A method and apparatus for the inspection of specularly reflective surfaces

Abstract: A method and apparatus for the visual inspection of at least partly specularly reflective surfaces, such as metal surfaces, which are in continuous motion, by means of a receiver apparatus (5) and an image-analysis apparatus (6). In order to eliminate from the image produced in the receiver apparatus (5) the uneven illumination caused by variations in the height and orientation of the surface (4) being inspected, the view angle (39) of the receiver apparatus is adjusted in such a manner that the length of the image of the source of light, formed by means of a perforated lens (3) in front of the source of light (1) is greater than the diameter of the input pupil of the lens of the receiver apparatus (5).

Description: The invention relates to a method and apparatus for the automatic visual inspection, by means of a receiver and of image-analysis equipment, of at least partly specularly reflective surfaces, such as metal surfaces, especially those in continuous motion. US Patent 4,455,090 discloses an apparatus which measures surface properties based on the opacity of a material. The apparatus has an optical projection system for illuminating the sample and a light cell for measuring the light reflected from the sample. The apparatus is intended specifically for the testing of paper, and in connection with a copying paper not containing carbon the apparatus is used for measuring the density of a watermark in the paper in relation to the background. The apparatus according to US Patent 4,455,090 is thus advantageously used for testing material through which light directed to the material can pass at least in part. US Patent 3,976,382 describes an apparatus for inspecting the geometry of a surface, for example in the treatment of workpieces in the engineering industry. In the apparatus, a source of light transmits light to the surface of the object being inspected. In addition, there is a light-shade disposed between the source of light and the object, the shade producing a sharp boundary surface between the shaded and the illuminated parts of the surface. The thus illuminated part of the object is examined by means of a TV camera, for example. The information thus obtained regarding the geometry of the surface is further used for controlling an apparatus used for altering the geometry of the surface, such as a welding apparatus. According to US Patent 3,976,382, a boundary surface describing the geometry of the surface is obtained of the outline between the shaded surface and the illuminated surface. It is clear that a technique such as this cannot be applied to the inspection of a surface for the purpose of inspecting incidental surface defects in a surface. US Patent 4,547,073 relates to a surface inspection apparatus and method in which rays of light deflected so as to be parallel are directed by means of a lens onto the polished surface of a silicon wafer. According to the method, defects in the surface of the sllicon wafer produce, in the reflected rays of light, rays which deviate from the parallel, in which case surface defects will cause shaded areas in the inspection means. In this case, mere unevenness in the surface of the silicon wafer will produce shaded areas, and therefore the use of the method for surfaces substantially larger than a silicon wafer is not practical. GB Patent Application 20 20 415 discloses a surface inspection apparatus in which moving bands are illuminated in order to inspect longitudinal grooves in the bands. The apparatus arrangement is disposed in such a manner in relation to the band that the rays coming from the source of light will illuminate the band from both sides of the presumed groove. The apparatus according to GB Application 20 20 415 is thus intended for the examination of surface unevenness known in advance, and thus it is not possible to use the apparatus for detecting unexpected surface defects possibly present within the entire surface area of the band. DE patent specification 29 11 177 describes an arrangement, in which light deriving from a spot-like light source is directed to the surface being examined, and a unidimensional image is formed of the surface by means of a camera or similar, the light-sensitive sensor of the camera being linear. There is no description of the focusing point of the ligth source, but it can be concluded from the figure that the light source has been focused on the surface being examined. In such an arrangement, which uses a spot-like light source, the diametre of the pupils of the various optics is without significance, and thus they are not defined in this specification. The arrangement is certainly usable for examining a mobile surface, however, the distance and position variations of the surface relative to the light source and/or camera entail considerable variations of the information provided by the sensor, thus causing fundamental errors in the outcome. US patent specification 2 315 282 describes arrangements, in which the light-sensitive member is spot-like, i.e. consists of one pixel only. Such an arrangement cannot be used for examining a moving surface, since it does not enable to perform gapless scanning of the entire surface. The principle described in the specification does not involve the use of a camera, i.e. there are no such optics between the surface and the light-sensitive member, by means of which the image of the surface would be formed at the light-sensitive member. Such a camera would be useless in the case of a one pixel-sensor, i.e. a zero-dimensional sensor. In terms of the specification, the aperture of the light source has been focused on the surface being examined or on a separate plate. The focusing point of the light source itself or its image size are not mentioned. In any case, such an illuminating arrangement results in that distance and position variations of the surface being examined affect the measurement result significantly. US patent specification 3 385 971 describes a provision for detecting holes in a fabric by using a zero-dimensional, i.e. spot-like light source and a spot-like, light-sensitive sensor. As a one pixel-implementation this arrangement is not suitable for examining moving surfaces, since it does not enable to perform gapless scanning of the surface. In terms of the specification, the picture of the light source is located at a reflecting surface immediately behind the surface being examined, in other words, virtually at the surface being examined. The consequence of this arrangement is also that distance and angular variations of the surface being examined relative to the light source and/or the light-sensitive member have a fundamentally detrimental effect on the measurement result. The pupil size is not mentioned at all in this connection, as it does not have any noticeable impact on the outcome. The object of the present invention is to eliminate drawbacks in the state of the art and to provide a method and apparatus, better than previous ones, for the visual inspection of specularly reflective surfaces, in which method and apparatus the light coming from at least one source of light and reflecting from the surface being inspected is used for inspecting the obtained image with the aid of light-detecting means and of image-analysis apparatus. The essential characteristics of the invention are given in the accompanying claims. When the invention is being applied, an image of the core of at least one source of light is produced on the receiver apparatus so that from a flawless specularly reflective surface, such as a metal sheet, the light hits the input pupil of the receiver apparatus, and can be seen as substantially bright in the image on the receiver apparatus. Flawed areas, on the other hand, will scatter light away from the receiver apparatus and are seen in the image as dark areas. According to the invention the length of the image of the core of the source of light in the vertical direction is greater than the diameter of the input pupil of the lens of the receiver apparatus; this allows the view angle of the receiver apparatus to be varied, thereby eliminating from the image any uneven illumination caused by variations of the surface height and surface orientation. According to the invention, the view angle of the receiver apparatus can be adjusted by altering the length of the source of light or, for example, the adjustable aperture in front of the source of light. It is also possible to place a shade in the adjustable aperture, or substantially in its vicinity, so that the shade preferably covers the central part of the source of light. Thus the shade will prevent direct specular reflection to the receiver apparatus, and by altering the width of the shade it is possible to regulate the view angle of the receiver apparatus in relation to the light specularly reflected from the surface being inspected. Furthermore, by using a shade which divides the light coming from the source of light into two beams of light, it is possible, in the image formed on the receiver apparatus, to compensate for the effect of variations in the evenness of the surface being inspected. When the invention is being used it is advantageous to have the burning position of the source of light be substantially horizontal, since such a burning position will lengthen the burning period of the source of light. For this reason there is placed between the adjustable aperture and the surface being inspected a mirror by means of which the rays of light coming from the source of light in a substantially horizontal burning position can be directed through a lens, such as a Fresnel lens, onto the surface being inspected and then be reflected in an advantageous view angle to the receiver apparatus. In the apparatus according to the invention it is also possible to use advantageously a plurality of sources of light and an equal number of lenses in front of the sources of light. In such a case, each source of light is offset in the lateral direction in relation to the lens which corresponds to the source of light, in such a way that each lens will produce on the receiver apparatus an image of one of the sources of light via the illuminated surface. It is possible, when necessary, to use shades between the sources of light to prevent the access of the light coming from a source of light to the adjacent lenses. By using several sources of light and several lenses it is possible to increase the image-forming area inspected or to use smaller-size lenses, the manufacture of which is simpler and less expensive and which are more available than large-sized lenses. Furthermore, in imaging dim scattering surfaces such as steel sheets, a stronger illumination on the surface is obtained than by using one source of light and one lens. By using a plurality of sources of light and of lenses it is, furthermore, possible to control the orientation of the light hitting the surface and thereby improve the visibility of flaws and the evenness of the light. The invention is described below in greater detail with reference to the accompanying drawings, in which Figure 1 depicts a schematic side elevation of a preferred embodiment of the invention, Figure 2 depicts a schematic side elevation of another preferred embodiment of the invention, Figure 3 depicts a schematic plan view of one more preferred embodiment of the invention. According to Figure 1, rays of light coming from a source of light 1 are directed to a mirror 2, which deflects the rays of light further through a Fresnel lens 3 onto the surface 4 being inspected. The rays of light reflected from the surface 4 forms the image of the light source onto the camera 5. The image of the surface is received by the camera 5. The image received by the camera 5 can be further analyzed by means of an image-analysis apparatus 6. According to the invention, the view angle 39 of the camera can be regulated, for example, by adjusting the length of the incandescent bulb serving as the source of light 1, or also by means of an adjustable aperture 7 in front of the source of light 1. Figure 1 furthermore depicts a curved mirror 8, which is disposed behind the source of light 1 and by means of which it is possible advantageously to increase the luminosity arriving at the input pupil 41 of the lens of the TV camera 5. In the embodiment according to Figure 2, there is placed in front of the source of light 10 a shade 11, which divides the rays of light directed to the mirror 12 into two light beams 13 and 14. The light beams 13 and 14 are further directed via a Fresnel lens 15 onto the surface 16 being inspected. From the surface 16 the light beams 13 and 14 are reflected towards the camera 17 and the image analysis apparatus 18. The magnitude of the view angle 40 of the camera 17 can be regulated advantageously by altering the width of the shade 11. To improve the luminosity of the source of light 10 there is disposed behind the source of light a curved mirror 19 and in front of the source of light limiters 20, which help the regulation of the width of the light beams 13 and 14 produced with the help of the shade 11, and at the same time the adjustment of the view angle 40. Figure 3 depicts an arrangement according to the invention in which the light is directed from a plurality of sources of light onto the surface being inspected. In front of the sources of light 21, 23, 25, 27 and 29 there are placed, in a row-like arrangement, Fresnell lenses 22, 24, 26, 28 and 30, respectively. The optical axes of these lenses are depicted by dashed lines 31, 32, 33, 34 and 35. Of the surface to be imaged, that part which is hit by the rays of light coming from the sources of light and from which they are reflected is depicted by dashed line 36. According Figure 3, the sources of light 21, 23, 25, 27, 29 are offset in the lateral direction in relation to the optical axes 31, 32, 33, 34 and 35 of the respective lenses so that each lens images one of the sources of light via the illuminated surface onto the camera 37.

Claims: A device for the automatic visual inspection of at least partly specularly reflective surfaces, such as metal surfaces, which are in continuous motion, comprising an illuminating member with at least one light source and a lens member placed in front of the light source, which directs the light beam deriving from the light source to the area of the said surface being examined, as well as a receiver apparatus which includes a light sensitive part directed to the area being examined illuminated by the light source and the lens member, whereby the average directing angle with respect to said surface of the light beam and the average directing angle of the receiver apparatus are essentially equal, characterized in that the receiver apparatus consists of an immobile camera equipped with optics and forming by means of this a bidimensional image out of the surface being examined, in that the image of the light source formed by the lens members through the surface being examined is produced at the receiver apparatus and in that the size of the light source is such that the smallest diameter of its image is greater than the diametre of the input pupil of the said optics. A device according to claim 1, characterized in that the illuminating part consists of several light sources, each of which sending a beam to be reflected through the surface being examined, and to form an image of the light source at the image receiving apparatus, and in that the light sources with the lens members have been placed perpendicularly to the optical axis of the receiver apparatus and essentially parallel close to each other aligned with the surface being examined, so that the surface being examined is evenly illuminated in the lateral direction. A device according to claim 2, characterized in that each source of light has been laterally displaced at least perpendicularly from the optical axis of the lens member cooperating with it, so that the lens member forms a light source through the surface of the image being examined to the point of the receiver apparatus. A device according to claim 1, characterized in that the light source or sources consist of a light bulb, the light emitting part of which is of a relatively great length and in that this length is disposed transversely to the optical axis of the light source in order to form a light source that is essentially greater than the input pupil of the optics transversely to the surface being examined. A device according to claim 1, characterized in that the illuminating part contains an edge shade, by means of which the diametre of each light source and/or the said length of the light source can be varied with regard to the light orientation and thus the specularly reflective portion and the scattered portion of the beam reaching the receiver apapratus, in order to be set appropriately for the type and variation of the surface being examined. A device according to claim 1, characterized in that the illuminating part contains a central shade or optinonally two lamps at the corresponding distance, by means of which the light specularly reflected to the receiver apparatus through the surface being axially examined can be delimited with regard to the specurlary relfective portion and the scattered portion of the beam reaching the receiver apparatus in order to be set approriately for the type and variation of the surface being examined. A device according to claim 6, characterized in that the central shade is a tape-like strip, the length of which is perpendicular to the optical axis of the light source and essentially in the direction of the surface being examined and this longitudinal direction extends over the entire width of the light source in this direction and in that the width perpendicular to the length of the strip is the measure to be set producing the said delimitation. A device according to claim 1, characterized in that the angle or angles between the average radiation axis of the beam specularly reflected from the surface being examined of the beam coming from a light source and the optical axis of the optics of the receiver apparatus joining the surface being examined are adjusted by means of an edge shade and/or central shade placed in front of the light source in the illuminating part. A method for automatic visual inspection of at least partly specularly reflective surfaces such as metal surfaces, by using the device of claim 1, comprising an illuminating part which contains at least one light source and a lens member placed in front of the light source, which directs the beam coming from the light source to the area of the said surface being examined, and a receiver apparatus, which comprises a light sensitive part being directed to the said area, of the surface being examined illuminated by a light source or via a lens member, whereby the average directing angle of the beam and the average directing angle of the receiver aparatus are essentially equal, characterized in that the images of the light source or sources are produced on the receiver apparatus by letting the beams coming from the light source specularly reflect from the surface being examined to the receiver apparatus by setting the position and direction of the light sources and respective lens members, whereby the dimensions of the image of the light source or sources perpendicularly to the axis of the beam is greater than the diametre of the input pupil of the optics belonging to the receiver apparatus and this image is situated at the receiver apparatus, and in that the image of the surface being examined has been focused on the two dimensional light sensitive image surface of the receiver apparatus by means of the optics of the receiver aparatus, and in that in the image of the mobile strip thus obtained, possible surface defects are searched by means of an image analyzing equipment.

Inventors: PIIRONEN TIMO HEIKKI, PIIRONEN, TIMO HEIKKI
Applicants: RAUTARUUKKI OY
Application year: 1989
Main classification: G01N21
IPC codes: G01N21