Note: this article only applies to the old Corona Legacy Material. Fresnel reflections are realistic, so they cannot be disabled in the new Corona Physical Material.
Fresnel IOR is a parameter present in both Corona Legacy and Physical Materials that controls how reflective a material appears based on the angle of incidence. For example, if you look straight down into a pool of water, you can see through it easily but if you look at the water from a low angle, it becomes more reflective, like mirror. Depending on the Fresnel IOR value, materials generally appear more or less reflective.
In Corona 7 and newer, the new Corona Physical Material is available, designed to replace the older Corona Legacy Material. The Corona Physical Material offers more realistic and physically accurate results, making it the new standard material for most situations. However, if you are working with older scenes or have some specific needs, the Corona Legacy Material might still be relevant for you.
There is no way to completely disable Fresnel IOR for Corona materials but setting the Fresnel IOR value in the Corona Legacy Material to 999 will make the object almost uniformly reflective. In real world, materials are neither completely non-reflective, meaning would absorb 100% of the light, nor are they 100% reflective, meaning they would reflect all incoming light. Both of these extreme cases are unphysical and are not observed in real world.
Examples (Corona Legacy Material only)
1. Fresnel IOR = 1.1
Pure black diffuse color:
With diffuse color:
2. Fresnel IOR = 1.52 (default value)
Pure black diffuse color:
With diffuse color:
3. Fresnel IOR = 6
Pure black diffuse color:
With diffuse color:
4. Fresnel IOR = 32
Pure black diffuse color:
With diffuse color:
5. Fresnel IOR = 999 (virtually uniform reflection level)
Pure black diffuse color:
With diffuse color:
Note: this article only applies to the old Corona Legacy Material. Fresnel reflections are realistic, so they cannot be disabled in the new Corona Physical Material.