reflectivity on a black material covered with a clear one

[CodeHD]

Same here, just had a look and not quite sure what the problem is.
The scene looks fine to me.

One interpretation of your question I could see is that you want - in total - only 0.1% refelction

As you have a glass surface at the front, you will always have the 4%-component from that.

From both my understanding Dade's answer and my own test on your scene, the exterior volume IOR has no effect for the glossy material.

You hence get a clear 4% specular reflection fromt he front glass surface, and a glossy component from the intermediate surface, which will be slightly diffuse because glossy has a minimum 0.001 roughness. Depending on the view angle and and the glass IOR there will also be a small refractive shift of the glossy component.

[lighting_freak]

I know about the round about 4% from first surface. That is true and right.

According to Dade's answer there will be another 5% of specular/glossy from interaction with black material.
That's how luxcore treat this situation.

This are my thoughts:
Having two clear materials connected by an intermediate surface there will be a reflection based on the difference if IOR between (relative IOR) both of them. I wonder whether this is going to be same with this black surface in real world. In that case it would be the 0,1 %.
Luxcore handles this with volume priority.

My work is all about car tail lamps. Their outer lens is often made from a multi-shot-mold. This means there are red acrylics meeting yellow acrylics meeting clear acrylics and from time to time also some black acrylics.
I wonder whether I could use luxcorerender to detect stray light and light leakage on those outer lenses.
That's the reason for trying to understand how this interaction works.

Thank you and with best regards.

[CodeHD]

ok, now the situation is much clearer.

Yes, a real smooth black surface will also show some reflection. There is no perfect black, and if it is smooth it will be glossy/specular.

I've attached a modified scene with a camera and mesh light at a shallow angle (don't try this with Sun, an inifinite light doesn't show it the same way).

In preview render (render area should be pre-set), observe the behaviour when you change the IOR of the volume. Also try the glass material on intermediate instead of glossy.

With the glass, you can see that the second reflection disappears when IOR_Glass = IOR_Volume (= 1.8 in this case), which does not work for glossy, in accordance with Dade's description.

[lighting_freak]

Hi,

thank you for the file, very impressive, and very well visible interaction.

Now, since I see that I could get everything, I'd like to know what's the truth...

... I did already a little web research but I didn't find anything useful.
Is, by coincidence, someone there who know abut the real world reflectivity of such a material combination?
Let's assume both, the black and the clear component are a PMMA based acrylic.
Both come up with an IOR of 1.492... what might be the "real" setting for luxcore?

Thanks a lot for all your support.

With best regards

[CodeHD]

This is very difficult to answer, as it will likely depend on the specific materials, how they are manufactured, geometries, and illumination.

If the materials really have the same IOR, and they are well bonded (i.e. without air gaps or bubble inclusions), then you will get no surface reflection. So question a) would be: How different is the IOR depending on the added color? In any case, I would expect the difference to be low, so little specular reflection from the surface.
This brings me to question b), being how much SSS do you get, also in the black acrylic? This could add to the appearance.
In your earlier post you write about light leakage. This sparks a separate question c), how thick is the black layer and will it even fully absorb incident light?

For car tail lamps, it sounds very detailed to me, and I wonder if the black acrylic in the situation you describe would produce any noticable stray light.
In the last years, I have worked in the design and testing of a telescope with very high stray light reduction. When you go really dark, you may find that effects of diffraction will dominate over geometrical optics.

[lighting_freak]

Hello,

Question a: I'll gonna try to organize some measurement of this situation. Maybe we could clear that up...
Question b: There should be no recognizable transmission of the black material. So no SSS is expected for this type of analysis. In fact you might be right in pointing out that some SSS may occur on very thin material slices.
Question c: This type of light leakage doesn't take into account any type of transmission through the black. It's assumed to be 100% opaque, but see b).

This is more or less the situation I am looking for.

I'll come back to you as soon as I've collected some more data and/or information.

BR