The white furnace test is a popular rendering debug tool that initially seemed mysterious and abstract to the author. It involves showcasing empty renders to demonstrate the concept of a 100% reflective object becoming indistinguishable from its environment. The author compares this effect to a whiteout in fresh snow, where the surroundings blend seamlessly with the sky. The purpose of the white furnace test is to ensure that a physically based renderer accurately preserves energy and to detect any errors in the implementation. It serves as a crucial sanity test and helps validate both the integration and BRDF parts of rendering.
https://lousodrome.net/blog/light/2023/10/21/the-white-furnace-test/