Pair Production in Photon-Photon Collisions

General formulas for the absorption probability from the process $\gamma +{\gamma }^{\prime }\to e^{+}+e^{-}$ are given for high-energy photons traversing isotropic photon gases having various spectra. Asymptotic formulas are derived and general results are given in graphical form for photon spectra having blackbody and power-law form. The results are applied in a following paper to calculate the absorption probability for high-energy photons traversing cosmic distances.