Method for determining the UV transmittance of water in a water-perfused UV disinfection system, whereby the UV disinfection system comprises - a plurality of radiator arrangements with respectively -- a UV radiation source, -- a cladding tube surrounding the UV radiation source, comprising an annular end face at an open end, and -- with a UV-C sensor which detects the UV radiation emitted from the cladding tube without the influence of the water, - and with at least one further UV sensor which is arranged spaced apart from the cladding tubes of the radiator arrangements, whereby the method comprises the following method steps: a) Taking a measurement of the UV radiation output emitted from the cladding tube by means of the UV-C sensor for at least one radiator arrangement, wherein the UV-C sensor is in optical connection with the end face of the cladding tube, so that the sensitive surface of the UV-C sensor detects the UV radiation emitted from the end face of the cladding tube; b) Taking a measurement of the transmitted radiation output by means of the at least one further UV sensor spaced apart from the UV radiation sources; c) Determining the transmittance of the water by means of the measurement of the emitted radiation output and the measurement of the transmitted radiation output. - a plurality of radiator arrangements with respectively -- a UV radiation source, -- a cladding tube surrounding the UV radiation source, comprising an annular end face at an open end, and -- with a UV-C sensor which detects the UV radiation emitted from the cladding tube without the influence of the water, -- a UV radiation source, -- a cladding tube surrounding the UV radiation source, comprising an annular end face at an open end, and -- with a UV-C sensor which detects the UV radiation emitted from the cladding tube without the influence of the water, - and with at least one further UV sensor which is arranged spaced apart from the cladding tubes of the radiator arrangements, whereby the method comprises the following method steps: a) Taking a measurement of the UV radiation output emitted from the cladding tube by means of the UV-C sensor for at least one radiator arrangement, wherein the UV-C sensor is in optical connection with the end face of the cladding tube, so that the sensitive surface of the UV-C sensor detects the UV radiation emitted from the end face of the cladding tube; b) Taking a measurement of the transmitted radiation output by means of the at least one further UV sensor spaced apart from the UV radiation sources; c) Determining the transmittance of the water by means of the measurement of the emitted radiation output and the measurement of the transmitted radiation output. a) Taking a measurement of the UV radiation output emitted from the cladding tube by means of the UV-C sensor for at least one radiator arrangement, wherein the UV-C sensor is in optical connection with the end face of the cladding tube, so that the sensitive surface of the UV-C sensor detects the UV radiation emitted from the end face of the cladding tube; b) Taking a measurement of the transmitted radiation output by means of the at least one further UV sensor spaced apart from the UV radiation sources; c) Determining the transmittance of the water by means of the measurement of the emitted radiation output and the measurement of the transmitted radiation output.