Ferranti Plc

Graham M Clarke

John Bedford

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

To distinguish between different types of fault detected by laser scanning of a moving web two or more detectors are arranged to collect light reflected in different ranges of angles in direction of web motion. By combining outputs of these in a logical way representing increases or decreases in the sums and differences of the signals they produce, the faults can be classified as absorbing, scattering, or their opposites, or deflecting.

Inspection apparatus

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 surface inspection system has a light beam repetitively scanning a surface and a receiver located to receive light specularly (directly) reflected. The receiver has an enclosure with a wall containing a plurality of slits by which reflected light can reach a detector in the enclosure. The slits are arranged to cause the detector to produce a series of pulses throughout the scan, each pulse providing an instantaneous picture of the surface gloss in terms of an intensity distribution, the half width of each pulse is measured to give an indication of any variation from the half width of the distribution for a known surface gloss.

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Edinburgh, United Kingdom

Graham M Clarke