Spectral measurements in the visible range of the plasma sheath ionization degree evolution on the plasma focus device PF-400J are presented. The measurements were done with temporal and spatial resolution in a plasma focus device of low stored energy: PF-400J (176-539 J, 880 nF, 20-35 kV, quarter period similar to 300ns) [1]. An ICCD was attached to a 0.5 m focal length visible spectrometer, which enabled the acquisition of time resolved spectrum with 20 ns integration time throughout the whole current pulse evolution. The spatial resolution was attained using a set of lenses which allowed the focusing of a small volume of the plasma sheath in different positions of the inter-electrode space. Discharges were carried out in mixtures of Hydrogen with gases in different proportions: 5% Neon, 5% Krypton and 2% Nitrogen. Discharges using Neon as an impurity showed no ionization of the gas, just a very low intensity emission of Ne I at times much larger than the maximum current. Nitrogen, on the other hand, showed a high ionization reaching N V (N4+) at the end of the axial phase, with a distinctive evolution of the ionization degree as the plasma sheath moved towards the end of the electrodes. A mixed result was found when using Krypton, since the ionization degree only reached levels around Kr II/III even though it has an ionization potential lower than Neon.