A transmissometer is an instrument that measures the transparency of the atmosphere by sending a beam of light across a fixed distance and detecting how much of that light arrives at the receiver. Airports use this measurement to determine runway visual range (RVR), which tells pilots how far down the runway they can see.
How It Works#
A transmissometer has two main components: a transmitter and a receiver. The transmitter sends a steady, calibrated light beam across a known baseline, typically 75 to 300 meters. The receiver measures the fraction of light that survives the journey through the air.
The ratio of received light to transmitted light is called transmittance. When fog, rain, or haze scatters or absorbs the light beam, transmittance drops. The instrument converts that drop into a measurement called extinction coefficient, which describes how quickly the atmosphere degrades visibility.
The airport's weather system feeds this data into a formula to calculate RVR. Multiple transmissometers are placed along a runway, usually at the touchdown zone, the midpoint, and the rollout end. Together, they give controllers and pilots a picture of visibility along the entire runway length.
Example in Aviation#
An aircraft is on approach to a fog-covered airport. The tower reports RVR 1,200 feet for Runway 28L. That number comes directly from the transmissometer at the touchdown zone. The pilot checks the published instrument approach procedure, which lists a visibility minimum of RVR 1,800 feet. Because actual RVR is below minimums, the crew cannot legally continue to a landing under 14 CFR §91.175.
The transmissometer is updating that RVR reading every 60 seconds. If visibility improves to 1,800 feet before the missed approach point, the crew may continue the approach.
Why It Matters#
Pilots rely on RVR to make legal go/no-go decisions on low-visibility approaches. Without an accurate, real-time visibility measurement, those decisions would depend on human observation alone, which is far less precise, especially in rapidly changing conditions.
Understanding where RVR comes from helps pilots trust the number and interpret it correctly. A transmissometer reading reflects conditions at a specific point on the runway, not the entire airport. Conditions can vary significantly from one end to the other.
Key Takeaways#
- A transmissometer measures atmospheric transparency using a calibrated light beam.
- Airports convert transmissometer data into RVR for pilot and controller use.
- Multiple units are placed along each runway for end-to-end coverage.
- RVR from a transmissometer updates frequently, reflecting rapidly changing conditions.
- Pilots use RVR to determine whether an approach meets published visibility minimums.