Depending on scene scale, you may need to use very high intensity values.
On mac it may be necessary to use v7, since the values slider/box was limited to a value of 100 or less; I reported this to mcneel, but am not sure if it is still the case in the most recent v6 (I generally stay one or two builds behind, for compatibility). I just confirmed it is possible to input larger values on the current v7.
I have done a bunch of work in this area in 20.13, so you will likely have to re-adjust light intensity values.
I was able to find enough data in the Rhino lights to compensate output by document scale, and scaling applied to individual lights, such that they should now be roughly equivalent in output to what you get in Rhino’s rendered or raytraced viewport modes: