Abstract: An irradiator includes a support structure for supporting and rotating a cylinder of the type used for treating fluids. The cylinder is hollow and is mounted for rotation about an inclined axis for permitting fluids such as blood constituents to flow by gravity downward along the internal peripheral wall of the cylinder. The angle of inclination, the drive speed and the thickness of the fluid film on the wall of the cylinder are controlled and variable in order to achieve consistent and predictable results during various treatment processes, and for critical in controlling the precise exposure of the fluid to the lamp. A unique collection cup is provided at the discharge end of the cylinder for receiving the discharged, treated fluid and the collection cup is designed to receive and seat one end of the lamp assembly. The cylinder is mounted on roller bearings having a resilient bearing surface for isolating potential external vibration.