Selection of appropriate multispectral camera exposure settings and
  radiometric calibration methods for applications in phenotyping and precision
  agriculture
Vaishali Swaminathan J. Alex Thomasson Robert G. Hardin Nithya Rajan
Radiometric accuracy of data is crucial in quantitative precision
agriculture, to produce reliable and repeatable data for modeling and decision
making. The effect of exposure time and gain settings on the radiometric
accuracy of multispectral images was not explored enough. The goal of this
study was to determine if having a fixed exposure (FE) time during image
acquisition improved radiometric accuracy of images, compared to the default
auto-exposure (AE) settings. This involved quantifying the errors from
auto-exposure and determining ideal exposure values within which radiometric
mean absolute percentage error (MAPE) were minimal (< 5%). The results showed
that FE orthomosaic was closer to ground-truth (higher R2 and lower MAPE) than
AE orthomosaic. An ideal exposure range was determined for capturing canopy and
soil objects, without loss of information from under-exposure or saturation
from over-exposure. A simulation of errors from AE showed that MAPE < 5% for
the blue, green, red, and NIR bands and < 7% for the red edge band for exposure
settings within the determined ideal ranges and increased exponentially beyond
the ideal exposure upper limit. Further, prediction of total plant nitrogen
uptake (g/plant) using vegetation indices (VIs) from two different growing
seasons were closer to the ground truth (mostly, R2 > 0.40, and MAPE = 12 to
14%, p < 0.05) when FE was used, compared to the prediction from AE images
(mostly, R2 < 0.13, MAPE = 15 to 18%, p >= 0.05).