1. A method of calibrating a 3D camera system for use with a radio therapy treatment apparatus, the method comprising: positioning a calibration phantom with the phantom's center at an estimated location for the iso-center of a radio therapy treatment apparatus; irradiating the calibration phantom using the radio therapy treatment apparatus and determining the relative location of the center of the calibration phantom and the iso-center of the radio therapy treatment apparatus by analyzing images of the irradiation of the calibration phantom; utilizing the 3D camera system to obtain images of the calibration phantom positioned at the estimated location for the iso-center of the radio therapy treatment apparatus; processing the obtained images of the calibration phantom positioned at the estimated location for the iso-center of the radio therapy treatment apparatus to generate a 3D computer model of the surface of the calibration phantom positioned at the estimated location for the iso-center of the radio therapy treatment apparatus; applying an offset corresponding to the determined relative location of the center of the calibration phantom and the iso-center of the radio therapy treatment apparatus to the generated 3D model of the surface of the calibration phantom positioned at an estimated location for the iso-center of the radio therapy treatment apparatus; relocating the calibration phantom by applying an offset corresponding to the determined relative location of the center of the calibration phantom and the iso-center of the radio therapy treatment apparatus to the calibration phantom; utilizing a 3D camera system to obtain images of the relocated calibration phantom to which the offset has been applied; processing the obtained images of the relocated calibration phantom to which the offset has been applied to generate a 3D computer model of the surface of the repositioned calibration phantom; and comparing the generated 3D computer model of the surface of the repositioned calibration phantom with the 3D computer model of the surface of the calibration phantom positioned at the estimated location of the iso-center of the treatment apparatus to which an offset has been applied to set the co-ordinate system of the 3D camera system relative to the iso-center of the radio therapy treatment apparatus.