CN111830043A - A surface defect detection device and detection method based on spatial frequency shift regulation - Google Patents

Abstract

The invention discloses a surface defect detection device and a detection method based on spatial frequency shift regulation. According to the detection method, the illuminating light with higher wave vector is used for moving the higher-frequency components of the sample in the frequency spectrum space, so that when the detection is carried out in a far field, the high-frequency components reflecting defect information can enter a lens field of an image acquisition system, and the defects are effectively detected. The method can realize the amplification effect on the tiny defects which are difficult to obtain in the traditional illumination mode, so that the tiny defects can be obtained through a simple image obtaining system, and the characteristic of high-resolution imaging is realized.

Description

A surface defect detection device and detection method based on spatial frequency shift regulation

technical field

The invention relates to the fields of machine vision and surface defect detection, in particular to a surface defect detection device and detection method based on spatial frequency shift regulation.

Background technique

The heat dissipation film of the mobile phone is at the bottom of the mobile phone display, which can effectively dissipate the heat of the mobile phone screen. However, when there are defects on the surface of the film, it will seriously affect the display effect and heat dissipation effect of the mobile phone screen. In addition, small defects on the polarizer and display panel of the mobile phone will also seriously affect the display effect of the screen. At present, the detection of surface defects of such films can be realized by machine vision. With the increasing requirements of mobile phone manufacturers for screen quality, it is required to effectively detect defects on the surface of the film smaller than 45 microns. This is the current traditional machine. What is not possible with visual methods. The invention proposes a small defect detection method based on spatial frequency shift regulation. The method uses illumination light with a higher wave vector to move the higher frequency components of the sample in the spectral space to realize far-field detection, so that the defect information can be reflected. High-frequency components can enter the field of view of the lens of the image acquisition system, so that defects can be effectively detected. The method can amplify the small defects that are difficult to obtain by traditional illumination methods, so that the small defects can be obtained through a simple image acquisition system, and the characteristics of high resolution are realized.

SUMMARY OF THE INVENTION

The invention discloses a method for regulating the frequency shift amount of spatial light, and a device and method for detecting surface defects of thin films based on the method. A line scan camera or an area scan camera is used to obtain the reflected light image of the surface of the sample to be tested to detect the surface defects of the film, which solves the problems of low efficiency and low machine detection accuracy of the existing manual visual inspection methods.

The technical scheme adopted by the present invention to solve its technical problems is:

A surface defect detection device is characterized in that the device comprises a light source 1 , a sample transmission mechanism 2 , a sample to be tested 3 , a line array or area array camera 4 , an image acquisition card 5 and a computer 6 .

The light source is a bar light source.

The length of the strip light source should be greater than 3 times of the sample to be tested.

The light wavelength of the light source selects short wavelength light to obtain a large transverse light wave vector, and the wavelength range is 330-700 nm.

The light source irradiates the surface of the sample to be tested for oblique incidence, and the incident angle ranges from 10 to 80 degrees.

When the light source illuminates the sample to be tested, the reflected light should be placed in the imaging field of view of the camera at the junction of light and dark.

A surface defect detection method is characterized in that, comprising: realizing high-resolution imaging by adjusting the frequency shift amount of input light, increasing the transverse wave vector of illumination light, so that during far-field imaging, the high frequency of reflected light after passing through the sample The space part can enter the field of view of the camera lens.

By increasing the transverse wave vector of the incident light, the defects of the sample to be tested are amplified, thereby realizing super-resolution imaging.

The high-frequency spatial part of the reflected light after the illumination light passes through the sample can enter the field of view of the camera lens.

When the light source illuminates the sample to be tested, the light and dark boundary of the reflected light should be within the imaging field of view of the camera.

The invention has the beneficial effects of good integration, mass production and easy operation. It has the characteristics of large field of view, fast imaging and ultra-high resolution in super-resolution imaging. The invention can obtain the clearest defect image by selecting the incident light with a suitable incident angle and reasonable wavelength, and when the light source irradiates the sample to be tested, so that the light-dark junction of the reflected light is within the imaging field of view of the camera, so that the clearest defect image can be obtained, and the efficiency is high and accurate. high rate.

Description of drawings

FIG. 1 is a diagram showing the relationship between the transverse wave vector of incident light and the incident angle of the present invention.

Figure 2 is a graph of calculated image contrast under different illuminations.

FIG. 3 is a block diagram of a measuring apparatus according to an embodiment of the present invention.

FIG. 4 is a comparison diagram of defect detection effects of illumination light of different wavelengths.

Figure 5 is a comparison diagram of defect detection effects under different incident light angles.

Figure 6. Comparison of light and dark and uniform field illumination.

Detailed ways

The embodiment of the present invention provides a film surface defect detection device and method, which realizes detection of film surface defects by acquiring a reflected light image from the surface of a sample to be tested by a line scan camera or an area array camera, which solves the problem of existing manual visual inspection. The method has problems such as low efficiency and great influence by human factors.

In order to make the objectives, features and advantages of the present invention obvious, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, but the protection of the present invention should not be limited by this. scope.

According to the Abbe imaging principle, an object is a collection of a series of different spatial frequency information, and these spectral information are displayed on the back focal plane of the lens through the lens. These spectral components include DC components with the same frequency as the incident light and high-frequency components containing detailed information about the object. The surface defects of the object to be tested are all reflected by high-frequency information. When a light with a wavelength of λ is incident on the surface of the sample to be measured, assuming that there is a defect with a width of d on the surface of the sample, the transverse wave vector of the reflected light after the light passes through the defect can be expressed as: The frequency information can be expressed as:

为入射光的波矢，且有

为待测样品缺陷引起的反射光的波矢，且有

根据衍射极限可知，当成像镜头的数值孔径为NA时，相机能接收的物体的频率成分满足如下关系：In the above formula

is the wave vector of the incident light, and has

is the wave vector of the reflected light caused by the defect of the sample to be tested, and has

According to the diffraction limit, when the numerical aperture of the imaging lens is NA, the frequency components of the object that the camera can receive satisfies the following relationship:

可知，采用短波长的光或增大入射角(入射光与检测面法线夹角，将增加入射光横向方向上的波矢，从而达到放大待测样品表面缺陷而实现微小缺陷检测的目的。横向波矢与入射光波矢及角度的关系如图1所示。In the above formula, the sum of the two wavevectors on the left exceeds the frequency receiving range of the imaging lens, and the subtracted part of the two wavevectors can be received by the lens and imaged in the camera. cut-off frequency of light. It can be seen from equation (2) that when the transverse wave vector of the incident light is increased, the wave vector of the reflected light of the sample can be increased, that is, its spatial frequency can also be increased. An increase in the spatial frequency of the sample means a decrease in the size of the sample, which is equivalent to amplifying the defects. according to

It can be seen that using short wavelength light or increasing the incident angle (the angle between the incident light and the normal line of the detection surface will increase the wave vector in the lateral direction of the incident light, so as to achieve the purpose of amplifying the surface defects of the sample to be tested and realizing the detection of small defects. The relationship between the transverse wave vector and the incident light wave vector and angle is shown in Figure 1.

其中I是信号强度，I_b是背景强度。通过计算可以发现明暗相间处的对比度最高，如图2所示。从图2可知，左边为光场图，右边为局部对比度图，光场图为正弦分布的光场，可以看出在峰和谷间隙的A、B点处，光场变化最剧烈，在对比度图中，A、B点处的对比度是峰值，位于强度峰或谷处的点的对比度等于零。峰和谷间隙处的光场即为明暗相间处的光场。Contrast and sharpness are as important as resolution if defects are to be clearly visible. Contrast: refers to the measurement of different brightness levels between the brightest white and the darkest black in the light and dark areas of an image. The greater the difference range, the greater the contrast. Sharpness: is the sharpness of edge changes in image detail. At the edge of image details, the sharper (faster change) and sharper (larger contrast) changes in optical density or brightness with position are, the clearer the edge of the details and the higher the degree of discernment. For local details, sharpness, not contrast, should be decisive. Sharpness here can be defined as the local contrast, that is, the size of the contrast in a small range. According to Weber's law,

where I is the signal intensity and I _b is the background intensity. Through calculation, it can be found that the contrast between light and dark is the highest, as shown in Figure 2. As can be seen from Figure 2, the left side is the light field map, the right side is the local contrast map, and the light field map is the light field with sinusoidal distribution. It can be seen that at points A and B between the peak and valley gaps, the light field changes most drastically, and the contrast In the figure, the contrast at points A and B is the peak, and the contrast at the point at the intensity peak or valley is equal to zero. The light field at the gap between the peaks and valleys is the light field between light and dark.

Based on the above analysis, the present invention provides a detection device and method for detecting film surface defects. As shown in FIG. 3, the detection device includes a light source 1, a sample transmission mechanism 2, a film sample to be tested 3, a linear array or an area array Camera 4, Frame Grabber 5 and Computer 6.

The light source 1 is incident on the sample to be tested at a certain angle, and the reflected light after the incident light is reflected by the sample enters the camera 4 to realize the imaging of the sample to be tested by the camera, and the camera 4 transmits the obtained image of the sample to be tested to the image acquisition. Defects on the surface of the sample to be tested are automatically determined in the card 5 and through the image processing algorithm in the computer 6 . During measurement, the sample to be tested is moved by the transmission mechanism, so that the camera can effectively image all areas of the sample, so as to obtain a complete image of the sample.

The light source 1 of the detection device is a strip light source. In order to achieve the best imaging effect, the length of the selected strip light source should be 3 times longer than that of the sample to be tested.

The core of the line or area scan camera 4 is the lens. The basic function of the lens is to realize beam transformation (modulation). In the machine vision system, the main function of the lens is to image the target on the photosensitive surface of the image sensor. The quality of the lens directly affects the overall performance of the machine vision system. Reasonable selection and installation of the lens is an important part of the design of the machine vision system. When evaluating the quality of industrial lenses, it is generally judged from several practical parameters such as resolution, sharpness and depth of field: 1. Resolution: Also known as the discrimination rate and resolution, it refers to the ability of the lens to clearly distinguish the fiber details of the subject. , The reason that restricts the resolution of industrial lenses is the diffraction phenomenon of light, that is, the diffraction spot (Airy spot). The unit of resolution is "line pairs per millimeter" (lp/mm). 2. Acutance: Also known as contrast, it refers to the contrast between the brightest and darkest parts of an image. 3. Depth of Field (DOF): In the scene space, the scene located within a certain distance before and after the focusing object plane can also form a relatively clear image. The depth distance between the above-mentioned objects located in front of and behind the focusing object plane that can form a relatively clear image, that is, the spatial depth range of the scene that can obtain a relatively clear image on the actual image plane, is called the depth of field. 4. Maximum relative aperture and aperture coefficient: relative aperture refers to the ratio of the diameter of the incident light aperture (represented by D) to the focal length (represented by f) of the industrial lens, namely: relative aperture=D/f. The reciprocal of the relative aperture is called the aperture scale, also known as the f/stop or aperture number. The relative aperture of general lenses can be adjusted, and the maximum relative aperture or aperture factor is often marked on industrial lenses, such as 1:1.2 or f/1.2. If the shooting scene is dark or the exposure time is short, you need to try to choose an industrial lens with a larger maximum relative aperture. Comprehensive consideration, the VST VS-L10028/F model of the Japanese VST industrial lens VS-L(F) series is selected.

The light wavelength of the light source selects short wavelength light to obtain a large transverse light wave vector, and a reasonable wavelength range is 330-700 nm. Figure 4 shows the difference in the defect detection effect of illumination light with different light wavelengths. As shown in Figure 4, when the light source emits ultraviolet light, the wavelength range of the ultraviolet light used is 330-400 nanometers, and the detected defect image is clear. When the light source emits red light, the wavelength range of the red light used is 620-700 nanometers. , the detected defect image can be seen, and it can be seen that when the ultraviolet light with a shorter wavelength is used, a larger transverse light wave vector can be obtained.

When the light source irradiates the surface of the sample to be tested with oblique incidence, a large transverse light wave vector can also be obtained, and the reasonable incident angle range is 10-80 degrees. Figure 5 shows the difference in the defect detection effect of different illumination light incident angles. As shown in Figure 5, when the light source uses ultraviolet light, the defect image photo when the incident angle is 10 degrees, it can be seen that the water ripple defect is faintly visible, and the defect image photo when the incident angle is 80 degrees, it can be seen that the water ripple defect is faintly visible, When the incident angle is 45±5 degrees, it can be seen that the water ripple defect is clear.

As shown in Figure 6, it is a comparison diagram of light and dark and uniform light field illumination. As can be seen from the figure, the defect images obtained when illuminated by light and dark light fields are more clearly visible.

During detection, the light source and the camera are installed on a movable mechanical structure body, and at the same time, they need to be installed on a mechanical structure body that can realize angle adjustment such as pitch and yaw angle. The two mechanical structure bodies are nested and installed in each other.

High-resolution imaging is achieved by adjusting the frequency shift amount of the input light. The transverse wave vector of the illumination light is increased, so that the high-frequency space part of the reflected light after passing through the sample can enter the field of view of the camera lens during far-field imaging.

In order to improve the contrast of defect identification, when the light source illuminates the sample to be tested, the light and dark boundary of the reflected light should be within the imaging field of view of the camera.

Although the illustrative specific embodiments of the present invention are described above so that those skilled in the art can understand the present invention, the present invention is not limited to the scope of the specific embodiments. As long as such changes fall within the spirit and scope of the present invention as defined and determined by the appended claims, all inventions and creations utilizing the inventive concept are included in the protection list.

Claims (10)

1. A surface defect detection device, characterized in that the device comprises a light source 1, a sample transmission mechanism 2, a sample to be tested 3, a line array or area array camera 4, an image acquisition card 5 and a computer 6. 2 . The surface defect detection device according to claim 1 , wherein the light source is a strip light source. 3 . 3. The surface defect detection device according to claim 2, the length of the strip light source should be greater than 3 times of the sample to be tested. 4 . The surface defect detection device according to claim 1 , wherein the light wavelength of the light source selects short wavelength light to obtain a large transverse light wave vector, and the wavelength range is 330-700 nm. 5 . 5 . The surface defect detection device according to claim 1 , wherein the light source irradiates the surface of the sample to be tested with oblique incidence, and the incident angle ranges from 10 to 80 degrees. 6 . 6 . The surface defect detection device according to claim 1 , wherein when the light source irradiates the sample to be tested, the junction of light and dark of the reflected light should be within the imaging field of view of the camera. 7 . 7. A surface defect detection method, characterized in that, comprising: realizing high-resolution imaging by regulating the frequency shift amount of input light, and increasing the transverse wave vector of illumination light, so that when far-field imaging is performed, the reflected light after passing through the sample is not detected. The high-frequency spatial portion can enter the field of view of the camera lens. 8 . The surface defect detection method according to claim 7 , wherein the enlargement of the defects of the sample to be tested is realized by increasing the transverse wave vector of the incident light, thereby realizing super-resolution imaging. 9 . 9 . The surface defect detection method according to claim 7 , wherein the high-frequency space part of the reflected light after the illumination light passes through the sample can enter the field of view of the camera lens. 10 . 10 . The surface defect detection method according to claim 7 , wherein when the light source illuminates the sample to be tested, the light and dark boundary of the reflected light should be within the imaging field of view of the camera. 11 .

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