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This is a night scene I modeled with some windows in the building.

Normally all the windows should be lit. But the light does not pass through the window if the camera is not perpendicular to the it.

enter image description here enter image description here

What should I do to let the light pass at any angles?

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By the way, I am using Blender 2.91

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    $\begingroup$ Check that your normals are facing the correct way and that your glads panes have ‘thickness’ (ie, they aren’t just a single face - add a Solidify modifier or extrude to give them physical thickness). $\endgroup$ Jan 11 at 7:47
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@Allen Simpson' s answer inspired me. Now I found the solution by simply setting the IOR value of the window glass to 1.0 or slightly larger (1.45 is the default setting).

Here is the result. enter image description here

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    $\begingroup$ ... but @Rich Sedman's answer is strictly the more physically correct one. (That may not matter, here) $\endgroup$ Jan 11 at 11:58
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The light is getting through, but it's going straight through like a prism because of the light source.

In real life the window would be receiving bounced lighting from all directions inside the room. The easiest way to recreate this is probably to bump up the transmission roughness value.

enter image description here

Note that while a realistic IOR value would be around 1.5 for most types of glass, I personally find an IOR of close to 1 helpful for rendering thin panes of glass.

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  • $\begingroup$ Bumpping up the transmission roughness value seemed not to work. But you inspired me to adjust the IOR value down to 1. And suddenly every window is lit! $\endgroup$ Jan 11 at 6:02
  • $\begingroup$ Right, I forgot to mention that. For thin panes I find an IOR of 1 to be helpful. $\endgroup$ Jan 11 at 6:03
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    $\begingroup$ In real refraction, the light would be bent one way on entering the glass, and back again on exiting it. For Blender's refraction model to be realistic, the glass has to have thickness. The ray must encounter front and back-facing surfaces on the way from the world, through the glass, and back again. $\endgroup$ Jan 11 at 12:03
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Fixing the normals of and adding thickness to the glass are the best solutions.

Thanks for @Rich Sedman and @Robin Betts's comments. It works by simply setting the IOR value of the window glass to 1.0 or slightly larger (1.45 is the default setting), but it is not a physically correct solution. Check their comments. I am adding this since they didn't post it in an answer.

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