Optics: measuring and testing – Inspection of flaws or impurities – Transparent or translucent material
Reexamination Certificate
1999-10-07
2001-05-29
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Transparent or translucent material
Reexamination Certificate
active
06239869
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a device for detecting diffusely scattering impurities in containers that have a transparent wall. The containers are illuminated by the dark-field method and are investigated for impurities by means of an optical detecting apparatus.
FIELD OF THE INVENTION
Homogeneous, structureless impurities in transparent containers, e.g. glass bottles, are very difficult to detect by existing methods of investigation. Examples of impurities are, in particular, residues of emulsion paints, rust, minerals and other semitransparent substances. If the containers are illuminated by the bright-field method, it is scarcely possible to detect the structureless impurities, as they merely cause a fluctuation in the transmission behaviour. There are neither changes in contrast nor a sufficiently pronounced decrease in image brightness, as the mean scattering angle is at times only very small. Using an optical detecting device, such as a CCD camera, it is therefore not possible for such impurities to be detected reliably, even if additional methods of image processing, e.g. contrast intensification, are employed.
The dark-field method known from EP-A-0 429 086 is used in particular for detecting transparent plastics films, for example cigarette packaging, in glass bottles. Scraps of plastics film are detected by the optical detecting device (CCD camera with polarizing filter) as image elements with strong contrast, at least in places. Homogeneous, structureless impurities, such as residues of emulsion paints, however, under dark-field illumination only produce image elements that are themselves also structureless and without contrast. The intensity of the light received by the optical detecting device does indeed increase with the scattering power of the impurities, i.e. with the layer thickness, provided absorption effects do not predominate. The increase in intensity of the amount of scattered light detected by the optical detecting device therefore becomes greater relative to containers that are free from impurities, depending on the layer thickness of the impurity. Detection of homogeneous, structureless impurities therefore requires measurement of the absolute intensity. However, experience has shown that with detecting devices there is considerable uncertainty in such measurements.
SUMMARY OF THE INVENTION
The basic aim of the invention is to make it possible for homogeneous, structureless, diffusely scattering impurities to be detected reliably in containers with a transparent wall.
According to the invention this aim is achieved in that the light beam or beams of the light source have a spatial intensity distribution with a contrast in an intensity at one point at least within their cross-section.
This one point, at least, where there is an intensity contrast, is generally the lateral boundary of the cross-section of a light beam. This assumes that the diameter of the light beam is much smaller than the container wall that is to be examined. It is sufficient if the intensity contrast along the light beam is present at the point where the wall to be investigated is located. Preferably, therefore, the light beam is focused on the transparent wall of the container.
It is also possible to use a beam of parallel, converging or diverging light rays or some other kind of illumination, which produce a high-contrast bright/dark distribution of intensity in the wall on which any impurities present are to be detected. In particular, highly collimated light beams such as laser beams are suitable for illumination. It is, preferable to use a chequered or matrix-like arrangement of point light sources or a striped light pattern. It is also possible to produce a sharp image on the container wall of a pinhole diaphragm or slit aperture, which is located at the light source. It is also possible to scan the wall of the container with a scanner. Since homogeneous, diffusely scattering impurities arise for example through large-area evaporation of a solvent—in the beverage industry, as a rule this is water—the contrast pattern can be scanned alt several points that are some distance apart. This greatly improves the reliability of detection of impurities in the form of large-area deposits. The distance between the individual contrast points or light points is then preferably much smaller than the anticipated extent of the deposit.
If no impurity is present on the transparent wall, the light beams pass essentially in a straight line through the wall and, because of the dark-field arrangement, they go past the optical detecting device. If, on the other hand, there is a diffusely scattering impurity on the transparent wall, e.g. a thin film of emulsion paint, the light beam is scattered by the impurity, so that some of the light is received by the optical detecting device. On account of the intensity contrast of the light beam within the transparent wall, the image picked up by the optical detecting device also contains at least one place with a high intensity contrast. This place with high intensity contrast can be detected very reliably, largely independently of the level of the absolute intensity value.
The detecting device according to the invention can also be equipped with a single light source and two or more optical detecting devices. The light source can for example emit several light rays and be arranged so that some of the light rays impinge on the bottom surface of the bottle, whereas others impinge on the side wall. It is then possible to use two CCD cameras, one inspecting via the mouth of the bottle through the bottom, whereas another laterally arranged CCD camera inspects the side wall of the glass bottle. Basically it is possible to operate both with one or more light sources and with one or more optical detecting devices.
The contrast structures detected by the optical detecting device can be brought out clearly by standard image-processing techniques for intensifying contours or contrast, so that reliable distinction between containers with impurities and those without impurities is possible. Due to the sensitivity of the method, even very slight deposits can be detected. For the detection of large-area homogeneous deposits it is therefore sufficient in principle to have a single light beam and therefore a single light spot in the transparent wall, and it is only necessary for instance to check the impingement point of the light spot.
When inspecting a large number of identical glass bottles, which inevitably have an identically shaped bottom of the bottle, the bright spot or spots of lights always appear at the same place, so that these places are examined selectively for the detection of structureless, homogeneous impurities.
In plants that operate with dark-field illumination for detecting structured defects (cracks, flaws in glass, polarization-altering defects, e.g. plastics films), the device according to the invention can be inexpensively installed, by using a source of illumination that has a pattern of high-contrast in the intensity distribution in the transparent wall. The image signals produced can be investigated for the presence of diffusely scattering defects or impurities with the same optical detecting device (CCD camera) that is used for the detection of structured defects. The same measurement principle can also be employed for checking diffusely scattering surfaces (etched or sandblasted surfaces) of otherwise transparent walls and for distinguishing between diffusely scattering surfaces and clear surfaces.
The device according to the invention can also be combined with a device operating according to the bright-field principle. Especially when inspecting the side wall of glass bottles, operation is generally based on the bright-field principle, i.e. the glass bottles pass in front of a large-area light source and are examined for the presence of foreign bodies using a CCD camera. Such an inspection device, working exclusively according to the bright-field principle, is unable to detect diffusely scattering impurities. If, how
Heuft Bernhard
Roesel Christoph
Schoening Bernd
Font Frank G.
Gardner Carton & Douglas
Heuft Systemtechnik GmbH
Stafira Michael P.
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