The maximum distance in the direction of takeoff or landing at which the runway, or specified lights or markers delineating it, can be seen from a position above a specified point on its centerline at a height corresponding to the average eye level of pilots at touchdown (ICAO). A height of approximately 16 ft (5 m) is regarded as corresponding to the average eye level of pilots at touchdown. In practice, the RVR cannot be measured directly from the position specified in the definition but is an assessment of what a pilot would see from that position. The system consists of a transmissometer projector along with related items, a transmissometer receiver (detector) and related items, a recorder, a signal data converter with its related items, and a remote digital or remote display programmer. The projector and the receiver are mounted on towers either 250 or 500 ft (75 or 150 m) apart. A known intensity of light is emitted from the projector and is measured by the receiver. Any obscuring matter such as rain, snow, dust, fog, haze, or smoke reduces the light intensity arriving at the receiver. The resultant intensity measurement is converted to an RVR value by the signal data converter. These values are displayed by readout equipment in the associated air traffic facility and updated approximately once every minute for controller issuance to the pilots. The signal data processor receives information from high-intensity runway lights, transmission values from the transmissometer, and the sensing of day or night conditions. From these values, appropriate RVR values are computed. An RVR transmissometer established on a 250-ft (75 m) baseline provides digital readout to a minimum of 600 ft (200 m), whereas that with a 500-ft (150 m) baseline is restricted to a minimum value of 1000 ft (300 m). Various types of RVRs arei
. Touchdown RVR. The RVR visibility readout values obtained from RVR equipment serving the runway touchdown zone.ii
. Mid-RVR. The RVR readout values obtained from RVR equipment located midfield of the runway.iii
. Rollout RVR. The RVR readout values obtained from RVR equipment located nearest the rollout end of the runway.