RVR is reported via the use of a transmissometer and reported as TDZ,MID and Rollout. Usually all 3 are required but exception may be made for RO for the Rollout section only depending on your companies policy. These instruments provide a readout to ATC, and ATC provide them to the pilot. Depending on the type of Approach (II, IIIA or B), certain minima are required before proceeding past the Approach Ban Point (ABP) which will normally be either 1000' or G/S intercept depending upon the State. You must always check the RVR before passing the ABP. If minima are less than required before the ABP you can not conduct the Approach. If you have passed the ABP you can continue to the relevant minima.