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The potential of using water-vapor plasmas excited by microwaves as a ultraviolet (UV) light source has been investigated by using various pressures and input powers. The UV irradiation power increased and saturated at a pressure range dependent on the input power. On the other hand, other visible and infrared emissions corresponding to four atomic lines, i.e., the Balmer series of hydrogen at 486.1 nm ( Hbeta) and 656.3 nm (Halpha) and oxygen atoms at 777.2 and 844.6 nm, were clearly decreased with an increase in the total gas pressure. It was found that pressures (1.4-2.0 kPa) near the saturated water-vapor pressure were found to give the most intense UV irradiation. With a vapor pressure of 1.6 kPa and a total microwave power of 300 W, the power density of UV (Gammauv) was measured to be 10.5 muWmiddotcm-2 at a distance of 30 cm from the center of the discharge tube as measured through an optical viewing port on the cavity discharge applicator. This value for (Gammauv) is comparable to that for a mercury lamp. However, the (etauv) efficiency was estimated to be considerably lower than that of a mercury lamp.