Black Sky in Point in Time Luminance

Hello All,

I am getting black sky in a point in time luminance analysis I am performing.

The view is looking towards the south and the roof is a huge skylight. So in this view, sun will be visible. I am not able to figure out why this is happening. Following is what I have tried already;

  1. I changed VLT to 40% and it gave following result. The results were comparable to expectations.

  2. I have tried using the latest Radiance version. That is 5.2.91bcc23df3 NREL 2018-06-20

  3. I have tried this on another machine to see if I can recreate the error. The problem remains. @sarith tried on this machine and could not re-create the error surprisingly.

  4. I have explored the radiance parameters as mentioned on this page.

Here the link to download the whole radiance folder, rhino and grasshopper file. Any help in understanding the issue will be appreciarted.


because the vlt of your glass is so low, the -lw .05 parameter is set so that view rays to the sky are terminating at the glazing (the sun gets through because it is a direct source). if you lower the -lw setting you are good to go (delete your ambient file if your process does not do that automatically). I would also suggest searching here and the rpict man pages for general guidance on properly setting -lw, but I think in general it should always be smaller than 1/ad.

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Hi Devang,

how is the skylight modeled? Is a trans, BSDF, glass applied to it?
There is something looking like a set of blinds, is that located on top
of the skylight?

Cheers, Lars.

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Here’s the output of 40% VLT

I know the image in the original post gives an impression that there are louvers on the top but there aren’t. The skylight is modelled as Glass in radiance. Thanks for looking into this.

That’s brilliant! That indeed solved it. Thanks a lot!