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A transmissometer or transmissiometer is an instrument for measuring the extinction coefficient of the atmosphere and sea water, and for the determination of visibility. 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 transmittance and/or extinction coefficient. (2025). 9780123877826, Elsevier. ISBN 9780123877826 In a transmissometer the extinction coefficient is determined by measuring direct light transmissivity, and the extinction coefficient is then used to calculate visibility range.
Atmospheric extinction is a wavelength dependent phenomenon, but the most common wavelength in use for transmissometers is 550 nanometer, which is in the middle of the visible waveband, and allows a good approximation of visual range.
Transmissometers are also referred to as telephotometers, transmittance meters, or hazemeters.
Transmissometers are also used by and to measure the optical properties of natural water. 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.
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