In-situ visibility monitoring by optical transmission

Visibility monitoring is used to control the tunnel ventilation at normal operation. If and with how much power artificial ventilation by jet fans is operated depends on the measured visibility.

Visibility is stated in the form of an extinction coefficient that corresponds to the light attenuation caused by air pollution.

The monitoring system consists of a light emitting and receiving sensor and an optical reflector. The sensor determines the light transmission along a 10 m measuring path. Visibility and extinction coefficient are calculated based on the determined transmission.

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t/VIS-TR Visibility monitoring system – transmission

  • Visibility measurement by optical transmission method
  • Reliable measurement along a path of 10 m parallel to the tunnel axis
  • Parametrisation of the sensor via Modbus RTU
  • External cable length of 1000 m to tunnel control system
  • Signal output either via analogue and relay outputs or RS-485 with MODBUS RTU protocol
  • Visible LEDs show sensor status


Product info DE (PDF, 356 kB)
Product info EN (PDF, 356 kB)


System setup


Visibility sensor

The sensor is mounted on the tunnel wall and aligned to the reflector in parallel to the tunnel axis.



The reflector is mounted on the tunnel wall in a distance of approximately 10 m from the sensor and aligned with it. The distance is an important parameter for the measurement.


Terminal box
The terminal box connects the sensor to the power supply and the tunnel control system.


Connection to tunnel control system

The connection can be established either via analogue outputs and relay contacts or via the RS-485 MODBUS RTU interface.