An amplified optical beam is provided to a region that receives a target including target material, an interaction between the amplified optical beam and the target converting at least some of the target material from a first form to a second form to form a light-emitting plasma; first data comprising information related to the amplified optical beam is accessed; second data comprising information related to the light-emitting plasma is accessed; and an amount of the target material converted from the first form to the second form is determined. The determination is based on at least the first data and the second data, and the second form of the target material is less dense than the first form of the target material.