Patent ID: 7391043

Claim:
A system for detecting passage of transition edges of a moving web where the transition edges change the energy transmissivity of the web, comprising: an emitter capable of emitting energy directed at the web, wherein the emitted energy is at least one of passed through or reflected by the web; an edge sensor positioned to receive the emitted energy following interaction of the emitted energy with the web, and for providing an output signal corresponding to an energy level received from said emitter; a reference sensor positioned to receive the emitted energy following interaction of the emitted energy with the web, and for providing an output signal corresponding to an energy level received from said emitter, wherein said reference sensor has a broader field of view than said edge sensor in the direction of the moving web; a bias that limits the reference sensor output signal to be more than a low level and less than a high level of the edge sensor output signal; and a comparator in communication with said edge sensor and said reference sensor and receiving respective signals therefrom, said comparator determining from the signals received from said edge and said reference sensors the transition edges on the web.