Abstract: In a surface inspection system a detector responds to light received from the surface to give a detector signal including disturbances due to surface faults, and includes an equalizer circuit to control the gain of a photomultiplier, and therefore signal amplitude, in response to a peak level detector, to control the level of the signal in relation to the amplitude of the disturbances and a band-pass filter to compensate for variations in the signals due to changes in the optical gain of the detector. The equalizer circuit is an amplifier and the control is achieved by way of feedback and feedforward control loops. In a modification the system may comprise a row of detectors and the equalizer circuit may be arranged to accommodate changes in the amount of light received at the junctions of detectors.

Abstract: A circuit arrangement for use with a scanned-beam moving-surface blemish detector uses the detection of leading-and trailing-edges of the surface in each scan to provide in the following scan a margin or margins, the separation of which provides a measure of the surface width over which detection can take place without the edges being detected as faults. In a modification the margins are extended to the direction of surface movement by introducing a minimum width, between leading-and trailing-edge margins, which must exist before fault detection is permitted.

Abstract: A discrimination circuit arrangement operable to discriminate between signals in the form of d.c. pedestals and superimposed thereon disturbance signals in the form of voltage variations, comprises high pass filter means to remove the d.c. pedestal, except adjacent the leading edge where a decaying impulse is produced by the filter means. A rectangular pulse is generated, triggered by the discontinuity of the pedestal and having an amplitude related to that of the pedestal; the pulse is passed through further high-pass filter means having the same time constant as the input signal filter means to produce a replica signal of the filtered input signal in the form of a decaying impulse, free of disturbance signals. The two decaying impulse signals are compared in comparison means, with a threshold signal, and disturbance signals exceeding the threshold signal produce an output signal.

Abstract: A detector of blemishes in a surface comprises a transmitting station from which a beam of light is caused to scan a moving surface to produce a raster pattern thereon. Light specularly reflected from, or transmitted by, the surface is collected at a receiving station after being diffused by a sheet of translucent material disposed in the path of this light. The diffused light in the receiving station is collected and directed by reflective surfaces towards a photodetector such that when the beam engages a blemish in the surface, the amount of light detected falls. The detector combines the sensitivity of collecting large amounts of specularly reflected light with a simple optical system associated with the collecting of diffused light.