times as described will give perfect correction at all speeds of travel if the delay line employed has infinite resolution clocked delay lines by their very nature are stepwise in operation and the smallest possible increment of correction is one step of the sampling process in the examples just described the departures from the ideal correction correspond to within plus or minus 0 05 foot for paint and plus or minus 0 10 foot for bead correction at any practical speed of application up to 60 mph there distinct advantages to chopping the input signal of low level signal processing devices such as photo detectors the chopping process modulates the optical input signal at a rapid rate and when the output of the device is ac coupled the ac component may be amplified separately from any static or slowly changing high level background signal level when the photodetector is a multi stage photomultiplier tube requiring anode and dynode voltages on the order of 1000 volts or greater it is advantageous by ac coupling to de couple the relatively small signal from the high dc potential the chopping process also automatically establishes a fixed background black level reference for the output signal shown in fig1 is one embodiment of my invention that uses optical chopping of the input signal to the monochromator 62 shown in fig5 for the purposes just described but also for additional reasons as well the portion of the optical image focused on the entrance mask 52 that passes through its slit has been described as a transverse section of the roadway beneath the detector 33 when the device.