Hi @hlkocalioglu - You can create a new modifier by clicking on the Add button and selecting Mirror.
Edit the material and press OK.
You should see the material being added to the model.
Finally, you can assign it to a face from the face properties menu.
Keep in mind, that the mirror modifier in Radiance will only be applied to the front of the face. You can use the Dir command in Rhino o check the normal direction, and Flip it if needed.
