GB2054835A - Scanning Apparatus for Flaw Detection - Google Patents

Scanning Apparatus for Flaw Detection Download PDF

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Publication number
GB2054835A
GB2054835A GB7926428A GB7926428A GB2054835A GB 2054835 A GB2054835 A GB 2054835A GB 7926428 A GB7926428 A GB 7926428A GB 7926428 A GB7926428 A GB 7926428A GB 2054835 A GB2054835 A GB 2054835A
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Prior art keywords
grating
beams
radiation
means
light
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Granted
Application number
GB7926428A
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GB2054835B (en )
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SIRA INSTITUTE
SIRA INSTITUTE Ltd
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SIRA INSTITUTE
SIRA INSTITUTE LTD
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Classifications

    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/84Systems specially adapted for particular applications
    • G01N21/88Investigating the presence of flaws or contamination
    • G01N21/89Investigating the presence of flaws or contamination in moving material, e.g. running paper or textiles
    • G01N21/8901Optical details; Scanning details

Abstract

Test apparatus and method for testing for faults in, for example, transparent sheet material in which two parallel beams of light separated by a distance "nw" where n is odd are scanned across the material and then passed through a grating having, light transmitting and absorbing areas of equal width "w" in the direction of scan, so that as the two beams are scanned across the object and the grating the light output through the grating is generally constant. If, however, there are faults which displace one beam with respect to the other, then the spacing between the beams will change so the light output will vary from the norm. Furthermore, if there is a fault which would cause attenuation of the two beams, then the light output will again vary from the norm. In alternative embodiments, light may be reflected from the material, the grating may have reflecting and transmitting areas and a grating may be provided for each beam. <IMAGE>

Description

SPECIFICATION Optical Test Apparatus for Examining an Object The present invention relates to test apparatus for testing an optical system.

According to one aspect the invention comprises optical test apparatus for examining an object comprising means for providing two beams of radiation, grating means for each beam, each grating means being identical and comprising alternate areas which influence the radiation in different ways, successive areas being of substantially the same width in the direction of scan, the beams and/or the grating means normally being spaced relative to one another by a total distance in the direction of scan equal to an odd number of widths, means for scanning the two beams across their respective grating means and across the object, and collector means for collecting the beams of radiation after they have been influenced by both grating means and the object.

The alternate areas of the grating means may comprise radiation absorbing and radiation transmitting areas or radiation absorbing and radiation reflecting areas or radiation transmitting and radiation reflecting areas.

Effectively the beams and gratings are arranged so that so long as the object under test does not divert one beam relative to the other then the radiation collected by the radiation collector will be generally constant. The reason for this is that since the beams and/or the grating means are normally displaced relative to one another by a total distance in the direction of scan equal to an odd'number of widths, as one beam enters a first one of the alternate areas then the second beam will enter a second one of the alternate areas. If, however, the object under test influences the two beams so as to vary the distance in the direction of scan between them or by attenuating the beams, for example because of faults in the object, then this will result in a change of radiation received by the radiation collector.

Thus the faults may be detected by considering the radiation collected by the collector. It will, however, be understood that what might be considered to be a fault in one application, may not be considered so in another application, but for the sake of clarity these differences from norm will be referred to hereafter as faults.

The respective grating means for each beam may be provided by a single grating or by two gratings in phase with one another, or by two gratings out of phase with another for example, by a distance equalling the width.

The source of radiation preferably comprises a laser.

The invention also provides a method of examining an object comprising passing two beams of radiation so as to be influenced by the object and by respective grating means, each grating means being identical and comprising alternate areas which influence the radiation in different ways, successive areas being of substantially the same width in the direction of scan, the beams and/or the grating means normally being displaced relative to one another by a total distance in the direction of scan equal to an odd number of widths, scanning the beams of radiation across both the object and the grating means, and collecting and analysing the radiation influenced by both the grating means and the object.

By analysing the radiation collected it is possible to detect faults in the object and in some circumstances identify them.

The object may comprise, for example, a transparent material in which case the radiation may be passed through the material before passing to the grating means or may comprise reflective material in which case the radiation may be reflected from the material before being passed to the grating means.

Such optical test apparatus may be used in manufacturing processes to inspect material and to reject, for example, faulty material.

The collector means may take various forms. If it is convenient, the radiation passing through the grating may be collected from behind the grating means. Alternatively, reflective, diffuse or retroreflective means may be provided behind the grating means to reflect radiation back through the grating means and means may be provided to separate the incident radiation from the reflective radiation by a suitable beam splitter. Because of the diffuse nature of the radiation beam reflected back through the grating means, the grating means has no overall effect apart from minor attenuative effect.

Throughout the specification the term "radiation" is intended to not only include visible wavelengths which will be referred to as light but also ultra-violet, infra-red and other wavelengths.

The term "optics" and "optical" should be similarly interpreted.

Various embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows a front diagrammatic view of an apparatus according to the invention, Figure 2 is a side view of the apparatus of Figure 1, Figure 3 is a diagram illustrating the principle of the invention, Figure 4 is a diagram of the output signal produced by the apparatus, Figures 3 and 4 being vertically aligned, and, Figures 5 to 9 show diagrammatically various relative arrangements of beam and grating means.

The invention will be most readily understood with reference to Figures 3 and 4. In Figure 3 it will be appreicated that two generally parallel beams 11, 1 2 of radiation are scanned across an object under examination in the form of a transparent sheet 1 3.

Various faults may be present in the sheet 13 such as bubbles, surface damage, and absorption defects, the faults being exaggerated in Figure 3 so as to be clear. Mounted below the sheet 13 is a pattern in the form of a grating 14. The grating 14 comprises a regular linear grating of alternate radiation transmitting areas 1 5 and radiation absorbing areas 1 6 of the same width "w" and at least at the grating 14 the two beams 11, 12 are spaced apart by the distance 'to".

At the extreme left side of Figure 3, the beams 11, 12 pass through a normal part of the sheet 13 and as they are scanned from left to right the two beams 11, 12 pass alternately through the light transmitting areas 1 6 and the light absorbing areas 17 of the grating 103. The two beams will have a cross section comprising a portion of the distance "w" (for example, the-beams might have a cross section in the range w/1 0 to w/2 and preferably w/2) and so as one beam passes into the light transmitting area 16 the other beam will simiiarly be passing into a light absorbing area 1 7 and the net amount of radiation passed through the grating will remain substantially constant.

Thus the light collected by the light detector 18 is substantially constant and this produces a generally constant output signal as is indicated in .Figure 4 below the extreme left portion of the sheet 13.

Considering now the fault 19 shown in Figure 3, this distortion will produce a change in the path of the beams 11, 12 as they pass through the fault. As the first beam reaches the fault it will be distorted away from its original position and because the two beams are not now spaced by distance "w" light from both beams 11, 12 or from neither beam 11, 12 will pass through the grating 14 and fall on the light detector 18.

During the passage of the two beams 11, 12 across the fault they will generally not be spaced by the distance "w" (or by an odd multiple thereof) and thus either both beams will pass through the grating or neither will pass through the grating at least at some time. The distortion fault 19 will therefore be readily indicated by observing and measuring the variation of output of the light detector 1 8 as is indicated in the middle of Figure 4.

A second type of fault often encountered is a light absorbing area indicated at 21 in Figure 3 and in this area 21 light from neither beam will pass through the sheet 13 or at least will be attenuated and the output of the light detector will fall from its normal value. This is indicated on the right hand side of Figure 4.

At this stage it will be understood that according to the principle of the invention the two beams should be spaced apart with respect to their grating means by the width of one of the light absorbing or transmitting areas.

The arrangement so far described is shown with reference to Figure 5 which shows the grating 14 and the two beams 11, 12 spaced on the left hand side of Figure 5 by the distance "w" and in an alternative arrangement at the right hand side by a distance equal to 3w.

Other arrangements are clearly possible. In Figure 6 it will be seen that two cases sirpilar to those in Figure 5 are shown except that the two beams 11, 12 are displaced at right angles to the ine of scan 25. There are certain advantages in carrying out the method in this way.

The arrangement in Figure 7 is the same as in Figure 6 except that the grating has been split into two portions 1 4A and 1 4B which are displaced at right angles to the line of scan.

Clearly the two portions of the grating can be displaced by as great a distance as is required.

In the arrangements described so far and, in particular in Figure 7, it will be understood that the two grating portions 1 4A, 1 4B are maintained so that the distance between any two corresponding areas in the direction of scan is zero. However, it is not necessary to maintain this relationship. For example in Figure 8 one grating portion 1 4A has been displaced in the direction of scan with respect to the other grating portion 1 4B by the width w. In these circumstances it is then necessary to displace the two beams 11, 12 by the distance w so as to maintain the same relevant relationship between the two beams and the grating portions. In practice, of course, the two grating portions may be displaced relative to one another by any odd number of widths. In other words, one width, three widths, five widths and so on.

Figure 9 shows another possible case in which the two grating portions have been displaced by half of a width and the two beams 11, 12 have also been displaced by half a width so as to again maintain the same relative relationship between the grating and the beams. The total displacement between the two grating- portions and the two beams must be equal to the width w or an odd number of widths, (2n+1)wwhere n is any number. Thus in a further arrangement, not shown, one might displace the two grating portions by some part of a width w and the two beams must be displaced by a distance which meets the above relationship, that is, that total relative displacement of the two grating portions and the two beams must be an odd number of widths.

Figures 1 and 2 illustrate the preferred apparatus of the invention which corresponds to Figures 3, 4 and 5. The apparatus is particularly adapted to examine for faults a strip of transparent material being produced by a manufacturing process.

The apparatus comprises a laser (not shown) for producing a beam which is split by a beam splitter to form the two essentially parallel beams 11,12.

The two beams 11, 12 are reflected from a mirror drum scanner 22 onto a cylindrical mirror/beam splitter 23. The cylindrical mirror 23 reflects the light from the mirror drum scanner 22 so as to scan the beams 11, 12 transversely across the sheet 13 under inspection.

in Figure 1 the length of the strip or sheet 13 extends out of the plane of the paper and the sheet moves in a direction along its length. The mirror drum scanner 22 thus causes the light beams 11, 12 to -scan from one side of the sheet 13 to the other as shown in Figure 1.

The light transmitted by the sheet 1 3 is passed through the linear grating 14 on to a retroreflective layer 24 which extends behind the grating 14. The light incident on the retroreflective layer 24 is retroreflected (that is is reflected back along the incident path with slight scatter) and passes back to the grating 14.

However, because of the scattering effect of the retroreflective sheet the light passes through the grating without any similar effect of that of the incident beam, the grating merely slightly attenuating the reflected light. The reflected light is passed back to the cylindrical mirror/beam splitter 23, through a cylindrical lens 26 to be focused on to a photomultiplier 27.

The grating 1 4 is arranged so that its alternate light transmitting areas and light absorbing areas 16, 1 7 extend outof the plane of the paper in Figure 1 and parallel to the plane of Figure 2. The use of a cylindrical mirror 23 allows for focusing of the light beams in the plane of Figure and separate focusing in the plane of Figure 2 so that the surface inspected sheet 1 3 are at conjugate points in the plane of Figure 2.

The mode of operation of the apparatus of Figures 1 and 2 will be readily apparent after consideration of Figures 3 and 4. By rotating the mirror drum scanner 22 the beams 11, 12 are traversed across the inspected sheet 13 in the plane of Figure 1 whilst the inspected sheet 13 moves at right angles to the plane of Figure 1 and the signal produced at the photomultiplier 27 by the light received is generally of the form shown in Figure 4.

By focusing the light at the photomultiplier 27 by means of the cylindrical lens 26 the effect of ambient light can be much reduced. In place of a retroreflective material in the arrangement of Figures 1 and 2 a matt white surface can be used although of course not so much light is reflected.

In alternative arrangements in place of a planar retroreflective layer 24 and planar grating 14 there may be provided a curved retroreflective layer and a curved grating, the centre of curvature of the layer and grating being at the mirror drum scanner 22 (taking into account the folding of the optical axis). The reason for the curving of the retroreflective layer and the grating is that, in Figure 1, as the light beams are scanned across the linear grating and linear retroreflective layer, by virtue of the different angle of incidence, the distance between the beams "w" when measured in the plane of the grating will increase and thus the width of the light transmitting and absorbing areas of the grating must be varied. With a curved grating and a curved retroreflective layer this is unnecessary as the angle at which the two beams reach the grating is always normal to the grating.

Furthermore, the beams are in focus accurately at all points along the scan when they meet the grating.

In another arrangement to overcome the problem of the distance between the beams when measured in the plane of the grating increasing as the beams are scanned across the grating, one may arrange for the two beams to be slightly nonparallel and to be-directed so as to converge so that, in the absence of the retroreflective material, they would meet at a point behind the retroreflective material. The point to which they converge should be the same distance from the grating as the distance between the grating and the mirror drum scanner 22. The effect of the two beams converging as they move away from the mirror drums compensates for the increase in distance between the two beams as they scan across the grating almost exactly so that the width between the beams in the plane of the grating remains almost constant as they are scanned across the planar grating.

In another arrangement, the grating, which may be curved may be combined with a cylindrical mirror in place of the retroreflective layer to reflect the beams back in the incident direction. The cylindrical mirror may in this instance form a grating by including areas of non reflective material corresponding to the absorbing area 1 7 and the mirror may be constructed of a flexible material such as glass or plastics which is flexed about two supports as disclosed in our British patent 1,441,386. A greater light intensity is achieved utilising this arrangement but it is more difficult to set up.

In another arrangement the radiation which is passed through the grating 14 is not reflected back along the same path by a retroreflective layer or by a mirror but is collected by suitable optical components from the side of the grating 14 opposite the sheet 13. This may be achieved by providing a cylindrical lens, the longitudinal axis of which is transverse to the plane of the scanned light and the width of which enables all of the light from the grating 14 to be collected from one end of the scan to the other. The cylindrical lens focuses the light via a second cylindrical lens having a longitudinal axis at right angles to the first cylirtdncal lens on to a photomultiplier. In this arrangement beams of radiation only pass through the grating once.

It may be arranged that the two beams are brought to a focus at the object under test since in this way even quite small defects in the object will displace the beams. It may thus be arranged that the beams are each brought to a focus at the object and are defocused to the extent that their cross section is w/2 at the grating.

The invention is not restricted to the details of the foregoing example.

Claims (17)

Claims
1. Optical test apparatus for examining an object comprising means for providing two beams of radiation, scanning means for scanning said beams across respective grating means for each beam and across the object, each grating means comprising alternate areas which influence the radiation in different ways, successive areas being of substantially the same width in the direction of scan, the beams and/or the grating means being spaced relative to one another, in the absence of faults (as hereinbefore defined) in the object, by a total distance in the direction of scan equal to an odd number of widths, and collector means for collecting the beams of radiation after they have been influenced by both grating means and the object.
2. Optical test apparatus as claimed in claim 1 in which the alternate areas of the grating- means comprise radiation absorbing and radiation transmitting areas.
3. Optical test apparatus as claimed in claim 1 in which the alternate areas of the grating means comprise radiation transmitting and radiation reflecting areas.
4. Optical test apparatus as claimed in any of claims 1 to 3 in which the collector means is arranged to collect radiation passing through the grating.
5. Optical test apparatus as claimed in any of claims 1 to 3 in which reflective, diffuse or retroreflective means is provided behind the grating means to reflect radiation back through the grating means and means is provided to separate the incident radiation from the reflected radiation.
6. Optical test apparatus as claimed in claim 1 in which the alternate areas of the grating means comprise radiation absorbing and radiation reflecting areas.
7. Optical test apparatus as claimed in any of claims 1 to 6 in which the respective grating means for each beam are provided by a single grating in phase with one another.
8. Optical test apparatus as claimed in any of claims 1 to 6 in which the respective grating means for each beam are provided by two gratings in phase with one another.
9. Optical test apparatus as claimed in any of claims 1 to 6 in which the respective grating means for each beam are provided by two gratings out of phase with another by a distance equalling the width or a multiple thereof.
10. Optical test apparatus as claimed in any of claims 1 to 9.in which the source of radiation preferably comprises a laser.
11. Optical test apparatus as claimed in any of claims 1 to 10 for inspecting a transparent object in which the radiation is passed through the object before passing to the grating means.
1 2. Optical test apparatus as claimed in any of claims 1 to 10 for inspecting a reflective object in which the radiation is reflected from the object before being passed to the grating means.
1 3. Optical test apparatus as claimed in claim 1 substantially as hereinbefore described with reference to the accompanying drawings.
14. A method of examining an object comprising scanning two beams of radiation so as to be influenced by the object and by respective grating means, each grating means comprising alternate areas which influence the radiation in different ways, successive areas being of substantially the same width in the direction of scan, the beams and/or the grating means normally being spaced relative to one another, in the absence of faults(as hereinbefore defined) in the object by a total distance in the direction of scan equal to an odd number of widths, and collecting and analysing the radiation influenced by both the grating means and the object
1 5. A method as claimed in claim 14 for inspecting a transparent object in which the radiation is passed through the object before passing to the grating means.
16. A method as claimed in claim 14 for inspecting a reflective object in which the - radiation is reflected from the object before being passed to the grating means.
17. A method as claimed in claim 14 substantially as hereinbefore described.
GB7926428A 1979-07-30 1979-07-30 Scanning apparatus for flaw detection Expired GB2054835B (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB7926428A GB2054835B (en) 1979-07-30 1979-07-30 Scanning apparatus for flaw detection

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB7926428A GB2054835B (en) 1979-07-30 1979-07-30 Scanning apparatus for flaw detection

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GB2054835A true true GB2054835A (en) 1981-02-18
GB2054835B GB2054835B (en) 1983-07-20

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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0152894A2 (en) * 1984-02-20 1985-08-28 Siemens Aktiengesellschaft Device for optically detecting local inhomogeneities in the structure of tested objects
EP0182471A2 (en) * 1984-09-24 1986-05-28 Sira Limited Inspection apparatus with a scanning beam of radiation
EP0244102A1 (en) * 1986-04-28 1987-11-04 Sira Limited Inspection apparatus
EP0408337A1 (en) * 1989-07-13 1991-01-16 De La Rue Systems Limited Sheet inspection method and apparatus
US5168322A (en) * 1991-08-19 1992-12-01 Diffracto Ltd. Surface inspection using retro-reflective light field
US5206700A (en) * 1985-03-14 1993-04-27 Diffracto, Ltd. Methods and apparatus for retroreflective surface inspection and distortion measurement

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0152894A2 (en) * 1984-02-20 1985-08-28 Siemens Aktiengesellschaft Device for optically detecting local inhomogeneities in the structure of tested objects
EP0152894A3 (en) * 1984-02-20 1987-05-06 Siemens Aktiengesellschaft Berlin Und Munchen Device for optically detecting local inhomogeneities in the structure of tested objects
EP0182471A2 (en) * 1984-09-24 1986-05-28 Sira Limited Inspection apparatus with a scanning beam of radiation
EP0182471A3 (en) * 1984-09-24 1988-09-21 Sira Limited Inspection apparatus with a scanning beam of radiation
US5206700A (en) * 1985-03-14 1993-04-27 Diffracto, Ltd. Methods and apparatus for retroreflective surface inspection and distortion measurement
EP0244102A1 (en) * 1986-04-28 1987-11-04 Sira Limited Inspection apparatus
EP0408337A1 (en) * 1989-07-13 1991-01-16 De La Rue Systems Limited Sheet inspection method and apparatus
US5084628A (en) * 1989-07-13 1992-01-28 De La Rue Systems Ltd. Sheet inspection method and apparatus having retroreflecting means
US5168322A (en) * 1991-08-19 1992-12-01 Diffracto Ltd. Surface inspection using retro-reflective light field

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Publication number Publication date Type
GB2054835B (en) 1983-07-20 grant

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