A uniform atmospheric pressure dielectric barrier discharge is operated in helium with an admixture (0.45%) of nitrogen. The discharge is ignited in the gas gap between a driven and a grounded electrode and propagates along the dielectric surface outside the gap. Plasma conditions are characterized with current and voltage measurements and by application of absolutely calibrated optical emission spectroscopy, with a focus on nitrogen molecular emission. Plasma parameters, namely electron density and reduced electric field are determined with spatial and temporal resolution in the frame of collisional-radiative model by application of calibrated intensified CCD camera and Abel inversion of measured images. The density of helium metastable states is calculated using the measured plasma parameters and successfully compared with values measured using tunable diode laser absorption spectroscopy.