Bidirectional effect on a spectral image sensor for in‐field crop reflectance assessment

Abstract

Varying illumination geometry affects spectral measurements of a target reflectance and the intensity of solar radiation is the most important factor for in‐field spectral measurements. This paper reports the effect of bidirectional electromagnetic radiation on an image‐based reflectance sensor designed for plant nitrogen assessment. The results show the nonlinearity of reflectance as a function of the solar zenith angle. Ambient illumination was analysed and compensated for using fixed nadir‐view positions of a solar radiometer and a 3‐charge‐coupled device (CCD) multispectral imaging sensor (MSIS). A compensation algorithm was developed to correct for the nonlinearity of both sensors. The compensated reflectance remained linearly consistent with varying the solar zenith angle throughout the daytime within a maximum standard deviation of 0.62% at all three (green, red and near‐infrared) spectral channels, when testing with a 20% reflectance panel. The consistent reflectance was recovered under both sunny and cloudy conditions.

Acknowledgements

We thank the USDA‐ARS Water Management Research Unit in Fort Collins for allowing us to use the goniometer.