DE2206126A1 - Device for detecting defects in a surface - Google Patents

Description

2206125

PATENT LAWYERS

DIPL.-ING. LEO FLEUCHAUS DR.-ING. HANS LEYH

Munich 71,

Melchiorstrasse 42. 8.2.1972

Our reference: A 12 388 / Lh / fi

PERPANTI LIMITED

Hollinwood Lancashire

England

Device for detecting defects in a surface

The invention relates to a device for finding or determining of "spots or defects in a surface.

Devices of the type mentioned are known which use high-power lamps and photodetectors. The strength of these lamps Usually xenon lamps are often so high that the energy consumption is on the order of kilowatts, while the sensitivity required to locate small defects, even if these can still be seen with the naked eye, is not given. It is known to be the light of a lamp focus a narrow beam impinging on a moving surface, the beam being transverse to the moving surface their direction of movement scans. The light reflected from the surface is captured and scanned by a photo amplifier tube, wherein a change in the sensed level indicates a change in the intensity of the reflected light which is from originates from a defective surface.

- 1 - At

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.When a surface that scatters light or diffuse reflected is represented by the photomultiplier on σ-3 catch · "light is only a sample or a sample of the entire urface of the ^ ^K che is diffusely reflected light * 'for a small. light ¹ -en ^ e is thus collected in order to determine whether a ^ rror the reflected light changes significantly, this error or ¹⁷¹ IeCk same Grcfenordnung should have, like the ^p Liche the light image, or the face of the light image or the light point should be of the same order of magnitude as the ^p Smile Friend of the smallest Dehler, still found to v / ground.

Even when using a high power light source, for example a xenon lamp, it is only possible to focus a small part of the light on a small area of a surface. It is therefore necessary to receive light from a "relatively large area of the surface, for example from about 2ιγτί", so that the detector emits an output signal that is greater than the electrical bulk of the amplifier tube. Although an area or ¹ ^ eId of about 2raiT is required to give an output signal that is distinguishable from the noise, the edges of the image are not properly delimited and it is impossible to distinguish between a large flaw or spot and several small spots . A smaller error with an area of about 0.01mm therefore does not provide sufficient contrast for it to be detected with certainty. However, a black spot on a white surface with a size of, for example, 0.01 mm can easily be detected with the naked eye.

The invention is based on the object of providing a device of To create the type mentioned at the beginning, with which smaller errors or spots can be detected than was previously possible.

According to the invention this is achieved by a sampling stage that is movable relative to the surface to be scanned and the one

- 2 - laser

BAD ORiGSNAL 209837/1056

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Has a laser that emits a continuous beam of light, furthermore by a scanning device with a reflector which is rotatable in the path of this beam and deflects the beam, see above that it scans the surface perpendicular to its direction of movement, a detector device for receiving that from the surface diffusely reflected radiation, the detector providing an output signal that is dependent on the intensity of the radiation received emits, further through a lens system to focus the beam on a small point on the surface.

The netektor works in the visible part of the Spectrum, but it can also advantageously use a laser that emits infrared or ultraviolet radiation, where corresponding detector devices for this radiation are provided. For the sake of simplicity, in the description the radiation emitted by the laser is referred to as light.

A suitable method for determining the above-mentioned errors or rieckan is to focus the light emitted by the laser on the surface moving relative to the scanning stage, so that the light scans the surface perpendicular to its direction of orientation, whereupon the reflected light from the surface light au ^ ^ n is fan and iade ^v change in the intensity of the diffusely reflected light is measured by the detector, whereupon the? emits in output signal in the ^T 7ellenform? IGT 'anToz this change.

T-eisra:? in addition to the drawing, in the

Fla. "1 in ¹ cut is a preferred embodiment of ^{1 of} the ¹ door opening time.

Fig. 2 shows in section the detector of Fig. 1 along the line H-II.

- 3 - T = ¹ Ig.

20333 '//, 0S _{6 BAD0R} , _elNAL

2206121

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Fig. 3 shows the waveform of a typical one of the detector means noted Sianales.

Fig. 4 shows in section a modified embodiment of the invention.

FIG. 5 is a section along line V-V of FIG. 4.

FIGS. 6a, 6b and 7 show lens arrangements in connection with FIG Device according to FIG. 4.

8 shows a further preferred embodiment in section the invention.

FIGS. 9, 10, 11a and 11b show mirror arrangements in connection with the device according to FIG. 8.

Fig. 12 shows waveforms of signals from the detector means at various stages of measurement of a number of Mistakes.

The detector according to Fig. 1 'comprises a laser 1 and a photo amplifier 2, which is arranged adjacent to one end of a scanning stage 3, at the other end of which a right-angled mirror 4 is arranged, which can be linearly adjusted by turning a screw V attached to the scanning stage is attached, can be moved towards and away from the laser. In the lütte of the scanning stage and opposite an inlet-outlet opening 5, a plane mirror 6 and a scanning device in the form of a rotatable mirror 7, which has twelve facets, are arranged. Coherent monochromatic light passes from the laser 1 through a lens system 8 comprising lenses 8a and 8b between which the mirror 4 is arranged and it is reflected by the mirror 7 onto a surface 11 which moves below the scanning stage.

- 4 - Through

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By rotating the mirror 7, the beam scans over this surface and meets this surface 11 along a line RS in a plane perpendicular to the axis of rotation of the mirror 7, and between the limits, which are shown by the dash-dotted lines leading to the points R and S.

Monochromatic light that is emitted by the laser in the form of a narrow beam or that passes through a narrow opening shows interference phenomena and edge effects when it is focused on a surface through a lens. The picture is made from a central area of light that is alternately surrounded by light and dark rings. One can show that the area of the central area of light is inversely proportional to the angle opposite the image and through the perimeter of the Beam is given to the lens. By making the incident beam sufficiently divergent to fill the lens area and thereby this angle is brought to a maximum value, a small area is obtained for the central light area, which represents the effective area of the light focused on the surface. A divergence of the substantially parallel through the Laser emitted beam is achieved through the first lens 8 a of the lens system 8. This lens can be one with a low negative Exponent (concave), or with a high positive exponent (convex) that focuses the beam, whereupon it comes from the focal point diverges towards the second lens 8b. This second lens is also a convex lens and its focal length and its opening are chosen so that after the reflection of the beam by the mirror 7 a focused image is generated on the surface. The mirror 4 represents a device for adjusting the length of the light path between the two lenses 8a and 8b and serves to to focus the beam on the surface 11. With such a long path between the lens 8b and the surface 11 that has System has a long focal length so that the image is essentially unaffected by movement of the surface perpendicular to its

- 5 - walking level

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Running level remains and / or due to the increased track length of the incident Beam at the ends of the scanning path.

The laser beam is, although it diverges for optimal focus but a ray with a smaller cross-sectional area than a beam of equal intensity produced by a common lamp and it can be focused on a smaller area than that Beam from a conventional lamp. Such a small area is required when successive scans are contiguous. The high sensitivity due to the focusing of the beam on a small area and due to the high Intensity of the narrow beam on the surface to be scanned gives the possibility to scan only part of the total area of this surface. This gives you reliable information or a reliable count of both large and small defects or spots on the entire surface, since they are generally sufficiently small Errors are present so that the scanning of a tail of the area is sufficient, while the larger area of the larger error the probability their finding increased.

Laser 1 has a 1 milliwatt output and it emits light in the red part of the spectrum. The beam is focused on an area of 0.01 ram on the surface to be scanned, which corresponds approximately to a diameter of 0.1 mm. The mirror 7 rotates with its twelve facets at a speed of 24,000 revolutions per minute and thus delivers 4,800 scans per second over a scanning angle of 60 °. Since the system works with continuous adjoining scans, the speed of the area passing under the scanning stage can only be about 48 cm per second or 30 m per minute used about 300 m per minute.

The light reflected diffusely from the surface 11 is via the

- 6 - mirror

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Mirror 6 captured by the photo amplifier. Fig. 2 shows this crosswinds 12 and 13, which are used to prevent stray light from entering the detector device. This helps to distinguish between the diffusely reflected Laser light and scattered light by narrowing the observation field of the detector device in the direction of movement of the surface, the indicated by part 43. An alternative device, not shown, may, for example, be a black area which is above the scanned area, except in the area of the scanning beam. These possibilities can alternatively or additionally to be used as a filter over the photo amplifier.

If the detector is used in cases in which the surface 11 does not strongly scatter the light, for example in the case of metal or Glossy paper in which a large part of the light is reflected back in a specular manner, then becomes a light diffuser (not shown) arranged in the path of the reflected light between the surface 11 and the photo amplifier 2.

If the detector is used on a surface that reflects the light mainly diffusely, but which, for example, if the light spot hits the surface 11 at a point halfway between the mirrors 6 and 7, results in a specular reflection, so the scanning device 3 can be pivoted or inclined so that the plane of the scanning radiator is not perpendicular to the. surface 11 stands when the specularly reflected light is directed away from the detector.

In Fig. 3 is the waveform of a typical output signal of the detector device for a single scan over a white paper surface is shown enlarged. The signal level falls towards the ends of each scan to about half the height in the middle. This drop occurs accordingly

- 7 - the

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the inverse square law of the increasing path length of the light at the ends of the scan and due to the angle of the normal to the surface on which the reflected light is captured by the detector device, with that of the detector device The amount of light received is proportional to the cosine of this angle. The end walls 14 and 15 in Fig. 1 can be reflective made v / ground so that the light reflected to the mirror 6 increases progressively towards the end of the scan, one 'Song frequency component of the output signal to the feed input of the detector device can be fed back to the level or to increase the level of the signal obtained at the ends of the scan and by, over the entire scan, an output signal level with a constant subsurface.

The drop in the output signal can be compensated for by other means e.g. by varying the sensitivity of the detector device in synchronism with the scanning, e.g. Generation of a repeated signal of suitable characteristic which is applied to the detector device. Alternatively can be suitable molded opaque objects into the path of light between the surface to be scanned and the detector means be brought to more. Collect light in the middle of the scan than at the end parts of the scan. Or the level or the response threshold at which the error is detected should be varied by using a filter with graduated opacity or blackening is brought into the path of the light reflected from the surface.

At the beginning of each scan, blanking signals are triggered that are used to adapt to different scan lengths in order to prevent that the edges or edges of the surface are measured as defects or that signals are emitted from outside these edges will. In certain circumstances and especially when an exceptionally wide surface is to be scanned,

- 8 - is

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is that received by the photo amplifier at the ends of the scan The amount of light is so small that the output signal of the photo amplifier is masked by the noise.

The sampling stage can, as shown in FIG. 4, by an additional Light receiving device can be modified in the form of a cylindrical lens 6 which lies above the surface 11 and whose longitudinal axis lies in the same plane as the scanning line RS of the light beam.

The lens system 8 has different focusing strengths in the direction of the scan and in a direction perpendicular to the scan direction, so that in the scanning direction the light, as described above, is focused on a point on the surface. In the direction perpendicular to the scanning direction, the light is guided through the lens 16, as shown in Fig. 5, before it is on the Surface is focused. The light that is scattered and diffusely reflected from the surface and captured by the lens, is focused on the sensitive surface of the photo amplifier via the mirror 6.

The reason for the penetration of the incident beam and the light reflected from the surface through the cylindrical one Lens 16 is explained with reference to FIGS. 6a and 6b.

Fig. 6a shows a different embodiment than Figures 4 and A light beam 17a, which comes from a stationary light source, not shown, is deflected perpendicular to the plane of the drawing and meets the surface 11a at a point 18a without passing through the cylindrical lens 16a which is that of the point 18a collects reflected, scattered light and focuses it on the sensitive surface of the detector device (not shown) o If the axis of the beam 17a is shifted somewhat relative to the lens, as indicated by dash-dotted lines, either in parallel

- 9 - or

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JO

or inclined at an angle to the original axis of the ray, so is the light intercepted by point 19a and lens 1 ^ a not focused in the same place, but away from the detector device .

Fig. 6b shows the arrangement according to Fig. 5, before sin diverging incident beam 17b is focused on a point 18b of the surface 11b by the cylindrical winch 16b, the scattered light reflected from the point 18b collected by the lens 16h and onto the sensitive surface the detector device, not shown, is focused. If the ray 17b is restricted, for example to the ray delimited by the lines P- and D, and its axis is shifted, either parallel or at an angle to its original axis, then only the part of the by the lines B runs and C limited beam through the lens 16b, but this part is still focused on the point I ⁵ ^ b, so that the light reflected from the surface is still focused on the sensitive area of the detector device, albeit with reduced intensity. 7, the incident beam reaches the lens along a line Pn which is inclined at an angle to the desired scan line along the longitudinal axis of the lens, the beam is refracted by the lens so that it crosses the surface 11 along the desired line RS reached parallel to the longitudinal axis of the lens, so that the light scattered by the surface 11 is focused on the sensitive surface of the detector device, not shown.

The lens 16 can be plano-convex or biconvex and circular Have cross-section, and they can be made of glass or in the form of a rod Be made of plastic. It is possible that dust or permanent Marks on the lens surface that interrupt the beam, mistakenly mistaken for defects in the surface to be scanned will. The effects of such markings can be eliminated by an electronic circuit that has a memory will.

- 10 - marriage

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Before the scattered light from the surface, which is collected by the lens 16, is focused on the sensitive surface of the detector device, it is necessary that the incident light on the lens IG is divergent in the plane perpendicular to the plane of the drawing. That in ¹⁷¹ Xg. The lens system 3 'shown in FIG. 4 therefore has a low negative exponent in this plane. However, a lens system with a high positive exponent can also be provided, the desired divergence of the beam being obtained after it passes through a focal point before it reaches the lens 16. The view according to FIG. 5 shows the focusing in the orthogonal plane.

Alternatively or additionally, the amount of light collected can be increased are by the side walls 12, 13 highly reflective Get inner surfaces, as shown in FIG. 8, or also highly scattering inner surfaces (not shown). In Fig. 9, the light source is the focused point 21 on surface 11 and the photo amplifier is denoted by 22. The light from the light source is emitted evenly in all angles of the plane and reaches the Photo amplifier or the detector via successive ReSections from the side walls 12 'and 13 · as well as on direct Path.

Losses occur with every reflection and the photo amplifier effectively sees a direct image (the light source) and also faint images 23 on each side that expand infinitely. The outer images are weaker both because of the losses that occur with each reflection and because of the insensitivity the sensitive area of the photo amplifier at large angles relative to the normal of this area.

The two parallel opposing reflective surfaces of Figures 9 form a simplified shape of an elliptical mirror. F, in such a mirror is ideal insofar as the light from the light

- 11 - source

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source to the photo amplifier with just a single peflection so that less light is lost. Fin form compromise between the expensive and elaborate elliptical Mirroring and the relatively weak sensitivity in the case of plane parallel mirrors is achieved by the arrangement according to FIG. 10, in which the opposite planes mirrors 12 'and 13 * are arranged somewhat conically in Richtxmg to the light source. With this arrangement, the images 23 appear on the surface of a cylinder 24, i.e., they are tight around the main image and around groups the most sensitive angle to the face of the photo intensifier.

The downside is that these powders are farther away from the photo intensifier than the plane of the main image. In order to eliminate the higher-order images, but while maintaining the simple design of parallel opposing vunae, only the lower parts 25 can be made conical, as shown in FIG. 11a, or elliptical parts 26 can be used, as shown in FIG. 11b while keeping the upper parts parallel.

One application of the detector, for example, is to detect all errors in different size ranges over a certain period of time to count on a white paper. The font contrast between the defects and the paper in which it is desired that the Counting errors can be adjusted by changing the sensitivity of the detector and it is usually one 30% font contrast between black / arz and white. The count itself is easy to carry out using a well-known digital technique, which is not described in detail here, but reference is made to Figures 3, 12a and 12b, the waveforms of FIG Show output signals picked up by the detector means at various stages of the count. Low frequency Changes are filtered out from the signal, with error pulses 27 and, if necessary, background pulses 28 remain, as Fig. 12a shows. All changes under

- 12 - one

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a level that is set to accommodate the background or background pulses off are eliminated in a Kenparator, the pulses 29 of constant height but different widths are generated for different error sizes (FIG. 12b). These output pulses are then used as indicators for the total count and for the large error count. A total count is achieved by giving the output of the comparator to a decade counter that shows the total number of errors in a period of, for example Show 10 seconds. Large defects or spots, for example with a diameter of 0.3mm, can be counted by the output of the comparator is blocked for a suitable period of time before it is put on the decade counter. The appropriate period of time is that which the focused beam image needs, to cross or go through a pillar with 0.3mm. By suppressing this output, the only impulses are which reach the counter and register as errors, those generated due to errors greater than 0.3mm diameter. For example, the scanning takes place at a distance above the paper surface of 50 cm. At a speed of The point of light moves 24,000 revolutions per minute at a speed of 2.4 mm per microsecond. An error of 0.3 mm thus appears as a 0.125 microsecond pulse at the output of the comparator. By suppressing this output for 0.125 microseconds, the only pulses are registered as errors those produced by defects larger than 0.3 mm in diameter displayed as a total number (Fig. 12). An analog output can be taken from the counters, which is suitable for permanent written recording.

The detector described above can be used with or without the aforesaid Ground modifications placed on a stationary recording strip. The registration strip contains a number of Lines and is placed under the beam so that the counter is the product of the number of lines on the strip and the number

- 13 - eggs

2 U ¹ J 'υ j 7 / j Ü b 6

Λ 12 388

to determine samples per second. ^ ai AnVOn- ¹ Un /; of a working interval obtained by the scanning speed, the correct sensitivity can easily be determined. No count is registered if the sensitivity is too high. ! The right attitude lies between these limits.

A iTebenwirkung in Vervrendung a Tionochrooatischen radiation is that token a red laser can not detect any red ^w ehlar, but the number is / inwendungs ^ Mllen in which errors of a certain color occur far outweighed by the benefits of the system, "on may use ^c oner fine laser having the same arbsn an output of two "or two lasers of different color, whose beams impinge au ^r the same rotating mirror.

The detector is not limited to finding faults on paper and similar flat surfaces, but it can also be used for three-dimensional diffuse reflective surfaces, e.g. for finding cracks in beers or for optical extraction of data of various forms.

The detector can also be used to detect errors in see-through Determine surfaces, whereby the light is passed through the surface here. In this case the detector is located under the arranged to be scanned surface.

BATH ORIGINAL

Claims (2)

220612g A 12 388 Expectations Device zuir. detection of defects or spots in a surface with a scanning stage which ^{is movable relative to the w} pool and has a laser that emits continuous radiation, a scanning device with a "reflector, which is rotatable in the path of the laser beam, around it to direct the surface in the x ^ esentlichen except to the relative movement, a detector device for receiving from the face diffusely reflected or transmitted radiation, the ^ etektoreinrichtung abaibt an output signal which is dependent ^r inaig of changes in the intensity of the er.ofangenen radiation changes, characterized by a lens arrangement (8) for focusing the beam on a small point on the surface to be scanned. 2. Device according to claim 1, characterized in that that the radiation is in the visible part of the spectrum lies. 3. Device according to Ansoruch 1 or 2, characterized in that the reflector is designed in the form of a rotatable mirror with a plurality of reflective facets. 4. Vorrichtuna according to claim 1, 2 or 3, characterized in that the lens arrangement (8) has two in the Light path has arranged spherical lenses that further diverges through the first lens of the beam and onto the second Lens is directed. 5. Apparatus according to claim 4, characterized gekennzeic.h- n e t that the length of the light path between the two lenses 2206121 A 12 3R8 away is adjustable. ζ . Device according to claim 4 or 5, characterized in that the two lenses are arranged adjacent to one another and that a right-angled mirror is provided for reflecting the light and for shifting the path of the light beam, which is adjustable in a direction parallel to the optical axis of the two lenses. 7. Device according to one of claims 4 to 6, characterized in that the two lenses with such more positive. Are exponents. 8. Device according to one of the preceding claims, characterized characterized in that the / ibtast stage is a cylindrical Lens ir.it has positive exponents whose long axis extending parallel to the surface along the path of the scanning beam, further including a focusing lens is provided. S. A device according to einsin of the preceding would appeal, characterized ge ^v e η η ζ ei C ₁ h η et that the sampling stage is provided at each side and at each end of the scan path with reflective surfaces extending between the surface to be scanned and the detector device extend. 10. Device according to one of claims 1 to 8, characterized in that the scanning stage on each side and at each end of the .Abtastweges diffuse areas, which are located between the area to be scanned and the detector device extend. 11. Device according to one of the preceding claims, characterized in that a plurality of side-by-side - 16 - 209837 / - «υ ü ö fr 220612 A 12 388 andar arranged scanning stage for a larger number of Samples are provided. 12. The device according to claim 11, characterized in that that the scanning stages transverse to the direction of movement of the surface to be scanned are arranged in order to obtain a shorter scanning length for each stage. 13. A method of detecting or locating defects or spots in a surface, one being emitted by a laser continuous light beam is directed onto a surface movable relative to the laser, that furthermore the light beam is directed across the direction of movement across the surface and the light reflected diffusely from the surface is captured is that any change in the intensity of the reflected light detected by a detector device and by a change in the waveform of an output signal of the detector means is displayed, characterized in that that the light beam is focused on the surface to be scanned by means of a lens arrangement. 2 ü yb : ■> 7 / ί υ 5 ts