Confocal chromatic metrology for euv source condition monitoring

A light source includes a rotatable drum to be coated with xenon ice and illuminated by a laser beam to produce a plasma. The drum may also be translatable. The light source further includes a confocal chromatic sensor to measure distances from the confocal chromatic sensor to the rotatable drum. The confocal chromatic sensor may include a sensor head to focus light onto the rotatable drum and to detect reflected light from the rotatable drum. The sensor head and the rotatable drum may be disposed within a vacuum chamber.