This page gives some basic details about the Reflection Index Render Element and how it is used in compositing.
The Reflection Index Render Element returns an image with floating-point values that correspond to the Fresnel IOR values of scene objects' materials.
This render element can be used to enhance or reduce Fresnel IOR effects in the final composite. Note that the values generated in this render element can often be above 1, so that the image may appear to simply be black and white. Sampling the pixel values in the white areas will return the precise floating-point value stored, which represent the Fresnel IOR numbers for the rendered object.
||Render Setup window|| > Render Elements tab > Add button > VRayMtlReflectIOR
This render element is enabled through the Render Elements tab of the Render Setup window in 3ds Max and displays its parameters in a rollout at the bottom of the window:
VrayVFB – When enabled, the render element appears in the V-Ray Virtual Frame Buffer.
Deep output – Specifies whether to include this render element in deep images.
The VRayMtlReflectIOR Render Element is useful for changing the appearance of Reflective elements after rendering in a compositing or image editing software.