Photon-photon pair production ($E_1{E}_2(1-\cos \alpha )\ge 2m_e^2{c}^4$)

Two photons meeting at angle $\alpha$ can produce an electron and a Positron when their combined four-momentum has sufficient invariant mass. For a head-on collision the threshold is $E_1{E}_2=m_e^2{c}^4$. Threshold means zero relative momentum in the centre-of-momentum frame, not necessarily in the laboratory.

Two head-on photons can create an electron-positron pair at the limiting product $E_1{E}_2=(m_e{c}^2{)}^2$. A target at the peak of the frequency-density blackbody spectrum has $E_1=2.82144k_{B}T$, whereas the wavelength-density peak corresponds to $E_1=4.96511k_{B}T$. At $T=3\mathrm{K}$ these conventions give incident thresholds approximately $3.58\times 10^{14}\mathrm{eV}$ and $2.03\times 10^{14}\mathrm{eV}$ respectively. A real blackbody has a continuum of target energies and angles, so an absorption calculation integrates the spectrum and cross section rather than imposing a sharp peak-photon opacity boundary.