WO1997043624A1

WO1997043624A1 - Device and method for detecting defects

Info

Publication number: WO1997043624A1
Application number: PCT/SE1997/000750
Authority: WO
Inventors: Svante BJÖRK
Original assignee: Svante Björk AB
Priority date: 1996-05-13
Filing date: 1997-05-06
Publication date: 1997-11-20
Also published as: SE9601800D0

Abstract

A device and method for improving a visualization of detected defects (1) in a substantially planar transparent strip (70), most preferably in a strip of a film. The device is arranged with a light source (90, 92) in order to substantially illuminate the width of the transparent strip (70), focusing means (50, 52) provided with an optical axis (100), in order to focus the light which is coming from the transparent strip (70), a detector (10, 12), in order to detect defects (1) on the transparent strip (70) and means (80) for displacing the transparent strip (70) in its longitudinal direction. The transparent strip (70) is arranged in a first plane between the light source (90, 92) and the focusing means (50, 52), and where the optical axis (100) is arranged in a second plane in order to focus the incoming light to the detector (10, 12) by means of the focusing means. The light from the light source is inclined with respect to the lateral direction of the transparent strip (70) and the light source (90, 92) is arranged in the second plane in order to obtain a high intensity.

Description

TITLE:

Device and method for detecting defects.

FIELD OF THE INVENTION:

The present invention relates to a device and a process for detecting optically refracting defects in a transparent strip, a strip of a polymer film.

STATE OF THE ART:

It has been known for a long while how occurrences of optically refracting defects can come about. The occurrence of optically refracting defects in a transparent film, for example of polymer can be brought about with the help of a device which illuminates a film from below through a light table provided with an illuminated opening (a slit), where said illuminated opening is limited in the direction of movement of the film. The film can be viewed from above by a detector, for example by means of a linear array. At the start and the end of the passage of a defect, that part of the detector onto which the actual part of the film is projected, will not be illuminated equally intensely since the start and end of the optically refracting defect temporarily refracts into the image of the non-light-emitting surface just behind and, respectively, in front of the light-emitting opening in the light table.

An alternative embodiment can be a similar device provided with a scanning light beam (e.g. laser light) which illuminates a film and where the light beam is viewed from the other side by a detector, e.g. by a photo-detector. This detector is arranged so that the detection only checks whether light is coming through the film. In the case that no light is detected, the light beam has therefore been refracted so that the light beam either lies in front of, or behind, the detector due to the defect which is present on the film.

One way of detecting defects in a transparent plastic film is described in US-Patent 4,038,554.

The defects are measured by a laser beam impinging on to a movable mirror, where the mirror makes a scanning movement over the film so that the laser beam can search the whole width of the film. Beneath the plastic film there is a light-collecting rod which is provided with a light-spreading strip in the length direction of the rod and a photo-diode on one short side of the rod.

Another way in which defects can be detected on a transparent glass plate is described in US-Patent 3,989,387. The defects are measured by illuminating a glass plate with a light beam, where the light beam which has passed through the glass plate is reflected on a mirror. The light beam can therefore basically pass through the glass plate one more time and is detected by a photoelectric cell. In order to able to obtain a suitable reflection angle, the light beam is led through the glass with an angle of up to 30° with respect to a normal at right angles to the length direction of the glass.

However problems can arise with the solutions which are shown in US-Patent 4,038,554 and US-Patent 3,989,387 where the start and the end of a longer defect, for example of the order of l-100μm is actually registered as two smaller defects, i.e. the sides of the defects are invisible since the light beam which is refracted due to the defect either lies in front of, or behind, the detector which is detecting the light beam. A solution for detecting optically refracting defects in transparent glass plates which takes account of long and short defects is described in US-Patent 4,492,477 and is used for the preamble of claim 1 and 6. The defects can be detected by illumination with a thin and linear light source over the width of a glass plate, which is displaced along a horizontal plane. The light source can be arranged with an angle of 45° with respect to the direction of movement of the plate and is parallel with a vertical plane which is provided with an optical axis from a convergent optical system and a linear network with photo-diodes. By turning the light source 45° the contrast will increase with respect to the threshold level of the light. Moreover the light source is given a low intensity in order to achieve a more diffuse light and is positioned at a distance from the vertical plane so that the linear network is outwardly tangential with the periphery of a blurred real image which has been achieved by the optical system. In order to maintain the light source at a constant distance from, and parallel with, the vertical plane, there are means arranged in connection with the linear network having photo-diodes in order to regulate this distance.

This solution, which however appears unnecessarily complicated in US-Patent 4,492,477, due to the fact that the distance from the vertical plane with respect to the light source has to be constant the whole time in order to obtain a high measurement accuracy and thus regulate this distance. Moreover the position has to be relatively high since the light source has a low intensity.

SUMMARY OF THE INVENTION:

An object of the present invention is therefore to provide a process for improving visualization of detected defects.

Another object is to facilitate the process during the detection of defects.

In accordance with the present invention the above-mentioned objects are achieved by providing a device in accordance with claim 1 and a method in accordance with claim 6.

Preferred embodiments of the method and the device in accordance with the invention are defined in detail in the respective dependent claims.

DESCRIPTION OF THE DRAWINGS:

The invention will be described in more detail below where preferred embodiments are shown as examples with reference to the accompanying drawings, in which:

Fig. 1 shows the principal of the present invention in an optical diagram;

Fig. 2 shows a preferred embodiment of a device for detection in accordance with the present invention; Fig. 3 shows how a light source can be placed with respect to the longitudinal direction of a strip, in accordance with the present invention;

Fig. 4 shows realistic images of defects;

Fig. 5 shows images of defects which are taken by a device for detection; and

Fig. 6 shows images of defects which are taken by a device for detection in accordance with the present invention.

PREFERRED EMBODIMENTS:

With reference to figure 1, a convergent optical system or lens 52 is shown, which is placed between a detector, which in this case can be constituted by a CCD 12, and a transparent strip 70, which in this case is constituted by a polymer film. There is also an optical axis 100 which passes at right angles through the centre of the optical system 52 and extends in a plane which lies beyond the focus F of the optical system, so that a real image Id can be achieved by a possible defect 1 which is positioned within or on the transparent strip 70. The possible defect 1, which can occur in the transparent strip, is such that the visible field from the detector 12 is refracted in such a way that no background illumination can appear and consequently the defect 1 will stand out as a dark mark on the detector 12.

Figure 2 shows an embodiment of a device for detecting defects in a substantially planar transparent strip 70, preferably a strip of a polymer film, where said device is arranged with a light source 90, for example a scanning laser or a diffuse light source, in order to substantially iUuminate the width of said transparent strip 70. In order to direct the light which comes from the light source, an opening, preferably a slit (not shown), can be arranged in front of said light source. Additionally the device is provided with focusing means 50, which for example include a lens system and an optical axis in order to focus the light which passes the transparent strip, as well as a detector 10 provided with one or more detector means (not shown), which for example can be constituted by a CCD-camera or one single detector means (e.g. a photo-diode) which detects light from a mirror which rotates substantially over the width of the film, in order to detect defects on said transparent strip 70. This detector 10 can be of a scanning type, i.e. the detector can substantially sweep over the light which passes the transparent strip, for example by mounting the detector on a movable shaft and setting this in motion either manually or by means of a motor (not shown). The CCD (Charge-Couples Devices) in the camera can, in a basic form, consist of a tightly packed matrix of light-sensitive MOS-transistors provided with electrodes, on a semiconductor substrate of silicon.

The detector 10 can produce an image of a defect which is present and the light which goes through the strip. Moreover this transparent strip 70 can be displaced in its length direction by displacement means 80. Other detectors 10 than CCD-cameras can of course be used, e.g. photo-diodes and other types of photo-cells can be used. With the aid of these detectors a grey scale can moreover be obtained of the image which is achieved.

The focusing means can also include, apart from the optical lens system, a control unit (not shown), provided with for instance a motor and control electronics in order to allow an automatic adjustment of focus.

The detector 10 can further include an electronic unit which is provided with a mains unit (for power supply to the detector), a first means for controlling the light source, a second means for controlling the sensitivity in the detector. The first means can e.g. regulate the intensity of the light source, indicate errors in the lamp etc. In order that the focusing means 50 is able to obtain the same amount of light, independent of the thickness of the strip, the first means can thus automatically or manually regulate the intensity of the lamp so that the same amount of light enters through the focusing means 50. The second means can additionally send signals to a marker 60, for example of the ink-jet type, in order to mark where the defect is on the strip. The sensitivity /detectability can be changed by the second means in order to be able to increase or decrease the detectability of a defect with different accuracy. The detector 10 can also be arranged with a focusing indicator 20 and an exposure indicator 40. In order to measure a defect, a reference voltage can for example be set to a predetermined level. In the case where a CCD-camera is used, where the substrate on the CCD is P-doped, an applied positive reference voltage on a metal electrode will cause charges in the surroundings to collect at this electrode. By alternatingly applying an increased reference voltage on each of three adjacent electrodes, a charge produced by light can be displaced in the desired direction. For example, if an electrode has a reference voltage of 5V, another has an increased reference voltage of 10V and a third has a reference voltage of 5V, the third electrode or the first electrode (depending on which direction it is desired to displace the charges in) will thus obtain a voltage of 15V when a charge produced by light is present. In this case the electrodes will thus obtain a voltage of 15V when light impinges on the light-sensitive part of the MOS-transistor. Where a defect occurs, i.e. when the light is blocked, the voltage on the electrodes, on the MOS-transistors which are no longer impinged upon by light, will fall for a certain time (for as long as the defect is present during a displacement of the strip) and thereafter return to the same level as the electrodes had before the blocking of light. Depending on how much and for how long the voltage was changed relative to the voltage which the electrodes had before the defect, the size of the defect can be determined. In order to achieve the best effect with this measurement, the lamp is placed in such a way that the CCD-camera obtains a high intensity, preferably as high as possible.

Figure 3 shows an example of how the transparent strip 70 can be arranged in a first plane, for example in a horizontal plane, between said light source 92 and said focusing means 52, and where said optical axis 100 can be arranged in a second plane, for example in a vertical plane, in order to focus the incoming light which passes the transparent strip to said detector through said focusing means 52. In order to be able to detect defects, which are either long and narrow or many and smaller which lie close to each other, the light from said light source can be inclined with respect to the lateral direction of said transparent strip at an angle α, for example between 30-60°, preferably 40-50°, and most preferably substantially 45°, in accordance with the present invention. Based on this inclination, a side-refracting effect can be used, which arises when the light hits a defect on strip 70, and thereby obtain a reliable image of the defect by means of the detector 10. Moreover the light source 92 can be arranged in said second plane in order to obtain a high intensity since the most light-intense part from the light source 92 is used there.

This side-refracting effect can also be achieved where said first plane is arranged to be inclined towards the longitudinal direction of the optical axis and/or towards said second plane.

Another variation is that the light varies along the slit, for example as a sinus shape or as a ramp over the whole length which results in that an intensity difference arises if a defect is present on the film which it is intended to check. Moreover the illuminated opening of this embodiment means a lesser dependence on how large the width of the opening is, since the light can vary over the width of the opening.

An additional embodiment can be to combine these two embodiments in order to obtain even safer detections.

The defects which can arise with manufacture of a transparent film can basically be divided into three groups; substantially spherical or round (see figure 4a), substantially cylindrical (see figure 4b) and so-called "arrow heads" (see figure 4c). Other types of defects can also be present, for example drip-shaped or similar. The defects generally appear within the transparent strip. The so-called "arrow heads" can be described as a substantially cylindrical shape which is bent.

Figure 5a-c shows the defects which are shown in figure 4 detected by light which is not inclined with respect to said transparent strip's lateral direction, i.e. light which lies at right angles (90°) with respect to said transparent strip's lateral direction. Figure 6a-c shows the defects in figure 4 where the side-refracting effect of the defects is used, i.e. which are detected by light which in this case is inclined at 45° with respect to said transparent strip's lateral direction. From figure 6a-c it is thus clear that an improved visualization is obtained of the defect's shape and appearance, upon a comparison with figure 5a-c, by inclining the light in accordance with the present invention. When these defects are illuminated, these or at least the edges of these will refract the light like a lens, due to the defects normally having rounded edges.

Although the depicted embodiments of the present invention have been described in detail with reference to the accompanying figures, it must be understood that the invention is not limited to these specific embodiments and that different changes or modifications can be achieved by a skilled man without deviating from the scope which is defined by the appended claims. For example, a parallel detector can be used instead of a serial one like the way the CCD-camera operates for example.

Claims

108041 MH - English translation 1997-08-21

-CLAIMS: 1. Device for detecting defects (1) in a substantially planar transparent strip (70), preferably a strip of a film, where said device is arranged with:

a) a light source (90,92), for illuminating substantially the width of said transparent strip;

b) focusing means (50,52) provided with an optical axis (100) in order to focus the light which passes said transparent strip (70);

c) a detector (10,12), in order to detect defects (1) in or on said transparent strip (70); and

d) means (80) for displacing said transparent strip (70) in its longitudinal direction,

whereby said transparent strip (70) is arranged in a first plane between said light source

(90,92) and said focusing means (50,52), and where said optical axis (100) is arranged in a second plane in order to focus the incoming light to said detector (10,12) by means of said focusing means (50,52), characterized in that the light from said light source (90,92) is arranged inclined with respect to the lateral direction of said transparent strip (70) and where said light source (90,92) is arranged in said second plane in order to obtain a high intensity.

2. A device for detecting defects (1) according to claim 1, characterized in that an opening, preferably a slit, is arranged in front of said light source (90,92), in order to incline the light which comes from the light source (90,92). 3. A device for detecting defects (1) according to claim 1 or 2, characterized in that the light from said light source (90,92) is inclined substantially 30-60°, preferably 40-50°, and most preferably 45°, with respect to the lateral direction of said transparent strip (70).

4. A device for detecting defects (1) according to any one of claims 1-3, characterized in that the detector (10,12) preferably a CCD-camera, is constituted by a scanning detector system.

5. A device for detecting defects (1) according to any one of the previous claims, characterized in that said first plane is arranged to be inclined towards the longitudinal direction of the optical axis (100) and/or towards said second plane.

6. Method for detecting defects (1) in a substantially planar transparent strip (70), preferably a strip of a film, by

a) illuminating substantially the width of said transparent strip (70) by means of a light source (90,92);

b) focusing the light which passes said transparent strip (70) through focusing means (50,52) provided with an optical axis (100);

c) detecting defects (1) in or on said transparent strip (70) by means of a detector (10,12); and

d) displacing said transparent strip (70) in its longitudinal direction by means of displacement means (80),

whereby said transparent strip (70) is placed in a first plane between said light source (90,92) and said focusing means (50,52), where said optical axis (100) is placed in a second plane and focuses the incoming light to said detector (10,12) by means of said focusing means (50,52), characterized in that the light from said light source (90,92) is inclined with respect to the lateral direction of said transparent strip (70) and where said light source (90,92) is placed in said second plane in order to obtain a high intensity.

7. A method for detecting defects (1) according to claim 6, characterized in that an opening in front of said light source (90,92), preferably a slit, inclines the light which comes from said light source (90,92).

8. A method for detecting defects (1) according to claim 6 or 7, characterized in that the light from said light source (90,92) is inclined substantially at 30-60°, preferably at 40-50°, most preferably 45°, with respect to the lateral direction of said transparent strip (70).

9. A method for detecting defects (1) according to any one of claims 6-8, characterized in that the focused light is detected, preferably by a CCD-camera of a scanning detector system.

10. A method for detecting defects (1) according to any one of claims 6-9, characterized in that said first plane is inclined towards the longitudinal direction of the optical axis (100) and/or towards said second plane.

AMENDED CLAIMS

[recei ved by the Internat i ona l Bureau on 12 September 1997 ( 12 .09.97 ) ; ori g i na l cl a ims 1 and 6 amended ; rema i n ing c l aims unchanged ( 4 pages ) ]

— 1. Device for detecting defects (1) in a substantially planar transparent str_ip (70), preferably a strip of a film, where said device is arranged with:

b) a detector (10,12)^, in order to detect defects ( 1 ) in or on said transpareni strip (70);

c) focusing means (50,52) provided with an optical axis (100) and a focus (F), in order to achieve a real image of an object, and where the focusing means (50,52) is arranged between the strip (70) and the detector (10.12); and

whereby said transparent strip (70) is arranged in a first plane between said light source (90,92) and said detector (10, 12), and that the light from said light source (90,92) is arranged inclined with respect to the lateral direction of said transparent strip (70), characterized in

- that the optical axis (100). the light source (90,92 ) and the detector (10.12) is arranged in a second plane, where the optical axis (100) goes through the light source (90,92) and the detector (10,12), in order to focus the incoming light from the light source (90,92) to said detector (10.12) by said focusing means (50.52) and achieve a high intensity on the detector (10.12), and - that the strip (70) and the focusing means (50,52) are arranged on a distance from each other so that a defect is detected beyond the focus (F) of the focusing means (50,52) and achieves a real image of the defect on the detector (10, 12).

2. A device for detecting defects (1) according to claim 1, characterized in that an opening, preferably a slit, is arranged in front of said light source (90,92), in order to incline the light which comes from the light source (90,92).

3. A device for detecting defects (1) according to claim 1 or 2, characterized in that the light from said light source (90,92) is inclined substantially 30-60° , preferably 40-50° , and most preferably 45° , with respect to the lateral direction of said transparent strip (70).

b) detecting defects (1) in or on said transparent strip (70) by means of a detector (10,12) and focusing means (50,52) provided with an optical axis (100) and a focus (F), where said focusing means (50,52) placed between the detector (10,12) and the strip (70); and

c) displacing said transparent strip (70) in its longitudinal direction by means of displacement means (80),

whereby said transparent strip (70) is placed in a first plane between said light source (90,92) and said detector (10,12), and that the light from said light source (90,92) is inclined with respect to the lateral direction of said transparent strip (70), characterized by

- focusing the incoming light from the light source (90,92) by said focusing means (50,52) and achieves a high intensity on the detector (10,12), by placing the optical axis (100), the light source (90,92) and the detector (10,12) in a second plane, where the optical axis (100) goes through the light source (90,92) and the detector (10,12), and

- detecting the defect beyond the focus (F) of the focusing means (50,52), in order to achieve a real image of the defect on the detector (10,12).

8. A method for detecting defects (1) according to claim 6 or 7, characterized in that the light from said light source (90,92) is inclined substantially at 30-60° , preferably at 40-50° , most preferably 45° , with respect to the lateral direction of said transparent strip (70).