Cycles Principled BSDF violating energy conservation?

[CarlG]

I did something similar a while back:

Principled furnace test fail with diffuse roughness

I checked, this is also the case in 2.79. I don’t have renderman, can anyone check this there? Basically, a shader setup is said to be energy conserving when it survives the furnace test. A furnace test is just a fully white background with no extra lights. Test using filmic as anything above 1 will actually show up without measuring. Tests: Result1 (correct): Vanilla regular Lambertian diffuse is ok. Result2 (unsure): Vanilla Oren-Nayar diffuse gets just slightly darker, which I can live wi…