Abstract: A web of paper or foil to be investigated is optically scanned line by line transversely to its direction of movement by a stationary scanning device and read off in reflection. The length of scanning line is greater than or equal to the width of the web of material under investigation. At least one time mark is formed electronically within a scanning line, which mark, starting from one end of the scanning range, is shifted forwards by a constant step with each scanning movement until the other end of the scanning range has been reached. The instantaneous value of reflected signal appearing in each case at the time mark is then averaged out over a cycle consisting of several scanning movements. A fault is only indicated if there is a deviation from the average value.