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 (usually a laser) 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.[1] In a transmissometer the extinction coefficient is determined by measuring direct light transmissivity, and the extinction coefficient is then used to calculate visibility range.[2]
Atmospheric extinction is a wavelength dependent phenomenon, but the most common wavelength in use for transmissometers is 550 nm, which is in the middle of the visible waveband, and allows a good approximation of visual range.[citation needed]
Transmissometers are also referred to as telephotometers, transmittance meters, or hazemeters.
Transmissometers are also used by oceanographers and limnologists to measure the optical properties of natural water.[2] In this context, a transmissometer measures the transmittance or attenuation of incident radiation from a light source with a wavelength of around 660 nm, generally through a shorter distance than in air, as water has a smaller maximum visibility distance.[citation needed]