According to aspects of the embodiments, there is provided an apparatus and method to maintain a clean and effective external heat roll surface in a printing system. Methods for determining a location of a transmitter without precise distance calculations. For each trace slice of which the identified event is a part, the identified event is added to an end of a record of the trace slice. One of the optical cavities and a wavelength-dependent structure are spectrally aligned, independent of another of the optical cavities.