function [Tr, c] = compute_transmittance ( Vsig, Vdark, Vref, path_length )
% COMPUTE_TRANSMITTANCE: calculate light transmittance products from raw engineering units
%
% USAGE: [Tr, c] = compute_transmittance ( Vsig, Vdark, Vref, path_length );
%
% PARAMETERS:
% Input:
% Vsig:
% the measured analog signal
% Vdark:
% the factory-supplied dark voltage offset for the instrument.
% Vref:
% the factory-supplied clean water offset
% path_length:
%
%
% Output:
% Tr:
% transmittance. Range 0 to 100 percent.
% c:
% attenuation coefficient. Units m-1
Tr = (Vsig - Vdark) / (Vref - Vdark);
c = -1 * log ( Tr ) ./ path_length;
%
% Convert to percent.
Tr = Tr*100;
return;
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