Immersion Factor of In-Water Radiance Sensors: an Analytical Method and its Experimental Assessment

The spectral immersion factor of in-water radiance sensors, If, quantifies the effects of changes in the sensor's response when operated in water versus in air. The values of If are currently computed with a relationship derived from a basic sensor model which only requires knowledge of the refractive indices of the water and of the material constituting the sensor's optical window in contact with it. Uncertainties in the computation of If are investigated in the 400-700 nm spectral range for a specific class of widely used multispectral radiometers. The analysis is made by comparing If values from the theoretical relationship currently in use, with: i. If from a new relationship based on an extended sensor model accounting for the actual solid angle field-of-view and, the reflectance and transmittance of the external and internal optical components; and ii. experimental If determined with sample radiometers having diverse optical windows made of materials with different refractive indices. Results highlight that the relationship derived from the basic sensor model introduces a 0.4% negative bias when applied to the considered class of radiometers having Fused Silica optical window, 13° in-air half-angle field-of-view and estimated detector reflectance of 0.15. Reference values of If for the specific class of radiometers, determined with the newly proposed relationship, are presented and their dependence on seawater temperature and salinity is discussed.JRC.H.5-Rural, water and ecosystem resource