Physical Fresnel (ND K) and the French Blue

Hello, sorry if my English is bad.

Personal work that ends with a diamond .
Bleu de France

The basic idea is simple: is it possible to realize a Fresnel function supporting the imaginary part with the nodes?

Assuming that ND and K are = or greater than 0, the function can then be recalculated, giving a node like this:
Physical Fresnel

I use this scene for the following examples

Basically, it produces the same result as the internal blender node.
Physical Fresnel

Blender Fresnel

The difference appears with the use of the imaginary part, I use the wavelengths 650,525,480 nm

The data used is taken from https://refractiveindex.info/ and .ior file

Metals:
Aluminium
Aluminium

Chromium
Chromium

Copper
Copper

Gold
Gold

Gold White
Gold White

Gold Rose
Gold Rose 9K

Gold Purple
Gold Purple

Iron
Iron

Silver
Silver

Zirconium
Zirconium

The imaginary part can be converted into an absorption coefficient, and used directly in cycle

In addition, the function supports total reflectance: simply invert the ND/K value pairs into, a shortcut is made using backfacing
Diamond whith and without total relfectance
Diamond(InterneReflectance)

Diamond(WithOutInterneReflectance)

We get this

Gemstone
Amethyst
Amethyst

Aquamarine
Aquamarine

Beryl Yellow
Beryl Yellow

Diamond
Diamond(InterneReflectance)

Emerald Ural
Emerald Ural

Garnet Orange
Garnet Orange

Sapphire (medium blue)
Sapphire Medium blue

Tourmaline Yellow
Tourmaline Yellow

Liquid

Biodisel
BioDisel

Water
Water

For the rest, I wanted to reproduce the blue diamond of France, the French blue
https://museumdiamonds.com/french-blue/
https://www.researchgate.net/figure/The-original-laser-scan-data-from-the-lead-cast-of-the-French-Blue-top-when-cleaned-of_fig3_274674367

The modelling was simple, I use the ND of the diamond and the K of the sapphire, I did not find the data of the Hope diamond.

HDRI textures come from https://polyhaven.com/hdris

Bleu de France side
Bleu de France Other side

the rendering is rather heavy, and the denoiser does not like it.

I hope I didn’t do anything wrong with the forum and my first post.

4 Likes

Looks interesting - can you make your node setup image larger - it doesn’t expand when you click on it and thus it’s difficult to see what nodes or node settings you have used.

Have you tried this setup with the new path guiding function available in 3.4 alpha?

Path guiding only works for diffuse brdf at the moment :
https://developer.blender.org/T92571

blender 3.3


blender 3.4 path guiding

Edite : I take back what I said: path guiding changes some elements
3.4 GPU

3.4 CPU Path Guiding

What we really need to see is what you added to the Node Group > U Physical Fresnel…in the node tree from above.


Thanks but that doesn’t really help…I don’t see what the K Complex IOR node has to do with your first node tree showing the Nd1 and K1 Nd2 K2 inputs…
and the K complex IOR also has several more group nodes nested with in it…