Reliable Measurements of UV Lamp Performance Using a Near-Field Technique

Abstract

Low pressure mercury arc lamps are an effective ultraviolet light source for disinfection of water. Critical lamp operating parameters such as the decrease in UV output with time, effect of water temperature on lamp output, and the effect of lamp dimming should be determined using a lamp in water. The reliable operation of a near-field test apparatus is described that is capable of measuring these quantities. The use of a radiometer with fiber optic light collection and standard reference lamp results in the reproducible measurement of these values using the near-field system. The stability of measurement is obtained using the reference lamp, which compensates for variations in the response of the fiber optic assembly. For sample lamps, a decrease of 11% output over 12,000 hours was observed. For the same lamp type, the relative calibrated output of the lamp increases between 5 and 50°C, with a ±10% change in lamp output relative to the reference temperature of 20°C. The apparatus samples the irradiance at a local point on the lamp, but the UV output of low pressure amalgam lamps as determined by the near-field technique is shown to be uniform over the entire lamp, except for within 1 cm of the lamp filament. This is shown for both new and 12,000 hour aged lamps, so that accurate lamp aging data can be determined. The change in UV output with water temperature change was demonstrated to be similar for both new and aged lamps in this study, except for the expected decrease in peak lamp UV output noted for the aged lamp.