A method carries out a soiling check of the measurement window of a measuring device for checking sheet material. A measuring device carries out the method. A device for processing a sheet material comprises the measuring device. The soiling check uses, only areas of the measurement window which correspond, in terms of width and position in the beam path of a light, to the areas of the checked sheet material which are checked during the checking of the sheet material. As a result, fewer cleaning steps are needed for the measurement window.