The standardization of water disinfection with polychromatic ultraviolet (UV) radiation has developed very rapidly during the past several years. The surveillance of irradiance and UV absorption in water by UV sensors is essential for safe water disinfection with medium pressure mercury UV lamps. In this study, calculations were carried out concerning the influence of the position of the sensor, the absorption spectrum of the water during the type testing and during the disinfection process in the water works, and the spectral sensitivity of the sensor on the measured values. This value represents a microbicidally weighted irradiance; this makes it possible to control the reduction equivalent fluence (REF), which is effective in the UV plant. It could be shown that an optimum path length of the water layer that is between the sensor and the UV source exists. This optimum is defined by equal dependence of the sensor signal on UV transmittance of the water compared to the dependence of the REF applied to the water on UV transmittance of the water.