JPH07104287B2 - Inspection method for minute defects of transparent object with curved surface - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of inspecting a transparent object having a curved surface for the presence of microdefects.

[0002]

2. Description of the Related Art For mass production, when a desired optical glass lens is manufactured only by press molding with a mold without polishing, minute defects (for example, size 5 μm to 100 μm) may occur on the surface, but such minute defects cannot be visually identified. Conventionally, the inspection is performed by visual observation with a microscope, but it is inconvenient to perform visual inspection for all mass-produced optical glass lenses from the viewpoint of productivity, so it must be a sampling inspection. I don't get it. However, this is not preferable for quality control.

Therefore, in order to carry out an automatic inspection, an inspection method by laser light scanning, which is used in the inspection of foreign substances on a semiconductor, can be considered. However, in the case of an optical glass lens, its own lens action and the surface are curved. Therefore, it is difficult to use the method using laser light scanning.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to automatically inspect a small defect of a transparent object having a curved surface having a lens effect such as an optical glass lens by a simple method with high precision and efficiently over a large area. To be able to do it.

[0005]

In the method according to the invention,
As shown in FIG. 1, in the vicinity of the curved surface 3a of the transparent object 3 to be inspected having a lens effect, a focus correction lens 6 having the same curvature and a similar lens effect is installed,
The transparent object 3 is irradiated with diffused light from the opposite side, and the light that has been transmitted by receiving the lens effect of the transparent object 3 is further transmitted in the vicinity of the transparent object 3 by the focus correction lens 6 with the same lens effect, and the transmitted light. Light is magnified by the magnifying optical system 8 and is received by the solid-state image sensor camera 7, and the pixel output thereof is digitally processed to detect a dark portion, thereby inspecting for the presence or absence of micro defects.

When the object to be inspected is an optical glass lens, an optical glass lens similar to that having no defect may be used as the focus correction lens.

Two pixel outputs from the solid-state image sensor camera 7
In the binarization, the threshold value is changed for each pixel according to the curvature of the curved surface. The curvature can be obtained from the gradation difference between pixel groups arranged in the scanning direction of pixel output.

[0008]

[Function] Since the surface of the transparent object to be inspected is a curved surface,
When the surface is imaged as it is with a solid-state image sensor camera, the distance from the center of the surface to the center of the light-receiving part of the solid-state image sensor camera is different, so if you focus the camera on the center of the curved surface, the peripheral part will appear. Blur. Therefore, the range that can be inspected at one time is naturally limited by the depth of field of the solid-state image sensor, and in the imaging that depends only on the depth of field of the solid-state image sensor, if one attempts to detect a minute defect, one transparent The inspection area of the curved surface of the object must be divided and the inspection must be repeated while adjusting the focus for each part, which is very inefficient.

Therefore, in the present invention, the light transmitted by the lens effect of the transparent object to be inspected is transmitted in the vicinity of the transparent object with the same lens effect by the focus correction lens. Here, the focus correction lens is installed in the vicinity of the curved surface of the transparent object to be inspected so that the transparent light is applied to the focus correction lens before the transmitted light from the transparent object is refracted by the lens effect to largely change the direction. This is because the focus correction is surely performed by transmitting all the transmitted light from the focus correction lens. Further, as the focus correction lens, a lens having substantially the same curvature as the curvature of the transparent object and having the same lens effect is used for effective focus correction. That is, it is assumed that the transparent object to be inspected is the optical glass lens (convex lens) 3 as shown in FIG. 1, and the focus correction lens 6 is the same optical glass lens (however, there is no defect). Since the focus of the solid-state imaging device camera 7 is set at the center of the curved surface of the optical glass lens 3, the portion where the focus correction is actually required is the peripheral portion of the curved surface of the optical glass lens 3. Optically, the speed of light that passes through the lens is inversely proportional to the refractive index of the lens, and even the light that passes through the same lens has a different focus speed on the imaging surface of the camera due to the difference in the speed at which it reaches the imaging surface of the camera. There are some places that do not match. The light transmitted through the central portion of the optical glass lens 3 is also transmitted through the central portion of the focus correction lens 6 near the optical glass lens 3 with little refraction, so that even if the focus correction lens 6 is installed, The transmitted light in the central portion has almost no influence on the focus on the image pickup surface of the camera. However, when the light transmitted through the peripheral portion of the optical glass lens 3 is transmitted through the focus correction lens 6, the light closer to the central portion is refracted more than the light transmitted through the optical glass lens 3 The speed at which the light is slowed is large, and the difference in the speed of the light transmitted from the central portion is reduced. This corrects the focus on the peripheral portion of the curved surface of the optical glass lens 3 on the imaging surface of the camera. In this case, since the focus correction lens 6 is the same as the optical glass lens 3, the focus correction can be performed evenly. Then, the focus-corrected image is magnified and then divided into pixels by the solid-state image sensor camera to be imaged.

Since the transparent object to be inspected has a curved surface,
When the curved surface is photographed by a solid-state image sensor, the image produces shading according to the curvature of the curved surface. That is,
When the optical axis of the solid-state imaging device camera is aligned with the center of the curved surface, if the curved surface is convex, the brightness decreases from the central portion to the peripheral portion. It is conceivable to optically perform the compensation, that is, the shading correction, but since the optical system becomes complicated, in the present invention, the binarization threshold value of the pixel output of the solid-state imaging device camera is set to the pixel according to the curvature of the curved surface. Shading correction is performed by image processing that is changed every time.

Since the pixel output before the binarization process shows a gradation difference according to the curvature due to the difference in the position of the curved surface, the curvature of the imaged curved surface is obtained by comparing the gradation difference. You can Then, if a variable binarization method of adjusting the binarization threshold value according to the obtained curvature is adopted, that is, if the pass / fail judgment level for binarization is not fixed, but changed according to the shading characteristics, Defect inspection can be performed without being affected by the positional deviation of transparent objects.

[0012]

Next, an embodiment of the present invention will be described.
FIG. 1 shows an overview of a device for carrying out the method according to the invention.
This device is a detection / handling device 1 and an inspection processing device 2.
Is roughly divided into The detection / handling apparatus 1 sets a transparent object to be inspected, for example, a press-molded optical glass lens (convex lens) 3 at a predetermined inspection position, and diffuses light from the diffused light source 4 from below the inspection position. Irradiate 3. As the diffused light source 4, for example,
A structure having a diffusing plate for diffusing light from a fluorescent lamp that is turned on at a high frequency by the stabilized power supply 5 is used.

The light transmitted through the optical glass lens 3 to be inspected is transmitted through the focus correction lens 6 disposed in the vicinity above the optical glass lens 3 and further above the magnifying glass of the CCD camera 7 with a magnifying glass (for example, a zoom microscope). CCD camera 7 through 8
Light enters the 2D image sensor. That is, the curved surface (upper surface) 3a which is the surface to be inspected of the optical glass lens 3 is subjected to focus correction by the focus correction lens 6 and magnified to a desired magnification by the magnifying glass 8, and then imaged by the CCD camera 7. As the focus correction lens 6, if an optical glass lens having no defects is selected from the same optical glass lenses as the optical glass lens 3 to be inspected and used, it is not necessary to prepare a dedicated lens.

The inspection processing apparatus 2 comprises a personal computer 2a which is an input / arithmetic / control means, an image processing circuit 9, a signal input / output circuit 10 and a monitor switch 11. The inspection processing device 2 is a detection / handling device 1.
The inspection is performed according to the inspection timing signal input from the signal input / output circuit 10 to the detection / handling apparatus 1 when the inspection result indicates that there is a defect. The pixel output from the two-dimensional image sensor of the CCD camera 7 is taken into the image processing circuit 9 in accordance with the image taking timing signal inputted from the detection / handling device 1 to the signal input / output circuit 10, and is subjected to image processing as described later. It Further, by switching the monitor switching device 11, the captured image of the CCD camera 7 which is not image-processed is displayed on the CRT 12 of the personal computer 2a.
It can also be displayed on the screen. Although the personal computer is used in this example in terms of operability, it goes without saying that the input / calculation / control may be performed by other means.

When the optical glass lens 3 to be inspected is a convex lens and the surface 3a to be inspected is a spherical surface as shown in FIG. 2, when this spherical surface is directly imaged by the CCD camera 7,
The inspectable range φ1 of the defect on the spherical surface is determined by the depth of field D1 of the camera 7 itself. Now, the inspection target range is wider than the inspectable range φ1 φ2 (φ1> φ2)
In this case, if the defects within the inspection target range φ2 are to be inspected at one time, the depth of field D of the CCD camera 7 itself
If it depends on 1, the portion between φ1 and φ2 will be blurred, and defects in this range cannot be detected.

However, in the present invention, since the focus correction lens 6 is installed near the surface 3a to be inspected of the optical glass lens 3 to correct the focus, the depth of field can be expanded from D1 to D2. As a result, defects within the inspection target range φ2 can be inspected at once.

FIG. 3 is a graph of measurement results obtained by experimentally examining the difference in depth of field between when the focus correction lens 6 is used and when the focus correction lens 6 is not used. In this experiment, the optical glass lens 3 to be inspected is a press-molded glass lens having a diameter of about 12 mm, a thickness of about 6.7 mm, and spherical surfaces with different curvatures on both sides, and one of the spherical surfaces having a foreign substance of about 50 μm. Was used as a sample, and a press-molded glass lens molded in a similar manner to this was used as the focus correction lens 6. Then, the S / N ratio on the surface 3a to be inspected was examined while moving the position of the CCD camera 7 with the magnifying glass toward the surface 3a to be inspected by 0.5 mm. As a result, when the focus correction lens 6 is not provided, the S / N ratio is 1.
The movement distance of the CCD camera 7 that is 5 or less is about 1.5 mm
On the other hand, when the focus correction lens 6 is provided,
The moving distance of the CCD camera 7 having an S / N ratio of 1.5 or less is doubled to about 3 mm.

FIG. 4 is a diagram in which one spherical surface of the optical glass lens 3 similar to the above sample is imaged by the CCD camera 7 through the magnifying glass 8 set at a magnification of 25 and projected on the CRT 12, and is indicated by an arrow. The dots show defects with a size of about 25 μm. As shown in FIG. 5, the pixel output from the two-dimensional image sensor of the CCD camera 7 was scanned along the line crossing this defect, and the luminance distribution was measured.
It was like. As can be seen from this figure, the defect portion becomes a dark portion and the brightness gradation value sharply decreases, and as the curved surface (convex surface) 3a of the optical glass lens 3 moves from the central portion to the peripheral portion, the curvature of the curved surface changes according to the curvature. The brightness gradation value is decreasing (shading). FIG. 7 is an enlarged view of the luminance distribution in the peripheral portion around the defect.

Therefore, in the present invention, when the captured pixel output is binarized by the image processing circuit 9, the curvature of the curved surface 3a is obtained from the gradation difference of the pixel groups arranged in the scanning direction of the pixel output, and according to the curvature. Set a binarization threshold. That is, in FIG. 7, the curved surface linear LN is obtained from the luminance distribution,
The shading correction is performed by setting the binarization threshold value SH along a line that is lower than the luminance forming the linear LN by a predetermined level. Then, a pixel whose gradation value is equal to or less than this binarization threshold value SH is a defective pixel, and when the number of defective pixels in the set area is a predetermined number or more, it is determined that there is a defect and an exclusion signal is output.

Although the examples in which the press-molded optical glass lens is the object of inspection have been described above, the present invention is not limited to such a lens, and other optical lenses having curved surfaces, glass products, or precision-processed. It can be widely applied to the inspection of minute defects in transparent plastic products having curved surfaces. Further, although the variable binarization method is adopted in the above embodiment,
A fixed binarization method may be used. Furthermore, photoelectric conversion means other than the solid-state image sensor camera may be used, and the output thereof may be subjected to analog processing to detect defects, and the light source is not limited to the diffused light source.

[0021]

As described above, the present invention has the following effects. A focus correction lens is installed near the curved surface of the transparent object to be inspected, and before the transmitted light from the transparent object is refracted by the lens effect to change the direction significantly, Since all the light is transmitted through the focus correction lens, focus correction can be surely performed. As the focus correction lens, a lens having the same curvature as that of the transparent object and having the same lens effect is used, so that the focus correction can be effectively performed, and the focus correction averaged over a wide area can be performed. Since the image whose focus has been corrected as described above is enlarged and then pixel-divided by the solid-state image pickup device camera to pick up an image, a minute defect can be detected accurately and efficiently over a large area.

When the object to be inspected is an optical glass lens as in claim 2, if an optical glass lens similar to that having no defect is used as the focus correction lens, it is not necessary to separately prepare a dedicated focus correction lens. Therefore, it is economical and the focus can be properly corrected.

If the binarization threshold value of the pixel output is changed for each pixel according to the curvature of the curved surface as in claim 3, shading correction can be easily performed by image processing without using an optical system.

According to the fourth aspect, if the variable binarization method is adopted in which the curvature of the curved surface is obtained from the gradation difference between the pixel groups arranged in the pixel output scanning direction and the binarization threshold value is adjusted according to the curvature,
Defect inspection can be performed without being affected by displacement of the inspection target.

[Brief description of drawings]

FIG. 1 is a schematic block diagram of an apparatus for carrying out the method of the present invention.

FIG. 2 is a diagram illustrating that an inspectable range for a curved surface of an optical glass lens to be inspected is determined by a depth of field of a CCD camera.

FIG. 3 is a graph of measurement results obtained by experimentally examining a difference in depth of field between when a focus correction lens is used and when the focus correction lens is not used.

FIG. 4 is a diagram in which a spherical surface of an optical glass lens having a defect is imaged by a CCD camera with a magnifying glass and displayed on a CRT.

FIG. 5 is a diagram showing scanning pixel outputs along a line that crosses a defect as above.

FIG. 6 is a graph in which a luminance distribution is measured by the same scanning.

FIG. 7 is a diagram for explaining a method of setting a binarization threshold value by enlarging a part of FIG. 6;

Claims (4)

[Claims] 1. A focus correction lens having the same curvature and a similar lens effect is installed in the vicinity of a curved surface of a transparent object to be inspected having a lens effect, and diffused light is transmitted from the opposite side to the transparent object. Light radiated and transmitted by the lens effect of the transparent object is further transmitted in the vicinity of the transparent object with a similar lens effect by the focus correction lens, and the transmitted light is magnified by the magnifying optical system to be used by the solid-state image sensor camera. It is characterized by inspecting the presence or absence of micro defects by receiving light and digitally processing the pixel output to detect dark areas.
A method for inspecting minute defects in a transparent object having a curved surface. 2. When the transparent object to be inspected is an optical glass lens, the same optical glass lens having no defects is used as the focus correction lens, and the transparent object having a curved surface according to claim 1 is used. Micro defect inspection method. 3. The binarization threshold value of the pixel output of the solid-state imaging device camera is changed for each pixel according to the curvature of the curved surface of the transparent object to be inspected.
A method for inspecting minute defects in a transparent object having a curved surface. 4. The curvature of a curved surface, which is the surface to be inspected, is calculated from the gradation values of the pixel groups arranged in the operation direction of pixel output, and the binarization threshold value is adjusted according to the curvature. The method for inspecting minute defects of a transparent object having a curved surface.