The invention provides a vacuum cleaner having a chassis, supporting wheels mounted on the chassis, drive means connected to the supporting wheels for driving the supporting wheels and a control mechanism for controlling the drive means so as to guide the vacuum cleaner across a surface to be cleaned. The top surface contains titanium carbide, vanadium carbide, zirconium carbide, niobium carbide, hafnium carbide, tantalum carbide, tungsten carbide, chromium nitride, molybdenum nitride, tungsten nitride, titanium nitride, vanadium nitride, zirconium nitride, or a combination thereof. The output signals are analyzed, including comparing the output signals with previously stored signal patterns of magnetic ink characters to determine if the magnetic characters can be recognized or not. The filter generates a filtered signal by filtering the first write signal from the diffracted laser beam.