Tandemrotor
My interpretation of APP1 to JAR-OPS 1.430 (h)(l) as mentioned by brimstone has always been that the reported TDZ RVR must be above minima unless the pilot assesses it to be lower in which case he cannot depart. However if the TDZ RVR isn't available then he can replace it with his own assessment. Some people interpret the rule as meaning that the pilot can go if he assesses the RVR to be above limits. However I would hope that he had a good lawyer if things went wrong
I don't have the relevant manual with me so can't be sure who is correct, but in pracical terms I would not want to take-off if I couldn't see the minimum number of required lights.
Airclues