Don't confuse no DH with no RVR, you still require 75m for a CATIII b no DH. So if you are outside the OM or above 1000' (if no OM exists) and you get a RVR report below 75m (very unlikely) then you can't continue the approach. If you are within the OM or below 1000' and the report is below 75m you can continue and have a look.