= Internal pair-production target energy in a relativistic blob
{title2=$\epsilon_{2,\rm obs}=D^2K/\epsilon_{1,\rm obs}$}
Let the relevant rest-frame <photon-photon pair production> <energy> product be $K=\epsilon'_1\epsilon'_2$ for a specified collision-angle prescription. For fixed observed high-energy <photon> <energy>, $\epsilon'_1=\epsilon_{1,\rm obs}/D$, so the target has $\epsilon'_2=DK/\epsilon_{1,\rm obs}$. Observing the same internally produced target population supplies a second <Doppler factor>, giving the displayed result. An external stationary <radiation> field requires its own angular transformation instead.
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