Self‐shading of in‐water optical instruments

The self‐shading of in‐water optical radiometers for measuring upwelling ( L u ) and irradiance ( E u ) is estimated from Monte Carlo simulations of the light field in the presence and absence of the instrument. It is found that the error ( ε ) induced by the presence of the radiometer is a function of its size and the absorption coefficient ( a ) of the medium, i.e. when ε 15–20% the error is independent of the scattering coefficient ( b ). Direct measurement of L u or E u with ε ≤ 5% places severe limitations on the instrument size, e.g. in the case of a cylindrical housing and small solar zenith angles (the worst case) the diameter of the instrument must be ≲ 1/30 a for E u and ≲ 1/100 a for L u . A correction method based on a simple model of ε is proposed and it is shown that when used these constraints on the diameter are reduced to 1/6 a and 1/30 a , respectively, for an after‐correction error of 5%. The self‐shading error is estimated across the spectrum as a function of the pigment concentration of case 1 waters, and it is found that it can be large for typical radiometers, especially at high pigment concentrations or wavelengths >600 nm.