Characterization of the Immersion Factor for a Series of In-Water Optical Radiometers

Spectral immersion factors, I f(lambda), account for the difference between the in-air and inwater absolute response of submersible radiometers and are required to properly apply the in-air absolute calibration of the sensor when used underwater. The use of the so-called typical values for a series of in-water radiometers is a source of uncertainty because of intrinsic instrument-to-instrument differences in the optics. To investigate this source of uncertainty, in addition to uncertainties associated with determining the immersion factors using a laboratory method, a sample of nine radiometers from the same series of instruments was characterized by three different laboratories. A comparison of the immersion factor characterizations from the three laboratories indicates average intralaboratory measurement repeatabilities ranging from 0.3% to 0.6%, which were evaluated using multiple characterizations of the same reference radiometer and defined by two standard deviations. Interlaboratory relative uncertainties, evaluated with I f(lambda) data from the nine sample radiometers, show average percent differences ranging from -0.5% to 0.6%. The dispersion of I f(lambda) values across all the radiometers show values up to 5% in the red part of the spectrum with a spectral average of 2% (defined by two standard deviations). Typical I f(lambda) values, computed with data from the so-called trusted radiometers (i.e., those not showing extreme outlier values), are also presented with their maximum uncertainties and a discussion on their spectral dependence.