GB2331359A - Glass defect detection - Google Patents

Abstract

In a system to detect the presence of microcorrugation defects on a face of a glass sheet, light from source 2 is reflected off the surface under investigation onto screen 4 and is viewed by light detection device 6 eg a camera. A layer of liquid having a refractive index close to that of the glass is deposited on the face of the sheet opposite to the face to be checked from a liquid source spaced apart from the said opposite face. The presence of the liquid layer prevents any stray reflections from the lower surface from creating a confusing or indecipherable image for inspection.

Description

1 Glass Defect Detection 2331359 The invention relates to the detection of

defects in glass sheet material. It is particularly concerned with detecting the presence of small transverse undulations known as "microcorrugation" on the surface of the glass, especially of thin sheets of glass.

Glass sheets produced by the float glass process are not perfectly flat, having small surface undulations resulting firom small waves on the tin surface. The undulations are generally less marked on the face that during manufacture had been in contact with the tin (the "tin face") than on the opposite face (the "air face"). The amplitude of the undulation is dependent on its wavelength. For long waves (>25 mm), the amplitude is of the order of 1 to 200 gm, giving sheets with a recognisably "rough" surface. For short waves (<2.5 mm), the amplitude is very small (<lO nm), classifying the sheets as "smooth" with a slight warpage.

Particular problems arise from microcorrugation in the case of thin glass sheets, e.g. with a thickness in the range of about 0.5 to 1.5 mm. Such thin glass sheets are employed in liquid crystal displays (LCD), for example in portable computers, calculators and personal electronic organisers, and for digital information carriers such as audio, video and computer ROM discs. The microcorrugations have intermediate wavelengths of about 2.5 to 25 mm and amplitudes in the approximate range of 20 to 500 nm and may cause undesired effects in the appearance and performance of the product including glass sheets.

Microcorrugation is thus regarded as a potential source of severe defects and is the subject of tight control specifications. In general the requirement is for the sheet to have a maximum undulation amplitude of 80 rim on the tin face.

Amplitudes of greater than 80 rim, described herein as "large amplitude microcorrugation", are generally considered to be harmful to the optical quality of the product.

When a sandwich-type of product is made from two thin sheets of glass and an intermediate layer, such as a layer of liquid crystal as in an LCD, large amplitude microcorrugation on the internal faces of the sheets leads to interference at the interfaces between the different media forming the product. The interference appears as visible optical distortions which reduce the quality of the product and at worst make it unusable.

2 2 One known technique to ensure the absence of large amplitude surface microcorrugation has been to conduct intensive polishing of the entire surface of the glass sheet, removing surface material to a depth equivalent to the estimated amplitude of the undulations. This is however a laborious and time- consuming technique which considerably increases the cost of producing the glass.

Careful control of the float glass process conditions can also produce a high quality product in which the size and extent of surface undulations are reduced to an acceptable level, but such a quality cannot be ensured for all the production.

Even for such a controlled product, a check for microcorrugation prior to despatching the product to customers is therefore desirable. The cheek permits rejection or further treatment, such as polishing, of any sheets that fall short of the required standards. It also ensures that needless further treatment of acceptable sheets can be avoided.

The objective is to provide a simple, reliable check which does not hinder the glass production process. It has been proposed to adopt an optical check in which a light beam is directed obliquely at the glass surface to be examined and the reflected image is inspected for defects. This check suffers from the problem that stray reflection occurs from the other surface of the glass and produces a confusing or indecipherable image for inspection. US patent 5602648 relates to a method and apparatus in which a light source obliquely illuminates a glass surface and forms a reflected image which is examined by means of a CCID camera. The stray reflection of the light on the opposite face of the sheet is neutralised by putting this face into contact with a material impregnated with a liquid of similar refractive index. The impregnated material may be for example a tissue or a resilient roller.

We have found that such a mechanical means of neutralising the stray reflections gives rise to delays and complications in the glass surface defect checking operations that are not compatible with the smooth continuous flow of sheet product from a float glass line. The problem therefore remained of finding a solution which is not susceptible of hindering the glass production process.

According to the invention there is provided a surface defects checkingunit for glass sheets, which unit comprises a light source, a screen and a light detection device, the light source being disposed to direct light onto a face of the sheet to be checked and by reflection therefrom to form on the screen an image for inspection by the light detection device, characterised in that the unit further comprises means for depositing on the face of the sheet opposite to the face to be checked from a liquid 3 source spaced apart from the said opposite face a layer of liquid having a refractive index close to that of the glass.

The invention further provides a method for checking surface defects on glass sheets, in which a light source is disposed to direct light onto a face of the sheet to be checked and by reflection therefrom to form on a screen an image for inspection by a light detection device, characterised in that the layer of liquid having a refractive index close to that of the glass is deposited indirectly on the face of the sheet opposite to the face to be checked.

The deposited liquid layer serves to neutralise the reflecting properties of the opposite face and thus to prevent the generation of stray reflections. Compared with prior proposals the layer and its means of indirect deposition present no mechanical hindrance to the passage of the sheets and allow the check to be conducted at a rate equal to that of the movement of the sheets from a float glass production line.

In one embodiment of the invention the sheet to be checked is placed on a conveyor system and moved through the checking unit past the light source and light detection device. It will generally be most convenient for the sheet to be conveyed horizontally through the checking unit.

The layer is preferably applied by means of a diffuser. The diffuser can either supply the liquid as vapour which condenses on the glass face to form the layer or as small droplets. In both cases the diffuser should be so disposed and operated as to create a layer of liquid over the whole of the area of the surface of the sheet opposite to the area of the surface to be checked.

Supply of the liquid as vapour is generally preferred. This can be achieved by an open reservoir of liquid located at a suitable point relative to the path of movement of the glass. The liquid in the reservoir can be heated in a suitable manner, for example by an electrical resistance, to produce the vapour. Supply as droplets may be achieved from a reservoir of liquid under pressure equipped with an outlet valve. Alternatively it can be achieved by an aerosol generator, for example using ultrasound.

The preferred qualities of the liquid to form the surface layer are that it should readily evaporate after the checking step and should have a refractive index which does not differ by more than about 20% from that of the glass to be checked. Water is particularly suitable because of its low cost, availability, non-toxic and nonpolluting qualities and its accurate refractive index. Indeed, the refractive index of the glass is of the order of 1.5 and the one of the water lies between 1.2 and 1.4.

4 The light source is preferably a point source since this permits the contrast quality of the screen image to be optimised. The light from such a source can illuminate the surface of the glass to be analysed so that the resulting two dimensional screen image can be observed by a matrix light-detection system.

Alternatively the light emitted by the point source can be configured as a pencil which progressively scans the sheet. In this case, the resulting mono- dimensional screen image can be observed by a linear light detection system. The scanning system can conveniently be used with a moving sheet, the pencil then scanning the sheet transversally to its direction of movement. Partial illumination and scanning options facilitate the checking of sheets of different sizes, including large sheets.

The power of the light source should be adapted to the quality of glass and to the sensitivity of the detection system. In general a lamp with a power of the order of W is sufficient. Suitable types of lamp include lamps with short arcs, mercury vapour lamps and xenon lamps. The light pencil for a scanning system can conveniently be provided by a laser source.

The screen can be a conventional white or silvered screen. It should be substantially free of any surface defects that would distort the image to be inspected.

In general the required extent of surface smoothness is most readily achievable by a planar screen.

The image formed by the reflection from a perfectly plane glass surface would be as uniform as the incident light. Flatness defects have the effect of concentrating the light at certain regions, the image formed by the reflection from a defective surface being composed of alternate light and dark bands. These bands correspond to the peaks and valleys of the surface, thereby giving a good representation of the surface condition of the sheet.

The light detection device can be a still camera, a video camera or other form of light detector, for example photo-detecting cells. Cameras are generally preferred as they have the advantage of facilitating precise identification and location of the defects. Most preferably the light detection device is an electronic camera, for example a CCD matrix camera, preferably with a number and size of pixels adapted to the required sensitivity of the measurement. Analysis of the image permits the observed defects in flatness to be quantified.

In some instances it may be convenient to employ more than one light detection device, each such device being aligned to observe on the screen a different 3 5 portion of light reflected from the sheet.

The nature of signal recorded by the light detection device(s) can be inspected or further processed to identify the type and magnitude of the defects. Once the nature and extent of the defects have been determined the glass sheets can be classified into those that are of sufficiently high quality to be used in the most demanding applications, those that can be used in less demanding applications, those that can be brought to the desired quality by such means as polishing and those that must be rejected.

The unit thus provides a simple effective means of classifying glass sheets to ensure that customers receive a quality of product sufficient to meet their needs, while avoiding additional and potentially expensive further processing of the glass for high quality duties.

The conveyor system can be provided by driven pairs of conveyor belts on which the sheets are placed, one belt being located at or near a side edge of the sheet (as viewed from the direction of travel) and supporting the sheet from below.

The testing unit of the invention can conveniently be included in a glass production line between the cutting station and any station for routine polishing of the glass or storage.

The invention is further described below with reference to the accompanying figure which is a schematic side view of a unit according to the invention.

The illustrated unit includes a 100 W mercury vapour lamp (2), a white screen (4) and a CCD camera (6). Rubber belts (7) and (8) running on rollers (9) provide a conveyor system to support the edges of a glass sheet (1) to be tested. The sheet (1) has an upper surface (11), corresponding to the tin face of the glass, and a lower surface (12).

The belts (7) and (8) and rollers (9) are driven by an electric motor (not shown) to move the sheets (1) in the direction away from the viewer. A deep trough (14) containing water (15) is located beneath the conveyor system and is provided with electrical heating elements (16) [only one of which is illustrated] to heat the water and produce steam. The sides of the trough (14) extend upwardly to just below the conveyor belts (7,8) so as to form a largely enclosed steam chamber (18) beneath the glass surface (12).

In use, a glass sheet (1) is conveyed through the unit by the driven belts in a direction away from the viewer. Steam from the chamber (18) condenses on the lower glass surface (12) and forms a continuous layer of water thereon. Steam also condenses on the conveyor belts (7,8) thereby extending the water layer to the edges 6 of the sheet (1). A light beam from the lamp (2) passes in the direction indicated by the lines with single arrows and strikes obliquely the upper surface (11) of the sheet (1). The beam is thereby reflected in the direction indicated by the lines with double arrows to form an image on the screen. The image is observed by the camera (6) over the field indicated by the lines with triple arrows and the digital signal so produced is fed to a process controller (not shown) to identify any microcorrugation of amplitude greater than 80 rim and thereby to classify the sheet into the categories of satisfactory, curable by post- treatment, or reject.

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Claims (18)

1. A surface defects checking-unit for glass sheets, which unit comprises a light source, a screen and a light detection device, the light source being disposed to direct light onto a face of the sheet to be checked and by reflection therefrom to form on the screen an image for inspection by the light detection device, characterised in that the unit further comprises means for depositing on the face of the sheet opposite to the face to be checked from a liquid source spaced apart from the said opposite face a layer of liquid having a refractive index close to that of the glass.

2. A unit as claimed in claim 1, which includes a diffuser disposed and operated as to create said layer of liquid over the whole of the area of the face of the 10 sheet opposite to the area of the face to be checked.

3. A unit as claimed in claim 2, in which the diffuser comprises an open reservoir of liquid.

4. A unit as claimed in claim 2, in which the diffuser comprises a valved pressurised reservoir of liquid.

5. A unit as claimed in any preceding claim, which includes a conveyor system on which the sheet to be checked is conveyed past the light source and light detection device.

6. A unit as claimed in any preceding claims, which comprises a light scanning system.

7. A unit as claimed in any preceding claim, in which the light detection device is a still camera.

8. A unit as claimed in claim 7, in which the light detection device is an electronic camera having a number and size of pixels adapted to the required sensitivity of the detection.

9. A unit as claimed in any preceding claim, comprising more than one light detection device, each such device being aligned to observe on the screen a different portion of the image of the sheet face to be checked.

10. A method for checking surface defects on glass sheets, in which a light source is disposed to direct light onto a face of the sheet to be checked and by reflection therefrom to form on a screen an image for inspection by a light detection device, characterised in that a layer of liquid having a refractive index close to that of the glass is deposited indirectly on the face of the sheet opposite to the face to be checked.

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11. A method as claimed in claim 10, in which the layer is applied by means of a diffuser disposed and operated as to c,-,:ate the layer over the whole of the area of the face of the sheet opposite to the area oi the face to be checked.

12. A method as claimed in claim 11 in which the layer is obtained by condensing vapour.

13. A method as claimed in claim 12, in which the vapour is produced by evaporating liquid in an open reservoir.

14. A method as claimed in claim 10, in which the layer is obtained by deposition of small droplets.

15. A method as claimed in claim 14, in which the droplets are supplied from a pressurised valved reservoir of liquid.

16. A method as claimed in claim 14 in which the droplets are supplied from an aerosol generator.

17. A method as claimed in any of claims 10 to 16, in which the liquid is water.

18. A method as claimed in any of claims 10 to 17, in which the light source provides a pencil of light which scans the glass surface.