Development of Mercury-free Light Source System

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We have developed mercury-free light source, which does not pollute the environment. Instead of the resonance line of mercury atom, we employed vacuum ultraviolet (VUV) radiations of xenon to excite phosphor. We made a VUV spectroscopic measurement system and measured VUV radiations from xenon fluorescent lamps; one of them had inner electrodes and the other had external ones. When pulsed voltage was applied to the lamps, radiations were observed two times per period; when the pulse turned on and turned off, the radiations occurred. In the case of inner electrode, the VUV radiated after current turned off (afterglow), thus it was found that the afterglow contributed to improvement of luminance and efficiency. By solving rate equations of excited xenon atoms, it was found that to control inner diameter of a lamp and gas pressure was necessary to control decay of afterglow. Further analysis, for example, investigation of relations between VUV radiations and the number of excited atoms are necessary.It was found that increasing an electrostatic capacity by decreasing thickness of glass tube of an external electrode lamp was favorable to improvement of luminance and efficiency. In the case of an inner electrode lamp, increasing the number of electrodes to promote diffusion of positive column is favorable.We also tried to improve an electronic ballast; the number of parts was reduced by replacing more than two switching elements in a general electronic ballast with one switching elements. Thus, it is able to reduce the size and the weight of electronic ballasts. The characteristics of the new ballast were discussed by solving circuit equations.We improved luminance of low-power lamps, such as backlight of liquid crystal display, as much as that of mercury fluorescent lamps. But further improvement of luminance and efficiency is necessary for general-purpose lamps in terms of developing new VUV source instead of xenon, for example.