F0 (Fresnel Reflectance at 0 Degrees)*
When light hits a surface straight on or perpendicularly (0 degree angle), there is a percentage of that light that is reflected back as specular. Using the Index of Refraction (IOR) for a surface, you can derive the amount that is reflected back and this is referred to as F0 (Fresnel 0) as shown in figure 09. The amount of light that is refracted into the surface is referred to a 1-F0.
The F0 range for most common dielectrics will be from 0.02 – 0.05 and for conductors the F0 range will be 0.5-1.0. Thus, the reflectivity of a surface is determined by the refractive index.
It is the F0 reflectance value that we are concerned with in regards to authoring our textures. Non-metals (dielectrics/insulators) will have a greyscale value and metals (conductors) will have an RGB value. With regards to PBR and from an artistic interpretation of reflectance, we can state that for a common smooth dielectric surface, F0 will reflect between 2% and 5% of light and 100% at grazing angles.
The dielectric (non-metal) reflectance values don’t actually change very drastically. In fact, when altered by roughness the actual changes in value can be hard to see. However, there is a difference in the values. In figure 11, you can see a chart that shows the F0 ranges for both metal and non-metal materials.
Notice that the ranges for non-metals do not deviate from each other drastically. Gemstones are an exception as they have higher values.
Substance values to Vray**
By default the reflectance at f0 for dielectric is 0.04. In SD we can modulate this value from 0 to 0.08 but in Painter it’s not yet possible.
So for non metal in SP, the reflectance is always 0.04.
Here is the formula to convert the f0 to 1/ior:[2.0/(sqrt(0.04)+1.0)]-1.0
Which is a constant in the case of SP since the reflectance is always 0.04: 1/ior = 0.66666
For metals we suppose the ior is close to infinite and the reflectance is defined by the basecolor.
So we set the 1/ior to something very low like 0.0001 to avoid the 0 division.
In the end the final 1/ior texture is
mix(0.6666, 0.0001, metallic)
We use a 1/ior texture because we don’t output an hdr map for the ior. When vray reads the texture it actually makes the inverse division to get the correct ior value back.
In the case of dielectrics: 1/0.6666 = 1.5
So basically if your using Substance Designer and Vray and you’ve exported your textures out of SD using their PBR converter (set to vray) to get your maps, then your all sorted, nothing to worry about.
If however you want to use the PBR fresnel values in the same way in Vray without SD then just use the formula: [2.0/(sqrt(0.04)+1.0)]-1.0 (replacing the 0.04 value with the material value your after) and then use this in your fresnel in vray.