Tandemrotor - I'm not a pilot so this reply comes with a health warning.
Appendix 1 to JAR-OPS 1.430 Aerodrome Operating Minima contains a note in the "RVR/Visibility for take-off" tables that says;
The reported RVR/Visibility value representative of the initial part of the take-off run can be replaced by pilot assessment.
This is a long and complex subject so you must read all of it.