Noun
photon sphere (plural photon spheres)
(of a black hole) A spherical limit around a black hole at such distance that any photon meeting it tangentially is constrained to travel in a circular orbit; (more loosely) the region between this limit and the event horizon.
Synonyms: photon circle, last photon orbit
The radius of the photon sphere is also the lower bound for any stable orbit.
For a Schwarzschild (non-spinning) black hole, the radius of the photon sphere is 1.5 times the Schwarzschild radius.