Abstract: A method and device for marking defects in or on a substantially planar strip (70, 170), preferably a strip of polymeric film. The defect is marked by means of firstly illuminating the width across the strip (70, 170) by means of a light source (90, 190) and detecting defects in or on the strip (70, 170) by means of a detector (10, 300). The strip (70, 170) is placed in a plane in connection with the light source (90, 190) and wherein the incident light is reproduced on the detector (10, 300). Thereafter, the defect is marked by means of a marker (200) which is placed in connection with the defect. The marker (200) is placed on a label (62, 162), which is attached onto the strip (70, 170).

Abstract: A system for detecting defects on a moving web having a sequential series of identical frames uses an imaging device to form a real-time camera image of a frame and a comparitor to comparing elements of the camera image with corresponding elements of an image of an exemplar frame. The comparitor provides an acceptable indication if the pair of elements are determined to be statistically identical; and a defective indication if the pair of elements are determined to be statistically not identical. If the pair of elements is neither acceptable nor defective, the comparitor recursively compares the element of said exemplar frame with corresponding elements of other frames on said web until one of the acceptable or defective indications occur.

Abstract: A device for monitoring a thread, for example the weft thread in a loom of the type in which the weft is driven through the shed of the loom with the aid of a jet, such as a gas jet, for example an air jet, and which has a reed with a longitudinal channel for the weft. An arm with a free end is placed in a position at which it is desirable to monitor the weft. The arm free end has at least one light source for illuminating the monitoring point and any weft located there. A number of light-sensitive elements on the arm free end are aimed towards the channel of the reed to sense the light reflected therefrom. Each light sensitive element generates an electric signal corresponding to the reflected light and is connected to an evaluator circuit for adding the signals regardless of whether the weft at the monitoring point results in an increase or a reduction of the signals obtained from the light-sensitive elements, to generate a signal corresponding to the absolute value of the signals.

Abstract: The invention relates to a method and a device for detecting impurities (F) in a fiber stream (1) of mainly textile fibers, wherein the fiber stream and at least one reference quantity (15) are artificially visually sensed. To enable even impurities which are difficult to detect to be removed with improved efficiency, the reference quantity is to be adapted at least periodically.

Abstract: An optical web defect detection system includes a light source, a detector disposed with respect to the light source so that the detector receives light modulated by the web, and a control mechanism in operative communication with the detector. The control mechanism receives an output signal from the detector to detect a defect in the web based thereon. The detector may be disposed so that it receives light modulated by the web and emitted by the light source at an oblique angle with respect to a plane defined by the web. A first detector may be disposed at a first distance from the web less than a second distance at which a second detector is disposed from the web. A shift mechanism may reciprocally remove the detector in a path parallel to a plane defined by the web.

Abstract: The present invention relates to a detector (1) of foreign fibres and to a corresponding detection method, which can be used for detecting foreign fibres and foreign materials in a measurement object (5) in the textile industry, for example in yarns, slithers and unformed fiber bands on, for example, weaving machines, spinning machines, bobbin winding machines, doubling machines, cards, drawing benches and bobbin frames . . . . The principle of the detection is based on the fact that a measurement object (5) and foreign fibres or materials which are situated in the measurement object (5) absorb the light emitted into the measurement volume (4) by a light source (2) to different extents. All the light in the measurement volume (4), or a representative sample thereof, is measured by photodetectors (3) and converted into a photoelectric signal.

Abstract: An inspection system for viewing substrate or coated substrate features, wherein a directed light source is projected through a coated substrate onto a viewing surface such that features or irregularities in the substrate or in the coating of the coated substrate are displayed. The inspection system includes a projection system including a light source positioned adjacent the second major surface of the optically transmissive substrate, and a viewing screen positioned adjacent the first major surface of the substrate. The substrate is positioned in optical alignment along an optical path between the light source and the viewing screen such that irregularities in the coated top surface of the substrate can be visually observed on the viewing screen by a user. In one aspect the substrate is coated. Optionally, the directed light source is a laser scanner assembly.

Abstract: A system or method for electronically simulating fabrics to assist in grading yarn and fabric qualities, in which the diameter or other qualities of one or more yarn samples along the total length of the samples is measured, and representations of the measured yarn qualities are evaluated in a fabric pattern to assist evaluation of the effects of the yarn quality variations on fabric which potentially would be produced from the sampled yarn.

Abstract: A small hole detection system which detects small holes in fast moving sheets of material. An array of LEDs is pulsed from an emitter which is aligned with an array of photocells positioned to detect light emitting from the emitter. The receiver sends a signal to a processing unit when light is detected. The small hole detection system may include an auto-shuttering mechanism which automatically adjusts the system according to the width of the material being scanned.

Abstract: The invention relates to a method and a device for detecting defects in textile webs. In order to rapidly adapt devices of this type to widely varying textile webs and to be able to operate such devices simply, brightness values are determined from the web and are supplied directly to a filter constructed as a neural network. The output results of the neural network can be displayed as grayscale values to indicate detected defects.

Abstract: An optical web defect detection system includes a light source and a detector disposed with respect to the light source so that the detector receives light modulated by a web moving proximate the light source. The detector outputs a signal corresponding to the modulated light. The detector may include a plurality of fiber optic elements having ends arranged in a group aligned generally parallel to the orientation of a defect in the web. A control mechanism is in operative communication with the detector to receive the signal therefrom. The control mechanism may include a signal processing stage and a defect detection stage that receives a signal modified by the signal processing stage and compares the modified signal to a predetermined criteria to determine whether a defect occurs in the web.

Abstract: A magnetic disk testing method is provided, which comprises a surface defect test step of detecting, as defect data, the size of surface defect of a magnetic disk, the continuity thereof, the number thereof and the position thereof by testing magnetic disks optically and a classification step of classifying the magnetic disks to first magnetic disks, second magnetic disk and third magnetic disks on the basis of the defect data obtained in the surface defect test step.

Abstract: A system and a method for determining the density in which fibers are disposed along a planar surface employs a source of irradiance and a detector of irradiance in optical communication. A planar surface is positioned slightly below the optical path between the source and the detector. The fibers disposed along the planar surface interrupt the transmission of irradiance between the source and the detector. This interruption causes a decrease in the quantity of irradiance reaching the detector, resulting in a corresponding drop in the detector's output voltage. The density in which fibers are disposed along the planar surface can be determined by comparing the detector's voltage output with a pre-established baseline relating voltage output to fiber density.

Abstract: An optical inspection system mounts on a cigarette making machine or like system and detects the presence of bands having irregular widths or spacings. In response to the detection of irregular bands, the optical inspection system instructs the cigarette making machine to reject cigarettes which will be subsequently made from portions of the cigarette paper containing the irregular bands. The optical inspection system includes a sensor which can accommodate different types of cigarette paper and band material having varying reflectance properties. In operation, sensor circuitry detects the range of voltages produced by a sensor detector, and formulates a peak value (or average peak value) for the range of voltages. The circuitry then takes a percentage of this peak value to form a threshold value. This threshold value is compared with the AC signal from the sensor detector. If the AC signal exceeds the threshold, then the circuitry asserts a signal to indicate that a band is present.

Abstract: An apparatus for inspecting a surface of a disk comprises an indexing table having chucks for holding said disks respectively, said chucks being arranged angularly at regular intervals on the indexing table. Each of disks held by the respective chucks is moved by indexing revolution of the indexing table sequentially from a loading station where the disk is loaded onto the chuck, to an inspection station where the disk is inspected for a surface of the disk and then to an unloading station where the inspected disk is unloaded to the outside of the apparatus. The disk loaded onto a chuck at the loading station is driven to rotate until the disk gets to a predetermined rate of revolution before the disk arrives at the inspection station. The disk which has already get to the predetermined rate of revolution can be immediately inspected for the surface of the disk in the inspection station by an inspection device.

Abstract: Inspection apparatus for determining defects in a planar light transmissive object. A diffuse light is projected through a mask toward an object under test at an inspection location. A digital video camera observes the inspection location and obtains images of the object. These images are examined and processed by a signal processor to obtain indications of various defects. The mask utilized is of a selected design having either multiple alternating narrow clear and opaque stripes, or a single narrow clear stripe straddled by opaque regions or a single narrow opaque stripe straddled by clear regions.

Abstract: The device includes a motor that move the web and an electronic camera which supplies a signal representing an image of the web formed on an image sensor having photosensitive points. The device has synchronization of the duration of a predetermined number of successive pitches of the web passing through the optical field of the camera and the duration separating the start of two successive imaging operations of the camera. Thus, an optical filtering is performed. From one image to the next, the same photosensitive point of the sensor corresponds to identical portions of threads and interstices of the web, so that any optical variation of the threads or interstice is detected. The signals representing the images produced by each photosensitive point during successive shots correspond to zones of material with similar designs.

Abstract: The invention relates to a method and apparatus for measuring and controlling optical properties of a moving web. Sensor heads are installed in the same location on the opposite sides of the web to be measured so that there is at least one means of illuminating the web and at least one means of detecting radiances on both sides of the web. At least one means of illuminating the web of at least one side is controlled so as to provide at least two different lighting states. Measuring the values in different lighting states with both means of detecting radiances simultaneously allows determination the optical properties of the web on its both sides.

Abstract: A small hole detection system which detects small holes in fast moving sheets of material. An array of LEDs is pulsed from an emitter which is aligned with an array of photocells positioned to detect light emitting from the emitter. The receiver sends a signal to a processing unit when light is detected. The small hole detection system may include an auto-shuttering mechanism which automatically adjusts the system according to the width of the material being scanned.

Abstract: A sheet-testing apparatus including kneading rolls, a frame feeder, sheet applicator, foreign material detector, a frame with a light transparent window and a cutter for cutting a sheet. A sheet thickness detection apparatus outputs an electric signal to a memory. A kneading apparatus has a pair of heating rolls separated by a gap. The directions and speeds of rotations of the rolls are adjustable. A sensor detects the gap and a bank sensor detects the condition of a bank in the gap. The distance between the rolls, their temperatures, rotations, and speeds are automatically controlled. A pair of blades peel a sheet from the sheet-winding roll. Material dropped through the gap between the heating rolls is returned to a point above the gap. A cutting device has a length of wire sprung onto a rolled sheet to sever the sheet. A heating roll cleaning device brings cleaning rolls into sliding engagement with the heating rolls.

Abstract: The invention provides methods and apparatus for machine vision illumination by moving a non-diffuse light beam over selected points on the object so that each is illuminated at substantially the same angle by multiple rays during acquisition of an image frame. An apparatus for illuminating an object to facilitate its being imaged by an image acquisition device, e.g., a camera, includes a radiation source that generates and transmits to a region of the a non-diffuse beam of imaging radiation having a non-point cross-section. A dithering element moves the beam so that at least a region of the object is illuminated at substantially the same angle by multiple rays during an image acquisition period.

Abstract: A device for use in detecting, characterizing and correcting flaws in optic fibers includes a light source for generating a beam of light at an optic fiber, a detector measurer for detectably measuring at least one of refracted light rays, diffracted light rays and scattered light rays from the beam and generating a signal in response thereto, a computer-based device operatively associated with the detector measurer for receiving the signal and manipulating the signal to produce an output indicating one of a normal area and a flawed area, a camera device operatively associated with the computer-based device and responsive to the output for visually recording the flawed area in response to indication of a flawed area and permitting display of thereof.

Abstract: A method and apparatus for detecting patches or areas of relatively high gloss on a moving web, and thereby associated wrinkles or other defects, and signalling the presence thereof comprises an analog and/or digital optical reflectivity sensor for sensing gloss on the surface of the moving web, a web marking device adjacent the edge of the moving web for marking the edge of the web with a dye, and a microcomputer for receiving the output signal from the sensor(s) and for controlling the operation of the marking device. The apparatus may also include a warning device such as a light or horn. The apparatus and method have particular applicability in association with paper web re-reeling equipment.

Abstract: A method of processing fiber includes the following steps: removing fiber tufts from a series of fiber bales by a tuft removing device of a bale opener traveling along the bale series; pneumatically conveying the removed fiber tufts in a duct in a direction of conveyance; detecting foreign substances by forming images of the conveyed material by an optical sensor device as the material passes through a location of the duct downstream of the bale opener as viewed in the direction of conveyance; determining the location of each fiber bale in the bale series; determining the degree of lightness, the degree of contamination or the color of the fiber tufts by an image processing device analyzing the generated images; and associating the degree of lightness, the degree of contamination or the color of the fiber tufts with the fiber bale from which the material passing through the duct location originates.

Abstract: A disc defect inspection apparatus which can detect a defect of a disc, such as an optical disc, with high accuracy. A disc defect inspection apparatus includes a rotating portion for rotating a disc, laser radiating optical portion for radiating laser lights on the disc rotated by the rotating portion under predetermined scanning conditions, sensors for detecting reflected laser lights from the disc, and a memory in which output signals of the sensors are stored together with address data generated in unison with the scanning of laser lights. Sensor outputs indicative of address positions of the disc are stored in the memory and sensor outputs near the address positions are read out and a defect of the disc is detected on the basis of a spatial difference among the nearby sensor outputs and the sensor output of the center address position.

Abstract: An optical inspection module detects defects on an active surface of a substrate in an integrated process tool system. The optical inspection module includes an enclosure, a substrate holder, a light source, a light beam path, a lens and a photodetector array. The light source has a light beam port. The light beam path extends from the light beam port to the substrate holder and has a grazing angle of incidence with respect to the active surface of the substrate. The light beam path illuminates substantially the entire active surface. The lens is oriented to collect non-secularly reflected light scattered from the light beam path by any defects on the active surface. The photodetector array has a plurality of pixels which are positioned within a focal plane of the lens. Each pixel corresponds to an area on the active surface, and the plurality of pixels together form a field of view that covers substantially the entire active surface.

Abstract: A surface testing device, operating by reflection, for linearly moving, tape-like materials, for example paper or polymer films, is suitable for automatic online registration of linear irregularities transverse to the direction of movement. The testing device includes a suitable test distance, at least two optical inspection units and at least one electronic circuit arrangement for detecting both the individual signals of the inspection units and the coincidence signals. In addition, a magnetic information medium wound in the form of a roll on a hub is tested by the surface testing device and carries a machine-readable and/or visually readable quality identification.

Abstract: A lighting control apparatus for illuminating a moving variable-speed web for inspection with a camera is provided. The apparatus is comprised of a detecting device for detecting the positioning of the web; a lighting device, aligned with the camera, for emitting a light to illuminate the web; a calculating device for calculating the speed of the web; and a control device which controls the emission of light by the lighting device as a function of the speed of the web.

Abstract: A lighting control apparatus for illuminating a moving variable-speed web for inspection with a camera is provided. The apparatus is comprised of a detecting device for detecting the positioning of the web; a lighting device, aligned with the camera, for emitting a light to illuminate the web; a calculating device for calculating the speed of the web; and a control device which controls the emission of light by the lighting device as a function of the speed of the web.

Abstract: An apparatus is provided for processing items moving along a transport path of a document processing system. The apparatus comprises a number of light sources located on one side of the transport path. A number of light sensors is located on the other side of the transport path and is provided for (i) sensing light from the light source transmitted through a variable first location of at least one item moving along the transport path and providing a first sensor output signal indicative of the extent of light transmitted, and (ii) sensing light from the light sources transmitted through a redefined second location of the at least one item moving along the transport path and providing a second sensor output signal indicative of the extent of light transmitted.

Abstract: An optical web defect detection system includes a light source and a detector disposed with respect to the light source so that the detector receives light modulated by a web moving proximate the light source. The detector outputs a signal corresponding to the modulated light. An adjustment mechanism is in operative communication with the detector to adjust the position of the detector with respect to the web. A control mechanism is in operative communication with the detector and the adjustment mechanism. The control mechanism receives the signal to detect a defect in the web based thereon and adjusts the position of the detector to optimize reception of the modulated light by the detector.

Abstract: An apparatus for monitoring a moving filamentary product having a device for monitoring the centering of the filamentary product relative to the coating, and a device for monitoring the quality of the coating. The centering monitoring device includes two light sources positioned at 90.degree. to each other in a plane perpendicular to the filamentary product. The coating quality monitoring device includes an optical receiver disposed between the two light sources in the same plane, and at 45.degree. to each of them, to receive the light beams reflected by the coating.

Abstract: A video inspection device using a field mode CCD camera. In order to inspect the printing on a moving web, a field mode CCD camera is provided with a zoom lens and strobe light. The image obtained is processed so that only alternate pixels of the image are stored. When reading out the stored image, the image is blown up by a factor of two and the missing pixels are interpolated horizontally and vertically. The resultant image is displayed in a monitor for the operator to inspect.

Abstract: A control device (3) for monitoring the travel of two superimposed paper webs (1, 2) in a paper processing machine has a thin stationary plate (4) arranged at the edge of the paper webs (1, 2) that extends between both paper webs in the direction of travel of the paper. Reflection-sensitive photoelectric sensors (5, 6) are arranged on opposite sides of the plate (4) so that the light rays (5a, 6a) from both sensors are projected from opposite directions upon the corresponding paper web (1, 2) and the plate (4) located behind the paper web.

Abstract: The present invention provides an optical detection system for detecting defects in an optical fiber. The system includes a light source for coupling light into the secondary coating of an optical fiber at a preselected angle with respect to the longitudinal axis of the fiber and an optical detector positioned adjacent the fiber at a preselected distance from the point at which the light is coupled into the fiber coating. In accordance with one embodiment, the light is coupled into the fiber coating at a sufficiently shallow angle with respect to the longitudinal axis of the fiber to cause the light to travel through the coating in a direction substantially parallel to the axis of the optical fiber for some distance before exiting the coating.

Abstract: A structure and yarn sensor for fabric directly determines pick density in a fabric thereby allowing fabric length and velocity to be calculated from a count of the picks made by the sensor over known time intervals. The structure and yarn sensor is also capable of detecting full length woven defects and fabric. As a result, an inexpensive on-line pick (or course) density measurement can be performed which allows a loom or knitting machine to be adjusted by either manual or automatic means to maintain closer fiber density tolerances. Such a sensor apparatus dramatically reduces fabric production costs and significantly improves fabric consistency and quality for woven or knitted fabric.

Abstract: A small hole detection system which detects small holes in fast moving sheets of material. An array of LEDs is pulsed from an emitter which is aligned with an array of photocells positioned to detect light emitting from the emitter. The receiver sends a signal to a processing unit when light is detected. The small hole detection system may include an auto-shuttering mechanism which automatically adjusts the system according to the width of the material being scanned.

Abstract: The present invention provides an optical detection system for detecting defects in an optical fiber. The system includes a light source for coupling light into the secondary coating of an optical fiber at a preselected angle with respect to the longitudinal axis of the fiber and an optical detector positioned adjacent the fiber at a preselected distance from the point at which the light is coupled into the fiber coating. In accordance with one embodiment, the light is coupled into the fiber coating at a sufficiently shallow angle with respect to the longitudinal axis of the fiber to cause the light to travel through the coating in a direction substantially parallel to the axis of the optical fiber for some distance before exiting the coating.

Abstract: A method and apparatus for probing with backside emission microscopy for locating defect sites in an integrated circuit specimen, where a first microscope positioned for observing the front side of the specimen and a second microscope positioned for observing the backside of the specimen. A carrier is provided for supporting the specimen in a plane perpendicular to the plane of the first and second microscopes. A white light camera is mounted on the first microscope for acquiring an illuminated image of the front side of the specimen, and generates its image as corresponding to site indicia of the backside of the specimen. A light emission sensitive camera mounted on the second microscope acquires a light emission image of the backside of the specimen in response to electrical test signals applied at terminals of the specimen by test probes. The light emission sensitive camera generates the emission image for superposing with the illuminated image to identify the location of defect sites in the specimen.

Abstract: The invention is directed to a machine for detecting anomalies in parts. A strip of parts (10) is transported through a sensor unit having light sources and photodetector pairs (118, 116; 132, 134; 136, 138) to detect pilot holes and the presence and position of parts. At least one of the light sources (136) is directed at an angle to detect out of plane deformations. Simple light sources and photodetectors are used to detect and identity misaligned contacts without the need for complex vision system components.

Abstract: The invention relates to a yarn sensor for scanning a yarn (2), which is moving in its longitudinal direction in a measuring gap (3), with a light beam from a light source (4), having a first receiver (7) for directly transmitted light, at least one second receiver (5, 6) for light reflected by the yarn and one element each (8, 9, 10, 11) for transmitting the light between the measuring gap, the light source and a receiver. To keep the overall size and the external dimensions as small as possible, the optical axes (12-15) of the elements for transmitting the light are disposed together in one plane lying at least approximately at right angles to the yarn.

Abstract: The position of a transverse crease line of a moving packing web is sensed in that light reflected from the web and emanating from a laser directed perpendicularly to the web is received by photo detectors from two directions in the direction of movement of the web and under an angle of for example 60.degree. in relation to the light beam directed to the web. The electrical measuring signal emanating from the photo detectors is compared in an electronic signal processor with a reference signal, which is stored in the processor and is representative of a reference crease line. A crease line is indicated when the value of a correlation signal, which constitutes a calculation of conformity between the measuring signal and the reference signal, reaches a maximum. In a corresponding way, the position of a longitudinal crease line can be sensed.

Abstract: The present invention provides an apparatus for inspecting a fluorescent screen, including a line light source for radiating light onto an external surface of a panel formed at an internal surface thereof with a fluorescent screen, a line sensor disposed facing to the internal surface of the panel for detecting light emitted from the line light source and transmitted through the fluorescent screen, a device for varying the relative positional relationship between the panel and a combination of the line light source and the line sensor in a direction perpendicular to a length-wise direction of the line sensor, and a processor for comparing linear patterns detected by the line sensor to a reference pattern. The apparatus automatically inspects an entire fluorescent screen, and further automatically makes judgement as to whether the fluorescent screen is good or bad in quality.

Abstract: The invention relates to a method and an apparatus for detecting a yarn wind on a rotating roll partially looped by the yarn and associated with a yarn processing apparatus, and which includes a light source for directing to the monitoring range of the roll a light beam, which is imaged as a light strip along a surface line of the roll, and a light sensor with an evaluation device. The sensor detects the light reflected from a yarn wind, which is converted into an electric signal and may then be further processed.

Abstract: There is disclosed a web of record members of the composite label web, tag and linerless types with cracks or fissures through the webs so that the position of the web can be detected by an optical detector. Apparatus can be used to convert the web into more usable form which includes for provision of optically detectable cracks or fissures in the web. These cracks are made by crushing the web locally between one or more crushing cutters and a back-up roll which cooperates with zero clearance. The method and apparatus can produce a reliably detectable web at low cost. The optical detector includes a source of light and a bicell detector between which the web with the cracks or fissures passes. The bicell detector produces a first output signal representing the amount of light impinging on a first portion of the detector and a second output signal representing the amount of light impinging on a second portion of the detector.

Abstract: Apparatus for detecting defects in a web of textile material as it is being drawn through an inspection zone. A header is arranged to project a line of light on the running web and a reflected light image from the web is sensed and analyzed for defects. The header includes a fiber bundle for producing a line of light and an optical element for collimating the light upon the web as it passes through the inspection zone.

Abstract: An analyzer is employed for detecting a cloud in an optical disc having a transparent surface layer atop a substrate and data pits therebetween. A light beam is projected at an acute incidence angle atop the surface layer to define a light spot covering a plurality of the pits. Light scattered from the pits below the spot is detected obliquely to both the incident light beam and a specular reflection thereof. The light spot is scanned over the surface layer, and variation of detected light intensities over the disc is determined for detecting a cloud. In a preferred embodiment, the scattered light is detected from two different oblique directions to increase contrast of the cloud.

Abstract: An apparatus and method for inspecting pieces of glass and other transparent sheets for optical defects is provided. The inspection apparatus can be integrated directly into the production line. The inspection apparatus includes an electro-optic inspection of the total viewing area for each transparent sheet as the sheets are conveyed at the end of the production operation. The apparatus includes a plurality of laser light sources positioned on one side of the sheet of glass or other transparent material, and a corresponding plurality of lenses and photosensitive position detectors on the opposite side. Light beams are directed from the light sources through the transparent sheets to the lenses and the detectors. The angular deviation of each light beam which occurs as the light beam passes through the sheet generates a signal at each of the detectors.

Abstract: An extraneous substance inspection apparatus includes a level conversion circuit for converting a level of an extraneous substance detection signal obtained from a position in a first chip into one of multi-valued levels. A judging circuit determines the existence or absence of an extraneous substance by comparing a signal indicative of one of the multi-valued levels with another signal indicative of a converted level obtained by converting a detection signal detected at a similar position in another chip adjacent to the first chip.

Abstract: An apparatus for optical monitoring of at least one thread for irregularities, in particular for filament breaks in multifilament yarns, with a radiation source, a radiation detector, and a ray beam transmitted by the radiation source and received by the radiation detector and directed past the thread perpendicular to and at a defined distance to it, whereby the radiation source and radiation detector are situated next to the thread, characterized in that the radiation source and radiation detector are situated on the same side of the thread and that on the opposite side of the thread means for displaced reflection of the ray beam are situated such that the transmitted ray beam is directed past the thread on one side and the displaced reflected ray beam is directed past the thread on the other side, whereby the transmitted ray beam and the displaced reflected ray beam are as equidistant as possible from the thread and are at least approximately parallel to one another is disclosed.