Principled Shader IOR Formula (Blender 3.5 – Cycles)
This animation describes the Fresnel effect, which is calculated using a refraction value. The Fresnel describes the intensity of the specular (gloss) of a surface depending on the viewing angle.
Andrew Price described in a BCON 2022 talk that you should not change the specular value of Blender's Principled Shader. This is not correct in principle, because the specular value controls the Fresnel of the surface.
Note: Every material (opaque or transparent) has an Index Of Refraction (IOR). The IOR does not only describe the refractive index for transparent surfaces.
ARNOLD, for example, uses the refraction index to automatically control the Fresnel. In Blender we have the artistic freedom to decide even if it is physically incorrect. For a physically correct surface, we can have the Specular value for a correct Fresnel calculated from the IOR of a material:
SPEC = ((IOR -1) / (IRO +1))^2 / 0.08
This formula can be mapped very easily using math nodes. We use a value node to control the IOR and the resulting specular value.
The output values in the image are visualized via Geometry Nodes (String).
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Sources:
Price, Andrew. (2022, 28. October). The Secrets of Photorealism [Video]. Blender. YouTube. • The Secrets of Photorealism
Seven, Metin. (2018, 05. November) Blender 3D tip — Realistic Specular value in Principled shader. BlenderNation. https://www.blendernation.com/2018/11...
Blender 3.4 Reference Manual. (2023, 05. January). Principled BSDF. https://docs.blender.org/manual/en/la...
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