I just purchased an interior scene from chocoviz for the cycles render engine.
When I do a test render in the viewport it renders, but is much darker than in the final render.
What could be the cause of this issue? How can I resolve it?
In interior scenes and in scenes that have lots of translucent materials it is common for them to be dark until the camera has sampled enough light. If it still seems too dark after 100 samples try duplicating the lights and disabling the render setting on them. This will most likely give better results if not the most accurate.
This might not answer OP question, but...
There are some limitations regarding to viewport rendering
I think the main problem is in viewport, they lack of light bounces. So adding a light (but discard on render) seems to solve the problem.