The black boxes pictured above are transmissometers that will be attached to different sampling devices. A beam of light travels through the open space on the front of the box, from a source at the top to a detector at the bottom. When particles in the water pass through the beam, some of the light is scattered or absorbed before it reaches the detector. The quantity of light missing is calculated and used to estimate the amount of particle in the water. Today the signal from each tranmissometer was calibrated using a voltmeter.