It then follows that an RVR for one runway cannot be substitued for another runway, just for convenience.
No one is substituting anything.
Let's look at a typical runway - 18/36 with CAT 1 ils on each end.
Runway 18/36 has 3 RVR transmissometers. One placed at a point abeam the touchdown point runway 18, the next abeam the mid-point and the next abeam the touchdown point runway 36.
They indicate RVRs that are deemed to be representative of the RVR at that point on the runway.
If starting at the 36 threshold the RVR's are 100m, 300m, 600m then 99.999% of the worlds pilots will want to fly the ILS 18 because the RVRs for 18 are clearly 600, 300, 100m.
You on the other hand would be diverting unless the airport had installed a second set of transmissometers right beside the first ones. :-)
The only case I can think of where the RVR at the stop end of 36 will be different from that for the touchdown of 18 is when the lighting intensity is different in each direction since the reported RVR does take lighting intensity into account. However, most runways have bi-directional lights and thus the lighting intensity will be the same in both directions unless lights are u/s.
So while the posibility exists, it has nothing to do with the RVR sensors being directional.
