The present invention relates to a laser apparatus system and method for determining a depth of a focus point of a laser beam. An interface device is coupleable to the laser apparatus and has an applanation element comprising a front surface and a back surface. A laser beam having a predefined shape is focussed through the applanation element at a focus point. A superimposed image of a spurious reflection which is reflected from the front surface of the applanation element with a standard reflection which is reflected from the back surface of the applanation element is detected. The spurious reflection is then filtered out of the superimposed image. Based on the remaining standard reflection the depth of the focus point of the laser beam can be determined.
