The invention relates to an apparatus and a method for determining the energy of a laser. In particular, the invention relates to an initial determination of a laser energy and the monitoring of the laser energy preferably of an excimer laser for use in a refractive laser system for treatment of any eye. An apparatus for determining an energy of an excimer laser comprises a tool comprising an area being ablated with a plurality of laser pulses of said excimer laser using at least one predetermined multi spot ablation pattern, said ablation area comprising a specific ablation area being as large as the ablation area or smaller, an image capturing means for capturing at least one image comprising at least said specific ablation area of the tool; an analyzing means for analyzing said at least one image, wherein the size of the specific ablation area provides a measure of the energy of the excimer laser.