Photons looping around photon shells in black hole spacetimes form a sequence of higher order images -- the photon ring. Unlike in the case of the direct image, the size and shape of the photon ring reflects properties of spacetime, aka "clean geometry" with only limited impact of the configuration of the emitting source, aka "dirty astrophysics". Thus, they enable unprecedented robust observational tests of strong gravity, possibly including estimates of black hole spin, or constraining deviations from the Kerr metric. For at least 1 black hole in the Universe, M87*, the photon ring can be characterized with space radiointeferometry at mm wavelengths. I will talk about the ongoing plans and efforts to do it in the next decade with the

proposed NASA Black Hole EXplorer (BHEX) space VLBI mission.