Meshlights darker in final render It seems like the emission shader lights up other objects stronger in the preview render than in final renders.



I'm using mesh lights in Blender 2.78. The error is reproducible by creating a plane with an emission shader and a subject with any kind of material, that's lit by the plane.



As I didn't find any reports of this issue, I'm wondering if I'm the only one experiencing it, and if so, what settings might cause this.





EDIT:

After resetting the preferences in the System tab this is the render:


It is a bit closer to the preview, but then I noticed that both of the lights are reflected two times. (Which was visible before, but I didn't realize it.)

I also tried to render with CPU, and the result was identical to the preview.

So it's definitely a problem with the GPU (Nvidia).
It's unlikely a driver problem since I noticed the problem some weeks ago.
I tried Delagone's comment without success,


  Have you applied the scale to your mesh lights?


but later I could fix the problem by also applying Rotation and Location.

I don't know what the GPU thinks the lamps looked like, but without all three transformations applied, the rendered image was wrong in some way.