Abstract: A light source apparatus includes a gas discharge stage including a three-dimensional body defining a cavity that is configured to interact with an energy source, the body including at least two ports that are transmissive to a light beam having a wavelength in the ultraviolet range; a sensor system comprising a plurality of sensors, each sensor is configured to measure a physical aspect of a respective distinct region of the body of the gas discharge stage relative to that sensor; and a control apparatus in communication with the sensor system. The control apparatus is configured to analyze the measured physical aspects from the sensors to thereby determine a position of the body of the gas discharge stage in an XYZ coordinate system defined by an X axis, wherein the X axis is defined by the geometry of the gas discharge stage.