  In this letter, we show the Unruh temperature of the photon sphere for an
AdS$_4$-Schwarzschild black hole can be determined holographically from the
retarded Green's function and is proportional to its circumference according to
a boundary observer. We then sharpen the conjecture that the photon sphere, as
seen by a boundary observer, is the spatial Fourier transform of the response
function. The conjecture is found to be in excellent agreement with our
numerics after certain long-lived excitations -- associated with geodesics
traveling between boundary points -- are removed from the response, which is
then controlled by the short-lived excitations of the AdS black hole.
