Control marking detector

摘要

A detector 10 used to sense the presence of control markings 20 on a moving web 12. The detector responds to abrupt changes in intensity of electromagnetic radiation rather than merely to absolute intensity values. Gradual changes in or a constant level of detected radiation or detector temperature do not change the magnitude of the abrupt changes in radiation intensity to which the detector responds. This capability allows the detector to distinguish between electromagnetic radiation emitted by wavelength-shifting control marks 20 and changes in web background and/or ambient radiation. A filter 38 is interposed between a photo diode 36 in the detector 10 and the web 12 to further enhance detector performance.