I am trying to render a parabolic trough collector (essentially a concave shaped mirror). But the rendered image has an area that is rendered completely black. I have tried different variations for shading and they are listed below along with the results:

  1. Glossy BSDF, Roughness=0: Render Image #1
  2. Glossy BSDF, Roughness=1: Render Image #2
  3. Principled BSDF, Metallic=1: Render Image #3
  4. Principled BSDF, Metallic=0.462: Render Image#4
  5. Principled BSDF, Metallic=0: Render Image#5

Rendered Images can be found here (the image name can be read from Google Photos Image info bar) Render Image 1 Render Image 2 Render Image 3 Render Image 4 Render Image 5

From what I read on the internet, this is most probably caused by mirror reflecting it self (owing to it's curved shape). And the advice on most of these threads was to increase the value of indirect clamping. But changing the value of indirect clamping does not seem to have any affect (I went up to 100) on the render. For reference, the uploaded pics were rendered at indirect clamping value of 5.00

Using Principled BSDF and reducing the metallic value seems to to give somewhat better result, but the dark area still exist, although the intensity is quite low. However, due to reduced metallic value, the mirror effect gets suppressed.

Any help in this matter will be highly appreciated

The .blend file can also be downloaded from here



  • 3
    $\begingroup$ Possible duplicate of Black reflections with Cycle's Glossy Material $\endgroup$
    – cgslav
    Apr 10, 2019 at 23:11
  • $\begingroup$ @cgslav Hi, I already had a look at the post you are referring to. But the problem is that my scene does not have any normal maps and is purely based on shading. Moreover the black reflection effect in the thread you mentioned is quite low and not as drastic as in my scene $\endgroup$
    – Uzair
    Apr 11, 2019 at 3:38
  • $\begingroup$ Update: If I remove smooth shading from the mirror, the effect goes away but the mirror is quite rough looking $\endgroup$
    – Uzair
    Apr 11, 2019 at 3:52
  • 1
    $\begingroup$ Hi. Please upload any relevant images using the built-in uploader. See: meta.stackexchange.com/questions/75491/… $\endgroup$ Apr 11, 2019 at 12:11

1 Answer 1


It seems that it was a problem with the mirror object mesh. The curved surface was created in SolidWorks, and exported in .stl format.

I created another mesh taking reference from the imported .stl file and the problem was resolved.

Latest render


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