  Based on the geometry of the codimension-2 surface in a general spherically
symmetric spacetime, we give a quasi-local definition of a photon sphere as
well as a photon surface. This new definition is the generalization of the one
by Claudel, Virbhadra, and Ellis but without reference to any umbilical
hypersurface in the spacetime. The new definition effectively rules out the
photon surface which has noting to do with gravity. The application of the
definition to the Lemaitre-Tolman-Bondi (LTB) model of gravitational collapse
reduces to a problem of a second order differential equation. We find that the
energy balance on the boundary of the dust ball can provide one appropriate
boundary condition to this equation. Based on this key investigation, we find
an analytic photon surface solution in the Oppenheimer-Snyder (OS) model and
reasonable numerical solutions for the marginally bounded collapse in the LTB
model. Interestingly, in the OS model, we find that the time difference between
the occurrence of the photon surface and the event horizon is mainly determined
by the total mass of the system but not the size or the strength of
gravitational field of the system.
