Calibration of dual‐beam spectroradiometric data

Abstract

An experiment to determine the most accurate and repeatable method for generating instrument inter‐calibration functions (ICFs) is described, based upon data collected with a dual‐beam GER1500 spectroradiometer system. The quality of reflectance data collected using a dual‐beam spectroradiometer system is reliant upon accurate inter‐calibration of the sensor pairs to take into account differences in their radiant sensitivity and spectral characteristics. A cos‐conical field‐based method for inter‐calibrating dual‐beam spectroradiometers was tested alongside laboratory inter‐calibration procedures. The field‐based method produced the most accurate results when a field‐derived ICF collected close in time was used to correct the spectral scan. A regression model to predict the ICF at a range of wavelengths was tested, using inputs of solar zenith angle, cosine of solar zenith angle and broadband diffuse‐to‐global irradiance ratios. The linear multiple regression model described up to 78% of the variability in ICF; the remainder of the variability was most likely due to complexities of instrumental behaviour in response to warm‐up time, ambient temperature and environmental conditions at the time of measurement. Collection of ICFs using a stable laboratory source was shown to provide unsatisfactory results due to differences between lamp outputs and the field‐measured solar spectrum. Consequently, the most practical and accurate method of deriving inter‐calibration functions is to use field‐derived ICFs, collected close in time and space to the data requiring correction.

Acknowledgements

The authors are pleased to acknowledge the contribution of David Emery, Bill Damon and Dr Charles Kerr to this research. The spectroradiometers and associated laboratory calibration facilities were provided by the NERC Equipment Pool for Field Spectroscopy. Referees are thanked for their constructive comments on the original manuscript.