Beam attenuation is measured with a device called a transmissometer that has a light source at one end and a detector at the other end, in one plane. The amount of light transmitted to the detector through the water is the beam transmission, and the amount of light lost is the beam attenuation. Beam attenuation is essentially the opposite of light transmission. Clearer water with a low beam attenuation coefficient will have high light transmission, and more turbid water with a high beam attenuation coefficient will have low light transmission. Beam attenuation is used as a proxy for particulate organic carbon in oligotrophic waters like the open ocean.