Abstract: An alphameter provides an indication of the volume absorption coefficient d the small angle forward scattering coefficient to give an indication of underwater visibility. A laser beam is alternately pulsed through the surrounding water and a reference fluid which fills a telescoping chamber. A photodiode is illuminated by the alternating pulses and representative signals are fed to a feedback loop where a comparison is made with signals emanating from the laser. If the representative signals exceed or fall below the predetermined threshold range, a DC motor enlarges or lessens the interior volume of the telescoping chamber causing more or less of the reference fluid to be contained therein. When the representative signals fall within the predetermined threshold range the telescoping chamber is no longer displaced and a readout indicates what the transmissivity of the surrounding water is.