Originally Posted by
Gonzo
Counting the runway lights is always the more accurate method of determining Visibility along the runway.
IRVR is a nice piece of kit, but it only takes a very localised reading at each site, and the algorithm converting the light reading into RVR always assumes a level of contamination (dirt, oil etc) on the AGL light fittings. Hence if your airport has a decent AGL cleaning regime, then your IRVR is probably probsbly reporting lower RVRs than is actually the case.
At Farnborough at certain times of year, the setting sun shines straight into the transmissometer and gives a false low reading.