An apparatus for irradiating blood or blood products, preferably with ultra violet or visible light, to reduce contaminants in the blood or blood products. A removable radiometer having light integrating chambers detects the light intensity, allowing the radiation characteristics of the apparatus to be calibrated. A control circuit uses the measurements to control the delivery of an effective dose of illumination to blood or blood products in a bag or container. One or more light integrating optical chambers in the radiometer allow a single light sensor to sense light across an entire field. Thermistors in the irradiating apparatus or the radiometer or both sense the temperature of photo sensors. The control circuit compensates for temperature-dependant variations in the output of the photo sensors.