During a periodic calibration, there is a process where the reference strip is scanned and {HEAD ~ the measured output} from the scan is compared to {TAIL ~ a reference value} that represents the expected value corresponding to the reflectivity of the reference strip 
During a periodic calibration , there is a process where the reference strip is scanned and { HEAD ~ the measured output } from the scan is compared to { TAIL ~ a reference value } that represents the expected value corresponding to the reflectivity of the reference strip
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH