A method for checking the soiling of a window of a measuring apparatus for checking sheet material that is transported past the measuring apparatus along a transport path in a transport direction (x), wherein the measuring apparatus - has respectively one sensor row (3a-d) for at least two different spectral channels (R, G, B, IR), said sensor row having several detection elements along a y direction transversal to the transport direction (x) of the sheet material, to detect light emanating from a sheet material, when a sheet material is present in the capture area of the sensor row, and - has at least one window arranged between the sensor rows (3a-d) and the transport path of the sheet material, and wherein for checking the soiling of the window the intensities of the light impinging on the sensor rows (3a-d) through the window are detected when no sheet material is present in the capture area of the sensor rows (3a-d), characterized in that for checking the soiling of the window - at least two of the sensor rows (3a-d) with different spectral channel (R, G, B, IR) detect, in respectively at least one y position along the y direction, the intensity (I R, I G, I B, I IR) of the light impinging on the respective sensor row (3a-d) through the window respectively for several detection times (t 1, t 2,...), when no sheet material is present in the capture area of the sensor rows (3a-d), and - from the intensities (I R, I G, I B, I IR) detected by the at least two sensor rows (3a-d) with different spectral channel (R, G, B, IR) at a detection time (t), an intensity (I K (t)) is determined of a combined spectral channel, wherein the intensity (I K (t 1), (I K (t 2),...) of the combined spectral channel is determined individually respectively for several of the detection times (t 1, t 2,...), and - a statistical evaluation is carried out for the intensities (I K (t 1), I K (t 2),...) of the combined spectral channel determined for the several detection times (t 1, t 2,...), and - on the basis of the results of the statistical evaluation a soiling check of the window of the measuring apparatus is carried out. - has respectively one sensor row (3a-d) for at least two different spectral channels (R, G, B, IR), said sensor row having several detection elements along a y direction transversal to the transport direction (x) of the sheet material, to detect light emanating from a sheet material, when a sheet material is present in the capture area of the sensor row, and - has at least one window arranged between the sensor rows (3a-d) and the transport path of the sheet material, and - at least two of the sensor rows (3a-d) with different spectral channel (R, G, B, IR) detect, in respectively at least one y position along the y direction, the intensity (I R, I G, I B, I IR) of the light impinging on the respective sensor row (3a-d) through the window respectively for several detection times (t 1, t 2,...), when no sheet material is present in the capture area of the sensor rows (3a-d), and - from the intensities (I R, I G, I B, I IR) detected by the at least two sensor rows (3a-d) with different spectral channel (R, G, B, IR) at a detection time (t), an intensity (I K (t)) is determined of a combined spectral channel, wherein the intensity (I K (t 1), (I K (t 2),...) of the combined spectral channel is determined individually respectively for several of the detection times (t 1, t 2,...), and - a statistical evaluation is carried out for the intensities (I K (t 1), I K (t 2),...) of the combined spectral channel determined for the several detection times (t 1, t 2,...), and - on the basis of the results of the statistical evaluation a soiling check of the window of the measuring apparatus is carried out.