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
A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction p ...
beam of energy (usually a
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...
) through the propagation medium. A narrow
field of view
The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation.
Human ...
receiver at the designated measurement distance determines how much energy is arriving at the
detector
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
, 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.
Atmospheric extinction is a
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...
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.
Transmissometers are also referred to as telephotometers, transmittance meters, or hazemeters.
Transmissometers are also used by
oceanographer
Oceanography (), also known as oceanology and ocean science, is the scientific study of the oceans. It is an Earth science, which covers a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dynamic ...
s and
limnologist
Limnology ( ; from Greek λίμνη, ''limne'', "lake" and λόγος, ''logos'', "knowledge") is the study of inland aquatic ecosystems.
The study of limnology includes aspects of the biological, chemical, physical, and geological characterist ...
s to measure the
optical properties The optical properties of a material define how it interacts with light. The optical properties of matter are studied in optical physics, a subfield of optics. The optical properties of matter include:
*Refractive index
* Dispersion
*Transmittance a ...
of natural water.
In this context, a transmissometer measures the
transmittance
Transmittance of the surface of a material is its effectiveness in transmitting radiant energy. It is the fraction of incident electromagnetic power that is transmitted through a sample, in contrast to the transmission coefficient, which is t ...
or
attenuation
In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at variabl ...
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.
EMOR - Extended MOR Technology
Latest generation transmissometer technology makes use of a co-located forward scatter visibility sensor on the transmitter unit to allow for higher accuracies over an Extended Meteorological Optical Range or EMOR. After 10,000 meters the accuracy of transmissometer technology diminishes, and at higher visibilities forward scatter visibility sensor technology is more accurate. The co-location of the two sensors allows for the most accurate technology to be used when reporting current visibility. The forward scatter sensor also enables auto-alignment and auto-calibration of the transmissometer device. Hence it is very useful for oceanography and water optics study.
See also
*
Runway visual range
In aviation, the runway visual range (RVR) is the distance over which a pilot of an aircraft on the centreline of the runway can see the runway surface markings delineating the runway or identifying its centre line. RVR is normally expressed in met ...
References
Spectroscopy
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