Today most airports use Instrumented Runway Visual Range or IRVR, which is measured by devices called transmissometers which are installed at one side of a runway relatively close to its edge. Normally three are provided, one at each end of the runway and one at the mid-point.
The term ‘visibility’ is variously defined, but generally indicates the distance to which human visual perception is limited by atmospheric conditions. The physical mechanisms that influence visual perception during the night in distinguishing lights differ from those in the day time in distinguishing objects illuminated by daylight. Basically, however, visibility describes the transparency of the air in the horizontal direction and represents the maximum distance that
one can see in the atmosphere at any given time.
A transmissometer is used for measuring the atmospheric transmittance at visibility deterioration (fog, rain etc.). This instrument is mainly installed at the runways of airports in order to determine the visual range for the flight control safety service. The double baseline unit is new generation equipment especially suited to render exact visual range data for flight operation at CATII to CAT-III(b) type airports.
A transmissometer is an instrument that transmits a beam of light (laser) to a receptor, the amount of energy received determines visibility. Usually there are several arrays installed to give information at several parts for each runway.
Before this an observer would have to personally go to the threshold and see how many of the runway lights he could observe to then give a calculated Visual Range.
Some aircraft have infrared sensors to aid identifying a runway, these don't determine visibility but do aid when visibility is reduced.
These units transmit a beam of light (usually using laser) to a detector. In some case the transmitter and detector are in the same unit and a reflector is used. The ratio of the transmitted energy and the detected energy is a measure of how much light is absorbed in the fog. From this the RVR can be calculated.
A minimum visibility report is needed before a takeoff or approach can be cleared, these values will vary depending on the runway lights installed, type of approach, aircraft capabilities and crew training.
Runway Visual Range (RVR) is defined to be the horizontal distance a pilot can see on the runway. RVR is the maximum distance that a pilot 15ft above the runway in the touchdown area can see marker boards by day and runway light by night.
RVR is reported in either feet or meters. In the United States, you can assume it is in feet.
This may seem like a no brainer, but RVR is only reported at airports with an RVR system installed. If you only see visibility reported in statute miles on the METAR, the airport probably doesn’t have RVR.
**Note
RVRs are needed to support increased landing capacity at existing airports, and new airport construction. Each RVR system consists of the following functional elements: Visibility Sensor, Ambient Light Sensor, Runway Light Intensity Monitor, Data Processing Unit and Controller Display(s).
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