  To separate the energetic photons produced in hard scattering processes from
those from other sources, measurements impose isolation requirements which
restrict the hadronic radiation inside a cone around the photon. In this paper,
we perform a detailed factorization analysis of the QCD effects associated with
photon isolation. We show that for small cone radius $R$, photon isolation
effects can be captured by a fragmentation function describing the decay of a
parton into a photon accompanied by hadronic radiation. We compute this
fragmentation function for different isolation criteria and solve the
associated renormalization group equations to resum logarithms of $R$. For
small isolation energy, the cone fragmentation function factorizes further,
into collinear functions describing energetic quarks and gluons near the cone
boundary and functions encoding their soft radiation emitted into the cone.
Based on this factorization we also resum the non-global logarithms of the
ratio of the photon energy and the isolation energy, so that we control all
logarithmically enhanced terms in the cross section. Finally, we provide a
simple formula to convert NNLO cross section results from smooth-cone isolation
to fixed-cone isolation.
