Selection of appropriate multispectral camera exposure settings and radiometric calibration methods for applications in phenotyping and precision agriculture

Abstract

Exposure time and gain are camera‐related parameters that affect the radiometric accuracy of unmanned aerial vehicle (UAV)‐based multispectral images used in quantitative precision agriculture. This study quantified the potential radiometric errors from the conventionally used autoexposure settings and compared the agronomic implications of fixed exposure and autoexposure settings. The exposure and gain in the auto‐exposed UAV images varied scene‐to‐scene based on the reflectance range of objects on the ground. Hence, to capture multispectral images with fixed exposure, the ideal exposure ranges were determined to prevent loss of spectral detail from overexposure or underexposure of the canopy and soil. Reflectance measurements from the fixed exposure orthomosaic had a higher coefficient of determination (R2 = 0.97–0.99) and lower mean absolute percentage error (MAPE = 3.07%–5.97%) than those from autoexposure (R2 = 0.79–0.96 and MAPE = 7.42%–25.06%), indicating better radiometric uniformity and accuracy, respectively. Calibrating images with reflectance targets captured with different exposure settings resulted in MAPE < 5% for the blue, green, red, and near infrared bands and <7% for the red‐edge band when exposure settings were within the ideal ranges; outside of those ranges MAPE increased exponentially. These observations highlighted the challenges in appropriately calibrating canopy and soil reflectance values subjected to exposure setting variabilities. Finally, early‐season plant nitrogen uptake (g/m2) from 2 years was estimated somewhat better with vegetation indices derived with fixed exposure (R2 = 0.32–0.40 and MAPE = 13%–14%) than with autoexposure (R2 = 0.00–0.19 MAPE = 15%–18%).