Dielectric materials (non-metal) can have their Fresnel effect rendered based on their refractive index alone. For metals, however, their reflectance curve also depends on other variables, and in order to achieve a precise Fresnel effect for a given metal (e.g. gold, copper etc.), you can use Complex IOR instead of base and edge color. When enabled, the metal reflection will no longer be specified by the default base and edge color of the material. Instead, the reflection is defined by a complex index of refraction, which is given by, n*(η-eta) and k**(κ-kappa) values.
Complex IOR as seen in the Advanced Options of the Corona Physical Material
Note: Complex IOR can only be enabled in the Corona Physical Material when Metalness is set to Metal. For non-metal materials, this parameter will be greyed out.
- *n stands for the refractive index value for RGB wavelengths
- **k stands for the extinction coefficient value for RGB wavelengths
ComplexIOR – Practical Examples
Below you will find some practical examples of how Complex IOR can be utilized in applying a proper base and edge color for physically correct metals, in pure form (unoxidized, unweathered). For starters, you have to go to https://refractiveindex.info/ select from the first rollout 3D - selected data for 3d artists, select your preferred metal, and input each of the following RGB wavelengths (0.70 μm for Red, 0,55 μm for Green, 0.45 μm for Blue) to get the corresponding n,k values.
Pure Copper(Cu)
- For Red, at 0.70 μm, n=0.212 - k=4.1
- For Green, at 0.55 μm, n=0.676 - k=2.624
- For Blue, at 0.45 μm, n=1.31- k=2.29
Pure Silver(Ag)
- For Red, at 0.70 μm, n=0.167 - k=4.28
- For Green, at 0.55 μm, n=0.145 - k=3.19
- For Blue, at 0.45 μm, n=0.135- k=2.38
Pure Gold(Au)
- For Red, at 0.70 μm, n=0.169 - k=3.88
- For Green, at 0.55 μm, n=0.421 - k=2.343
- For Blue, at 0.45 μm, n=1.373 - k=1.770
Note: There is definitely no need to use the Complex IOR option for all metal materials in the scene. The base color and edge color properties can be used instead, as they offer more flexible control over the material. Using Complex IOR without the reference N and K values is not recommended.