A visual docking method for relaying optical modules with precision turntable modules

Abstract

The application discloses a visual docking method for relaying optical modules and a precision turntable module, and solves the problem of low docking efficiency of the relaying optical modules and the precision turntable module in the prior art.According to the coaxial characteristics of light receiving and light emitting of the relaying optical modules, the principle of optical self-centering is used to firstly transfer the pitch axis line of the precision turntable module to the center normal line of the star-hole mirror in the star-hole reflection tool, and the visual docking of the two relaying optical modules and the precision turntable module is completed through real-time monitoring of a fine tracking camera and a light spot analyzer.

Description

Technical Field

[0001] The present invention relates to an optical module docking method, and in particular to a visual docking method for a relay optical module and a precision turntable module. Background Art

[0002] With the changing trend of mass development of intersatellite laser communications in my country, it is particularly important to improve the assembly and adjustment accuracy and efficiency of laser communication optical terminal systems. Figure 1 As shown, the laser communication optical terminal system consists of three parts: a relay optical module 1, a precision turntable module 2 and a telescopic optical module 3. The relay optical module 1 is connected to one end of the precision turntable module 2 through the rear end flange 4 of the pitch axis and does not rotate with the pitch axis. A trimming pad is provided at the connection end face; the telescopic optical module 3 is connected to the other end of the precision turntable module 2 through the front end flange 5 of the pitch axis and rotates with the pitch axis.

[0003] like Figure 2 As shown, the relay optical module 1 mainly includes a communication transmitting unit 11, a communication receiving unit 12 and a precision tracking camera 13. A central light exit hole 14 connected to the precision turntable module 2 is provided at one end of the relay optical module 1 close to the precision turntable module 2; the optical axis passing through the communication transmitting unit 11, the communication receiving unit 12 and the precision tracking camera 13 can pass through a folding mirror inside the relay optical module 1 and converge into a coaxial optical path at the central light exit hole 14; the communication transmitting unit 11 can emit a parallel light beam with a wavelength band of 1550nm and an aperture of Φ6mm, and the precision tracking camera 13 can image the parallel light beam in the 1550nm band.

[0004] like Figure 3 As shown, the precision turntable module 2 includes an azimuth axis 21 and a pitch axis 22 perpendicular to the azimuth axis 21 ; the relay optical module 1 and the telescopic optical module 3 are respectively connected to both ends of the pitch axis 22 .

[0005] The essence of docking the relay optical module 1 with the precision turntable module 2 is to adjust the coaxiality of the parallel light beam emitted from the central light exit hole 14 of the relay optical module 1 with the pitch axis 22 of the precision turntable module 2. The traditional docking method first calibrates the axis system of the precision turntable module 2 separately. The optical calibration of the rotation axis is completed with the help of a reflector cross-reticle fixture, guided by the human eye monitoring the autocollimation through-center image in the external theodolite. This process requires one person to monitor and another to adjust, which seriously reduces the efficiency of the installation. Summary of the Invention

[0006] In order to solve the technical problem of low docking efficiency between a relay optical module and a precision turntable module in the prior art, the present invention provides a visual docking method for a relay optical module and a precision turntable module.

[0007] The inventive concept of the present invention:

[0008] The present application utilizes the principle of optical self-centering, and first transfers the pitch axis of the precision turntable module to the center normal of the star-hole mirror in the star-hole reflection tool, and then completes the visual docking of the two relay optical modules and the precision turntable module through real-time monitoring of the fine tracking camera and the spot analyzer.

[0009] In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme:

[0010] A visual docking method for relay optical modules and precision turntable modules, which is characterized by comprising the following steps:

[0011] Step 1: placing a corner cube prism assembly outside the light outlet of the relay optical module, the relay optical module emits parallel light beams, the parallel light beams return along the original path after being affected by the corner cube prism assembly, and a first circular spot is formed on the focal plane of the fine tracking camera in the relay optical module, and the position of the first circular spot is recorded as position 1;

[0012] Step 2: connecting the relay optical module to one end of the pitch axis of the precision turntable module through the pitch axis rear flange, and installing a star-hole reflection tool at the other end of the pitch axis;

[0013] Step 3: the parallel light beams emitted by the relay optical module pass through the pitch axis and return along the original path after being incident on the star-hole reflection tool, and a second circular spot is formed on the focal plane of the fine tracking camera, and the second circular spot draws a circle on the focal plane when the pitch axis is rotated, and the star-hole reflection tool is adjusted to minimize the amount of circle drawing, and the position of the center point of the minimum circle drawing is recorded as position 2;

[0014] Step 4: calculating the inclination of the chamfering ring arranged at the pitch axis rear flange according to position 1, position 2 and the outer diameter of the pitch axis rear flange;

[0015] Step 5: grinding the chamfering ring according to the inclination, so that the position 1 coincides with the position 2;

[0016] Step 6: setting up a spot analyzer outside the end of the star-hole reflection tool away from the precision turntable module, the parallel light beams emitted by the relay optical module form a third circular spot on the target surface of the spot analyzer, and the third circular spot draws a circle on the target surface when the pitch axis is rotated, and the star-hole reflection tool is adjusted until the position of the third circular spot remains unchanged when the pitch axis is rotated, and the position of the third circular spot is recorded as position 1';

[0017] Step 7, disassemble the star hole reflection tool, or disassemble the star hole mirror frame and the star hole mirror in the center of the star hole mirror frame, then the parallel light beam emitted by the relay optical module forms a fourth circular spot on the target surface of the spot analyzer, and the position of the fourth circular spot is recorded as position 2';

[0018] Step 8, keep the precision turntable module stationary, translate the relay optical module along the radial direction of the pitch axis, so that the position 1' coincides with the position 2', then the optical axis of the parallel light beam emitted by the relay optical module coincides with the rotation axis of the pitch axis, and the visual docking of the relay optical module and the precision turntable module is completed.

[0019] Further, step 1 is specifically:

[0020] Step 1, place a corner cube prism assembly outside the center light exit hole of the relay optical module, the communication transmitting unit of the relay optical module emits a laser beam, the laser beam is converted into a parallel light beam and exits from the center light exit hole, returns along the original path after being affected by the corner cube prism assembly, and forms a first circular spot on the focal plane of the fine tracking camera of the relay optical module, the spot diagram of the first circular spot is collected and the position of the first circular spot on the focal plane of the fine tracking camera is interpreted, and the position is recorded as position 1.

[0021] Further, step 2 is specifically:

[0022] Connect the relay optical module to one end of the pitch axis in the precision turntable module through the pitch axis rear flange, and install the star hole reflection tool at the other end of the pitch axis through the pitch axis front flange;

[0023] The star hole reflection tool includes a cylindrical star hole reflection shell, a star hole mirror frame installed in the star hole reflection shell through a compression ring, and a star hole mirror installed at the center of the star hole mirror frame; the center of the star hole mirror is provided with a center hole, and the star hole mirror frame is provided with an inclination adjustment knob for adjusting the inclination angle of the star hole mirror; the star hole reflection shell is provided with a translation adjustment knob for translating the star hole mirror along the radial direction of the star hole reflection shell.

[0024] Further, step 3 is specifically:

[0025] The parallel light beam emitted by the relay optical module exits from the center light exit hole and enters the star hole reflection tool through the pitch axis, returns along the original path after being reflected by the star hole mirror in the star hole reflection tool, and forms a second circular spot on the focal plane of the fine tracking camera, and the second circular spot draws a circle on the focal plane by rotating the pitch axis, and the inclination adjustment knob on the star hole reflection tool is adjusted to minimize the amount of drawing a circle, so that the center of rotation of the pitch axis is parallel to the normal line of the star hole mirror, and the position of the minimum circle center point is recorded as position 2.

[0026] Further, step 4 is specifically:

[0027] The angle Δθ between the parallel light beam emitted from the center light exit hole of the relay optical module and the axis of the elevation shaft is calculated by the pixel difference between position 1 and position 2, and the inclination K of the trimming ring is calculated according to the diameter d of the rear flange of the elevation shaft by the following formula:

[0028] K = d sin Δθ.

[0029] Further, step 6 is specifically:

[0030] The spot analyzer is arranged outside the end of the star-hole reflection tool away from the precision turntable module, the laser beam emitted by the communication transmitting unit is converted into a parallel light beam, the parallel light beam is emitted from the center light exit hole, passes through the center small hole on the star-hole mirror, and is emitted to the spot analyzer to form a third circular spot, the elevation shaft is rotated, the third circular spot rotates on the target surface of the spot analyzer as the elevation shaft rotates, the third circular spot position is recorded on the target surface of the spot analyzer, and the third circular spot position from the center small hole is the rotation center of the elevation shaft.

[0031] Further, step 7 is specifically:

[0032] The star-hole reflection tool is disassembled, or the pressing ring on the star-hole reflection tool is disassembled, the star-hole mirror frame and the star-hole mirror in the center of the star-hole mirror frame are taken out, the parallel light beam emitted by the relay optical module forms a fourth circular spot on the target surface of the spot analyzer, and the position 2' of the fourth circular spot is recorded.

[0033] Advantages of the present application:

[0034] 1. The visual docking method for the relay optical module and the precision turntable module provided by the present application not only integrates the calibration process of the shaft system of the precision turntable module and the docking process of the precision turntable module and the relay optical module, but also provides a visual docking method, so that the docking efficiency of the relay optical module and the precision turntable module is increased by more than one time, and the number of debugging personnel required in the docking process is reduced by half.

[0035] 2. According to the coaxial optical characteristics of the relay optical module, the rotation axis angle of the precision turntable is visualized by the star-hole mirror tool, the rotation autocollimation principle, and the self-calibration method without the aid of external observation equipment, the optical axis angle of the relay system is visualized by the corner cube prism self-calibration method, the shaft system center is visualized by the principle of rotating the small hole diaphragm, and the shaft system center and the relay exit light beam center are visualized by the spot analysis, so that the docking is more accurate. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 is a structural schematic diagram of a laser communication optical machine terminal system;

[0037] Figure 2 is a structural schematic diagram of a relay optical module;

[0038] Figure 3 is a structural schematic diagram of a precision turntable module;

[0039] Figure 4 is a structural schematic diagram of a star hole reflecting tool adopted in an embodiment of the present application;

[0040] Figure 5 is a structural schematic diagram of a star hole reflecting tool adopted in an embodiment of the present application;

[0041] Figure 6 is a structural schematic diagram of a corner cube prism assembly adopted in an embodiment of the present application;

[0042] Corresponding reference signs: 1-relay optical module, 11-communication transmitting unit, 12-communication receiving unit, 13-precision tracking camera, 14-central light emitting hole, 2-precision turntable module, 21-azimuth axis, 22-elevation axis, 3-telescopic optical module, 4-elevation axis rear end flange, 5-elevation axis front end flange, 6-star hole reflecting tool, 61-star hole reflecting shell, 62-pressing ring, 63-star hole reflecting mirror frame, 64-star hole reflecting mirror, 65-inclination adjusting knob, 66-translation adjusting knob, 67-central small hole, 7-corner cube prism assembly;

[0043] Figure 7 is a structural schematic diagram of an embodiment of the present application when the visual docking method for the relay optical module and the precision turntable module is carried out to step 6;

[0044] Corresponding reference signs: 8-light spot analyzer. DETAILED DESCRIPTION

[0045] The technical solutions of the present application will be described clearly and completely in combination with the drawings and embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments of the present application, all the other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0046] The visual docking method for the relay optical module and the precision turntable module provided by the embodiment of the present application comprises the following steps:

[0047] Step 1, placing a corner cube prism assembly 7 outside the central light emitting hole 14 of the relay optical module 1, the structure of the corner cube prism assembly 7 is as shown in Figure 6As shown, the communication transmitting unit 11 of the relay optical module 1 emits a laser beam, which is converted into a parallel light beam and exits from the central light exit hole 14, returns along the original path after the action of the corner cube prism assembly 7, and forms a first circular light spot on the focal plane of the fine tracking camera 13 of the relay optical module 1. The light spot pattern of the first circular light spot is collected and the position of the first circular light spot on the focal plane of the fine tracking camera 13 is judged by the centroid interpretation software, which is recorded as position 1.

[0048] Step 2, connect the relay optical module 1 to one end of the elevation shaft 22 of the precision turntable module 2 through the rear flange 4 of the elevation shaft, and install the star hole reflection tool 6 in the mounting star hole of the other end of the elevation shaft 22 through the front flange 5 of the elevation shaft;

[0049] Combined Figure 4 And Figure 5 As shown, the star hole reflection tool 6 is composed of a cylindrical star hole reflection shell 61, a star hole reflection mirror frame 63 installed on the inner wall of one end of the star hole reflection shell 61 through a compression ring 62, and a star hole reflection mirror 64 installed at the center of the star hole reflection mirror frame 63. One side of the star hole reflection mirror 64 is a reflective surface coated with a reflective film, and the center has a Φ0.5mm center hole 67, which is the light exit port of the star hole reflection tool 6; the tilt of the star hole reflection mirror 64 can be adjusted through the two inclined adjustment knobs 65 on the star hole reflection mirror frame 63; the radial translation of the star hole reflection mirror 64 can be adjusted through the four translation adjustment knobs 66 evenly distributed around the star hole reflection shell 61. For its specific internal structure, please refer to “Research on Inter-satellite Laser Communication Pointing and Acquisition Mechanism” [D]. University of the Chinese Academy of Sciences (Zhang Furui. Xi'an Institute of Optics and Fine Mechanics, Chinese Academy of Sciences), 2019. DOI: 10.27605 / d.cnki.gkxgs.2019.000031.

[0050] Step 3, the laser beam emitted by the communication transmitting unit 11 is converted into a parallel light beam, which exits from the central light exit hole 14 and enters the star hole reflection mirror 64 of the star hole reflection tool 6 through the elevation shaft 22 of the precision turntable module 2. After reflection by the star hole reflection mirror 64, the reflected light again passes through the elevation shaft 22 of the precision turntable module 2, enters the fine tracking camera 13 of the relay optical module 1 from the central light exit hole 14, and forms a second circular light spot on the focal plane of the fine tracking camera 13. When the elevation shaft 22 is rotated, the second circular light spot traces a circle on the focal plane of the fine tracking camera 13. By adjusting the inclined adjustment knobs 65 of the star hole reflection tool 6, the amount of tracing is minimized or stationary. The light spot pattern of the minimum tracing or the second circular light spot is collected, and the precise position of the minimum tracing center point or the second circular light spot on the focal plane of the fine tracking camera 13 is judged by the centroid interpretation software, which is recorded as position 2. At this time, the normal line of the elevation shaft 22 and the star hole reflection mirror 64 is strictly parallel;

[0051] Step 4: Calculate the angle Δθ between the parallel light beam emitted from the central light exit hole 14 of the relay optical module 1 and the axis of the pitch axis 22 using the pixel difference between position 1 and position 2. Calculate the tilt K of the trimming circle using the following formula based on the outer diameter d of the flange 4 at the rear end of the pitch axis:

[0052] K = dsinΔθ;

[0053] Step 5: According to the tilt amount K, the trimming circle at the connection between the relay optical module 1 and the precision turntable module 2 is repaired so that position 1 and position 2 coincide with each other. At this time, the parallel light beam emitted by the central light exit hole 14 is parallel to the axis of the pitch axis 22, and the parallelism accuracy is less than 10".

[0054] Step 6: Figure 7 As shown, a spot analyzer 8 is set up on the outer side of the end of the star hole reflection tooling 6 away from the precision turntable module 2. The spot analyzer 8 can receive the spot and locate the center position of the spot. The communication transmitting unit 11 in the relay optical module 1 emits a laser beam, which is converted into a parallel beam and sequentially passes through the central light exit hole 14 and the pitch axis 22, and then is emitted from the central small hole 67 of the star hole reflector 64 to the target surface of the spot analyzer 8 to form a third circular spot. The pitch axis 22 is rotated, and when the central small hole 67 When the midpoint of the third circular light spot is not on the axis of the pitch axis 22, as the pitch axis 22 rotates, the third circular light spot draws a circle on the target surface of the light spot analyzer 8. By adjusting the translation adjustment knob 66 ​​on the star hole reflection fixture 6 until the position of the third circular light spot on the target surface of the light spot analyzer 8 does not change when the pitch axis 22 rotates, the position of the third circular light spot emitted from the central small hole 67 represents the rotation center of the pitch axis 22, and the position 1′ of the third circular light spot on the target surface of the light spot analyzer 8 is recorded;

[0055] Step 7: Loosen the pressing ring 62 on the star hole reflector tool 6, remove the star hole reflector frame 63 and the star hole reflector 64 at the center of the star hole reflector frame 63, and the light spot analyzer 8 receives the fourth circular light spot formed by the Φ6 mm parallel light beam emitted by the communication transmitting unit 11 and records the position 2′ of the fourth circular light spot;

[0056] In other embodiments, the star hole reflection tooling 6 may also be directly removed from the pitch axis 22 .

[0057] Step 8: Keep the precision turntable module 2 stationary and translate the relay optical module 1 radially along the pitch axis 22 until the position 1′ of the third circular light spot coincides with the position 2′ of the fourth circular light spot. At this time, the optical axis of the outgoing light beam of the relay optical module 1 coincides with the rotation axis of the pitch axis 22.

[0058] Complete the visual docking between the relay optical module 1 and the precision turntable module 22.

[0059] The parallel light beam optical axis of the relay optical module 1 and the rotation axis of the pitch axis 22 can be coincided with an accuracy of 0.01 mm by the above method.

[0060] The above merely provides a specific implementation of the present application, but the protection scope of the present application is not limited thereto, any change or replacement within the technical scope disclosed by the present application should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for visualizing the interface of a relay optical module and a precision turntable module, characterized in that, Comprising the following steps: Step 1, placing a corner cube prism assembly (7) outside the light exit of a relay optical module (1), the relay optical module (1) emits parallel light beams, the parallel light beams return to the original path after the action of the corner cube prism assembly (7), and form a first circular light spot on the focal plane of the fine tracking camera (13) in the relay optical module (1), and the position of the first circular light spot is recorded as position 1; Step 2, connecting the relay optical module (1) to one end of the elevation shaft (22) in the precision rotary table module (2) through the elevation shaft rear flange (4), and installing a star hole reflection tool (6) at the other end of the elevation shaft (22); Step 3, the parallel light beams emitted by the relay optical module (1) pass through the elevation shaft (22) and return to the original path after being incident on the star hole reflection tool (6), forming a second circular light spot on the focal plane of the fine tracking camera (13), rotating the elevation shaft (22) to make the second circular light spot circle on the focal plane, and adjusting the star hole reflection tool (6) to make the circle amount to a minimum value, and recording the position of the minimum circle center point as position 2; Step 4, calculating the inclination of the chamfering ring arranged at the elevation shaft rear flange (4) by position 1, position 2 and the outer diameter of the elevation shaft rear flange (4); Step 5, grinding the chamfering ring according to the inclination to make the position 1 and the position 2 coincide; Step 6, erecting a light spot analyzer (8) outside the end of the star hole reflection tool (6) away from the precision rotary table module (2), the parallel light beams emitted by the relay optical module (1) form a third circular light spot on the target surface of the light spot analyzer (8), rotating the elevation shaft (22) to make the third circular light spot circle on the target surface, and adjusting the star hole reflection tool (6) until the position of the third circular light spot remains unchanged when the elevation shaft (22) rotates, and recording the position of the third circular light spot as position 1'; Step 7, disassembling the star hole reflection tool (6), or disassembling the star hole reflection mirror frame (63) and the star hole reflection mirror (64) at the center of the star hole reflection mirror frame (63), then the parallel light beams emitted by the relay optical module (1) form a fourth circular light spot on the target surface of the light spot analyzer (8), and the position of the fourth circular light spot is recorded as position 2'; Step 8, keeping the precision rotary table module (2) stationary, translating the relay optical module (1) along the radial direction of the elevation shaft (22) to make the position 1' and the position 2' coincide, then the optical axis of the parallel light beams emitted by the relay optical module (1) coincides with the rotation axis of the elevation shaft (22), and the visual docking of the relay optical module (1) and the precision rotary table module (22) is completed.

2. The method for visualizing the interface of a relay optical module with a precision turntable module according to claim 1, wherein, Step 1 is specifically: Step 1, placing a corner cube prism assembly (7) outside the center light exit hole (14) of a relay optical module (1), the communication transmitting unit (11) of the relay optical module (1) emits a laser beam, the laser beam is converted into a parallel light beam and emitted from the center light exit hole (14), and the parallel light beam returns to the original path after the action of the corner cube prism assembly (7), forming a first circular light spot on the focal plane of the fine tracking camera (13) in the relay optical module (1), collecting the light spot diagram of the first circular light spot and judging the position of the first circular light spot on the focal plane of the fine tracking camera (13), which is recorded as position 1.

3. The method for visualizing the interface of a relay optical module with a precision turntable module according to claim 1 or 2, characterized in that, Step 2 is specifically: The relay optical module (1) is connected to one end of the pitch shaft (22) in the precision rotary table module (2) through the pitch shaft rear end flange (4), and the star hole reflection tooling (6) is installed at the other end of the pitch shaft (22) through the pitch shaft front end flange (5); The star hole reflection tooling (6) comprises a cylindrical star hole reflection shell (61), a star hole reflection mirror frame (63) installed in the star hole reflection shell (61) through a pressing ring (62), and a star hole reflection mirror (64) installed at the center of the star hole reflection mirror frame (63); the star hole reflection mirror (64) is provided with a central small hole (67) at the center, and the star hole reflection mirror frame (63) is provided with an inclination adjustment knob (65) for adjusting the inclination angle of the star hole reflection mirror (64); and the star hole reflection shell (61) is provided with a translation adjustment knob (66) for translating the star hole reflection mirror (64) along the radial direction of the star hole reflection shell (61).

4. The method of visual interfacing a relay optical module with a precision turntable module of claim 3, wherein, Step 3 is specifically: The parallel light beam emitted by the relay optical module (1) is emitted from the central light exit hole (14) and enters the star hole reflection tooling (6) through the pitch shaft (22), is reflected by the star hole reflection mirror (64) in the star hole reflection tooling (6), and returns to the original path, thereby forming a second circular light spot on the focal plane of the fine tracking camera (13); rotating the pitch shaft (22) makes the second circular light spot circle on the focal plane; by adjusting the inclination adjustment knob (65) on the star hole reflection tooling (6), the circle amount is minimized, and then the center of rotation of the pitch shaft (22) is parallel to the normal line of the star hole reflection mirror (64); and the position of the minimum circle center point is recorded as position 2.

5. The method of visual interfacing a relay optical module with a precision turntable module of claim 4, wherein, Step 4 is specifically: The angle Δθ between the parallel light beam emitted from the central light exit hole (14) of the relay optical module (1) and the axis of the pitch shaft (22) is calculated by the pixel difference between position 1 and position 2; and the inclination K of the trimming ring is calculated according to the diameter d of the pitch shaft rear end flange (4) by the following formula: K = d sin Δθ.

6. The method of visual interfacing a relay optical module with a precision turntable module of claim 5, wherein, Step 6 is specifically: A light spot analyzer (8) is erected outside the end of the star hole reflection tooling (6) away from the precision rotary table module (2); the laser beam emitted by the communication transmitting unit (11) is converted into a parallel light beam; the parallel light beam is emitted from the central light exit hole (14) and then emitted from the central small hole (67) on the star hole reflection mirror (64) to the light spot analyzer (8) after passing through the pitch shaft (22), thereby forming a third circular light spot on the light spot analyzer (8); rotating the pitch shaft (22) makes the third circular light spot circle on the target surface of the light spot analyzer (8); by adjusting the translation adjustment knob (66) on the star hole reflection tooling (6), the position of the third circular light spot on the target surface of the light spot analyzer (8) remains unchanged when the pitch shaft (22) rotates, and then the position of the third circular light spot emitted from the central small hole (67) is the center of rotation of the pitch shaft (22); and the position of the third circular light spot on the target surface of the light spot analyzer (8) is recorded as position 1′.

7. The method of visual interfacing a relay optical module with a precision turntable module of claim 6, wherein, Step 7 is specifically: When the star-hole reflection tool (6) is disassembled or the pressing ring (62) on the star-hole reflection tool (6) is disassembled, the star-hole mirror frame (63) and the star-hole mirror (64) in the center of the star-hole mirror frame (63) are taken out, and the parallel light beam emitted by the relay optical module (1) forms a fourth circular light spot on the target surface of the light spot analyzer (8), and the position 2' of the fourth circular light spot is recorded.

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