Conventional backlight for liquid crystal display (LCD) uses mercury which leads to environmental pollution. In this study, characteristics of AC coplanar type mercury-free plasma flat lamp have been studied. Pollution-free Xe-He was adopted as a discharge gas system. Since the Xe gas has a lower efficiency in generating vacuum ultraviolet (VUV) than mercury, the study on the improvement of luminance and luminous efficiency of the Xe gas system is very important.
The subsequent fabrication of electrode, dielectric and phosphor layers on the bottom glass was carried out by using the screen printing method. Infrared (IR) emission characteristics of Xe-He gas mixture were analyzed by optical emission spectroscopy (OES). The Xe-He discharge gas generates 147, 150, and 173 nm VUV rays and 823, 828 nm IR rays. The intensity of 823 nm peak decreases whereas the intensity of 828 nm peak increases as the Xe concentration increases from 10% to 40%, indicating that the He atom incites the transition from higher Xe state to the metastable state but suppresses the transition to the resonance state. Luminance and luminous efficiency increase with increasing Xe concentration and gas pressure. However firing voltage and minimum sustain voltage also increase, and accordingly, a high voltage pulse generator and a high quality dielectric layer are needed. The effects of pulse shape, on-time, and pulse frequency on the luminance and luminous efficiency were examined. For Xe(5%)-He gas, the lamp exhibits higher efficiency with sharper pulse shape, higher peak voltage, and shorter pulse on-time (up to 2 ㎲). Higher efficiency and lower consumption of power could be obtained at 30 kHz than at 60 kHz. The collision of ions to bottom electrodes is a dominant factor to raise the lamp temperature. Therefore the high voltage and low current discharge system is needed to reduce the lamp temperature as well as to enhance the luminous efficiency.
