A transmissometer or transmissiometer is an instrument for measuring the extinction coefficient of the atmosphere and sea water, and for the determination of visual range. It operates by sending a narrow, collimated beam of energy through the propagation medium. A narrow field of view receiver at the designated measurement distance determines how much energy is arriving at the detector, and determines the path transmission and/or extinction coefficient. In a transmissometer the extinction coefficient is determined by measuring direct light transmissivity, and the extinction coefficient is then used to calculate visibility range.