Surfing around web I founded this one a bit old but compatible with CAD software and import IGES STEP and Brep and source available.
and it's Freeware ;)
On Linux WineHQ, seem not working on my notebook but Yes is version with GUI there is a little tutorial, but the render engine zip folder is dated March 9, 2017. a bit old..
In fact not too difficult the CadRays! :)
And ultra speed!
And can import directly iges & Step file from Moi!
I was affraid by the Command line...but not used it! :)
Just a little logo on the left corner...