The measurement of optical attenuation in sea water

The high performance of a submersible optical beam transmissometer, such as Chelsea Instruments´ Alphatracka, depends on an integrated approach to the design of the electronic, electro-optic, optical and mechanical components, both individually and as a system, to yield both long term stability and a low temperature coefficient (tempco) of calibration. This must be achieved together with an ability to retain the performance when exposed to the full ambient sunlight or when taken to full ocean depth (6000m). Many of the beam transmissometers currently on the market make measurements at a single wavelength (660 nm available from a red LED). As the LED output is not usually corrected for thermal and optical changes with time, it is not a trivial task to change the LED colour used. This paper describes a new instrument with a choice of operational wavelengths of 660, 565 and 470 nm using LED sources, which have been stabilised by sophisticated electronic methods. Specimen results from this transmissometer, obtained during rigorous tests, are also presented