retIrradiance_PerArea = RetIrradiancePerDegrees2AndEyeLengthToRetIrradiancePerArea(retIrradiance_PerDegrees2,eyeLength)
Convert retinal irradiance measured in units of Y/deg^2 to units of
Y/x^2, where x is a unit of distance (m, cm, mm, um, etc.) and
Y is a measure of light amount (Watts, Joules, quanta/sec, quanta, etc.)
Eye length should be passed in units of x.
The conversion assumes that we are in the small angle regime, where
degrees are essentially linear with retinal extent.
6/23/13 dhb Wrote it.