Cornell box
A simple boxed scene — two coloured walls and a light in the ceiling — used since 1984 to test whether a renderer gets indirect light right.
It was built as a physical model at Cornell's graphics program, photographed under controlled light and measured, so a render could be checked against the photograph rather than against taste. The red and green side walls are the whole point: a correct global-illumination solution bleeds a faint tint of each onto the nearby white surfaces — colour bleeding — something direct light alone never produces.
The box became the de facto proof for radiosity and later path tracing: if a renderer can't reproduce its soft shadows and colour bleed, it can't be trusted on anything harder. The scene's exact geometry and spectral measurements are still published today.