Ferranti Plc

John Bedford

Graham M Clarke

John Alexandre Sherrington

Correction for variations in scan period of optical image scanners includes measuring and storing the scan times at which optical events occur to cause image detector signals, detecting an optical marker near the end of the scan and the scan time of its detection, determining the difference between this scan time and a stored scan period, comprising the nominal scan time for marker detection, which difference represents the scan period error, dividing the difference by the stored scan period to provide a scan time unit error and multiplying each stored measured scan time by the unit error to provide a correction for each measured scan time, thereafter adding the correction to the stored measured scan time to give a corrected scan time.

Correction for scan period variation in optical image scanners

A discrimination circuit operable to discriminate between an input signal in the form of a pedestal and disturbance signals superimposed thereon has an A-D converter to digitize a disturbance-free reference pedestal signal, circulating shift register to store the digitized reference signal, a D-A converter to produce a pedestal signal from the reference signal, a controlling clock to shift the reference signal in the store in synchronism with the incoming signal such that the two pedestal signals are available together, and a comparator to compare the reference signal with the input signal to produce an output caused by differences due to disturbances. The circuit forms part of an optical surface inspection apparatus.

Discrimination circuit arrangements

A high-speed pulse detection circuit comprises a Schmitt trigger stage connected to a second stage to form a bistable feedback loop having a working characteristic with an infinite gain loop.

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Gorebridge, United Kingdom

John Bedford