Apparatus ( 10 ), systems and methods are disclosed for treating a biological fluid with light. A container ( 206 ) of biological fluid is introduced into a fluid treatment chamber ( 40 ) where it is contacted with light provided by one or more light sources ( 60, 70 ) in proximity to the fluid treatment chamber ( 40 ). A light sensing system ( 650 ) senses the intensity of illumination of the light. A radiometer ( 460 ) may be inserted into fluid treatment chamber ( 40 ) to calibrate the light sensing system ( 650 ). An electronic control system ( 600 ) utilizes an interface circuit board ( 606 ) to interface a computer circuit board ( 602 ) to a display panel ( 37 ), a user interface panel ( 39, 39 a), a relay circuit board ( 640 ), light sensors 404 and various other sensors ( 649 ). A detector ( 385 ) senses agitating movement of a tray ( 90 ) that contains biological fluids. Methods include calibrating ( 781-785 ), sensing ( 770-773 ) and correcting ( 774-775 ) light intensity measurements, and determining the length of treatment ( 776 ) to reach a desired illumination dose. A radiometer ( 460 ) is equipped with a plurality of light sensors ( 469 ) disposed on both sides to measure light intensity in chamber ( 40 ) and to provide a reference for calibrating light sensing system ( 650 ).
