Characterising Spectroradiometer Instrumental Spectral Performance and Its Impact on Retrieved Reflectances.

The accurate characterisation of the instrumental spectral performance of field spectroradiometers acquires particular importance in view of the instruments' widespread use for calibrating and validating airborne- and satellite-based optical sensor systems. Typically, spectral calibration relies on the basic assumption of a Gaussian instrumental spectral response function (ISRF). We test alternative ISRF parameterisations and examine the resulting changes in spectral performance of four ASD field spectroradiometers. We then use a MODTRAN-simulated spectrum to compare an ideal Bottom Of Atmosphere (BOA) reflectance with the reflectances derived from ASD-convolved radiances using nominal (per ASD specifications), Gaussian and Symmetric Super Gaussian parameterisations of the spectroradiometers. We highlight the impact of the small but significant differences by retrieving the Photochemical Reflectance Index (PRI) values, which change by a few percentage points compared to the reference PRI.