A method of manufacture comprising the following steps: (a) during a respective indexing portion of a respective work cycle, advancing a fastener tape made of flexible thermoplastic material along a process pathway, the fastener tape not advancing during a respective dwell time of the respective work cycle; (b) during each dwell time, forming or attaching a respective structural feature of a type on the portion of the fastener tape that is resident at a first fixed station situated along the process pathway, the structural features of that type being spaced at intervals along the portion of the fastener tape that is downstream of the first fixed station; (c) while the fastener tape is advancing along the process pathway during the indexing portion of each work cycle, transmitting light toward a volume of space at a second fixed station, the process pathway intersecting and passing through the volume of space; and (d) photodetecting at least portions of the transmitted light after some or all of the transmitted light has entered and then exited the volume of space at the second fixed station, the photodetected portions of the transmitted light being converted into electrical signals.