, Kenneth Voss, Heidi M. Dierssen, , , Zhehai Shang
Global downwelling plane irradiance is a necessary variable to normalize the water-leaving radiance measurements, reducing the magnitude and spectral variabilities introduced by the incident light field. As a result, the normalized measurements, known as remote sensing reflectance, have higher correlation with the inherent optical properties of the water body and so to the composition of optically active water components. For in situ measurements, the global downwelling plane irradiance can be estimated from the emergent radiance of sintered polytetrauoroethylene plaques or other diffuse reflectance standards. This allows to use a single spectrometer to measure all necessary variables to estimate the remote sensing reflectance, reducing cost in acquisition and maintenance of instrumentation. However, despite being in use for more than 30 years, the uncertainty associated with the method has been only partially evaluated. In this study, we use a suite of sky radiance distributions for twenty four atmospheres and nine Sun zenith angles in combination with full bidirectional reflectance distribution function determinations of white and grey plaques to evaluate the uncertainties. The isolated and interactive effects of bidirectional reflectance distribution,shadowing and tilt error sources are evaluated. We found that under best performing geometries of each plaque and with appropriate estimation functions, average uncertainty ranges from 1% to 6.5%. The simulated errors are shown to explain both previous empirical uncertainty estimates and new data collected during this study. Those errors are of the same magnitude as uncertainties of plane irradiance sensors, which supports the continued use of the plaque method in hydrologic optics research and monitoring. Recommendations are provided to improve the quality of measurements and assure that uncertainties will be in the range of those calculated here.
Downwelling Irradiance, Diffuse Plaque, Remote Sensing, Uncertainty