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A transmissometer measures light attenuation through air or water to determine visibility and turbidity. It uses a light source and detector to measure the proportion of light reaching the detector. The atmospheric extinction coefficient varies based on the wavelength of light and can be affected by pollutants. Transmissometers are used by the EPA to monitor visibility and on airport runways. They can also be used in power plant stacks and to measure water turbidity.
A transmissometer is a device for measuring the attenuation of light as it travels through a medium, usually 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 may be a lamp, laser or light emitting diode (LED), and a detector located at a known distance. The detector converts the light reaching it into an electrical voltage 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 registered by the detector.
The attenuation of light 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 to scattering and absorption per unit distance. To obtain a value for this, the light intensity measured by the detector must be compared to what would be expected if no gas or particles were present between the source and the detector. The extinction coefficient will vary based on the wavelength of light being measured, as different gases and particles absorb and scatter different wavelengths of light to varying degrees. For air visibility, a wavelength of 550 nanometers (nm) is commonly used, as it is in the middle of the visible light spectrum. When measuring the turbidity of water, a wavelength of 665 nm is typical. In some circumstances, it is possible to make separate measurements for different wavelengths.
Pollutants in the atmosphere can seriously reduce visibility. Light can be scattered by some substances, such as sulphates and nitrates, and absorbed by others, such as smoke, soot and nitrogen dioxide, which are responsible for the brown haze often seen around urban areas. To measure 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 provide what is known as a long path measurement. Therefore, visibility can be monitored on an ongoing basis for certain areas. In the United States, transmissometers have been used extensively 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 transmissometers are specifically designed for use in power plant stacks to measure the opacity of the smoke plume. In the ocean or in fresh water, light can be impeded by suspended silt, disturbed sediments, 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 approximately one meter long. This can be suspended in the water to the required depth, so that the pipe fills up with water.
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