to be accounted for accurate retrievals.