This paper deals with an optical emission spectroscopy study of a diffuse coplanar surface barrier discharge (DCSBD) in air at atmospheric pressure. The main aim of this study was to verify the areal homogeneity of the generated plasma, which is important for many applications like treatment of nonwoven fabrics, glass, metals, polymers, foils, and so on. Optical emission spectra of DCSBD plasma in air were measured for three different frequencies (15, 30, and 50 kHz) of the applied voltage. Comparison of the calculated rotational and vibrational temperatures was carried out, and areal homogeneity of plasma was proved. Electrical parameters of discharge such as the plasma power using the area of the Lissajous figures and energy transfer efficiency to the discharge were also investigated. The effective thickness of plasma layer as a function of the input power was measured.