Abstract: To determine machine-direction variations in measurements of a travelling sheet during production, the sheet is repeatedly traversed with a scanning sensor and, during each traverse, measurements are taken at a plurality of locations. Then, a series of reference slice locations are selected which extend in the true cross-direction. For the reference locations, measurement values are estimated based upon actual measurements. Then, for each scan, the average of the estimated measurement values is calculated. Next, cross-directional variations in values at each slice location along selected scans are calculated by subtracting the average value from the estimated value at each location along the scan. During the next consecutive scans, machine-directional variations in the sheet property at slice locations are calculated by determining the difference between the measured value and the calculated cross-directional variation value for that slice location along the prior scan.

Abstract: An optical inspection system and method for detecting flaws on a diffractive surface such as a reticle or wafer, includes illuminating a surface to be inspected to generate a first scattered energy angular distribution in response to a flaw on the surface and a second scattered energy angular distribution in response to an unflawed surface; the first and second energy distributions are sensed and the minimum energy detection energy level is established; determining whether the minimum detected energy level is in a first or second predetermined energy range and indicating that no flaw is present when the minimum detected energy level is in the first range and a flaw is present when the minimum detected energy level is in the second range.

Abstract: Variations in the properties of a continuously running web are measured by reciprocating the scanning device across the web. The movement of the scanning device is so time controlled that this movement is out of phase with the variation in the paper properties in the machine direction of the web.

Abstract: An optical web monitoring device is described for which a directed illumination is realized by means of a mirror strip which fully illuminates the pupil of the camera objective of a diode row camera with the image of the illuminating pupil. All contrasting methods can thus be used also for the finding of surface faults in running webs. Several cameras or one camera with pupil division and two diode rows can be simultaneously illuminated using one illuminating device.

Abstract: A method of detecting tape defects according to a light amount signal obtained from the surface of a tape by scanning the width of the tape surface with a light beam is disclosed. The light beam is caused to scan beyond the tape edges. The light amount signal is differentiated to obtain a light amount differential signal. A tape defect in the vicinity of the tape edge is detected if the light amount differential signal has a level different from a predetermined reference level or if the point in time when a change in the level of the light amount differential signal is different from a predetermined point in time.

Abstract: Web-like planar structures 10 are tested by means of a light source 11 which directs a light beam 4 onto a rotating mirror wheel 5. The light beam is projected onto the web 10 to be tested via a stationary folding mirror 1 which is centered above the web to be tested. The folding mirror 1 is composed of at least two mirror surfaces 2, which form an obtuse angle .alpha. to each other. The light beam 4 coming from the polygonal surface 14 of the mirror wheel 5 covers successively all mirror surfaces 2 of the folding mirror 1 during the rotation of the mirror wheel 5. The number of scannings of the web-like planar structures 10 per polygonal surface 14 is increased, corresponding to the number of mirror surfaces 2.

Abstract: An optical scanning apparatus for plane surfaces (13) having a laser light source (20), a light deflecting means (16) illuminated by the laser beam, a telecentric image forming element (15) and a light receiving means. Parallel to the scanning line (11) generated by the light deflecting means (16) and the telecentric image forming element (15) there is arranged, at the angle of regular reflection, an image forming retroreflector (12; 38,39) which forms an image of the light bead running along the scanning line (11) on itself.

Abstract: This invention relates to a discriminating type flaw detector for detecting the types, sizes and locations of flaws existing in a light-transmitting plate material with high accuracy and high speed. A light-spot scanner (11) repeatedly scans a light-transmitting plate material with a light spot in the direction normal to the travelling direction of the light-transmitting plate material. When the light spot falls on a flaw in the light-transmitting plate material, the conditions of transmitted light, transmitted and diffused light, reflected light and reflected and diffused light change. These changes are detected by light receptors (12), converted into electrical signals in photoelectric converters (13), and processed into flaw data in a flaw data generating circuit (14). A flaw data acquisition circuit (15) collects flaw data, prepares a flaw pattern by combining the flaw data into flaw data blocks by flaw, and reads the addresses of the flaw data blocks as location data.

Abstract: To determine measurements such as basis weight and caliper of a travelling sheet during production, the sheet is repeatedly traversed with a scanning sensor and, during each traverse, measurements are taken at a plurality of slice locations. Then, a series of reference locations are selected which are spaced apart in the machine direction along the sheet surface. Then, for selected slices, measurement values are estimated based upon actual measurements taken at locations on the selected slices which are not spaced in the machine direction at the same spacing as reference locations.

Abstract: In a testing or inspecting apparatus for the detection of objects, which have differently formed surfaces and are moved relative to the testing apparatus, the object (12) to be tested is illuminated in a plane at right angles to the main movement direction (arrow 14) with substantially an identical intensity over a narrow zone extending over the width of the object. A detection device detecting the reflected brightness and/or color values of the object, responding to changes to the detected values in the illuminated plane, supplies an information signal, which is interpreted by a computer (FIG. 1).

Abstract: In a method for quantitatively analyzing an analyte in a liquid sample by applying the liquid sample on one of a set of analytical elements and measuring a color developed in the analytical element through reflection photometory in which said set of analytical elements are the same elements as a standard element except for deviation of sensitivity to the analyte, but said deviation of sensitivity is essentially equivalent to each other among the set of analytical elements, the calibration curve of the standard element with respect to said analyte being predetermined, an improvement involving the preparation of a linear equation for defining a relationship between an optical reflection density value OD.sub.x to be measured on an analytical element belonging to said set of the analytical elements and an optical reflection density value OD.sub.s being predetermined on the standard analytical element, in which the optical reflection density values OD.sub.x and OD.sub.

Abstract: A scanning system and method for optically measuring parameters such as dry basis weight, basis weight and moisture content of fibrous sheets during manufacture. The system includes a first track that extends generally parallel to one face of a traveling web in the cross direction; a stationary light source arranged to direct collimated light generally parallel to the first track; a first reflector that travels along the first track and focuses the collimated light against the adjacent face of the web; a second track that extends parallel to the first track adjacent the opposite face of the web; a second reflector that travels along the second track and collects and collimates rays that are transmitted through the web; and stationary light detectors that detect the intensity of the collected rays at least at two selected ranges of wavelengths to measure the absorption properties of the traveling web at selected locations.

Abstract: A method of testing security papers with respect to the positions of a safety thread and water marks to detect errors in the paper is performed by a linewise scanning of a finished paper web. The web has at least two point like measuring marks inserted into it in addition to the water marks, whereby impulses which are originated from these measuring marks are evaluated and the sheet width and the sheet distortion are calculated by comparing the x-y positions of the individual measuring marks with each other. The measuring marks are sharpe edged and rich in contrast. The given edge impulse is evaluated.

Abstract: A sheet inspection station including pneumatic means for guiding the sheet past one or more measurement sensors in non-contacting fashion, thus eliminating measurement error resulting from fiber/dust buildup associated with friction between the sheet and conventional guide rails. Air is directed from components, that extend from a sensor surface, onto the sheet surface.

Abstract: An apparatus for inspecting the surface of web material moving lengthwise at a constant speed by scanning the surface in the direction of the width of the surface with a light beam. The scanned beam modulated by the surface is received and converted into an electric signal which is divided into division signals representing divisions of the width of the web. A first memory capability is provided for storing division signals added together by division every N transverse scans. The added division signals are transferred to a second memory capability by division, for evaluating the surface defect distribution of the scanned surface division by division.

Abstract: The sharply bounded entry pupil (13) of an optical web monitoring apparatus is imaged by the system after passage through or reflection at the material web onto a dark field stop (16). This stop is provided at its periphery with light receiving surfaces having a relatively small area relative to the extent of the dark field stop (16). These light receiving surfaces conduct the received light to a photoconverter arrangement which is connected to an electronic processing circuit. In this way material faults are detected which bring about only small angular deviations of the scanning beam. In order to detect faults with greater light scattering a multiplier (23) is additionally mounted behind this receiver arrangement and receives light deflected beyond the light receiving surfaces.

Abstract: A method and apparatus for generating in real time full width formation profiles of a web of material which passes between a light source and a camera which generates signals corresponding to the light intensity across the entire width of the web as compared to single point or sample measurements. These signals are then processed to generate in real time at a full width formation profile of the web.

Abstract: An apparatus and method for measuring the distance between center lines in pairs of cords located in a web of cords. A laser beam passes through an expander which enlarges the beam. The enlarged beam is directed by mirrors to a prism and cylindrical lens through a cube beam splitter to form a line of light. The prism, cylindrical lens and beam splitter are mounted in a carriage. The carriage moves along a track which is orthogonal to the cords in the fabric, with the fabric either being stationary or moving with respect to the track. The line of light is directed against the cords by a lens and the reflected light is collected by the cube beam splitter and converted by an electronic detector into a wave form, amplified and has its apex detected by an analog picking circuit. A train of short pulses is generated, the apexes of which correspond to the center lines of the cords.

Abstract: A flatness measuring device for thin-gage strip in cold rolling mills comprises a rotating measuring roller (13) is arranged transversely to the strip travel direction (a) and across the strip width (b) and detects the rolled strip (6), with at least one surface groove (22) extending over the entire roller length (21), as well as a scanner (23) with one or more light beams (L4) for scanning strip wavinesses (24) that may occur over the upper section (31) of the surface groove (22) of the measuring roller (13) on account of a differing strip tension distribution over the strip width (b), which section (31) travels, on account of the rotational movement of the roller (13), over the entire length of the roller (13).

Abstract: In a timber testing apparatus, use is made of an optical process, in which there is a linewise scanning of the timber surface at right angles to the longitudinal direction of the timber. If an optical detection apparatus (18) indicates a fault, the particular piece of timber is stored as being faulty at this point, so that continuous operation and detection of faults is possible in piece-related and position-related manner, so that suitable parts of the timber with a given, desired fault-free length can be rapidly determined.

Abstract: A surface scanner useful for detecting defects on the surface of a water that is being processed in a vacuum equipment employs a light beam that scans the wafer surface in an arc. The scanning light beam impinges on the wafer surface at a low angle of incidence which permits the beam to pass under photosensing elements disposed in an arcuate array substantially congruous with the arc defined by the scanning light beam. The photosensing elements are patterned close to the wafer surface to receive light scattered by defects on the wafer surface.

Abstract: A laser-beam, pattern drawing/inspecting apparatus comprises a reciprocating stage for supporting a printed substrate having an etchant-resistant resist layer coated on it, a laser-beam scanning device having a laser for selectively emitting a first laser beam and a second laser beam less intense than the first laser beam, and an optical system for guiding the first and second laser beams to the printed substrate along the same optical path, the first laser beam scanning the resist layer to draw a pattern thereon, and the second laser beam scanning the resist layer, and a pattern inspection device for receiving the second laser beam reflected from the resist layer, thereby to optically inspect the pattern drawn on the resist layer.

Abstract: A process for locating weft thread defects in woven fabrics includes using a macro (close-up) lens and video camera to produce an enlarged image of the fabric, and then observing and evaluating the weft pattern from the video generated image. Transmissive and reflecting illumination may be used on the fabric to enhance the video image. Evaluation and measurement of weft thread positions is carried out by direct visual observation or by using computerized pattern recognition systems with micro processor techniques involving digitizing the video image. A relaxation defect of weft thread is determined by measuring variation between weft thread spacing when the fabric is viewed from two different viewing angles.

Abstract: In a method of inspecting a magnetic disk surface, a magnetic disk web is exposed to a light beam, information on the condition of reflection of the light beam by the magnetic disk web or passing of the light beam through the magnetic disk web is detected, and the condition of surface defects of a magnetic disk is judged based on results obtained by the detected information. The judgement is carried out based on the results at a portion of the magnetic disk web outside of a non-use region of the magnetic disk.

Abstract: An optical web monitoring apparatus has an illuminating arrangement which images the pupil of the source of illumination into the observing pupil of the camera objective (18) of a diode row camera using a strip-like concave mirror (14), or two strip-like concave mirrors (14, 15), and thus achieves the maximum attainable light yield. The construction is realized such that a fault seeking apparatus with a laser scanner can be used at the same time using the same mirror strips (14, 15). Thus two monitoring operations can be carried out simultaneously.

Abstract: A particle detection method for matching particles detected in two scans of a surface taken at different times in which particles having a light scattering intensity above a collection threshold are first detected and the measured position and scattering intensity therefor stored in a computer memory. Corresponding first and second measured positions from the respective first and second scans are determined by forming a triangle from selected first detected particles and finding those second detected particles which form a variant triangle with matching perimeter and area. From these matching first and second particles a transformation is found for mapping first measured positions to corresponding second positions and vice versa. Areas around corresponding positions of particles having a scattering intensity above a display threshold are examined for matching particles. If not found, the area is reexamined at a reduced threshold.

Abstract: A proposal for detecting flaws not defining normal stitches in a knitted fabric. Fabric detection signals are derived from three detectors arranged to correspond to intervals between lines of thread defining stitches. The detection signals are then added with predetermined weights, and the signals thus weighted are subjected to adding and/or subtracting operations. As a result, the flaws not defining normal stitches in the knitted fabric are detected. According to this proposal, reliable detection is made of not only large flaws such as circular holes but narrow flaws such as runs also.

Abstract: In a method of marking faults on rapidly moving material webs the material web is moved past a fault detection scanning device, which cyclically scans the web transverse to its longitudinal direction, and past a marking device at a defined distance therefrom. Signals coming from the fault detection scanning device (16) are analyzed one after the other to determine whether they are fault signals or structure signals. The fault signals which are detected are stored with their web coordinates. Checks are made at short time intervals as to whether a fault signal is present in the store (11) which corresponds to the longitudinal coordinate which is just moving past the marking device (13). A marking is effected if a fault signal is present at the corresponding web coordinates (FIG. 1).

Abstract: A flying spot generator is provided for changing a beam of x-rays into a flying spot moving in a linear direction over an object to be detected. The flying spot generator includes a rotating cylinder having even numbers of helical slots. A single long detector is provided behind the object for measuring the radiation characteristics of the object.

Abstract: Textile articles are inspected for size, defects by forming an image e.g. in a video camera, capturing data representing at least salient features of the image, comparing the data automatically with reference data, and effecting a selection process in accordance with the result of the comparison. Articles e.g. socks, presented as a succession of imperfectly matched items, can be sorted for size e.g. into categories of leg and foot length.

Abstract: Process for testing transparent material webs, particularly plate glass ribbons or the like, for material defects, in which the glass ribbon or the like conveyed in its longitudinal direction is scanned over its width in a scanning plane at right angles to its conveying plane and direction by means of a laser-produced flying light spot, the forward and back scattering produced by the inclusions is measured in each case one detection plane sloping with respect to the scanning plane, electrical signals are derived from the measured values obtained and said signals are processed for material defect identification, characterized in that a laser operating in the near IR-range is used as the light spot source, as well as apparatus for preforming this process.

Abstract: An optical projection system, e.g., an imaging infrared system, is compensated for errors caused by scan-to-scan jitter, framing jitter, vibrational effects, and scan nonlinearities. One or two closed control loops compensate for errors associated with mirrors (32, 52) of the optical projection system. In the scan control loop, a set of scan reference marks (75) in a reference plane (7) is superimposed onto a scan position detector (59) lying in a plane (10) containing a detector array (9), producing a scan error compensation signal (56). For wide scan fields of view (72), the scan position detector (59) comprises two pairs (94, 95) of triangular detectors, and means (86) for periodically switching therebetween.

Abstract: A light scanning device for determining the condition of a surface has an optical system which concentrates light beams leaving a scanning line in a specific direction at a concentration zone (16'). A light concentrator (11) with a light entry surface (12) and a desired light exit surface (13) arranged parallel thereto are arranged at the location of the concentration zone (16'). Convexly curved side surfaces (15) which are mirrored at the outside are present between the light entry surface (12) and the desired light exit surface (13) and ensure that light entering alongside a dark field stop (14) all falls onto a photomultiplier which adjoins the desired light exit surface (13) (FIG. 5).

Abstract: An optical fault seeking apparatus has a light scanning apparatus (21) and a light line receiving apparatus (14) which receive the light reflected from the material to be investigated and direct it onto a photoelectric converter. The light beam which forms the light line 11 impinges at an acute angle (.alpha.) onto the surface (13) of the article or web (18) under investigation. The image forming optical light line receiving apparatus (14), effectively consists of two light receivers (14, 14') positioned away from the normally reflected beam (23) and arranged to receive light from respective ones of two spaced apart strip-like regions (11', 11") extending parallel to and partly overlapping the line of light (11). In this way the device is able to detect faults (e.g. lumps or depressions) in fleece-type webs with irregular surface structures which are normally very difficult to monitor with optical fault seeking apparatus (FIG. 1).

Abstract: Apparatus for testing a printed wiring board comprising means for optically scanning a board to be tested in two orthogonal, X-Y axes such that for each scan in the Y direction it scans a narrow strip of the board of predetermined width, w in the X direction. The optical scanning means is arranged to provide n.m discrete signals for each scan in the Y direction, wherein n is a predetermined number of signals representative of characteristics of the board across the width w of the strip (typically 1000) and m is a number dependent upon the dimension of the board to be scanned in the Y direction. Processing means is provided for processing the signals in groups of n'.times.m' where n' is less than or equal to n and m' is less than or equal to m (typically n' and m' are 40) to provide signals representative of the characteristics of the board along said strip and for storing said signals.

Abstract: The inspection for surface deformation of flat glass is carried out by the flying-spot method wherein the beam of light is thrown by a rotating mirror 13 onto a folding mirror 6 which reflects the light onto the continuous sheet of glass 2 to be inspected. The reflected light beam 4 strikes a light-diffusing plate 1 disposed ahead of the receiver 11. The area of the light-diffusing plate 1 on which the beam of light impinges when the sheet of glass 2 is free of defects is opaque. On this opaque area there are disposed in a spaced relationship two photodiodes 7 between which a light bar formed by the scanning light beam extends. When the thickness of the flat glass 2 varies gradually, light will strike one of the photodiodes 7. The pulse so triggered controls a tracking mechanism 50 which adjusts the folding mirror 6 so that the light bar again extends between the photodiodes 7.

Abstract: An improved method and apparatus is disclosed for the inspection of continuous running webs such as those used in the printing trade. A rotating drum of mirrors, having a polygonal cross-section, allows inspection of a repeating image on the web, such that the repeating image on the moving web appears stationary to the viewer. The target portion of the web is oriented with respect to the rotating drum so that a straight line bisecting the target portion of the web passes through a locus of points having as its center the point halfway between the center of the drum and the viewing point on the drum. The sweeping scanning angle continually maintains correspondence to one repeat length of the web.

Abstract: A flaw detector for optically transmissive surfaces having a first light collector above the surface and a second light collector below the surface. A scanning light beam is directed into the first light collector through a beam entrance aperture and only light scattered from the surface is collected. Light specularly reflected from the surface exits the collector through the beam entrance aperture. Similarly, light passing through the surface enters the second collector, but the axial beam component is dumped through an opening in the second collector, while only diffracted light is collected. Preferably, two-stage light collectors are used with the first stage admitting the beam and generating a scattered or diffracted beam component, with the second stage admitting the scattered or diffracted beam component and integrating the component over a collection surface and sampling the integrated portion at a photoelectric detector. An electrical output signal from the detector may be displayed.

Abstract: The present invention concerns a procedure and a means for observing streaks in the machine direction in paper or in its coating, on a web (1) in continuous motion, a measuring box (2) being moved reciprocatingly across the paper web, from a light source (A) therewithin light being conducted against the paper web (1) in the form of separate light beams (5) spaced uniformly in the direction across the web, whereafter the intensities of the transmission beams from the paper or of the reflected beams from the coating are measured each separately by means of a detector (7) placed in the path of each light beam (5), the deviation of intensity found by the detector being automatically recorded in time and in space, whereafter the deviations of intensity coming later in succession from other detectors (7) are superimposed in time, whereby when similar intensity deviations come from the majority of the different detectors, the deviation must have been caused by a streak in the machine direction across which the equiv

Abstract: In an optical fault seeking apparatus for webs moved in their longitudinal direction there is provided an optical scanning arrangement which generates a light bead on the surface of the web which moves transversely to the direction of the movement of the web. The scanning arrangement generates several light traces (12, 13, 14) which are displaced in the direction of advance and are parallel to one another. A light receiving arrangement (15) for each light trace has a photoelectric receiver (16, 19; 17, 20; 18, 21). The light traces (12, 13, 14) either adjoin one another or have a distance such that after the passage of the web (22) past all the light traces (12, 13, 14) each web region has been detected once by one scanning light bead (FIG. 1).

Abstract: The eccentricity of an optical fiber core within a transparent fiber jacket is monitored by directing light towards the fiber and obtaining a returned light intensity profile as a function of the angle at which light is returned from the fiber. Any deviation from symmetry of the profile is an indication of eccentricity of the fiber core within the jacket. The method can be used for in-line monitoring when producing fiber with an extruded jacket. The technique is substantially simpler than known techniques which require the use of coherent light for generation of optical fringes.

Abstract: A flying spot scanner system uses a beam splitting mirror to produce adjacent flying spot images on a material to be analyzed, typically a moving web. The images from the flying spots are split by this beam mirror and sensed by separate photodetectors. The outputs from the photodetectors are algebraically combined on a real time basis, the resultant output signal indicative of web conditions, such as web edges, dirt spots, streaks, scratches and holes. The algebraic output signal amplitude is immune from rapid changes in the web illumination such as caused by changes in external ambient light levels. This signal is also highly immune to variations in the illumination level of the web along the path of the flying spots. The algebraic output signal, by not utilizing differentiation of the received information, has an amplitude independent of the flying spot speeds. The shape of the flying spots can be varied to enhance detection of particular types of defects.

Abstract: Apparatus for scanning a document includes an array of light emitting diodes, or optical fibers, arranged across the width of the document and extending further than its edges, a corresponding array of photo-diodes, or fiber optics connected to photo-diodes, for providing analog intensity signals indicative of the degree of transmission of light to each sensor, and an analyzing circuit responsive to each of the intensity signals to determine the condition of the document, and especially to determine the position of the edges of the document, and the size of any pin-holes or tears in the document. According to the invention, the analyzing circuit derives digital signals indicative of the relative intensity value of each analog signal with regard to three or more predetermined threshold values of intensity, each threshold level corresponding to a different degree of obscuration of a sensor by the corresponding area of the document.

Abstract: A data compression interface is characterized by a memory system having an architecture configured from a first and a second serial memory connected in parallel. One memory serves during alternate frames as a data collection memory while the other serves during that same frame as an output memory.

Abstract: An inspection system of the type having a rotating multifaceted mirror and a radiation collecting arrangement is characterized by a network in which the signal representative of the web being inspected is compensated for deviations superimposed on that signal due to variations in the reflectivity among the various mirror facets and normalized for variations in the optical efficiency of the radiation collecting arrangement.

Abstract: A method of correcting irregularities in the build-up of a mat during the manufacture of particleboard in which a plurality of individual measurements are taken substantially transverse to the mat as it moves through a measuring station. The measurements take the form of contactless measurements of the distance between a predeterminable measuring plane and the surface profile of the mat. The measured values that are obtained are further processed as representative values of weight per unit area for the positions at which the measurements are taken. The apparatus for effecting the contactless measurements is conveniently either optical apparatus or ultrasonic apparatus.

Abstract: A method and apparatus of photoelectrically scanning a moving web material is described which comprises scanning the web with a narrow light beam sweeping across and beyond the edges of the web, directing light reflected or transmitted by the web to a light receiver of a light measuring unit so generating a corresponding electrical signal, and optically or electronically simulating the optical reflexion or transmission property of the web for two auxiliary scanning zones adjacent the lateral edges of the web in such manner that the average strength of said electrical signal remains constant over substantially the whole scanning sweep.By use of this method abrupt changes in electronic signals received by the receiver when the scanning beam traverses the edges of the web are minimized. The effect of flare is also reduced.

Abstract: Fault detection apparatus for material webs comprising a light transmitting device for generating a line of light on the material web and an elongate light receiving device which extends parallel to the line of light and is so arranged that it can receive light which enters a predetermined reception plane from the line of light, with the predetermined reception plane lying at a specified angle to the plane normal to the material web and containing the line of light, and with the line of light being formed by a bead of light which scans the web point by point, line by line. The light receiving device (43) comprises a plurality of light receivers (1 to 30) which can be selectively switched in, with at least one of the light receivers being switched out at each particular instant.

Abstract: A defect inspection apparatus having a television camera which picks up an object to be inspected and produces a video signal for inspecting defects on the object, wherein the apparatus comprises a detection section which outputs a signal responding to the positive changes of the video signal with respect to time and a further detection section which outputs a signal in response to the negative changes of the video signal with respect to time, so that a defect detection signal is only output when, after one of the detection sections generates an output, the other detection section generates an output within a given time period.

Abstract: Disclosed is a surface defect detecting apparatus comprising a scanning device for scanning a rolled steel plate rolled and transferred by a pair of rolls at a fixed period over its full width to successively produce signals representing a surface condition of the rolled steel plate over the full width thereof, and a defect signal generating circuit for producing output signals representing the presence of a surface defect on the surface of the rolled steel plate in accordance with output signals from the scanning device.