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function irradiance = RadianceToRetIrradiance(radiance,radianceS,pupilAreaMM,eyeSizeMM)
% irradiance = RadianceToRetIrradiance(radiance,radianceS,pupilAreaMM,eyeSizeMM)
%
% Perform the geometric calculations necessary to convert a measurement of source
% radiance to corresponding retinal irradiance.
%
% Input radiance should be in units of power/m^2-sr-wlinterval.
% Input radianceS gives the wavelength sampling information.
% Input pupilAreaMM should be in units of mm^2.
% Input eyeSizeMM should be the length of the eye in mm.
% Output irradiance is in units of power/um^2-wlinterval.
%
% Light power may be expressed in watts or quanta-sec or in your
% favorite units. Indeed, it may also be passed as energy rather
% than power.
%
% This conversion does not take absorption in the eye into account,
% as this is more conveniently foldeded into the spectral absorbtance.
%
% See also: PupilAreaFromLum, IsomerizationsInEyeDemo.
%
% 7/10/03 dhb Wrote it.
% 11/06/03 dhb Fixed comments about units, as per Lu Yin email.
% 3/29/12 dhb Comment on output units was wrong. It said power/um^2-sec-wlinterval
% but the 'sec' part makes no sense given the 'power' in the numerator.
% Convert spectral units to power/sr-mm^2-wlinterval
radianceMM = radiance*1e-6;
% Define factor to convert radiance spectrum to retinal irradiance in watts/mm^2-wlinterval.
% Commented out code shows the logic, which is short circuited by actual code.
% but is conceptually convenient for doing the calculation.
% distanceToSourceMM = 100;
% fractionfSphere = pupilAreaMM/4*pi*distanceToSourceMM^2;
% pupilAreaSR = 4*pi*fractionOfSphere;
% sourceAreaMM = (distanceToSourceMM^2)/(eyeSizeMM^2);
% conversionFactor = pupilAreaSR*sourceAreaMM;
conversionFactor = pupilAreaMM/(eyeSizeMM^2);
irradianceMM = conversionFactor*radianceMM;
% Convert units to um^2 from mm^2 base.
irradiance = irradianceMM*1e-6;
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