GB2117897A

GB2117897A - Detecting surface defects

Info

Publication number: GB2117897A
Application number: GB08305847A
Authority: GB
Inventors: Robert Noel West; Geoffrey Bernard Dunn
Original assignee: Sira Ltd
Current assignee: Sira Ltd
Priority date: 1982-03-03
Filing date: 1983-03-03
Publication date: 1983-10-19
Also published as: GB2117897B; GB8305847D0

Abstract

A method and apparatus for detecting discontinuities in a radiation diffusing surface 100 in which radiation from the surface is collected 118A 119A from an angle 123A other than normal. In one arrangement, used to detect creases, if no creases are present, then according to Lambertian law, the maximum radiation will be radiated normal to the surface 100 and so if there are creases in the surface there will be an increase or decrease in the amount of radiation radiated at that angle delta to the normal. In an alternative arrangement, used to scan an image on a sheet which might include discontinuities 101 in the sheet, the above method is applied in two separate directions 123A, 123B, and the signals from the two separate directions compared 121, the signals being passed if they are identical or substantially identical and being attenuated if they are different. <IMAGE>

Description

SPECIFICATION Optical apparatus and method for examining an object The present invention relates to an optical apparatus and method for examining an object.

Many methods for examining objects optically have been proposed and they frequently comprise means for scanning radiation across the object under examination and collecting the radiation influenced by the object to determine any faults or flaws in the object.

One particular problem which arises with the manufacture of sheet material is that creases can occur in the material and these have hitherto been difficult, if not impossible, to detect by optical inspection apparatus.

Another problem arises in the production of newspapers. Before printing, a mock-up of the newspaper is produced which comprises a sheet of paper the size of the finished product and glued to this paper are a number of other pieces of paper with the various articles which are to appear on tha page. The reason they are produced separately is that they can be moved around to change the layout during the planning of that page of the newspaper. A problem arises in printing the newspaper from this mock-up in that where the separate pieces of paper are stuck on to the large sheet of paper there are provided discontinuities in the form of edges.This problem is particularly acute in a case where the image of that page is to be transmitted to a remote location in the form of a linear digital (or analogue) signal which is produced by scanning across the page and detecting, during the scan, the light signal reflected or otherwise influenced by the surface of the paper. Clearly one wishes to pass signals which correspond to the blank paper and to the print since that will reproduce the image but it is possible with this system that the discontinuous edges between the sheets of paper which have been stuck on to the large sheet of paper will also be reproduced and this is clearly undesirable.

The present invention provides, according to a first aspect, optical apparatus for detecting discontinuities in a radiation diffusing surface comprising means for passing radiation to the surface under examination, and radiation collection means arranged to receive the diffused radiation from a limited area of the surface, said radiation collection means including means to receive diffuse radiation from said limited area only in one or more predetermined directions other than normal to said surface and means for moving said limited area across said surface.

The present invention provides, according to a second aspect, apparatus for producing a signal corresponding to an image on a radiation diffusing surface comprising means for passing radiation to the surface under examination, and radiation collection means arranged to receive the diffused radiation from a limited area of the surface, said radiation collection means including two means to receive diffuse radiation from said limited area only in respective predetermined directions, means for comparing the signals produced by said radiation receiving means arranged to operate so that if the two signals are substantially identical then the signal is transmitted and if the two signals are different then the signal is not transmitted, and means being provided for moving said limited area across said surface.

The present invention provides, according to a further aspect of the invention, optical test apparatus for detecting creases in a radiation diffusing surface comprising means for passing radiation to the surface under examination, and radiation collection means arranged to receive the diffused radiation from a limited area of the surface only when there is a crease within said limited area, said radiation collection means including means to receive diffuse radiation from said limited area only in a direction other than normal to said surface and means for moving said limited area across said surface.

In normal circumstances, it is weil understood that radiation incident and reflected or transmitted by the surface of an object, in such a way that the intensity of the reflected or transmitted light is proportional to the cosine of the angle between the normal to the surface and the direction from which the light is viewed, this law being referred to as the Lambertian law. Thus the maximum intensity of light is reflected or transmitted in a direction normal to the surface and a minimum intensity is reflected or transmitted at right angles to the surface. In practice, this relationship relates only to a perfect diffuser or transmitter of light and is subject to variation depending upon the extent which, for example, in the case of reflection, the surface provides specular reflection, in which case the intensity of the reflected light will depend upon the angle of the incident light.

Further, through the specification we shall refer to "light" and "optical". The preferred arrangement of the invention uses visible light, but it is possible to operate the invention with radiation of other wavelengths such as infra red or ultra violet and the words light and optical should be interpreted accordingly.

The radiation passed to the object is preferably scanned across the object and may be in the form of a beam of radiation.

Radiation collection means may include radiation guides arranged to collect radiation from said second direction and the radiation guides may comprise louvres which define the angle from which radiation is collected or may comprise radiation guides of a transparent material such as polymethyl methacrylate (Perspex R.T.M.).

As an alternative, there is now available a light control film manufactured by 3M which will only transmit light at a predetermined angle to its surface and this forms a very convenient way of providing the louvres.

The present invention also provides a method for detecting discontinuities in a radiation diffusing surface comprising passing radiation to the surface under examination, collecting diffused radiation from a limited area of the surface in one or more predetermined directions other than normal to said surface, and moving said limited area across said surface.

The present invention provides according to a further aspect a method for producing a signal corresponding to an image on a radiation diffusing surface comprising passing radiation to the surface under examination, collecting the radiation diffused by a limited area of the surface in two predetermined directions, producing respective signals in response to the radiation received and comparing the signals, and transmitting a signal if the two compared signals are substantially identical and not transmitting a signal if the two signals are different, and moving said limited area across said surface.

The present invention also provides a method for detecting creases in a radiation diffusing surface comprising passing radiation to the surface under examination, collecting diffused radiation from a limited area of the surface in a direction other than normal and moving said limited area across said surface.

Preferred 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 in diagrammatic form incident light and reflected light and the effect of a crease in the sheet material, Figure 2 shows in diagrammatic form incident light and transmitted light and the effect of a crease in the sheet material, Figure 3 shows in diagrammatic form an apparatus for carrying out the invention, Figure 4 and Figure 5 show alternative arrangements of part of Figure 3, Figure 6 shows a plan view of part of the apparatus of Figure 3, Figure 7 shows examples of the signals detected, Figure 8 shows in diagrammatic form an alternative arrangement of part of Figure 3 comprising a second embodiment of the invention, Figure 9 is a view of the apparatus of Figure 8 at right angles thereto, Figure 10 shows typical signal outputs from the apparatus of Figures 8 and 9, Figure 11 shows in diagrammatic form, part of the apparatus illustrating the angle over which light is collected, Figure 12 shows a plan view, in diagrammatic form, of an alternative form of the apparatus according to the invention for use in scanning a mock-up of a newspaper page, and, Figure 1 3 shows a side view of the apparatus of Figure 12, with part of the electronic apparatus added diagrammatically.

There is illustrated in diagrammatic form one aspect of the invention in Figure 1. The object to be examined comprises a sheet 11 which may be paper, textile or the like having a light diffusing surface. The sheet 11 is scanned by a collimated beam of light 12 which is scanned in the direction of arrow 1 3. (In an alternative arrangement one might use a high intensity background source of light). The beam 12 therefore moves from left to right across Figure 1.

In the arrangement of Figure 1 the incident beam 12 is reflected by the sheet 11 and the sheet 11 is of a generally diffuse nature so that the reflected scattered light forms a profile 14 according to the Lambertian Law. Profile 14 Figure 1 is a plot of the reflected radiation against the angle of reflection. The distribution of the reflected light across the profile 14 reaches a peak on the axis 16 of the profile 14.

The sheet 11 illustrated in Figure 1 includes a crease 1 7 which extends generally across the scan line 13. As a result, the direction in which the axis 1 6 of the profile extends will vary as the beam 1 2 which is normal to the sheet is scanned across the crease. In the first position A, before the crease is reached, the axis 1 6 of the profile 14 is normal to the sheet 11.

In the position B where the beam 12 strikes the crease 17, the axis 16 of the profile 14 is displaced to an angle of approximately 450 although the angle will depend upon the angle of the sheet at that particular point.

In the position C the axis 1 6 extends in a direction generally opposed that of the axis 16 at position B and when the incident beam 12 has reached position D then the axis 16 has returned to the same direction as position A.

If reflected radiation were to be collected in such a way that only radiation reflected in the direction E were collected then a smaller amount of light would be collected when the beam 12 was in position A a minimum amount of light would be collected at position B, a maximum amount of light would be collected at position C, and a smaller amount of light would be collected at position D. The presence of a minimum and a maximum, which is present if there is some kind of a crease in the sheet material since there is one part of the crease which will direct reflected light away from the direction E and one part of the crease which will direct reflected light towards the direction E, will indicate the presence of a crease.

Figure 2 illustrates the same principle but in respect of light which is transmitted through the sheet rather than reflected from the sheet. In this case the profile 14 is similar to that of profile 14 in the case of reflected light, but is of course on the opposite side of the sheet. Once again, the maximum amount of light is transmitted at an angle which is normal to the sheet.

A preferred apparatus for carrying out the invention will now be described with reference to Figure 3 which is in diagrammatic form only.

The scanning beam 12 is provided by means of a laser 21 which produces a beam 22 of radiation directed at a rotating multi-facetted mirror drum 23, the beam thereby being scanned to produce the incident beam 12. The sheet material 11 moves in a direction indicated by arrow 24.

Disposed below the scan line 1 3 on the sheet 11 is a radiation or light collecting means 26 in the form of a plurality of louvres 40 arranged at an angle a of 450 to the plane of the sheet 11. The effect of this is that only light in the direction of approximately 450 will pass through the louvres 40 which are of blackened sheet material. Light passing through the louvres 40 reaches a light diffusing material 32 and light from the diffusing material 32 is collected by a light collector 33.

The light collecter 33 passes the light from the diffusing material 32 to a photosensitive detector 34, the output signal of which is passed to signal processing means 36. The light collector 33, detector 34 and signal processing means 36 may comprise a system similar to that illustrated in British Published Patent Application 201 8985 the light being collected by means of transparent light guides 22 and passed to a photodetector 36.

The apparatus described with reference to Figure 3 operates as foilows. As the sheet 11 moves in the direction 24, the rotating mirror drum 23 rotates in the direction of the arrow and the beam 12 is scanned transversely across the sheet 11 in the direction of arrow 13. Matters can be arranged so that all of the sheet 11 is inspected by the beam 12.

Under normal circumstances when there is no crease present, most of the light transmitted by the sheet 11 will fall on the louvres 40 and will be absorbed by the light absorbing walls of the louvres 40. Only that proportion of the light which is at 450 to the normal will pass through the louvres 40 and pass through to the diffuser 32 and hence to the light collector 33. Figure 7 shows the signals produced by the photosensitive detector 34 and during this stage when the beam 12 is scanning across a non-creased portion of the sheet 11 the signal level will be as indicated at 41.

However, as the beam passes across the crease 1 7 there will be an initial displacement of the axis 1 6 in the way shown in position B of Figure 2 in which case the axis 1 6 normal to that part of the sheet from which the light is being transmitted will be directed almost at right angles to the direction through the louvres 40 and hence a reduced amount of light will fall on the diffusing materials 32 and the signal produced by the photosensitive detector 34 will decrease as shown at 42 in Figure 7.

As the beam then passes further across the crease 17 there will be a displacement of the axis 16 in the way shown in position C of Figure 2 so that the axis 1 6 generally will be more likely to coincide with the angle a of the louvres 40 and hence an increased amount of radiation will pass through the louvres 40 to the diffusing material 32 and the signal produced by the photosensitive detector 34 will increase as shown by the increased level at 43 in Figure 7.

As the beam 12 scans across the crease further the axis 1 6 will move to the direction shown in position B once again (i.e. as it passes between positions C and D) so that in that position a reduced amount of radiation will pass through the louvres 40 and the signal level will drop as shown at 44 in Figure 7.

After the beam 1 2 has passed back to a normal part of the sheet 11, the signal level will return to its original level. In the signal processing means 36 there is provided two signal channels, one channel providing an output 46 when that instantaneous value is greater than the upper limit 47 of the normal level 41 and the other channel producing an output 48 when the instantaneous value is less than the lower limit 49 of the average value signal 41. Flaws in the sheet 11 which, for example, absorb or reflect the radiation (such as a grease spot), will cause only a reduction in the light transmitted through the sheet and will only produce the signal level drop 42 so that only the signal 48 will be produced and not the signal 46.

It requires a crease to provide, in juxtaposed relationship, signals 48, 46, and 48.

Alternative arrangements are illustrated in Figures 4, 5 and 6. In Figure 4 in place of the louvres 40 there are provided a plurality of transparent prisms 27 of polymethyl methacrylate (Perspex R.T.M.). Prisms 27 are each of identical shape and comprise identical skew rectangular prisms and are arranged so that their long faces abut one another to form a stack. A radiation absorbing layer is provided between each prism 27 which may be in the form of a matt black coating on the long face of each prism. Thus only the upper and lower surfaces of the prisms 27 need be transparent. This provides, effectively, louvres of zero thickness. The upper surfaces 28 of the prisms 27 form a continuous radiation collection surface 29.The angle p at which the light from the sheet material 11 strikes the continuous radiation collection surface 29 may be 450 in which case the angle of the prisms P is 620 to take into account the refraction of the light at the continuous radiation collection surface 29.

The bottom surface of the prisms 27 form a continuous transmission surface 31, below which is provided the diffusing material 32 and light transmitted by the diffusing material 32 is passed to the light collector 33 as before.

There are considerable practical advantages using prisms as shown in Figure 4 rather than louvres shown in Figure 3. It is complex and expensive to produce a large number of louvres 40 which are closely spaced from one another as is necessary for the apparatus to operate satisfactorily whereas the louvres can effectively be provided by the matt black surfaces between the prisms 27 in the arrangement of Figure 4 and a very large number of prisms can readily be provided by cutting and stacking together very thin sheets of perspex. Because the perspex sheet has parallel sides then there is no difficulty in maintaining the effective louvre surfaces between the prisms in parallel with one another whereas difficulties are experienced in the arrangement of Figure 3 in maintaining the louvres parallel to one another.

In the arrangement of Figure 4 the radiation leaving the surface 31 strikes the diffuser 32 at an angle. If it is required to eliminate this the bottom surface of each prisms 27 may be shaped as shown in Figure 5 so that the radiation strikes the diffusing material 32 normally.

Figure 6 shows a plan view of the louvres 40 or prisms 27 from which it can be seen that there are provided two sets of louvres 40 or prisms 27 disposed side by side, the louvres or joints between the prisms being displaced with respect to one another. This is arranged because it has been found in practice that there is some variation in the signal received as the light beam 12 is scanned across the sheet 11 for the reasons which become clear from Figure 11. In Figure 11 when the beam is in position H, then a large proportion of the light (i.e. that within the angle 61) will pass through the apparatus to the diffusing material 32.In the position J, however, light within the angle 62 will pass through the apparatus to the diffusing material 32 and clearly this angle is smaller than 61. As a consequence, there is a small variation of the light passing through the apparatus as the beam is scanned across the sheet 11, and this can be eliminated by arranging the louvres 40 or prisms 27 as shown in Figure 6, the light striking one line of prisms or louvres passing through at an angle 61, whilst the light striking the other line of prisms or louvres passing through at an angle 62. In this way the two lines of louvres 40 or prisms 27 compensate for one another to produce an even signal.

If the sheet 11 contains an internal structure (in the case of paper contains fibres) or a surface pattern which gives rise to successive dark and light area in the normal surface then clearly a signal of the type shown in Figure 7 might be produced. This can however be eliminated by the apparatus shown in Figure 8. In this case, in place of a single (or double) line of louvres 40 or prisms 27 extending in one direction there is provided a second line of louvres or prisms extending at the same angle with respect to the direction of scan but in the opposite direction. The first set of louvres/prisms is here indicated by 27A and the second line of louvres/prisms by 27B.

Each line of louvres/prisms has a separate light collector 33 and photo sensitive detector 34 and the output signal from the two photo sensitive detectors 34 is indicated in Figure 10, the output from 27A (channel A) being shown on the upper part of Figure 10 and the output from louvres/prisms 27B (channel B) being shown on the lower part of Figure 1 0. The output humps and troughs 42 and 43, 44 associated with a crease will of course be in opposite phase with respect to one another since an increase in the light passing through the prisms 27A in one direction will coincide with a decrease in the light passed through the louvres/prisms 27B in the other direction and vice versa.Subtraction of the signals in the two channels A and B will therefore produce the signal shown at the bottom in Figure 10 since under those circumstances the humps 43 and troughs 42 and 44 reinforce one another. Any internal structure or pattern, will however, create identical signals on the two signal channels e.g.

trough 50, which on subtraction of the two signals will be eliminated.

The principle described with respect to Figure 10 may be utilised in the manner illustrated in Figures 1 2 and 1 3. As described above there is a requirement to transmit the image on the surface of a mock-up newspaper page, normally by means of an electrical signal and it is desired to eliminate any effect on the signal produced by discontinuities in the form of edges between adjacent pasted up parts of the sheet. It will be well understood that as radiation is scanned across the sheet then a discontinuity in the form of an edge will produce shadows or highlights depending upon the relative dispositions of the radiated beam and the edge.

In Figure 12 there is illustrated a sheet 100 which comprises a mock-up of a page of the newspaper and pasted onto that sheet 100 are smaller sheets 101, 102 and as described above problems can arise with the edges 103, 104, 105, 106, 107 for example of the sheets 102 and 101. Line 110 illustrates the scan line in which a light beam such as a laser beam is scanned across the sheet 100 and the sheet 100 is moved in the direction of arrow 111 so that the whole of the surface of the sheet 100 will be scanned by successive scan lines 110. The point 112 illustrates the position of the beam of light at a particular instant.As can be seen, the beam 112 is directed downwardly as is clear from Figure 13 and may be scanned along the scan line 110 by means of a rotating mirror drum as is conventionally known. Mounted above the sheet 100 and parallel to the scan line 110 are two sheets of light control film 114 manufactured by 3M and which have alternate light and dark bands which act as louvres of the type shown in, for example, Figure 3. The preferred direction of transmission is shown by the lines 1 5 in Figure 12.The two films 1 4A and B are disposed so that, when viewed in Figure 13, light reflected from the scan line 110 and passing upwardly at an angle a of 45 will pass to the films 1 4A and 11 4B. The disposition of the "louvres" in the films 1 4A and 1 4B is arranged such that light reflected from the scan line and travelling, in Figure 12, at an angle E thereto will pass through the films 1 4A and 1 4B. Behind the films 1 4A and 1 4B are provided light collecting means in the form of perspex guides 1 8A, 1 8B having front faces coterminus with the films 1 4A, 1 4B respectively and rear faces coterminus with light detectors 1 9A, 1 9B respectively. Signals from the light detectors 11 9A and B are passed to a comparator circuit 121 and the output from the comparator circuit is passed along the output line 122.

The apparatus of Figures 12 and 13 operates as follows. In normal circumstances as the beam 112 is scanned along the scan line 110 the incident radiation is reflected according to the Lambertian law and thus the light passing from the surface of sheet 100 and passing to the films 11 4A and 1 14B along the paths 1 23A, 1 23B should be identical and thus the light detected by the light detectors 11 9A and 11 9B will be identical. When the scanned beam 112 reaches part of the image on the page, that is a printed letter, then the signal passed to the light detectors 11 9A, 11 9B will simultaneously reduce.The comparator circuit 121 compares the signals from the two light detectors 11 9A, 11 9B and so long as they are the same or substantially the same passes that signal to the output line 122.

However, when the beam 112 reaches a discontinuity formed by one of the edges 103 to 107 if for any reason there is produced a shadow or a bright spot, then because of the discontinuity in the surface the light reflected from that edge will not be of Lambertian nature and will therefore have a preferred direction. In that case, the light signal passed along the beam paths 1 23A and 1 23B will be different (it does not matter whether one is greater or less than the other) and thus the light received by the light detectors 11 9A and 11 9B will be different. The comparator circuit 1 21 will detect this difference and will not pass the signal to the output line but it may be arranged, for example, to maintain the same signal level as previously. Generally of course the signal level will be that for a plane area carrying no image and so that signal will continue to be passed to the output line 122.

Claims

1. Optical apparatus for detecting discontinuities in a radiation diffusing surface comprising means for passing radiation to the surface under examination, and radiation collection means arranged to receive the diffused radiation from a limited area of the surface, said radiation collection means including means to receive diffuse radiation from said limited area only in one or more predetermined directions other than normal to said surface and means for moving said limited area across said surface.

2. Apparatus as claimed in claim 1 in which scanning means is provided to scan the radiation across the surface.

3. Apparatus as claimed in claims 1 or 2 in which the radiation is in the form of a beam of radiation.

4. Apparatus as claimed in claim 3 in which the beam is directed at the surface, and the radiation collected is reflected from the surface.

5. Apparatus as claimed in claim 3 in which the beam of radiation is directed at the rear surface of the surface under examination and the radiation detected is transmitted through the surface under examination.

6. Apparatus as claimed in any of claims 1 to 5 in which the radiation collection means includes radiation guides arranged to eliminate normal radiation and collect radiation from said one or more directions other than normal.

7. Apparatus as claimed in claim 6 in which the radiation guides comprise louvres which define the angle from which the diffused radiation is collected.

-

8. Apparatus as claimed in claim 7 in which the radiation guides comprise transparent material.

9. Apparatus as claimed in claim 8 in which the transparent material is polymethyl methacrylate.

1 0. Apparatus for producing a signal corresponding to an image on a radiation diffusing surface comprising means for passing radiation to the surface under examination, and radiation collection means arranged to receive the diffused radiation from a limited area of the surface, said radiation collection means including two means to receive diffuse radiation from said limited area only in respective predetermined directions, means for comparing the signals produced by said radiation receiving means arranged to operate so that if the two signals are substantially identical then the signal is transmitted and if the two signals are different then the signal is not transmitted, and means being provided for moving said limited area across said surface.

11. Optical test apparatus for detecting creases in a radiation diffusing surface comprising means for passing radiation to the surface under examination, and radiation collection means arranged to receive the diffused radiation from a limited area of the surface only when there is a crease within said limited area, said radiation collection means including means to receive diffuse radiation from said limited area only in a direction other than normal to said surface and means for moving said limited area across said surface.

12. Apparatus as claimed in claim 1 substantially as hereinbefore described with reference to the accompanying drawings.

13. A method for detecting discontinuities in a radiation diffusing surface comprising passing radiation to the surface under examination, collecting diffused radiation from a limited area of the surface in one or more predetermined directions other than normal to said surface, and moving said limited area across said surface.

14. A method for producing a signal corresponding to an image on a radiation diffusing surface comprising passing radiation to the surface under examination, collecting the radiation diffused by a limited area of the surface in two predetermined directions, producing respective signals in response to the radiation received and comparing the signals, and transmitting a signal if the two compared signals are substantially identical and not transmitting a, signal if the two signals are different, and moving said limited area across said surface.

1 5. A method for detecting creases in a radiation diffusing surface comprising passing radiation to the surface under examination, collecting diffused radiation from a limited area of the surface in a direction other than normal and moving said limited area across said surface.

1 6. A method as claimed in claim 12 substantially as hereinbefore described.