JPH06109660A - Method for inspecting foreign matter - Google Patents

Abstract

PURPOSE:To properly and efficiently detect a foreign matter in a subject by inspecting with a focus of an image pick-up lens shifted from a portion to be inspected to a degree that an image of the foreign matter can be identified and by aligning a focus when the image of the different material is sensed. CONSTITUTION:A subject S is mounted on a deck 11, and a light amount of either or both of ring lights 14 or 23 is appropriately adjusted to illuminate the subject S. Then a recording lens 21 is focused so that a sharp image of a portion to be inspected can be obtained on a display for displaying an analog image. Then a focus of the lens 21 is adjusted vertically on the subject S or on a recorder 20, and it is shifted by a predetermined amount to be fixed. An image processing circuit is switched to a digital image, and a centralized controller is used to drive an XY stage 13 for scanning. If a signal exceeding a threshold of an A/D-converter appears on an output from a CCD element, a digital image is displayed on a corresponding position on a display. This image indicates that a foreign matter exists at the inspected position of the subject S. The display is switched to the analog image, and a focusing device is driven to make a focus on the portion to be inspected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for inspecting foreign matter in a specimen by an optical observation means, and more particularly to a method for eliminating foreign matter in a specimen efficiently by eliminating a noise image.

[0002]

2. Description of the Related Art For example, when metal particles or undispersed particles of various additives or voids are present in an insulating material such as a cross-linked polyethylene used as an insulating material of a power cable, these become the insulating material. Insulation performance is significantly reduced,
Impairs the reliability of the power cable. In a synthetic resin insulation molded product used not only in power cables but also in power / electronic devices, such contamination of impurity particles and voids must be strictly monitored. Foreign particles and voids generally contained in an object (hereinafter, these are collectively referred to as "foreign particles")
Various methods are known as methods for inspecting the presence of a. As a non-destructive method, there is a method of obtaining a shadow image of a foreign matter by irradiating an X-ray or other sample-transmitting electromagnetic wave. However, these methods require a large-scale apparatus and the detection accuracy is not always sufficient.

As another inspection method, a specimen is cut or polished into a plane, or sliced, and the specimen is irradiated with reflected light or transmitted light, and then it is optically observed with a magnifying glass, a microscope, a camera, a projector, or the like. There is a method of obtaining the image by using means. From this image, the presence of a foreign substance can be identified with the naked eye, and the difference between the electrically normal sample and the foreign substance is detected by converting it into an electric signal by a CCD element or the like, and the presence and the type of the foreign substance are detected. The shape, size, etc. can also be measured.

[0004]

In a method for inspecting a foreign substance in a specimen by using an optical observation means, usually, the observation means is focused on an inspection site of the specimen (focus),
A sharp image of the inspection area is obtained. According to this method, it goes without saying that it is possible to detect the presence of foreign matter in the specimen, but at the same time, fine scratches and the preparation that occurred on the surface when preparing the specimen by cutting, polishing, slicing, etc. The dust adhering to the surface of the sample later appeared as image noise, and was sometimes confused with the foreign substance originally present in the sample and misidentified. In particular, if an attempt is made to inspect a sample for foreign matter by an electronic means instead of observing it with the naked eye, an extremely complicated device is required to eliminate the above-mentioned image noise, and its reliability is poor.

The present invention has been made by paying attention to this point, and its object is very simple in inspecting a foreign substance in a specimen using an optical observation means without requiring any special device. Another object of the present invention is to provide a foreign matter inspection method capable of eliminating noise and accurately and efficiently detecting foreign matter in a specimen.

[0006]

[Problems to be Solved by the Invention] Such a problem is encountered when inspecting a foreign substance in a specimen by an optical observation means.
The problem can be solved by using a foreign substance inspection method which comprises inspecting by shifting the focus of the optical observation means from the inspection site to such an extent that the image of the foreign substance can be identified. Further, when the image of the foreign matter is detected by the above-mentioned inspection, it can be solved by a foreign matter inspection method comprising focusing and closely examining the image.

Here, the optical observation means means for converging the light transmitted or reflected by the inspection portion of the specimen with an optical lens to form a shadow image of the inspection portion on the screen including the retina. A magnifying glass, a microscope, various cameras, a projector, etc. are included in the optical observation means here.

[0008]

In order to inspect the foreign matter at the inspection site by the optical observing means, the focus is usually accurately set on the inspection site. However, the foreign matter inspection method of the present invention is
The first characteristic is that the inspection is performed by intentionally shifting the focus so that the image of the foreign matter can be distinguished from the background. In general, as shown in FIG. 2, when projecting images of two large and small subjects A and B on the same plane through a lens L onto a screen, if the screen is in the focal plane P of the lens L,
A sharp image A ', B'of each subject is obtained on the screen. This sharpness is brought about by the fact that the contour line of each subject is imaged on the screen with high contrast. When the screen is deviated from the focal plane P, for example, placed in the middle Q between the lens L and the focal plane P, the contour lines a ₁ and a ₃ of the subject A are a ₁ ′ and a ₃ ′, respectively.
The image is projected with a width like that, and the contrast with the background is reduced accordingly. This reduces the ability to distinguish the subject from the background, but subject A
Is sufficiently large, the main body part a _{2 of the} subject maintains a sufficient contrast as a ₂ ′ on the screen, so that the presence of the subject A can be discriminated from the background although it is unclear. This is called a "bokeh image with a core." On this same screen, the images of the small subject B are overlapped with the outline images, and as a whole, a low-contrast, cloudless blurred image with no "center". This phenomenon is the same when the screen is arranged outside the focal plane P.

When inspecting foreign matter at the inspection site by optical observation means, minute scratches caused by cutting or slicing on the surface of the specimen near the inspection site, and dust adhering thereafter (these are referred to as noise bodies hereinafter). ), Since these images are sharply imaged on the focal plane as well as foreign matter, it is difficult to distinguish the foreign matter from the noise body by both the naked eye judgment and the electrical judgment means, and they are confused and misidentified. I will end up. In the inspection method of the present invention, the focus is shifted from this inspection site to such an extent that the foreign matter can be discriminated, so the foreign matter is detected as the above-mentioned "bokeh image with a core". The image of a noise body (noise image) such as dust or fibers whose diameter is low or whose diameter is smaller than that of the foreign matter becomes unclear as a whole and becomes a blurred image without a "core" and melts into the background. Discrimination is easy, and these noise images can be easily eliminated by providing a threshold in the contrast recognition circuit even by the electrical discrimination means. The degree of the operation of shifting the focus from the inspection region to the extent that the foreign matter can be identified can be experimentally determined in consideration of the relationship between the variation range of the diameter of the foreign matter to be detected and that of the noise body.

Next, in the second method of the present invention, for the "blur image with a core" identified and detected as described above,
The image is sharpened by focusing, and the nature, shape, and size of the foreign matter are scrutinized. According to this method, compared to the conventional foreign matter inspection method in which foreign matter is selected and extracted from the noise body in a focused state from the beginning, since image noise is eliminated, the specimen surface can be inspected much more efficiently. Become.

The foreign matter inspection method of the present invention is used for inspecting foreign matter or voids appearing on a cut surface of an electrically insulating material or metal using a magnifying glass, a microscope, a projector, a still camera, a TV camera or the like as described above. Can be applied. This inspection can be performed by visual observation, but it is also applicable to automatic detection by electronic means, recording, or feedback of the information to the process. For example, the surface of the sample is photographed by a CCD camera, and at that time, the focus of the camera lens is shifted from the normal inspection site so that the "core" of the blurred image of the foreign matter can maintain the contrast in shade or color. By binarizing the output obtained from the CCD element by setting a threshold value, it is possible to detect whether or not a foreign substance exists at the inspection position [X, Y] of the inspection site. Further, if the camera lens is automatically focused on the basis of this detection signal, a clear image of the foreign matter can be projected or recorded on the screen.

[0012]

EXAMPLES The present invention will now be described with reference to examples. Figure 1
The example shown in is an example in which the present invention is applied to the inspection of foreign matter (foreign matter particles and voids) contained in the insulating layer of the power cable. A specimen S is obtained by thinly slicing the power cable without the conductor. In FIG. 1, reference numeral 10 is a support base that supports the sample S, and 20 is an imaging device.

The support base 10 includes a transparent deck 11 such as a glass plate on which the sample S is placed, and a frame 12 for horizontally supporting the deck.
The ring light 1 in which the relative position between the XY stage 13 that can electrically slide the frame 12 in the horizontal orthogonal axis XY direction and the imaging device 20 so as to illuminate the specimen S through transmission is fixed.
Have 4. Illumination light from the ring light 14 passes through the sample S and enters the photographing lens 21 of the photographing device 20.

The photographing device 20 has a photographing lens 21 whose focus can be adjusted by an electric signal, a CCD camera 22 which allows a light flux passing through this lens to form an image on a CCD element, and a photographing lens which reflects and illuminates a sample S. A ring light 23 is provided on the outer peripheral portion of 21. Although not shown, the CCD camera 22 is connected to a central control device having a display and an image processing device.

The centralized control device can display the signal from the CCD camera 22 on the display as an analog image through the television circuit, and a switch is provided in the middle of the circuit, and as a digital image through an A / D converter whose threshold can be adjusted. Can also be displayed.

(Embodiment 1) A sample S is placed on the deck 11 of the foreign matter inspection apparatus having such a configuration, and either or both of the ring lights 14 and 23 are illuminated by appropriately adjusting the mutual light amount, and an analog image is displayed. The taking lens 21 is focused so that a sharp image of the inspection region can be obtained on the display for displaying. Next, the focus of the photographing lens 21 is adjusted by vertically adjusting the sample S or the photographing apparatus 20, and is shifted and fixed by an amount of deviation determined in advance by an experiment. At this time, the optical center of the lens may be displaced in the direction of approaching the inspection site, or vice versa. If it is desired to eliminate noise mainly from dust adhering to the surface of the sample, it is preferable to shift the optical center of the lens in the direction toward the inspection site. This is because the amount of blurring of dust particles on the surface of the sample is further increased, and the image noise elimination performance is further improved. In such a state, the image processing circuit is switched to the digital image side, and the central controller controls the XY stage 13.
To drive and scan. When a signal exceeding the threshold of the A / D converter appears at the output from the CCD element, a digital image is displayed at the corresponding position on the display. This digital image shows that a foreign substance is present at the inspection position of the sample S. The scratches and dust on the surface of the sample are not displayed as a digital image because the image output does not reach the threshold of the A / D converter.

(Embodiment 2) The centralized control device which detects the foreign matter at the inspection site as in the embodiment 1 is the XY stage 13.
Then, the display is switched to an analog image, and the focusing device is driven to focus on the examination site. As a result, it is possible to confirm on the display what foreign particles in the sample the foreign particles are, or whether they are voids. When the observer sends a confirmation end signal, the centralized controller again shifts the focus of the taking lens 21 to the specified value, switches the display to the digital display side, drives the XY stage 13, and continues the foreign substance detection operation.

Thus, according to the method of the first embodiment,
The presence of foreign matter in the insulating layer of the power cable can be reliably and efficiently inspected by eliminating image noise without using special equipment. Furthermore, according to the method of the second embodiment, what kind of foreign matter is detected, its type, shape,
The size can be scrutinized.

[0019]

According to the foreign matter inspection method of the present invention, when inspecting a foreign matter at an inspection site by the optical observation means, the focus of the optical observation means is shifted from the inspection site to such an extent that the image of the foreign matter can be identified. Therefore, image noise due to scratches on the specimen surface or dust that has adhered can be easily eliminated without requiring a special device, and the presence of foreign matter at the inspection site can be efficiently inspected. Further, the second foreign matter inspection method of the present invention, when the image of the foreign matter is detected by the above-mentioned inspection, it focuses and closely examines it, so that the type, shape, and size of the foreign matter can be efficiently and precisely determined. Can be inspected.

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of the present invention.

FIG. 2 is a schematic diagram for explaining an optical action in the method of the present invention.

[Explanation of symbols]

10 ... Support stand, 20 ... Photographing device, 21 ... Photographing lens, S
… Sample.

Claims (2)

[Claims] 1. When inspecting a foreign matter in a sample by an optical observation means, the focus of the optical observation means is shifted from an inspection site to such an extent that the image of the foreign matter can be discriminated, and the foreign matter inspection is performed. Method. 2. When inspecting a foreign substance in a sample by an optical observing means, the focus of the optical observing means is shifted from an inspection site to such an extent that the image of the foreign substance can be identified, and the image of the foreign substance is inspected by this inspection. When a foreign matter is detected, the foreign matter inspection method is characterized by focusing and inspecting it.