Trinity House has visibility detectors at each site where we require the operation of a fog signal or light during restricted visibility. The detector consists of a light beam of a frequency close to infra red which reflects on any moisture particles in the air such as fog or mist. A detector set at 90 degrees to the beam measures any scattered reflections of the beam and this is a measure of the visibility in the area. It takes a sample every 20 minutes.
For Trinity House’s purposes if the detector records a reading of reduced visibility it automatically activates the operation of the light and/or fog signal.