Femtosecond laser processing system and three-dimensional surface topography online measurement method

Abstract

The present invention relates to the field of femtosecond laser technology, and to an online measurement method for three-dimensional surface topography, comprising: obtaining a photocurrent signal I0 of a processing point in a non-processing state; during the processing, receiving a femtosecond laser beam reflected from a component to be processed, and dividing the beam into two laser beams, which are respectively transmitted to a first photodetector and a second photodetector; decoding the photocurrent intensity in real time and decoding the photocurrent intensity I0 according to a pre-calibrated photocurrent intensity. AM The corresponding relationship between the axial depth Z and the axial depth Z is used to obtain the axial depth Z of the surface of the processing point; and the three-dimensional topography of the processing area of ​​the processing element is drawn.

Description

[0001] This patent is ZL202210141374X, and the invention name is: Divisional application for femtosecond laser processing system and three-dimensional surface topography and online measurement method. Technical Field

[0002] The present invention relates to the field of femtosecond laser technology, in particular to a femtosecond laser processing system and a three-dimensional surface topography online measurement method. Background Art

[0003] Femtosecond laser, with its ultrashort pulse width and extremely high peak intensity, has brought revolutionary changes to micro-nano processing of materials. It is widely used in cutting-edge processing, manufacturing and measurement fields related to national economy and people's livelihood, such as aerospace, quantum communications, new energy, and biomedicine.

[0004] The processing quality inspection of femtosecond laser processed components is generally carried out through two-dimensional offline measurement such as optical microscopes or scanning electron microscopes. This places high demands on the repeatable positioning accuracy of the processing component clamping and ultimately makes it impossible to guarantee the processing quality.

[0005] Traditional integrated machining measurement platforms typically incorporate a commercial CCD camera onto the machining platform, or couple an AFM and SEM to the platform. While adding a commercial CCD camera to the platform effectively allows for real-time observation of the machining status of components during processing, this approach is limited to 2D imaging. While coupling an SEM to the platform produces a strong 3D image, it is limited to 2D imaging and cannot achieve full 3D imaging. Furthermore, this approach is costly. Coupling an AFM to the platform also faces the same costly challenges.

[0006] Due to the lack of the function of online monitoring of three-dimensional surface morphology, it is time-consuming and labor-intensive to repeatedly search for processing points during measurement, making it impossible to monitor the processing quality online, and ultimately making it impossible to guarantee the reliability of processing and measurement. Summary of the Invention

[0007] The purpose of the present invention is to provide an integrated system for online measurement of processing and three-dimensional surface morphology of a femtosecond laser confocal system, so as to solve the problems of time-consuming and labor-intensive repeated search for processing points during traditional offline measurement, the inability to guarantee the positioning accuracy of repeated clamping of processing components, and the inability to perform online monitoring of the surface morphology of processing components.

[0008] The object of the present invention is achieved like this:

[0009] The online measurement method of three-dimensional surface topography processed by femtosecond laser includes the following steps:

[0010] (1) In the non-processing state, the femtosecond laser beam is positioned to the processing point of the component to be processed;

[0011] (2) Entering the processing state, the femtosecond laser beam is focused on the specified position of the processing component to perform ablation processing;

[0012] (3) During the processing, the femtosecond laser beam reflected from the component to be processed is received and split into two laser beams, which are respectively transmitted to the first photodetector and the second photodetector;

[0013] The first photodetector converts the detected light signal into a photocurrent signal I1;

[0014] The second photodetector converts the detected light signal into a photocurrent signal I2;

[0015] Real-time decoding of photocurrent intensity I AM =(I1-I2) / (I1+I2);

[0016] (4) Decoding I based on the pre-calibrated photocurrent intensity AM The corresponding coefficients a and b between the axial depth Z are used to obtain the axial depth Z=I of the surface of the processing point. AM *a+b;

[0017] Photocurrent intensity decoding I AM It is linearly related to the axial depth Z of the surface of the processed component;

[0018] (5) The processing element is displaced in the XY direction to realize point-by-point scanning of the XY plane, and the axial depth Z of each point in the XY plane is obtained;

[0019] (6) Summarizing the coordinate information and axial depth Z of the processing element in real time, and drawing the three-dimensional topography of the surface of the processing element.

[0020] The present invention also proposes a femtosecond laser processing system, comprising

[0021] XYZ three-coordinate linear displacement platform is used to fix the processing components and control the displacement of the processing components in the XYZ direction;

[0022] A femtosecond laser emitter, used to emit a laser beam;

[0023] a polarization beam splitter and a quarter-wave plate for modulating the laser beam into circularly polarized light and then transmitting the light to the first non-polarization beam splitter;

[0024] A first non-polarization beam splitter reflects the incident laser beam into the spatial light modulator;

[0025] A spatial light modulator changes the polarization direction and energy distribution of the incident laser beam as needed, and emits the laser beam to the first non-polarization beam splitter and then transmits it to the 4f system;

[0026] The 4f system performs low-pass filtering on the received laser beam and then passes it to the second non-polarization beam splitter;

[0027] The second non-polarization beam splitter splits the laser beam into N outgoing beams, which are then transmitted to the focusing objective lens according to a set method;

[0028] The focusing objective lens focuses the incident laser beam on the specified position on the processing component to perform ablation processing;

[0029] a third non-polarized beam splitter, receiving the laser beam reflected from the component to be processed, and splitting it into at least two laser beams, which are then transmitted to the first photodetector and the second photodetector respectively;

[0030] Before processing, the first photodetector and the second photodetector cooperate to record the initially detected light signal as a photocurrent signal I0;

[0031] During the processing, the first photodetector converts the detected light signal into a photocurrent signal I1;

[0032] During the processing, the second photodetector converts the detected light signal into a photocurrent signal I 2 ;

[0033] The computer calculates and draws the three-dimensional topography of the surface of the processing component based on the photocurrent signal I0, the photocurrent signal I1 and the photocurrent signal I2 in combination with pre-installed software.

[0034] Preferably, a first neutral grayscale mirror and a first focusing lens are provided between the third non-polarization beam splitter and the first photodetector, and a second neutral grayscale mirror and a second focusing lens are provided between the third non-polarization beam splitter and the second photodetector.

[0035] Preferably, the detection end of the first photodetector is in close contact with a first pinhole plate, the pinhole of the first pinhole plate corresponds to the focus of the first focusing lens; the detection end of the first photodetector is located on the focal plane of the first focusing lens and is offset from the focal center of the first focusing lens by an off-axis distance v d ;

[0036] The detection end of the second photodetector is closely attached to the second pinhole plate, the pinhole of the second pinhole plate corresponds to the focus of the second focusing lens, the detection end center of the second photodetector is aligned with the focus center of the second focusing lens, and the defocus plane has a defocus distance u d .

[0037] Preferably, the 4f system comprises a third focusing lens, a third pinhole plate and a fourth focusing lens arranged in sequence;

[0038] The third focusing lens and the fourth focusing lens cooperate to focus the incident laser beam first and then diffuse it, and then emit it to the second non-polarized beam splitter; a third pin plate hole is provided on the focal plane between the third focusing lens and the fourth focusing lens, and the pinhole of the third pin plate hole corresponds to the focal point of the third focusing lens.

[0039] Compared with the prior art, the present invention has the following outstanding and beneficial technical effects:

[0040] The present invention can monitor the depth of the processing point in real time and obtain the three-dimensional topography of the surface of the processing component through the designed differential method and the proposed signal processing method;

[0041] The present invention realizes real-time online measurement of the depth of the processing point of the processing element and can further obtain the three-dimensional morphology of the surface of the processing element, realizing the simultaneous ablation processing and morphology measurement using the same femtosecond laser light source.

[0042] The spatial light modulator of the present invention changes the polarization direction of the incident laser beam and the energy distribution of the laser. Combined with the polarization beam splitter and the quarter-wave plate, the emitted femtosecond laser beam is no longer reflected back into the laser resonant cavity, thereby avoiding cross-interference and protecting the femtosecond laser assembly.

[0043] The present invention abandons the design concept of installing expensive commercial measuring instruments in the processing optical path. Instead, it uses a processing femtosecond laser as the measurement light source. By adopting a measurement scheme of a confocal system with high axial tomography characteristics and using low-cost but highly mature photodiodes as measurement elements, the accuracy of three-dimensional imaging is guaranteed while greatly reducing costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a schematic diagram of the femtosecond laser processing and measurement integrated system of the present invention.

[0045] Figure 2 Decoding the photocurrent intensity I of the present invention AM and schematic diagram of the axial depth Z.

[0046] Figure 3 Schematic diagram of constructing the three-dimensional topography of the surface of the processing element of the present invention.

[0047] 1-Femtosecond laser emitter; 2-Plane mirror; 3-Polarization beam splitter; 4-Quarter-wave plate; 5-First non-polarization beam splitter; 6-Spatial light modulator; 7-4f system; 8-Second non-polarization beam splitter; 9-Focusing objective lens; 10-XYZ three-coordinate linear displacement platform; 11-Computer; 12a-Second neutral grayscale mirror; 12b-Second focusing lens; 13-Third non-polarization beam splitter; 14a-First neutral grayscale mirror; 14b-First focusing lens; 15-First pinhole plate; 16-First photodetector; 17-Second photodetector; 18-Second pinhole plate; 19-Femtosecond laser reflected from the processing point; 20-Femtosecond laser beam; 21-Workpiece to be processed;

[0048] 71 - third focusing lens; 72 - third pinhole plate; 73 - fourth focusing lens. DETAILED DESCRIPTION

[0049] The specific embodiments of the present invention are further described in detail below with reference to the accompanying drawings.

[0050] The three-dimensional surface topography and online measurement method of femtosecond laser processing includes the following steps:

[0051] (1) In the non-processing state, the femtosecond laser beam is positioned to the processing point of the component to be processed;

[0052] (2) Entering the processing state, the femtosecond laser beam is focused on the specified position of the component to be processed for ablation processing;

[0053] (3) During the processing, the femtosecond laser beam reflected from the component to be processed is received and split into two laser beams, which are respectively transmitted to the first photodetector and the second photodetector;

[0054] The first photodetector converts the detected light signal into a photocurrent signal I1;

[0055] The second photodetector converts the detected light signal into a photocurrent signal I2;

[0056] Decoding the photocurrent intensity I AM =(I1-I2) / (I1+I2);

[0057] (4) Decoding I based on the pre-calibrated photocurrent intensity AM The corresponding coefficients a and b between the axial depth Z are used to obtain the axial depth Z=I of the surface of the processing point. AM *a+b; the “*” in the formula is a multiplication sign; the corresponding coefficient a and the corresponding coefficient b are constants determined in advance, which involve the parameters of the optical system used, including the laser wavelength, the numerical aperture of the focusing lens, the focal length material, etc.

[0058] Photocurrent intensity decoding IAM It is linearly related to the axial depth Z of the surface of the processed component;

[0059] (5) The processing element is displaced in the XY direction to realize point-by-point scanning of the XY plane, and the axial depth Z of each point in the XY plane is obtained;

[0060] (6) Summarizing the coordinate information and axial depth Z of the processing element in real time, and drawing the three-dimensional topography of the surface of the processing element.

[0061] The high spatial coherence and high stability of femtosecond laser light sources make them ideal light sources for three-dimensional online measurement systems.

[0062] Overview of depth measurement principles and 3D topography construction: Figure 2 As shown, the laser beam is focused onto the surface of the processing component by a focusing objective lens. The femtosecond laser reflected from the processing point passes through a beam splitter and a neutral grayscale mirror, and is focused by a focusing lens onto a first photodetector and a second photodetector, respectively. The first and second photodetectors receive and convert the received signals into axially responsive photocurrent signals I1 and I2.

[0063] The signal processing method I proposed by the present invention AM , using the photocurrent signal I1 and photocurrent signal I2 detected during the processing, the decoding of the axial depth Z of the processing point of the processing component and the photocurrent can be realized, and finally the depth information Z1, Z2…Z of the processing point of the processing component can be realized. n Conduct real-time monitoring.

[0064] The XY plane is then scanned point by point through a three-coordinate displacement platform to obtain the depth information of each point in the XY plane. Finally, all the depth information and coordinate information are summarized to draw the three-dimensional morphology of the surface of the processed component.

[0065] like Figure 3 As shown, each processing point P1, P2, P3, ..., P n , through the above description, we can get the photocurrent signals I1(x1, y1), I2(x1, y1), I1(x2, y2), I2(x2, y2), I1(x3, y3), I2(x3, y3), ..., I1(x n ,y n ), I2(x n ,y n ), the depth information of each measurement point Z1(x1, y1), Z2(x2, y2), Z3(x3, y3), ..., Z can be obtained through signal processing and depth and photocurrent intensity decoding. n (x n ,y n ).

[0066] Finally, all the depth information and coordinate information are combined to draw the three-dimensional topography of the surface of the processed component.

[0067] This method is suitable for processing metal or non-metal materials with hard and brittle characteristics, such as silicon wafers. The melting point of single crystal silicon is 1693K.

[0068] In this embodiment, under the conditions of laser wavelength of 1040nm, focusing lens aperture of 0.42, and focusing material N-SF11, the corresponding coefficient a is measured to be 3.894 and the corresponding coefficient b is 0.7445.

[0069] The data set 0 in the table below represents the photocurrent signals detected by the first and second photodetectors, respectively, at room temperature (300K) and in the non-processing state (i.e., photocurrent signal I0). Starting from the data set 1, during actual processing, the photocurrent signals I1 and I2 detected by the first and second photodetectors are as follows: The actual detection results for point-by-point processing on the silicon wafer surface are as follows:

[0070]

[0071]

[0072] Femtosecond laser processing and measurement integrated system, including

[0073] The XYZ three-coordinate linear displacement platform is used to fix the processing components and control their displacement in the XYZ directions. By adjusting the displacement in the XYZ directions, the femtosecond laser beam can be focused on the specified position for ablation processing and measurement.

[0074] Femtosecond laser emitter 1, used to emit a laser beam; the emitted parallel light source is reflected by a plane mirror 2 to change the laser emission direction, and then passes through a polarization beam splitter 3 and a quarter-wave plate 4;

[0075] The polarization beam splitter 3 and the quarter-wave plate 4 are used to modulate the laser beam into circularly polarized light and then pass it to the first non-polarization beam splitter 5;

[0076] The first non-polarization beam splitter 5 reflects the incident laser beam into the spatial light modulator 6;

[0077] The spatial light modulator 6 changes the polarization direction of the incident laser beam and the energy distribution of the laser as needed. By adjusting the polarization state of the laser emitted by the spatial light modulator 6 and combining the polarization beam splitter 3 and the quarter-wave plate 4, it can prevent it from being reflected back into the laser resonant cavity of the femtosecond laser transmitter 1 to avoid cross-influence and protect the femtosecond laser component; the femtosecond laser beam emitted by the spatial light modulator 6 passes through the first non-polarization beam splitter 5 and is then transmitted to the 4f system 7; this design mainly uses the spatial light modulator to modulate the phase and polarization state of the initial laser to adapt to different processing requirements; half of the laser beam coming out of the spatial light modulator 6 passes through the first non-polarization beam splitter 5 into the 4f system, and the other half is reflected by the first non-polarization beam splitter 5 and reaches the quarter-wave plate again. At this time, the combination of the quarter-wave plate and the polarization beam splitter can effectively prevent this half of the laser from returning to the resonant cavity and affecting the light source.

[0078] 4f system 7, performs low-pass filtering on the received laser beam and then transmits it to the second non-polarization beam splitter 8;

[0079] The second non-polarized beam splitter 8 splits the laser beam into N outgoing beams, which are then transmitted to the focusing objective lens 9 according to a set method;

[0080] The focusing objective lens 9 focuses the incident laser beam on a designated position on the processing element to perform ablation processing;

[0081] The third non-polarized beam splitter 13 receives the laser beam reflected from the component to be processed and splits it into at least two laser beams, which are then transmitted to the first photodetector 16 and the second photodetector 17 respectively;

[0082] Before processing, the first photodetector 16 and the second photodetector 17 obtain a photocurrent signal I0 based on the initially detected light signal;

[0083] During the processing, the first photodetector 16 converts the detected light signal into a photocurrent signal I1;

[0084] During the processing, the second photodetector 17 converts the detected light signal into a photocurrent signal I2;

[0085] The computer 11 calculates and plots the three-dimensional topography and distribution of the surface of the processing element based on the photocurrent signal I0, the photocurrent signal I1 and the photocurrent signal I2 in combination with pre-installed software.

[0086] Preferably, a first neutral grayscale mirror 14a and a first focusing lens 14b are disposed between the third non-polarizing beam splitter 13 and the first photodetector 16, and a second neutral grayscale mirror 12a and a second focusing lens 12b are disposed between the third non-polarizing beam splitter 13 and the second photodetector 17. This effectively prevents damage to the photodetector caused by excessive femtosecond laser pulse energy and effectively increases the ablation threshold of the femtosecond laser pulse without worrying about affecting the photodetector, thereby expanding the application range of the integrated processing and measurement platform.

[0087] Preferably, the detection end of the first photodetector 16 is in close contact with the first pinhole plate 15, and the pinhole of the first pinhole plate 15 corresponds to the focus of the first focusing lens 14b; the detection end of the first photodetector 16 is located on the focal plane of the first focusing lens 14b and is offset from the focal center of the first focusing lens 14b by an off-axis distance v d This can more effectively block out-of-focus scattered light and improve the signal-to-noise ratio of the measurement; In fact, the light spot on the focal plane of the first focusing lens 14b is not a point, but a light spot of several microns; The off-axis distance v d The value range is 100 to 200 microns. Such a design can isolate more scattered light without changing the structure of the optical system, thereby improving the noise ratio.

[0088] The detection end of the second photodetector 17 is in close contact with the second pinhole plate 18. The pinhole of the second pinhole plate 18 corresponds to the focus of the second focusing lens 12b. The center of the detection end of the second photodetector 12b is aligned with the focus center of the second focusing lens. At the same time, there is a defocus distance u between the defocus plane and the detection end. d This defocus distance u d The adjustable range is 100 to 200 microns. By adjusting the defocus distance u d Can optimize I AM The signal processing mode is optimized to achieve a larger measurement range and better linearity of light intensity and depth.

[0089] Preferably, the center of the detection end of the second photodetector 12b is offset from the focal center of the second focusing lens 12b by an off-axis distance v d2 ; Further isolate more scattered light, thereby improving the noise ratio.

[0090] Preferably, the 4f system comprises a third focusing lens 71, a third pinhole plate 72 and a fourth focusing lens 73 arranged in sequence;

[0091] The third focusing lens 71 and the fourth focusing lens 73 work together to focus and then diffuse the incident laser beam before it exits to the second non-polarizing beam splitter 8. A third pinhole plate aperture is provided on the focal plane between the third and fourth focusing lenses 71, 73. The pinhole of the third pinhole plate aperture corresponds to the focal center of the third focusing lens. The light spot passing through the pinhole of the third pinhole plate 72 is equivalent to passing through a frequency-domain low-pass filter, which not only eliminates high-frequency noise in the light spot but also smoothes the femtosecond laser beam, facilitating subsequent processing and measurement.

[0092] The integrated processing and measurement platform designed in this invention can achieve both real-time measurement of the depth of the processing point of the processing component and real-time online measurement of the surface morphology of the processing component. Compared with traditional offline measurement, which requires time and effort to repeatedly search for the processing point, requires high positioning accuracy for repeated clamping of the processing component, and cannot guarantee processing quality, the processing and three-dimensional online morphology measurement method proposed in this invention combines the advantages of femtosecond laser processing technology without thermal damage and the three-dimensional imaging advantages of high axial tomography of confocal microscopy technology. It is a practical technical approach to achieve femtosecond laser processing and three-dimensional online non-destructive testing.

[0093] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for measuring three-dimensional surface topography by femtosecond laser processing, characterized in that: Applicable to silicon wafer materials; including the following steps: (1) In the non-processing state, the femtosecond laser beam is positioned to the processing point of the component to be processed; (2) Entering the processing state, the femtosecond laser beam is focused on the specified position of the processing component to perform ablation processing; (3) During the processing, the femtosecond laser beam reflected from the component to be processed is received and split into two laser beams, which are respectively transmitted to the first photodetector and the second photodetector; The first photodetector records the detected light signal as a photocurrent signal I1; The second photodetector records the detected light signal as a photocurrent signal I2; Real-time decoding of photocurrent intensity I AM =(I1-I2) / (I1+I2); (4) Decoding I based on the pre-calibrated photocurrent intensity AM The corresponding coefficients a and b between the axial depth Z are used to obtain the axial depth Z=I of the surface of the processing point. AM *a+b; Photocurrent intensity decoding I AM It is linearly related to the axial depth Z of the surface of the processed component; (5) The processing element is displaced in the XY direction to realize point-by-point scanning of the XY plane, and the axial depth Z of each point in the XY plane is obtained; (6) Summarizing the coordinate information and axial depth Z of the processing element in real time, and drawing the three-dimensional topography of the surface of the processing element.

2. The method for measuring three-dimensional surface topography of femtosecond laser processing according to claim 1, characterized in that: The corresponding coefficient a is 3.894, and the corresponding coefficient b is 0.7445.

3. Femtosecond laser processing and measurement integrated system, characterized by: include XYZ three-coordinate linear displacement platform is used to fix the processing components and control the displacement of the processing components in the XYZ direction; A femtosecond laser emitter, used to emit a laser beam; a polarization beam splitter and a quarter-wave plate, for receiving the laser beam from the femtosecond laser transmitter and modulating it into circularly polarized light, and then transmitting it to the first non-polarization beam splitter; A first non-polarization beam splitter reflects the incident laser beam into the spatial light modulator; A spatial light modulator changes the polarization direction and energy distribution of the incident laser beam as needed, and emits the laser beam to the first non-polarization beam splitter and then transmits it to the 4f system; The 4f system performs low-pass filtering on the received laser beam and then transmits it to the focusing objective lens; The focusing objective lens focuses the incident laser beam on the specified position on the processing component to perform ablation processing; a third non-polarized beam splitter, receiving the laser beam reflected from the component to be processed, and splitting it into at least two laser beams, which are then transmitted to the first photodetector and the second photodetector respectively; Before processing, the first photodetector and the second photodetector cooperate to record the initially detected light signal as a photocurrent signal I0; During the processing, the first photodetector converts the detected light signal into a photocurrent signal I1; During the processing, the second photodetector converts the detected light signal into a photocurrent signal I2; The computer calculates and draws the three-dimensional topography of the surface of the processing component based on the photocurrent signal I0, the photocurrent signal I1 and the photocurrent signal I2 in combination with pre-installed software.

4. The femtosecond laser processing and measurement integrated system according to claim 3, characterized in that: A second non-polarizing beam splitter is provided between the 4f system and the focusing objective lens; the second non-polarizing beam splitter splits the laser beam into N outgoing light beams again, and then transmits them to the focusing objective lens in a set manner.

5. The femtosecond laser processing and measurement integrated system according to claim 3, characterized in that: A first neutral grayscale mirror and a first focusing lens are arranged between the third non-polarization beam splitter and the first photodetector, and a second neutral grayscale mirror and a second focusing lens are arranged between the third non-polarization beam splitter and the second photodetector.

6. The femtosecond laser processing and measurement integrated system according to claim 5, characterized in that: The detection end of the first photodetector is closely attached to a first pinhole plate, and the pinhole of the first pinhole plate corresponds to the focus of the first focusing lens; The detection end of the first photodetector is located on the focal plane of the first focusing lens and is offset from the focal center of the first focusing lens by an off-axis distance v d ; The detection end of the second photodetector is closely attached to the second pinhole plate, the pinhole of the second pinhole plate corresponds to the focus of the second focusing lens, the detection end center of the second photodetector is aligned with the focus center of the second focusing lens, and the defocus plane has a defocus distance u d .

7. The femtosecond laser processing and measurement integrated system according to claim 6, characterized in that: The center of the detection end of the second photodetector is offset from the focal center of the second focusing lens by an off-axis distance v d2 .

8. The femtosecond laser processing and measurement integrated system according to claim 3, characterized in that: The 4f system includes a third focusing lens, a third pinhole plate and a fourth focusing lens arranged in sequence; The third focusing lens and the fourth focusing lens cooperate to focus the incident laser beam first and then diffuse it, and then emit it to the second non-polarization beam splitter; A third pin plate hole is provided on a focal plane between the third focusing lens and the fourth focusing lens, and a pin hole of the third pin plate hole corresponds to a focal point of the third focusing lens.

Images