In Probing the linear polarization of photons in ultraperipheral heavy ion collisions and Impact parameter dependence of the azimuthal asymmetry in lepton pair production in heavy ion collisions it is proposed to study the collision between linearly polarized photons via the azimuthal angle between $q_\perp$ and $P_\perp$ where: $$ q_\perp = p_{1\perp} + p_{2\perp} = k_{1\perp} = k_{2\perp} $$ and $$ P_\perp = (p_{1\perp} - p_{2\perp})/2 $$ Experimentally this is accessible as: $$ \Delta\phi = \Delta\phi[ (p_{1}^+ + p_{2}^-), (p_{1}^+p_{2}^-) ] $$ This observable is a bit hard to visualize, so I put together the widget below. The red line represents the vector sum $(q_\perp)$ and the blue line is $P_\perp$