Monte Carlo modeling of the performance of a reflective tube absorption meter.

A Monte Carlo model of the behavior of photons in a reflective tube absorption meter has been developed and used to investigate how well such a meter can measure the absorption coefficient in waters of different optical types. The apparent, i.e., measured, absorption coefficient (alpha(m)) is always greater than the true absorption coefficient (alpha). The ratio alpha(m)/alpha increases linearly with the ratio of scattering to absorption (b/alpha) at a rate that depends on the scattering phase function of the water. The excess attenuation is mainly due to the failure of forward-scattered photons to undergo reflection at the cylinder wall. To achieve the highest accuracy possible with the meter, one must maximize the reflectivity and detector acceptance angle. Performance is also improved by increasing the cylinder diameter.