  We study 2-to-2 scattering amplitudes of massless spin one particles in $d=4$
space-time dimensions, like real world photons. We define a set of
non-perturbative observables (Wilson coefficients) which describe these
amplitudes at low energies. We use full non-linear unitarity to construct
various novel numerical bounds on these observables. For completeness, we also
rederive some bounds using positivity only. We discover and explain why some of
these Wilson coefficients cannot be bounded.
