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I'm wondering how I can obtain a realistic partially transparent object using a spot light.

I see the effect of the light on the side directly lit by the spot light, but I see nothing on the other side.

How can I see the diffusion on the opposite side?

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  • $\begingroup$ You really need to upload your blend and add more details to your question, like screenshots. Otherwise, it will be very difficult for people to help you. $\endgroup$
    – aidan-j-rhoden
    Commented Dec 15, 2023 at 14:05
  • $\begingroup$ I need to follow a tutorial to send my screen captures !... $\endgroup$
    – Franck
    Commented Dec 15, 2023 at 20:16

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I believe what you are referring to is subsurface scattering. It simulates light leaking through thin objects. To get this to work, you need to tweak the Subsurface settings in the Principled BSDF.

Old Principled BSDF:

Shader 3.x

New Principled BSDF:

Shader 4.x

Basically, you need to use a nonzero value for Subsurface (old) or Weight (new). Additionally, you need to tweak the Subsurface Radius (old) or Radius (new) to determine the color of the light that leaks to the other side. If you want it to pass as is, all components of the Radius need to be set to one. Note that, in the old Principled BSDF, in EEVEE, the Subsurface Radius didn't affect the color.

There is one more thing to do depending on the render engine.

In EEVEE, you need to enable Subsurface Translucency in the material settings.

EEVEE

In Cycles, you need to add a Solidify modifier to give thickness to your plane. The effect won't show up on backfaces.

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  • $\begingroup$ I tested your ideas and I see my both faces now diffusing light but despite the fact taht my material is partialy transparent, the light ceased to be transmitted and my second plane screen is not lighted now! $\endgroup$
    – Franck
    Commented Dec 15, 2023 at 20:14
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    $\begingroup$ Then, I misunderstood your question. Please provide a reference of what you want to achieve. $\endgroup$
    – Mr A
    Commented Dec 15, 2023 at 22:11
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Use a Translucent BSDF.

enter image description here

In versions of Blender before 4.0 (and indeed I remember some old documentation that mentioned) it was my belief that the Add Shader gave more accurate representation, but in my testing today I was getting more consistent results with a Mix Shader. In either case, try them both in your scene and see if that's what you're looking for.

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