Electric field strength in diffuse coplanar barrier discharge in helium at atmospheric pressure, driven by AC voltage (amplitude 1.5 kV, frequency 10 kHz), was measured by optical emission spectroscopy. Two helium singlet lines at 667.8nm (He I 2 1 P-3 1 D) and 728.1nm (He I 2 1 P-3 1 S) were recorded within the discharge space and period with a high spatial (20 μm) and temporal (50 ns) resolution and analyzed with the line intensity ratio method developed by Ivković et al 2014. A novel experimental approach, based on a direct observation of the discharge by ICCD camera through two interference filters, was developed to obtain 2D spatially resolved distributions of the electric field. The obtained results proved the existence of several developing regions of high electric field, namely the cathode-directed and anode ionization waves. The electric field in the cathode-directed wave peaked at 32 kV/cm during the first contact of the wave with the cathode's dielectrics, followed by a decrease to values of 15-25 kV/cm above the electrode. The electric field in the anode-directed wave was lower, reading some 15 kV/cm. The performed 'sensitivity analysis' pointed out the crucial importance of precise temporal and spatial synchronization for the techniques based on the intensity ratio of two spectral lines. Artificially introduced 50 ns misalignment between the measured data of both spectral lines resulted in the substantial distortion of the electric field map. Reduction of the temporal resolution from 50 to 100 ns led to the decrease of electric field maxima by 30%. The reduction of the spatial resolution from 20 to 40 μm decreased the maximal field value by 10%.