Optics: measuring and testing – Inspection of flaws or impurities – Transparent or translucent material
Patent
1995-04-20
1997-01-28
Evans, P. L.
Optics: measuring and testing
Inspection of flaws or impurities
Transparent or translucent material
348131, 25055941, 25055946, G01N 2189
Patent
active
055982620
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and a device for inspecting transparent material and concerns the field of automatic industrial inspection by imaging at the exit from a production line.
In a general way, the inspection of a transparent material such as glass, for example, is intended to detect and to locate defects, also called "inclusions", which are generated in the course of the manufacturing phase of the glass. The impact of these defects on the quality of the glass produced depends upon their number, their shape and on the standards imposed on the glassmaker according to the application envisaged.
These defects are of various types, the main ones of which are of the blister type and of the stone type:
In the blister type, the defects are gas bubbles which have remained captive in the material when molten and the shape of which varies from a sphere to a very elongate filament having an aspect ratio exceeding ten.
In the stone type, the defects are defects of homogeneity of the basic constituents, entailing the formation of a visible particle in the glass.
The size and the shape of these inclusions vary depending on the material and the depth within the material. Their size is, in general, less than one millimetre, the maximum resolution reaching one tenth of a millimetre.
The automatic analysis of the quality of the material, i.e. the automatic detection of the defects of the material, reacts directly on its production. This analysis makes it possible to reject the defective products, but also to have reliable statistics available concerning the number and the size of the defects, in order consequently to modify the various production parameters of the material.
Known inspection devices make use of an observation system including a light source, for example a laser source, illuminating a transparent material to be inspected. After passing through the material, the light radiation is analysed by a detection device which is sensitive to the light radiation, for example a CCD camera, and permits the detection and the location, in depth, of any possible defects.
However, the use of artificial vision for the inspection of transparent materials and of glass in particular, is currently limited by a lack of effectiveness concerning the discrimination between surface perturbations, such as dust particles deposited on the surface of the material to be inspected, and inclusions which are actual defects of manufacture which are embedded within the mass of the material. In fact, the presence of dust particles on the surface of the material gives rise to confusion when an actual defect is localized within the mass of the material in proximity to the surface. The dust particles are illuminated in proximity to the light source, and may mask a possible defect situated in proximity to the surface.
On the other hand, certain defects of elongate form do not sparkle when they encounter the light beam and therefore are not detected.
The object of the invention is to alleviate the aforementioned disadvantages.
To this end, the object of the invention is a method for inspecting transparent material by illumination by means of at least one light source, and by observation of the thickness of the material by at least one camera, characterized in that it consists in uniformly illuminating a bright background placed relative to the camera behind the material so as to be viewed by transparency through the material, covering the field of view of the camera and serving as contrast reference, in laterally illuminating the surface of the material in order to distinguish defects included within the material from parasitic elements deposited on its surface, in viewing by transparency by the camera, placed in the vertical to the surface of the material, a sequence of contrasted images reproducing the thickness of the material, and in processing information acquired by the successive images which are representative of the material seen in its thickness, in order to detect and locate the defects included within the thickness of t
REFERENCES:
patent: 2042526 (1936-06-01), Hohmann
patent: 3519362 (1970-07-01), Cardno et al.
Jutard Yann
Sacre Jean-Jacques
"Thomson-CSF"
Evans P. L.
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