Effects of Distance, Temperature, and Relative Humidity on the Irradiance of Ultraviolet-C Germicidal Lamp: A Quantitative Study

Ultraviolet germicidal irradiation (UVGI) as an engineering control against pathogenic microbes necessitates a clear understanding of operational parameters and environmental effects on inactivation rates. Here, we investigated the variation laws of ultraviolet-C (UV-C) irradiance under the influence of distance, ambient conditions of temperature, and relative humidity (RH) in a dark chamber using 30-W low-pressure mercury lamps, and all data were analyzed with curve fitting methods. UV-C irradiances in each plane were measured as the distance adjusting between 0.5 and 1.2 m, and a threshold of 70 μW/cm² was utilized to calculate the effective irradiation area. For the temperature and RH, UV-C irradiances were measured at 1 m perpendicular from the lamp axis at the lamp midpoint, with the ambient temperature increasing from 15.5°C to 40°C and RH adjusting from 10% to 97%. Results showed that the UV-C irradiance and effective irradiation area exhibited a notable decrease as the distance increased, both corresponded to polynomial 2nd order fits. The UV-C lamps operate at maximum efficiency at 20°C. Temperature above or below the optimum value will decrease UV output, especially when the ambient temperature exceeds 38°C and the irradiance decreases by 16% compared to the observed maximum. However, the impact of RH on radiant power is negligible with the UV-C irradiance maintaining an overall steady state (84–91 μW/cm²) in the 10%–97% RH range. The use of the measurement and modeling techniques demonstrated in this study may help understand various ambient conditions that influence the irradiance of UV-C and improve reliability and working performance of UVGI systems through better design.