Variable Description

Variable:	Beam Attenuation Coefficient
Units	1/m
Description	The beam transmissometer measures the fraction of light, from a collimated light source, reaching a light detector a set distance away. Light which is absorbed or scattered by the media (usually water) between the source and the detector does not reach the detector. The fraction of light received is converted to the beam attenuation coefficient (usually called "c") by the formula c = -ln(T)/z where T is the fraction of light transmitted and z is the path length of the instrument in meters. Beam attenuation has units of 1/m. The Seatech beam transmissometer uses a red LED light source that has a wavelength of 670 nm, beam diameter of 15 mm, and a path length of 0.25 m. The depth rating of the instrument housing is 2000 meters of water. In general, the measurement of c at 670 nm (c670) is linearly related to the concentration of particulate material in the water, but the exact relationship varies with the type of particles present.