CN110006924B - Method for detecting two-dimensional profile of micro defect on surface of optical element - Google Patents

Method for detecting two-dimensional profile of micro defect on surface of optical element Download PDF

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CN110006924B
CN110006924B CN201910311988.6A CN201910311988A CN110006924B CN 110006924 B CN110006924 B CN 110006924B CN 201910311988 A CN201910311988 A CN 201910311988A CN 110006924 B CN110006924 B CN 110006924B
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王红军
武雄骁
田爱玲
刘卫国
王大森
朱学亮
刘丙才
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    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
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    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/8851—Scan or image signal processing specially adapted therefor, e.g. for scan signal adjustment, for detecting different kinds of defects, for compensating for structures, markings, edges
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    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
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Abstract

The invention relates to the technical field of optical surface defect detection, in particular to a method for detecting a two-dimensional profile of a tiny defect on the surface of an optical element. The problems of complex structure, low measurement precision, large data volume and long measurement time in the prior art are solved. The method comprises the following steps: 1) the laser beam forms convergent light through a beam shaping system, the convergent light reaches a CCD target surface after being reflected by a sample, the position and the posture of the sample are adjusted, so that a convergent point is positioned in the center of the target surface and is vertical to the CCD target surface, and the distance from the reflective point to the center of the target surface is L; 2) irradiating the sample surface defect area with light beams, and obtaining a scattering distribution diagram on the CCD; 3) calculating the central position of the scattering distribution diagram, and taking values along different directions by taking the central position as a reference point to obtain a one-dimensional light intensity distribution curve I (w); 4) and repeating the process of the third step, and taking values of the scattering map in different directions to obtain width values of the defect in different sampling directions, so as to fit a two-dimensional profile of the defect.

Description

Method for detecting two-dimensional profile of micro defect on surface of optical element
Technical Field
The invention relates to the technical field of optical surface defect detection, in particular to a method for detecting a two-dimensional profile of a tiny defect on the surface of an optical element.
Background
With the development of future science and technology, the optical system is required to adapt to the harsh working conditions of strong laser and high energy gathering or achieve ultrahigh resolution. These limitations of optical systems have increased the demand for surface quality of optical components. Therefore, the technical difficulty and requirement for defect detection will be very severe, and therefore, more accurate and efficient surface defect detection methods must be studied for various types of optical elements.
The domestic surface microdefect detection method comprises a visual method, a filtering imaging method, a frequency spectrum analysis method and the like, and the detection result has the problems of low efficiency, poor accuracy and the like. The visual method is still widely applied to domestic optical element detection as an original detection method. Under dark field illumination environment, an observer observes the surface of an optical element by using a magnifying glass or naked eyes and judges surface defects according to self experience. The method has the defects of very obvious subjectivity, easily influenced detection results by experience and fatigue of detection personnel, low detection efficiency and unstable precision due to different detection qualities, and limits the development of the detection method. The filter imaging method is similar to the visual method in basic principle, but different in that the defect is not directly observed by naked eyes, but is replaced by an optical sensor, and the size and the characteristics of the defect are judged by observing and testing the size and the brightness of the defect image. The spectrum analysis method is that scattered light caused by the defects on the surface of the optical element passes through a Fourier lens, the energy of the defect backward diffraction spectrum is obtained through the light intensity distribution of a back focal plane, and then the defect size and depth condition are obtained through energy integration and defect morphological treatment. Its disadvantage is that the surface area of the surface layer of the defect cannot be reflected due to the influence of the deep structure of the defect.
At present, surface defect detection related to machine vision has become a hot research field at home and abroad. Machine vision is a new detection technology, a computer vision technology and an image processing technology are integrated, digital image detection is used, an object is identified by a machine, useful information is extracted by an image processing method, the current industrial production process gradually tends to be automatic, the machine vision can exert own advantages, and the machine vision is applied to working environments which cannot be observed or are dangerous by human eyes, and has the advantages of real-time performance, flexibility, accuracy and the like. Although the surface defect detection technology of optical elements by machine vision has been greatly developed in recent years, the machine vision still has no capability of detecting tiny defects under the influence of the diffraction limit of an imaging system.
The scattering method is a new surface defect detection technology, the surface defects are detected through the scattering distribution of the surface defects in a scattering space, the brightest first-level stripes are found out in the defect detection of the scattering method, and the defect size is obtained through calculation according to the corresponding relation of diffraction. In the measurement process, the size of the defect is closely related to the size of the detection area, and for small defects, the measurement space needs to be increased, thereby increasing the complexity of the measurement system. The external noise pair in the measurement can also reduce the signal-to-noise ratio of the scattering signal, and the misjudgment of the fringe position and the defect distribution value occurs, so that the measurement precision is influenced. The detection of the whole surface shape also generates a great amount of data and consumes more measurement time.
Disclosure of Invention
The invention provides a method for detecting micro defects on the surface of an optical element, which aims to solve the problems of complex structure, low measurement precision, large data volume and long measurement time in the prior art.
In order to achieve the purpose of the invention, the technical scheme provided by the invention is as follows: a method for detecting micro defects on the surface of an optical element comprises the following steps:
step one, laser beams emitted by a laser form convergent light through a beam shaping system, the convergent light reaches a CCD target surface after being reflected by a sample, the position and the posture of the sample are adjusted, a convergent point of the convergent light is positioned at the center of the CCD target surface, a convergent optical axis is perpendicular to the CCD target surface, and the distance from the reflection point to the center of the CCD target surface is L;
step two, irradiating the light beam on the surface defect area of the sample, and obtaining a scattering distribution map on the CCD;
step three, calculating the center position of the scattering distribution diagram by using a gravity center method, taking the center position as a reference point, and taking values along different directions to obtain a one-dimensional light intensity distribution curve I (w), wherein the abscissa of the curve is the distance w between the point and the reference point, the ordinate is a gray scale image of the image, and the light intensity I corresponding to the point
By
Figure BDA0002031809890000031
Conversion of light intensity distribution curve to l (theta)) Converted into a two-dimensional matrix [ I, theta ]]Wherein I and theta are one-dimensional matrixes, and the number of elements is the number of samples N
Taking the maximum value of the light intensity I as Imax and the sampling interval as deltaImaxMinimum value Imin of light intensity I, sampling interval ΔImin. Setting the width range of the defect in the value direction as (d)min,dmax) The value interval is deltad
The width of the defect in the sampling direction is calculated as follows:
step 1: setting the wavelength λ of incident light, setting I0＝0，Ig＝0，Irms0＝0，d＝dmin；
Step 2: reading a two-dimensional matrix [ I, theta ];
step 3: substituting d, theta and lambda into
Figure BDA0002031809890000032
Obtaining a one-dimensional matrix alpha;
step4 reaction of alpha, I0And IgSubstitution into
Figure BDA0002031809890000033
Obtaining a one-dimensional matrix I1；
Step 5: calculating a root mean square value of 1rms＝rms(I1-I)；
Step 6: if Irms＜Irms0Then, Irms0＝Irms；d′＝d；
Step7：Ig＝Ig+ΔIminIf I isg＜IminGo to Step4
Step8：I0＝I0+ΔImaxIf I is0＜ImaxGo to Step4
Step9：d＝dmin、+ΔdIf d < dmaxGo to Step3
Step 10: d' is the calculated width of the defect in the sampling direction;
and step four, repeating the process of the step three, and taking values of the scattering map in different directions to obtain width values of the defect in different sampling directions, so as to fit a two-dimensional profile of the defect.
Compared with the prior art, the invention has the advantages that:
1. the invention uses the diffraction distribution diagram obtained by the CCD camera to identify the defects, obtains the light intensity distribution curve according to the image processing, simultaneously compares the light intensity distribution curve with the theoretical values under different parameters in a certain range, and determines the resolution ratio of the defect width by the parameter value interval, thereby achieving high resolution measurement and realizing the detection of the tiny defects.
2. The scattering method is characterized in that a scattering distribution diagram is obtained in a large scattering space, the data of light intensity distribution curves caused by different parameters are subjected to contrastive analysis and circular fitting to realize the optimization of structural parameters, and the optimal defect size is finally matched with the closest defect size, so that the two-dimensional characteristics of surface defects can be realized by utilizing images acquired by a CCD (charge coupled device) in a small detection area.
3. The equipment used by the detection method of the invention has simple structure, the light intensity is calculated according to the scattering distribution generated after the light passes through the small detection area, when the bright stripes can not be observed, the time can be consumed for the detection of each point in a large range for the whole surface type, the smaller the area is, the more time is saved, the generated data volume is small, and the measurement time is short.
Description of the drawings:
FIG. 1 is a schematic diagram of surface defect detection;
fig. 2 is a scatter profile.
The specific implementation mode is as follows:
the present invention will be described in detail below with reference to the drawings and examples.
A method for detecting micro defects on the surface of an optical element comprises the following steps:
step one, referring to fig. 1, a laser beam emitted by a laser forms convergent light through a beam shaping system, the convergent light reaches a CCD target surface after being reflected by a sample, the position and the posture of the sample are adjusted, a convergent point of the convergent light beam is positioned at the center of the CCD target surface, a convergent optical axis is perpendicular to the CCD target surface, and the distance from the reflection point to the center of the CCD target surface is L.
Step two, obtaining a scattering distribution diagram: if the surface of the sample is an ideal smooth surface, the reflected beam converges to the center of the target surface of the CCD as an impulse function. When there is a defect in the beam illumination area on the sample surface, scattered light will be formed in the hemispherical space of the reflection surface, and the scattering distribution shown in fig. 2 can be obtained on the CCD.
And step three, calculating the center position of the scattering distribution graph by using a gravity center method, taking the center position as a reference point, and taking values along different directions to obtain a one-dimensional light intensity distribution curve I (w), wherein the abscissa of the curve is the distance w between the point and the reference point, and the ordinate is a gray scale image of the image and corresponds to the light intensity I of the point.
According to the geometrical relationship of the system, the following are:
Figure BDA0002031809890000051
therefore, the light intensity distribution curve can be converted into l (theta) and converted into a two-dimensional matrix [ I, theta ], wherein I and theta are one-dimensional matrixes, and the number of elements is the number of samples N.
By
Figure BDA0002031809890000052
The light intensity distribution curve is converted into l (theta), and is converted into a two-dimensional matrix (I, theta)]Wherein I and theta are one-dimensional matrixes, and the number of elements is the number of samples N.
Taking the maximum value of the light intensity I as Imax and the sampling interval as deltaImaxMinimum value Imin of light intensity I, sampling interval ΔImin. Setting the width range of the defect in the value direction as (d)min,dmax) The value interval is deltad。
The width of the defect in the sampling direction is calculated as follows:
step 1: setting the wavelength λ of incident light, setting I0＝0，ig＝0，Irms0＝0，d＝dmin；
Step 2: reading a two-dimensional matrix [ I, theta ];
step 3: substituting d, theta and lambda into
Figure BDA0002031809890000061
Obtaining a one-dimensional matrix alpha;
step4 reaction of alpha, I0And IgSubstitution into
Figure BDA0002031809890000062
Obtaining a one-dimensional matrix I1；
Step 5: calculating the root mean square value Irms＝rms(I1-I)；
Step 6: if Irms＜Irms0Then, Irms0＝Irms；d′＝d；
Step7：Ig＝Ig+ΔIminIf I isg＜IminGo to Step4
Step8：I0＝I0+ΔImaxIf I is0＜ImaxGo to Step4
Step9：d＝dmin+ΔdIf d < dmaxGo to Step3
Step 10: d' is the calculated width of the defect in the sampling direction;
and step four, repeating the process of the step three, and taking values of the scattering map in different directions to obtain width values of the defect in different sampling directions, so as to fit a two-dimensional profile of the defect.

Claims (1)

1. A method for detecting two-dimensional contours of micro defects on the surface of an optical element is characterized by comprising the following steps: the detection method comprises the following steps:
step one, laser beams emitted by a laser form convergent light through a beam shaping system, the convergent light reaches a CCD target surface after being reflected by a sample, the position and the posture of the sample are adjusted, a convergent point of the convergent light is positioned at the center of the CCD target surface, a convergent optical axis is perpendicular to the CCD target surface, and the distance from the reflection point to the center of the CCD target surface is L;
step two, irradiating the light beam on the surface defect area of the sample, and obtaining a scattering distribution map on the CCD;
step three, calculating the center position of the scattering distribution diagram by using a gravity center method, taking the center position as a reference point, and taking values along different directions to obtain a one-dimensional light intensity distribution curve I (w), wherein the abscissa of the curve is the distance w between each point and the reference point in the sampling direction, the ordinate is a gray scale diagram of the image, and the light intensity I corresponding to the point
By
Figure FDA0002992610890000011
The light intensity distribution curve is I (theta), and is converted into a two-dimensional matrix (I, theta)]；
Taking the maximum value of the light intensity I as Imax and the sampling interval as deltaImaxMinimum value Imin of light intensity I, sampling interval ΔIminThe width of the defect in the value-taking direction is set to be (d)min，dmax) The value interval is deltad
The width of the defect in the sampling direction is calculated as follows:
step 1: setting the wavelength λ of incident light, setting I0＝0，Ig＝0，Irms0＝a，d＝dmin；
Step 2: reading a two-dimensional matrix [ I, theta ];
step 3: substituting d, theta and lambda into
Figure FDA0002992610890000012
Obtaining a one-dimensional matrix alpha;
step4: alpha, I0And IgSubstitution into
Figure FDA0002992610890000013
Obtaining a one-dimensional matrix I1；
Step 5: calculating the root mean square value Irms＝rms(I1-I)；
Step 6: if Irms＜Irms0Then, Irms0＝Irms；d′＝d；
Step7：Ig＝Ig+ΔIminSuch asFruit Ig＜IminGo to Step 4;
Step8：I0＝I0+ΔImaxif I is0＜ImaxGo to Step 4;
Step9：d＝dmin+Δdif d < dmaxGo to Step 3;
step 10: d' is the calculated width of the defect in the sampling direction;
and step four, repeating the process of the step three, and taking values of the scattering map in different directions to obtain width values of the defect in different sampling directions, so as to fit a two-dimensional profile of the defect.
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