Abstract: Device, apparatus, and method for the irradiation of fluids, especially physiological fluids such as blood. The device comprises a hollow casing having two end walls, at least one element adapted to transmit radiant energy disposed within said casing with an end thereof being secured in one of the end walls, and an inlet and an outlet in said casing between the end walls. The outer peripheral surface of the end portion of the element secured in the end wall is contacted by a material whose refractive index is equal to or less than the refractive index of the element. Preferably, the device employs a plurality of radiant energy transmitting elements in the form of fibers. Apparatus includes the aforementioned device and means for directing radiant energy onto the element(s) for transmitting radiant energy.