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.
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Document Type: Research Article
Northern Plains Agricultural Research Laboratory, US Department of Agriculture - Agricultural Research Service, Sidney, MT 59270, USA
Director, Moline Technology Innovation Center, John Deere Technology Center, Moline, IL 61265, USA
Publication date: 2007-01-01
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