We present the study of the evolution of the ionization degree of the plasma sheath in a low energy plasma focus device with time and spatial resolution, by means of visible spectroscopy. The measurements were developed in the low energy plasma focus device PF-400J (176-539 J, 880 nF, 20-35 kV, quarter period similar to 300 ns) [1], using an ANDOR Shamrock 500i visible spectrometer with an ICCD that enabled the acquisition of spectra with time resolution in the order of tens of nanosecond. The use of a lens system improved spatial resolution, allowing the study of the plasma sheath in different stages of the discharge dynamic. In this work we present results from the pinch volume to study the evolution of the ionization degree of the plasma. The results include discharges in pure Hydrogen and a mixture of noble gases and Hydrogen, such as Neon and Helium, which were introduced in the discharge by small percentages ranging from 2 to 5%.