In situ calibration for beam attenuation coefficient determination at short pathlengths

A laboratory model of a short pathlength transmissometer with an in situ calibration facility was built to demonstrate the feasibility of high precision attenuation coefficient measurements at short pathlength even in clear waters. The high precision is enabled by the in situ calibration by variation of pathlength which is needed to overcome errors introduced by various sources such as window staining due to biofouling, multiple reflections at the glass-water interfaces and changes in refractive index of the medium. The laboratory model consists of a narrow bandwidth light source (LED) with a peak wavelength of 650 nm and beamforming optics, a moveable prism for retroreflection of the probe beam and receiving optics with a PIN-photodiode detector and transimpedance amplifier. With this setup a short term precision of the order of 10/sup -4/ m/sup -1/ was achieved at a constant pathlength of 54.7 mm. Drifts on longer terms, including staining of the instrument windows, can be compensated by the in situ calibration which showed a repeatability in the order of 10/sup -3/ m/sup -1/.