  While the QED photon amplitudes at full momentum so far have been calculated
only up to the six-photon level, in the low-energy limit there are explicit
formulas for all helicity components even at the N-photon level, obtained by
Martin et al. in 2002. Here we use the worldline formalism to extend that
result to the N-photon amplitudes in a generic constant field. For both scalar
and spinor QED, we obtain compact representations for the low-energy limits of
these amplitudes involving only simple algebra and a single global proper-time
integral with trigonometric integrand.
