Pulsed-mode operation of the magnetron is achieved through certain necessary modifications in the circuit. The magnetic field is upgraded to enhance the microwave power, where an additional electromagnet is introduced around the magnetron cavity that confines the fast moving electrons. This modified microwave source is sufficient to achieve the breakdown in GLAST-II with improved plasma current of 5kA. GLAST-III is an upgraded version of the GLAST-I and GLAST-II designs which features a larger vessel diameter and a larger central bore for the placement of diagnostic tools such as Rogowski coils and flux loops. GLAST-III retained most of the diagnostics used in GLAST-I and GLAST-II, but a newly developed spectroscopic system based on linear photodiode array was installed on the upgraded GLAST-III for spatial and temporal characterization of hydrogen discharge through light emission. The spectral range of each silicon photodiode is from 300 nm to 1100 nm with response time of 10 ns and active area of 5 mm2 (circular). The light from the plasma is collected through holes along 4 line-of-sight channels with spatial resolution of about 5 cm passing from entire poloidal cross section. The photodiode's signals located at position of 10 and 14 cm from inboard side show fluctuations in the central plasma region. Moreover, the sequence of plasma lighting shows that plasma instigates from the central resonant field region and then expands outwards. At lower pressure, outboard movement of the plasma is slower suggesting better plasma confinement. In addition to photodiode array, an optical spectrometer (Ocean Optics HR2000+) has been used to record the visible spectrum over the selected range (597–703 nm) with a spectral resolution of 0.15 nm. The studies have been conducted during initial phase of plasma formation for two different hydrogen gas fill pressures. The triple probe is used to get time-resolved information on plasma parameters in the edge region. The time evolution of whole discharge including microwave pre-ionization phase and current formation phase has been demonstrated by temporal profiles of light emission and plasma floating potential.