A multi-stage space-polarization joint noise suppression optical system and method for thomson scattering measurement

Abstract

The application discloses a multistage space-polarization combined noise reduction optical system and method for Thomson scattering measurement, a central reference line penetrating through a front end and a rear end is used as an optical axis, a scattering volume, a multistage object-side diaphragm assembly, a first short-focus convex lens, a polarization selection assembly, a second convex lens, a diaphragm, a third convex lens and a fiber coupling assembly are sequentially arranged along the optical axis; the multistage object-side diaphragm assembly is arranged in a light path space between the scattering volume and the first short-focus convex lens; the polarization selection assembly is arranged in a light path behind the first short-focus convex lens; the second convex lens is arranged at an axial position behind the polarization selection assembly; the diaphragm is arranged at an imaging surface or a conjugate position of the second convex lens; the third convex lens is arranged on the optical axis behind the diaphragm; and a receiving end surface of the fiber coupling assembly is arranged at a secondary imaging surface position of the third convex lens. The application solves the problem of low signal-to-noise ratio of the Thomson scattering signal in a strong background radiation environment, and improves the accuracy and reliability of plasma parameter measurement.

Description

Technical Field

[0001] This invention belongs to the field of plasma diagnostics and relates to a multi-level spatial-polarization joint noise suppression optical system and method for Thomson scattering measurement. Background Technology

[0002] Pinch plasma is a typical high-energy-density plasma state, widely used in high-energy-density physics research, inertial confinement fusion-related experiments, and high-current pulse-driven plasma physics research. During Z-pinch, the plasma is radially compressed under the action of a large current, generating a high-temperature, high-density state, accompanied by strong continuous-spectrum radiation, spectral line radiation, and background radiation such as plasma autoluminescence.

[0003] Thomson scattering diagnostics has become an important method for measuring high-temperature plasma parameters due to its non-invasive nature and ability to directly measure parameters such as electron temperature and electron density. In this method, the incident laser undergoes elastic scattering with free electrons in the plasma, and the scattering spectral information can be used to invert plasma physical parameters.

[0004] However, in a Z-pinch plasma environment, due to the high intensity of spontaneous plasma emission, small plasma volume and steep gradient, limited experimental space and complex stray light paths, the Thomson scattering signal is often submerged in strong background radiation, resulting in a significant reduction in the signal-to-noise ratio and seriously affecting the measurement accuracy and stability.

[0005] Existing technologies typically employ methods such as time gating, narrowband filters, or single-stage apertures to limit the collection angle to suppress background noise. However, these methods often only suppress noise in a single dimension and cannot simultaneously take into account multiple factors such as spatial orientation selection and polarization selection. Consequently, their suppression capabilities are insufficient under conditions of strong background radiation. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a multi-level spatial-polarization joint noise suppression optical system and method for Thomson scattering measurement, which solves the problem of low signal-to-noise ratio of Thomson scattering signal under strong background radiation environment and improves the accuracy and reliability of plasma parameter measurement.

[0007] To achieve the above objectives, the present invention employs the following technical solution: A multi-level spatial-polarization joint noise reduction optical system for Thomson scattering measurement, with a central reference line running through the front and rear ends as the optical axis, includes a scattering volume, a multi-level object-side aperture assembly, a first short focal length convex lens, a polarization selection assembly, a second convex lens, an aperture, a third convex lens, and an optical fiber coupling assembly arranged sequentially along the optical axis. The multi-stage object-side aperture assembly is arranged in the optical path space between the scattering volume and the first short-focal-length convex lens; The polarization selection component is located in the optical path behind the first short focal length convex lens; The second convex lens is positioned axially behind the polarization selection component; The aperture stop is positioned at the imaging plane of the second convex lens or at its conjugate position. The third convex lens is positioned on the optical axis behind the aperture; The receiving end face of the fiber optic coupling assembly is positioned at the secondary imaging surface of the third convex lens.

[0008] Optionally, the scattering volume is the same as the axial distance of the first short focal length convex lens and the polarization selection component is the same as the axial distance of the first short focal length convex lens.

[0009] Optionally, the multi-level object-side aperture assembly includes at least two aperture structures spaced apart along the light propagation direction. The aperture size of each aperture structure changes gradually along the optical axis to form an involute light transmission structure. The geometric center of the aperture of each aperture structure coincides with the optical axis.

[0010] Optionally, in the multi-level object-side aperture assembly, the edges of the light-transmitting apertures of each level of the aperture structure form a geometric cone, the apex of the geometric cone is located in the scattering volume, and the cone angle of the geometric cone is within the effective receiving cone angle range of the first short focal length convex lens.

[0011] Optionally, the axial distance between the second convex lens and the polarization selection component and the aperture stop are the same as the axial distance between the second convex lens and the polarization selection component.

[0012] Optionally, the aperture is made of an opaque solid light-blocking plate, with a light-passing hole in the center of the aperture.

[0013] Optionally, the fiber optic coupling assembly includes multiple optical fibers arranged along a spatial direction, with the incident end faces of the multiple optical fibers closely aligned side by side to form a receiving array spanning the secondary imaging plane, and the end faces of the multiple optical fibers conjugate aligned with the imaging plane of the third convex lens.

[0014] A noise suppression method includes the following steps: The beam of light is emitted outward from the scattering volume and propagates backward to reach the multi-level object-side aperture assembly; The light beam passes through the aperture of the multi-stage object-side aperture assembly and reaches the first short-focal-length convex lens; The first short focal length convex lens refracts the light beam into a parallel light beam, which is then emitted. A parallel beam enters the polarization selection component, and the beam passing through the polarization selection component enters the second convex lens; The second convex lens refracts and converges the light beam, which is focused on the plane of the aperture and passes through the aperture. The light beam passing through the aperture enters the third convex lens, which refracts and performs secondary imaging on the light beam, projecting it onto the receiving end face of the fiber optic coupling component. The optical fiber inside the optical fiber coupling assembly receives the light beam that shines on the end face of the optical fiber.

[0015] Optionally, after the parallel beam enters the polarization selection component, the beam whose polarization direction is parallel to the transmission axis of the polarization selection component continues to propagate backward through the polarization selection component. A beam of light whose polarization direction is not parallel to the transmission axis of the polarization selection component is intercepted by the polarization selection component.

[0016] Optionally, the second convex lens converges the light beam onto the plane of the aperture to form a real image distribution; The target ray in the beam is focused within the central aperture of the aperture and passes through the aperture; Stray rays in the beam are focused outside the central light aperture and blocked by the solid light-shielding plate of the aperture.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention establishes a multi-dimensional joint suppression mechanism in the spatial angle domain, polarization domain, and image plane spatial domain by sequentially arranging a multi-level object-side aperture assembly, a polarization selection assembly, an aperture, and an optical fiber coupling assembly along the propagation path of scattered light. The multi-level object-side aperture assembly reduces background radiation from non-target directions entering the optical system by limiting the effective solid angle of scattered light collection. The polarization selection assembly selects scattered light with the target polarization direction according to the geometry of Thomson scattering, suppressing non-target polarized background light. A second convex lens, in conjunction with the aperture, defines the imaging area and blocks stray radiation from non-target imaging areas. A third convex lens efficiently couples the target light to the optical fiber coupling assembly. The spatially arranged optical fibers directly provide spatial resolution. Through the multi-level synergistic configuration of object-side angle selection, polarization selection, and image plane region selection, this invention can effectively improve the signal-to-noise ratio of Thomson scattering signals under strong background radiation conditions, enhance the accuracy and stability of plasma electron temperature and electron density measurements, and features a simple optical path, convenient assembly and adjustment, and good engineering applicability. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the multi-level spatial-polarization joint noise suppression optical system for Thomson scattering measurement according to the present invention.

[0019] Wherein: 1-scattering volume; 2-multi-level object-side aperture assembly; 3-first short focal length convex lens; 4-polarization selection assembly; 5-second convex lens; 6-aperture; 7-third convex lens; 8-fiber coupling assembly. Detailed Implementation

[0020] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0021] The following disclosure provides many different embodiments or examples for implementing various structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. These are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. In addition, examples of various specific processes and materials are provided in this invention, but those skilled in the art will recognize the application of other processes and / or the use of other materials.

[0022] like Figure 1 As shown, this embodiment discloses a multi-level spatial-polarization joint noise suppression optical system for Thomson scattering measurement. The system uses a central reference line running through the front and rear ends as the optical axis. The system includes, from front to back, a scattering volume 1, a multi-level object-side aperture assembly 2, a first short focal length convex lens 3, a polarization selection assembly 4, a second convex lens 5, an aperture 6, a third convex lens 7, and an optical fiber coupling assembly 8.

[0023] Scattering volume 1 is the source region for optical signals in this system. The incident laser and free electrons within the plasma undergo elastic scattering within this region, generating Thomson scattering signals with specific polarization directions and spectral information. Scattering volume 1 is also the region where strong background radiation, including plasma continuous spectrum radiation, spectral line radiation, and plasma autoluminescence, is generated. This mixed radiation propagates outwards from scattering volume 1 into the surrounding three-dimensional space.

[0024] A multi-stage object-side aperture assembly 2 is arranged in the optical path space between the scattering volume 1 and the first short-focal-length convex lens 3. The multi-stage object-side aperture assembly 2 includes at least two aperture structures spaced apart along the direction of scattered light propagation. Each aperture structure is made of an opaque material and has a central aperture for light to pass through. Starting from the first-stage aperture near the scattering volume 1 and progressing to the final-stage aperture near the first short-focal-length convex lens 3, the aperture size of each stage of the aperture changes progressively along the optical axis, forming a gradually opening optical structure that defines the solid angle of scattered light propagation. The geometric center of the aperture of each stage of the aperture coincides with the optical axis. The aperture of the multi-stage object-side aperture assembly 2 is matched to the aperture of the first short-focal-length convex lens 3. The lines connecting the edges of the apertures of each stage of the aperture form a virtual geometric cone, the apex of which is located in the scattering volume 1, and the cone angle is limited to the effective receiving cone angle range of the first short-focal-length convex lens 3. The multi-level object-side aperture assembly 2 is used to limit the effective collection solid angle from the scattering volume 1, and to intercept and absorb background radiation from non-target directions outside the range of the solid angle.

[0025] The object-side focal point of the first short-focal-length convex lens 3 is located near the scattering point, and this lens is positioned close to the scattering volume. The axial distance between the location of the scattering volume 1 and the surface of the first short-focal-length convex lens 3 is one focal length. f 1. The first short-focal-length convex lens 3 has a short focal length parameter, enabling it to capture a large solid angle at a relatively close distance, thus receiving the light signal from the scattering volume 1 with a large solid angle. The diverging light entering the first short-focal-length convex lens 3 is refracted by the internal medium of the lens and then exits. Since the scattering volume 1 is located at the object-side focal point of the first short-focal-length convex lens 3, the diverging scattered beam is transformed into a collimated beam parallel to the optical axis after refraction by the first short-focal-length convex lens 3, and propagates backward along the optical axis into space.

[0026] The polarization selection component 4 is positioned in the parallel optical path behind the first short-focal-length convex lens 3. The axial distance between the polarization selection component 4 and the first short-focal-length convex lens 3 is one focal length. f1. The polarization selection component 4 employs a linear polarizer or other optical elements with polarization selection functionality. The polarization selection component 4 has a specific polarization transmission axis, and its optical characteristics allow light wave components with polarization directions parallel to this transmission axis to pass through, while absorbing or reflecting light wave components with polarization directions perpendicular to this transmission axis. The polarization direction of the polarization selection component 4 is set according to the Thomson scattering geometry. The target Thomson scattered light generated by the interaction between the incident laser and free electrons has a definite linear polarization characteristic, and the transmission axis of the polarization selection component 4 is parallel to the polarization direction of the target Thomson scattered light. The plasma background radiation contained in the parallel beam is usually unpolarized light. When its various polarization components pass through the polarization selection component 4, only the components parallel to the transmission axis are allowed to pass through; the remaining orthogonal components are blocked by the polarization selection component 4. The polarization selection component 4 is used to select scattered light with the target polarization direction and suppress background light and stray light with non-target polarization states. The polarization selection component 4 can also be positioned in the optical path in front of the first short focal length convex lens 3, between the first short focal length convex lens 3 and the second convex lens 5, or between the second convex lens 5 and the aperture stop 6, depending on different optical path arrangement requirements.

[0027] The second convex lens 5 is positioned axially behind the polarization selection component 4, and the axial distance between the second convex lens 5 and the polarization selection component 4 is one focal length. f 2. The second convex lens 5 is used to image the scattered light into a real image. The parallel light beam, filtered by the polarization selection component 4, enters the second convex lens 5, and after refraction and converging by the lens glass medium, it exits and forms a convergent beam. The second convex lens 5 has an image-side focal length. f 2. After the parallel incident beam passes through the second convex lens 5, its image-side focal length is... f Focused imaging on the spatial plane where 2 is located.

[0028] The aperture stop 6 is positioned at the imaging plane or conjugate position of the second convex lens 5. The axial distance between the aperture stop 6 and the second convex lens 5 is one focal length. f 2. The aperture 6 is composed of an opaque solid light-shielding plate with a central light-passing aperture. The aperture 6 is either a single-stage adjustable aperture or a multi-stage tapered aperture, and the size and shape of its light-passing area match the real image scattered by the target. Target scattered light rays emitted from the location of scattering volume 1, after processing by the preceding components, are focused on the central light-passing area on the plane of the aperture 6 and pass through the aperture. Stray radiation emitted from other non-target areas in space, as well as background light rays deviating from the optical axis in the optical path, are projected onto the opaque light-shielding plate of the aperture 6 after being imaged by the second convex lens 5, with focal coordinates deviating from the central target real image area. The aperture 6 is used to block stray radiation around the real image area, allowing only light signals falling within the target scattered real image area to propagate backward.

[0029] The third convex lens 7 is positioned on the optical axis behind the aperture 6. The third convex lens 7 is used to re-image the light signal filtered by the aperture 6. The light beam passing through the light-passing area of ​​the aperture 6 enters the third convex lens 7 in a divergent state. The third convex lens 7 has its own preset focal length parameters, which are selected according to the specific requirements of the system for collection efficiency, imaging magnification, and spatial resolution. The divergent light beam entering the third convex lens 7 is refracted and converged inside it, and after exiting the third convex lens 7, it forms a convergent beam again. The third convex lens 7 projects the light spot shape of the object plane where the aperture 6 is located onto the secondary imaging plane behind it according to a preset magnification or reduction ratio, establishing a conjugate mapping relationship between the light-passing plane of the aperture 6 and the receiving plane behind it.

[0030] The receiving end face of the fiber optic coupling assembly 8 is located at the secondary imaging plane of the third convex lens 7. The fiber optic coupling assembly 8 includes multiple optical fibers arranged along a spatial direction, with the fiber end faces conjugate aligned with the imaging plane of the third convex lens 7. The optical fibers inside the fiber optic coupling assembly 8 are arranged in a one-dimensional straight line or a two-dimensional planar spatial array, with the incident end faces of the multiple optical fibers closely aligned side-by-side, forming a receiving array spanning the secondary imaging plane. The target image point formed by the third convex lens 7 falls on this receiving array. Image points at specific spatial coordinates are respectively incident into the core of their corresponding single optical fibers. The fiber end faces with different spatial coordinates collect light signals emitted from different spatial locations within the scattering volume 1. Multiple optical fibers receive scattered light signals from different spatial locations, achieving spatial resolution measurement. The fiber optic coupling assembly 8 transmits the received multi-point spatial light signals through its internal optical guide channel to an external detection instrument for subsequent spectral analysis.

[0031] The system employs multidimensional selective noise suppression of scattered light, and the specific process is as follows: During plasma generation, the laser pulse excites Thomson scattered light within the scattering volume 1, accompanied by a large amount of spontaneous plasma emission background light. The mixed radiation beam is emitted outward from the scattering volume 1 and propagates backward to the multi-stage object-side aperture assembly 2.

[0032] Large-angle background radiation rays deviating from the preset effective collection solid angle in the mixed beam are projected onto the opaque baffle surfaces of each stage of the aperture and are absorbed or reflected, unable to continue propagating. Rays within the cone angle range defined by the involute light-transmitting structure pass through each stage of the aperture and reach the first short-focal-length convex lens 3. The first short-focal-length convex lens 3 refracts and transforms this portion of the ray, converting it from a diverging beam into a parallel beam before it is emitted.

[0033] The outgoing parallel beam enters the polarization selection component 4. The target Thomson scattered light in the mixed beam has a linear polarization property in a specific direction, which is consistent with the transmission axis of the polarization selection component 4. Therefore, the energy of the target scattered light directly passes through the polarization selection component 4 and continues to propagate backward. The plasma background radiation light in the mixed beam is mostly unpolarized. When its various random polarization components pass through the polarization selection component 4, all components that are not parallel to the transmission axis are intercepted by the solid structure of the polarization selection component 4. In the beam output after passing through the polarization selection component 4, the intensity of the background light is reduced.

[0034] The parallel beam of light, after polarization filtering, enters the second convex lens 5. The second convex lens 5 refracts and converges the beam, forming a real image distribution on the plane where the aperture 6 is located. The target scattered light rays are focused in the central light-transmitting area of ​​the aperture 6 and pass through the aperture. Stray light rays from the background radiation that do not belong to the target scattering volume 1, after being imaged by the lens, have their focusing coordinates falling outside the central light-transmitting area and are directly blocked by the solid light-blocking part of the aperture 6.

[0035] The light beam passing through aperture 6 contains the target scattered signal that has been angularly limited, polarization-filtered, and image plane-space-limited. This light beam enters the third convex lens 7. The third convex lens 7 refracts and performs secondary imaging on the light signal, projecting it onto the receiving end face of the fiber optic coupling assembly 8.

[0036] Multiple optical fibers arranged in an array within the fiber coupling assembly 8 receive beams of light illuminating specific coordinate regions on their respective end faces. Each fiber guides the scattered light signals belonging to different initial spatial coordinate positions into its internal channel and transmits them to subsequent equipment for parameter calculation of plasma electron temperature and electron density, thus completing the noise suppression and collection work of the entire optical system.

[0037] The sequence numbers of the embodiments in this application are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0038] In the above embodiments of this application, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0039] In the several embodiments provided in this application, it should be understood that the disclosed technical content can be implemented in other ways. The device embodiments described above are merely illustrative; for example, the division of units can be a logical functional division, and in actual implementation, there may be other division methods. For instance, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the displayed or discussed mutual coupling, direct coupling, or communication connection may be through some interfaces; the indirect coupling or communication connection between units or modules may be electrical or other forms.

[0040] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0041] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

[0042] It should be understood that the above description is for illustrative purposes and not for limitation. Many embodiments and applications beyond the provided examples will be apparent to those skilled in the art upon reading the above description. Therefore, the scope of this application should not be determined by reference to the above description, but rather by reference to the foregoing claims and the full scope of their equivalents. For purposes of completeness, all articles and references, including patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the foregoing claims is not intended as a waiver of that subject matter, nor should it be construed as an indication that the applicant has not considered that subject matter as part of the disclosed inventive subject matter.

Claims

1. A multi-level spatial-polarization joint noise suppression optical system for Thomson scattering measurements, characterized in that, With the central reference line running through the front and rear ends as the optical axis, it includes a scattering volume (1), a multi-level object-side aperture assembly (2), a first short focal length convex lens (3), a polarization selection assembly (4), a second convex lens (5), an aperture (6), a third convex lens (7), and an optical fiber coupling assembly (8) arranged sequentially along the optical axis. The multi-level object-side aperture assembly (2) is arranged in the optical path space between the scattering volume (1) and the first short focal length convex lens (3); The polarization selection component (4) is located in the optical path behind the first short focal length convex lens (3); The second convex lens (5) is positioned axially behind the polarization selection component (4); The aperture stop (6) is set at the imaging surface or conjugate position of the second convex lens (5); The third convex lens (7) is arranged on the optical axis behind the aperture (6); The receiving end face of the fiber optic coupling component (8) is set at the secondary imaging surface of the third convex lens (7).

2. The multi-level spatial-polarization joint noise suppression optical system for Thomson scattering measurement according to claim 1, characterized in that, The axial distance between the scattering volume (1) and the first short focal length convex lens (3) and the axial distance between the polarization selection component (4) and the first short focal length convex lens (3) are the same.

3. The multi-level spatial-polarization joint noise suppression optical system for Thomson scattering measurement according to claim 1, characterized in that, The multi-level object-side aperture assembly (2) includes at least two aperture structures spaced apart along the light propagation direction. The aperture size of each aperture structure changes gradually along the optical axis to form an involute light-passing structure. The geometric center of the aperture of each aperture structure coincides with the optical axis.

4. The multi-level spatial-polarization joint noise suppression optical system for Thomson scattering measurement according to claim 3, characterized in that, In the multi-level object-side aperture assembly (2), the edges of the light-transmitting apertures of each level of the aperture structure form a geometric cone. The apex of the geometric cone is located in the scattering volume (1), and the cone angle of the geometric cone is within the effective receiving cone angle range of the first short focal length convex lens (3).

5. The multi-level spatial-polarization joint noise suppression optical system for Thomson scattering measurement according to claim 1, characterized in that, The axial distance between the second convex lens (5) and the polarization selection component (4) and the aperture stop (6) is the same as the axial distance between the second convex lens (5).

6. The multi-level spatial-polarization joint noise suppression optical system for Thomson scattering measurement according to claim 1, characterized in that, The aperture (6) is made of an opaque solid light-blocking plate, and a light-transmitting hole is provided in the center of the aperture (6).

7. The multi-level spatial-polarization joint noise suppression optical system for Thomson scattering measurement according to claim 1, characterized in that, The fiber coupling assembly (8) includes multiple optical fibers arranged along the spatial direction. The incident end faces of the multiple optical fibers are closely arranged side by side to form a receiving array that spans the secondary imaging plane. The end faces of the multiple optical fibers are conjugate aligned with the imaging plane of the third convex lens (7).

8. A noise suppression method based on the system according to any one of claims 1-7, characterized in that... The process includes the following: The beam is emitted outward from the scattering volume (1) and propagates backward to reach the multi-level object-side aperture assembly (2). The light beam passes through the aperture of the multi-stage object-side aperture assembly (2) and reaches the first short focal length convex lens (3); The first short focal length convex lens (3) refracts the light beam into a parallel light beam and emits it; A parallel beam enters the polarization selection component (4), and the beam passing through the polarization selection component (4) enters the second convex lens (5). The second convex lens (5) refracts and converges the light beam, which is focused on the plane of the aperture (6) and passes through the aperture (6). The light beam passing through the aperture (6) enters the third convex lens (7), and the third convex lens (7) refracts and performs secondary imaging on the light beam, projecting the light beam onto the receiving end face of the fiber optic coupling component (8). The optical fiber inside the optical fiber coupling assembly (8) receives the light beam illuminating the end face of the optical fiber.

9. The noise suppression method according to claim 8, characterized in that, After the parallel beam enters the polarization selection component (4), the beam whose polarization direction is parallel to the axis of the polarization selection component (4) continues to propagate backward through the polarization selection component (4); The polarization direction is not parallel to the axis of the polarization selection component (4), and the light beam is intercepted by the polarization selection component (4).

10. The noise suppression method according to claim 8, characterized in that, The second convex lens (5) converges the light beam onto the plane of the aperture (6) to form a real image distribution; The target ray in the beam is focused in the central aperture of the aperture (6) and passes through the aperture; Stray rays in the beam are focused outside the central light aperture and are blocked by the solid light-shielding plate of the aperture (6).

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