A transmissometer is a device for measuring the attenuation of light as it travels through a medium, normally air or water. It is generally used as a means of determining visibility in the atmosphere, and turbidity in the ocean. The instrument consists of a light source of known frequency and intensity, which can be a lamp, a laser or a light emitting diode (LED), and a detector located at a known distance. The detector converts the light reaching it into an electrical voltage that is proportional to the intensity of the light. The degree of transparency of the intervening air or water can be determined by measuring the proportion of light from the source that is recorded at the detector.
Light attenuation by gases and particles in the atmosphere can be expressed as the atmospheric extinction coefficient, which is a measure of the proportion of light lost by scattering and absorption per unit of distance. To obtain a value for this, the light intensity measured by the detector must be compared with what would be expected if there were no gases or particles present between source and detector. The extinction coefficient will vary according to the wavelength of light being measured, as different gases and particles absorb and scatter different wavelengths of light to varying extents. For air visibility, a wavelength of 550 nanometers (nm) is commonly used, as this is in the middle of the visible light spectrum. When measuring water turbidity, a wavelength of 665 nm is typical. In some circumstances, separate measurements may be taken for different wavelengths.
Pollutants in the atmosphere can seriously reduce visibility. Light can be scattered by some substances, such as sulfates and nitrates, and absorbed by others, such as smoke, soot and the gas nitrogen dioxide, responsible for the brown haze often seen around urban areas. For measuring air quality in terms of visibility, the transmissometer light source and detector can be placed 0.62-6.21 miles (1-10 km) apart to give what is known as a long path measurement. Thus, visibility can be monitored for given areas on an ongoing basis. In the USA, transmissometers have been widely used by the Environmental Protection Agency (EPA) to monitor visibility, as required by the Clean Air Act.
Transmissometers are sometimes used on airport runways to check visibility. Some types of transmissometer are specifically designed to be deployed in smoke stacks at power stations to measure the opacity of the smoke plume. In the ocean or in freshwater, light can be impeded by suspended silt, disturbed sediment and microorganisms. The turbidity of water can be measured using a transmissometer with the light source and detector positioned at opposite ends of a tube about one meter long. This can be suspended in the water at the required depth, so that the tube fills with water.