Surface plasmon polariton (SPP) excitation at a gold-vacuum interface via 800 nm light pulses mediated by a periodic array of gold ridges is probed at high lateral resolution by means of photoemission electron microscopy (PEEM). We directly monitor and quantify the coupling properties as a function of the number of grating ridges and compare the PEEM results with analytic calculations. An increase in the coupling efficiency of > 3 is observed when increasing the number of ridges from 1 to 6. We observe, however, that a further addition of ridges is rather ineffective. This saturation behavior is assigned to the grazing incidence excitation geometry intrinsic to a conventional PEEM scheme and the limited propagation distance of the SPP modes at the gold-vacuum interface at the used wavelength.