A method as well as a system are described for determining the travel time of at least one mobile user end device between a starting point and an end point that is spatially separated from it. The image processing circuit receives grayscale data, identifies a range in which a gray level of the grayscale data is located, and generates a reference signal based on the range. One example of a property that may be so indicated is an undefined instruction. A sheet of a material is translated horizontally on a surface of the melt and the sheet is removed from the melt. The bolster is rigidly supported from its underside, thereby eliminating the risk that the bolster may move downward during machining.