To provide a method for controlling a threshold level while monitoring broken filaments or loops in a multifilament yarn during processing.

This method of and apparatus (10) for monitoring the processing of a multifilament yarn (11) is provided. In this method, a multifilament yarn (11) is passed through a light beam (13) directed from a light emitting device (12) to a light receiving device (14) and the amount of light received by the light receiving device (14) is measured at predetermined time intervals. From the measured amounts of light over a predetermined time period, a measuring and computing device (16) produces a frequency distribution (30) and a threshold level (34) representative of an ideal yarn (11) is calculated from the frequency distribution (30). The number of measurements (23) that fall outside that threshold level (34) is recorded and is indicative of the number of filament breaks and loops in the yarn (11).