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

Abstract

Spectral immersion factors, If(?), account for the difference between the in-air and in-water 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 If(?) data from the nine sample radiometers, show average percent differences ranging from ?0.5% to 0.6%. The dispersion of If(?) 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 If(?) 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.