Tunnel Portal Photometers – choosing the right location

The Acoem LUMIOS-MkIII photometer is designed to measure the level of luminance created by natural light at road tunnel portals. The luminance data is supplied to the tunnel’s lighting control systems, these systems ensure the visual perception of drivers is maintained both day and night by adjusting the tunnel’s portal light levels. This creates a safer environment for the driver by reducing sudden variations in light levels and preventing black hole syndrome.

The LUMIOS-MkIII’s light receptor measures the average luminance within a fixed viewing angle, the standard instrument has a viewing angle of 20°, with a measurement range of 0 to 10,000 cd/m2, which can be scaled to suit end user requirements.


When selecting a position for the LUMIOS-MkIII installation, it is common for national and international standards, such as Commission Internationale de l´Eclairage (C.I.E.) publication 88 (2004), to have been adopted into the tunnel design. Where a location is not defined by the tunnel design specification, it is advised that the CIE publication 88 recommendations are followed.

These recommendations include;

  • Mount the monitor at a height of 3-5m, on a pole, or gantry, above the carriageway
  • Orientate the monitor in both horizontal and vertical planes such that it looks directly at the tunnel portal, centring on a point ~1.5m above the surface of the carriageway.
  • Mount a 20° viewing angle monitor (L20) at the driver’s safe stopping distance from the tunnel portal, based on the road’s maximum vehicle approach speed, and any national definitions of “safe stopping distance”.

Note: If it is not possible to fix the monitor at the recommended distance from the portal, then a monitor with a different viewing angle can be specified. The required viewing angle will scale with the inverse of the distance from the portal.

There is an increasing trend for tunnel lighting designers and tunnel operators to keep the daytime L20 measurement level as low as possible, this helps reduce the lighting intensity required at tunnel portals and therefore saving energy. The primary method is to reduce the amount of sky seen by the photometer, for example by growing trees or building walls. On existing tunnels the road’s approach speed can be lowered, this reduces the stopping distance so the photometer can be installed closer to the tunnel portal and will consequently see more tunnel infrastructure and road, and less sky.

If you would like to know more about Acoem’s road tunnel luminance monitoring solutions please do not hesitate to contact Graham Jennings or Charles Mawle.

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