Underwater spectral reflectance measurements: the reflectance standard submersion factor and its impact on derived target reflectance.

For Earth observation remote sensing, high quality reflectance spectra are necessary for model input, algorithm development, and validation of derived products. In the aquatic environment, a common approach for making spectral reflectance measurements involves using a calibrated reflectance standard such as a Spectralon plaque underwater. The manufacturer provides a National Institute of Standards and Technology traceable reflectance curve with each standard, measured in air. Here, we demonstrate how the reflectance factor changes when submerged in water based on the standard albedo and viewing geometry. Target reflectances calculated incorrectly with the air calibrated values are 10%-60% lower than those estimated with submerged plaque calibrations. We provide guidelines for proper use and calibration of standards underwater.