  A measurement of the production cross section for two isolated photons in
proton-proton collisions at a center-of-mass energy of $\sqrt{s}=8$ TeV is
presented. The results are based on an integrated luminosity of 20.2 fb$^{-1}$
recorded by the ATLAS detector at the Large Hadron Collider. The measurement
considers photons with pseudorapidities satisfying $|\eta^{\gamma}|<1.37$ or
${1.56<|\eta^{\gamma}|<2.37}$ and transverse energies of respectively
$E_{\mathrm{T,1}}^{\gamma}>40$ GeV and $E_{\mathrm{T,2}}^{\gamma}>30$ GeV for
the two leading photons ordered in transverse energy produced in the
interaction.The background due to hadronic jets and electrons is subtracted
using data-driven techniques. The fiducial cross sections are corrected for
detector effects and measured differentially as a function of six kinematic
observables. The measured cross section integrated within the fiducial volume
is $16.8 \pm 0.8$ pb. The data are compared to fixed-order QCD calculations at
next-to-leading-order and next-to-next-to-leading-order accuracy as well as
next-to-leading-order computations including resummation of initial-state gluon
radiation at next-to-next-to-leading logarithm or matched to a parton shower,
with relative uncertainties varying from 5% to 20%.
