High-temperature tension-torsion composite fatigue testing device integrated with time-gated DIC

Abstract

The invention relates to the technical field of high-temperature material mechanical test and non-contact optical full-field measurement, in particular to the technical field of fatigue test equipment, and discloses a high-temperature tension-torsion composite fatigue test device integrated with a time gating DIC, which comprises a mechanical loading measurement module and an imaging module working with the mechanical loading measurement module, the mechanical loading measurement module is provided with a clamping assembly and a transmission assembly which are used for clamping a sample and transmitting tension and torsion combined load, and a tension test assembly and a torsion test assembly which are respectively used for measuring axial deformation and torsion deformation of the sample; the imaging module comprises an active illumination unit, an image acquisition unit and a synchronous control unit, and the synchronous control unit is configured to synchronously trigger the active illumination unit and the image acquisition unit at a preset phase according to a fatigue load cycle signal so as to obtain a time sequence structured light image of the surface of the sample.

Description

Technical Field

[0001] This invention relates to the field of high-temperature material mechanical testing and non-contact optical full-field measurement technology, and particularly to the field of fatigue testing equipment technology. Background Technology

[0002] In the field of high-temperature mechanical property testing of materials, especially when simulating tensile-torsional combined load conditions experienced by key components such as aero-engines and gas turbines, accurate measurement of material deformation faces a dual challenge. On the one hand, the specimen 17 will generate strong broadband thermal radiation under high-temperature conditions, severely interfering with non-contact measurement methods based on optical imaging. This can easily lead to saturation and decreased contrast in speckle images acquired by techniques such as digital image correlation, making it difficult to stably obtain surface texture information with a high signal-to-noise ratio. On the other hand, although traditional contact extensometers or strain gauges can be used in room temperature or medium-low temperature environments, they are difficult to achieve simultaneous decoupled measurement of tensile-torsional deformation, and they suffer from problems such as complex installation, susceptibility to thermal interference, and limited range and durability under high-temperature conditions. Therefore, developing an integrated testing device that can achieve simultaneous and accurate deformation measurement and resist thermal radiation interference under high-temperature, combined load, and fatigue cycle conditions has significant engineering requirements and technical value.

[0003] Chinese patent document CN107255454B discloses an ultra-high temperature multi-scale multifunctional strain measurement system based on ultraviolet imaging DIC. In this technical solution, a monochromatic ultraviolet light source is used to actively illuminate the sample surface. By installing a narrow-bandpass filter matching the illumination wavelength in front of the camera, stray light of other wavelengths in the sample's thermal radiation is suppressed, thereby obtaining images usable for DIC analysis under high-temperature and even ultra-high-temperature conditions. The system also integrates a coaxial microscopic imaging optical path and a mechanical adjustment platform, supporting multi-field observation from millimeter to micrometer scales, and is suitable for high-resolution imaging of material surface deformation in high-temperature furnace environments.

[0004] However, although the aforementioned device achieves high-temperature DIC measurement based on spectral filtering, its technical solution still has significant limitations in application to dynamic, composite load fatigue testing scenarios. First, the system is primarily designed for static or quasi-static tensile observation; its optical measurement unit is independent of the loading mechanism and not integrated with the mechanical transmission structure of tension-torsion composite loading. It lacks dedicated mechanical interfaces and measurement components for synchronous decoupling and measurement of axial and torsional deformation under dynamic coupled loads. Second, its thermal radiation resistance mechanism relies on narrowband spectral filtering, which is a steady-state suppression strategy under continuous exposure mode. In fatigue tests, when thermal radiation intensity may fluctuate transiently with load and temperature, the robustness and image stability of this method remain insufficient. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, a high-temperature tensile-torsional composite fatigue testing device with integrated time-gated DIC is provided.

[0006] This invention is achieved through the following technical solution: a high-temperature tensile-torsional combined fatigue testing device with integrated time-gated DIC, comprising a mechanical loading measurement module and an imaging module that works in tandem; the mechanical loading measurement module is provided with a clamping assembly and a transmission assembly for clamping the specimen and transmitting the tensile and torsional combined loads, as well as a tensile testing assembly and a torsional testing assembly for measuring the axial deformation and torsional deformation of the specimen, respectively; the imaging module includes an active illumination unit, an image acquisition unit, and a synchronization control unit, the synchronization control unit being configured to: synchronously trigger the active illumination unit and the image acquisition unit at a predetermined phase according to the fatigue load cycle signal, so as to acquire a time-series structured light image of the specimen surface.

[0007] In a preferred embodiment of the present invention, the clamping assembly includes a clamping handle, a clamping spring, and a cutting edge; the cutting edge is mounted on the front end of the clamping handle, and the clamping spring is coaxially sleeved on the clamping handle, and the cutting edge clamps the sample by the clamping spring.

[0008] In a preferred embodiment of the present invention, the clamping assembly further includes an upper clamping sleeve and a lower clamping sleeve; the upper clamping sleeve is connected to the clamping tool holder, and the lower clamping sleeve is used to receive and position the transmission assembly.

[0009] In a preferred embodiment of the present invention, the transmission assembly includes an inner heat-insulating sleeve, an outer heat-insulating sleeve, a rolling ball, and a retainer; the inner heat-insulating sleeve and the outer heat-insulating sleeve are coaxially sleeved, and the rolling ball is disposed between the two through the retainer; an axial limiting structure is also provided between the inner heat-insulating sleeve and the outer heat-insulating sleeve.

[0010] In a preferred embodiment of the present invention, the axial limiting structure includes an axial limiting block disposed on the inner heat insulation sleeve and an axial limiting hole disposed on the outer heat insulation sleeve; the axial limiting block passes through the axial limiting hole and can move axially within the axial limiting hole.

[0011] In a preferred embodiment of the present invention, a rolling ball is embedded in a slide rail provided between the inner heat insulation sleeve and the upper clamp sleeve, and the inner heat insulation sleeve rotates relative to the upper clamp sleeve by means of the rolling ball.

[0012] In a preferred embodiment of the present invention, the torsion test assembly includes a code disk and an encoder circuit board; the code disk is mounted on the inner heat insulation sleeve, and the encoder circuit board is mounted at the port connected to the clamping assembly, for detecting the relative rotation angle between the code disk and the encoder circuit board caused by the torsional deformation of the sample.

[0013] In a preferred embodiment of the present invention, the tensile testing assembly includes an LVDT linear displacement sensor; the LVDT linear displacement sensor is mounted on the inner and outer heat insulation sleeves via a sensor support and is used to measure the axial relative displacement between the two.

[0014] In a preferred embodiment of the present invention, the active illumination unit includes a pulsed light source and a projection grating, used to project a high-frequency pulsed structured light pattern onto the surface of a sample under high-temperature conditions.

[0015] In a preferred embodiment of the present invention, the image acquisition unit includes a camera and a narrow bandpass filter disposed in front of the camera lens; the center wavelength of the narrow bandpass filter matches the illumination wavelength of the active illumination unit.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] A high-temperature tensile-torsional combined fatigue testing device integrating time-gated DIC combines a mechanical loading measurement module with a time-gated active imaging DIC module, thus solving the problem in traditional high-temperature fatigue tests of clearly imaging and synchronously measuring combined deformation under dynamic loads. This device can capture high-quality images suitable for DIC analysis during the fatigue process when the specimen's thermal radiation is strongest, through active synchronous control, at a specific phase with minimal thermal noise interference. It also decouples the measurement of axial and torsional deformation in real time, achieving precise spatiotemporal correlation between mechanical response and full-field deformation evolution.

[0018] Furthermore, the clamping assembly, through its self-tightening spring-blade structure, ensures that the clamping force will not decrease due to thermal relaxation or vibration under prolonged high-temperature cyclic loading, effectively preventing sample slippage in the fixture and providing reliable and stable mechanical boundary conditions for accurate deformation measurement. This design is particularly suitable for high-temperature fatigue testing, as traditional threaded or hydraulic clamping methods are prone to failure due to heat.

[0019] Furthermore, the upper and lower clamping sleeves enable a modular and standardized interface between the clamping unit and the transmission / measuring unit. This structure clearly defines the functional areas, facilitating assembly and maintenance. The upper clamping sleeve serves as the power input interface, while the lower clamping sleeve acts as a support and positioning reference, together ensuring the coaxiality of the load transmission path and the rigidity of the overall structure, which is fundamental to ensuring the accuracy of subsequent deformation measurements.

[0020] Furthermore, the inner and outer sleeves combined with the rolling ball structure allow for low-friction torque transmission while simultaneously enabling free axial displacement. The axial restraint structure ensures only pure axial relative movement between the inner and outer sleeves, creating conditions for independent measurement of tensile deformation. The sleeve structure itself also constitutes a thermal barrier, protecting external precision measuring components from the direct effects of the high temperatures inside the furnace.

[0021] Furthermore, the axial restraint structure ensures that the measurement reference will not introduce errors due to radial movement or additional rotation when transmitting complex loads, greatly guaranteeing the accuracy and purity of tensile displacement measurement results, and the structure is robust and durable. The cross-sectional shape of the restraint block closely matches the shape of the hole, allowing it to slide freely along the length direction of the hole (i.e., axial direction), but effectively preventing any movement and rotation in the plane perpendicular to the axis, achieving precise linear guidance.

[0022] Furthermore, the rolling ball achieves a low-friction, high-precision, and self-aligning rotary connection between the inner insulating sleeve and the upper clamp sleeve. It not only efficiently transmits torque but also absorbs minor misalignments caused by machining errors, thermal deformation, or off-center loads, ensuring smooth and stable rotation, reducing hysteresis and backlash, and improving the sensitivity and linearity of torsion measurements.

[0023] Furthermore, the torsion testing component can measure the absolute or relative torsion angle of a sample at high temperatures in real time and dynamically, with fast response and strong anti-interference capability. By converting the mechanical rotation angle into a photoelectric signal, it achieves long-distance, wear-free signal transmission with high accuracy and long lifespan.

[0024] Furthermore, the LVDT sensor possesses theoretically infinite resolution, excellent linearity, and repeatability. The tensile testing assembly can directly and in real-time measure minute axial relative displacements between the inner and outer sleeves, thereby accurately inferring the deformation of the gauge length of the specimen. The sensor itself is protected by a thermal insulation structure from high-temperature damage. This provides a contact-based, high-precision, and highly stable axial displacement measurement solution.

[0025] Furthermore, the active illumination unit overcomes the shortcomings of weak surface features and strong thermal radiation interference of high-temperature samples. By actively projecting specific patterns with strong light and dark contrast (such as speckle and grid lines), it provides the camera with stable, clear and easily recognizable texture information, which is a prerequisite for DIC analysis.

[0026] Furthermore, the image acquisition unit achieves selective enhancement of signals in specific wavelength bands through optical filtering. This suppresses the primary noise source in high-temperature environments (broadband thermal radiation), allowing only light signals of the active illumination wavelength to enter the camera. This is equivalent to "opening a narrow window" in extremely strong background light, thereby significantly improving the signal-to-noise ratio of the image and making it possible to acquire clear feature images at high temperatures.

[0027] Other features and advantages of the present invention will be disclosed in detail in the following detailed description and accompanying drawings. Attached Figure Description

[0028] The invention will be further described below with reference to the accompanying drawings:

[0029] Figure 1 A three-dimensional exploded view of a high-temperature tensile-torsional composite fatigue testing device with integrated time-gated DIC according to the present invention;

[0030] Figure 2 This is a schematic diagram of the imaging process of the imaging module of the present invention;

[0031] The annotations in the attached figures are explained as follows:

[0032] 1. Clamping handle; 2. Clamping spring; 3. Blade; 4. Lower clamping sleeve; 5. Upper clamping sleeve; 6. Rolling ball; 7. Cage; 8. Inner heat insulation sleeve; 9. Outer heat insulation sleeve; 10. Axial limiting block; 11. Axial limiting hole; 12. Code disk; 13. Encoder circuit board; 14. Sensor support; 15. LVDT linear displacement sensor; 16. Observation window; 17. Sample; 18. Camera; 181. Narrow bandpass filter; 19. Pulse light source; 20. Synchronization control unit. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be explained and described below with reference to the accompanying drawings. However, the following embodiments are only preferred embodiments of the present invention and not all of them. Other embodiments obtained by those skilled in the art based on the embodiments in the implementation methods without creative effort are all within the protection scope of the present invention.

[0034] In the following description, terms such as “inner,” “outer,” “upper,” “lower,” “left,” and “right” are used only to indicate orientation or positional relationship for the convenience of describing the embodiments and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0035] like Figures 1 to 2 As shown, a high-temperature tensile-torsional combined fatigue testing device integrating time-gated DIC (Digital Interchange Control) is characterized by the deep integration of a mechanical system for mechanical loading and deformation measurement with an optical non-contact full-field measurement system based on synchronous trigger control. This enables high-precision, multi-dimensional synchronous measurement of material deformation behavior under high-temperature and tensile-torsional combined fatigue loading conditions. The device is mainly divided into two parts: a mechanical loading measurement module and a synchronous imaging module. The mechanical loading measurement module is the main mechanical structure of the device, while the synchronous imaging module is an independent optical measurement system. The two work together to complete the combined deformation test under high-temperature conditions.

[0036] The mechanical loading measurement module is directly mounted or integrated into a testing machine capable of providing tensile-torsional combined fatigue loads, and is placed together with the specimen in a heating chamber (such as a high-temperature furnace) capable of providing a high-temperature environment. The main mechanical structure of this module is responsible for clamping the specimen, transferring the load, and directly measuring the deformation within the high-temperature furnace. The clamping assembly of this module is used to securely hold the specimen 17. Specifically, multiple blades 3 are mounted on the front end of the clamping handle 1, and a clamping spring 2 is coaxially sleeved on the column of the clamping handle 1. When the specimen 17 is installed, the clamping spring 2 is compressed, causing its stored elastic force to push the clamping handle 1, thereby causing the blades 3 to retract radially inward, thus firmly engaging the surface of the specimen 17 to form a self-tightening clamp, effectively preventing loosening and slippage at high temperatures. The clamping assembly also includes an upper clamping sleeve 5 and a lower clamping sleeve 4. The upper clamping sleeve 5 is connected to the clamping handle 1, forming the load input end; the lower clamping sleeve 4 serves as a basic support component, used for positioning and supporting subsequent transmission components.

[0037] The transmission assembly is the core component for decoupling and transmitting composite loads. It mainly consists of an inner heat-insulating sleeve 8, an outer heat-insulating sleeve 9, rolling balls 6, and a cage 7. The inner and outer heat-insulating sleeves are made of high-temperature resistant materials with low thermal conductivity, or coated with a heat-insulating coating, to provide thermal protection for internal precision components (such as code disks) and external sensors (such as the LVDT linear displacement sensor 15). They are coaxially fitted together, with the rolling balls 6 evenly distributed between them via the cage 7, forming a low-friction rotating pair that allows relative rotation between the inner and outer sleeves to transmit torque. Furthermore, an axial limiting block 10 is provided on the inner heat-insulating sleeve 8, and a corresponding axial limiting hole 11 is provided on the outer heat-insulating sleeve 9. The axial limiting block 10 passes through the axial limiting hole 11 and can slide freely along the sleeve's axial direction within it, but its movement is strictly limited to the radial and circumferential directions. This design ensures that only one degree of freedom of relative axial movement is maintained between the inner and outer heat-insulating sleeves. Furthermore, to facilitate the input of torque from the loading mechanism to the transmission assembly, the rolling ball 6 is also positioned within a specially machined arc-shaped slide rail between the inner heat-insulating sleeve 8 and the upper clamping sleeve 5. This allows the upper clamping sleeve 5 to drive the inner heat-insulating sleeve 8 to rotate via the rolling ball 6, while rolling friction ensures smooth movement and low hysteresis.

[0038] Based on the aforementioned transmission structure, tensile and torsional deformations can be measured independently. The torsional testing assembly includes a code disk 12 and an encoder circuit board 13. The code disk 12 is fixedly mounted on the inner heat-insulating sleeve 8 (or the outer heat-insulating sleeve 9) and rotates with it. The encoder circuit board 13 is fixedly mounted at the port of the relatively stationary upper clamp sleeve 5 or lower clamp sleeve 4. When the sample 17 undergoes torsional deformation, it will cause the code disk 12 and the encoder circuit board 13 to generate a relative rotation angle. The torsional angle of the sample 17 can be accurately measured through the photoelectric encoding principle. The tensile testing assembly includes an LVDT linear displacement sensor 15, which is mounted on the inner heat-insulating sleeve 8 and the outer heat-insulating sleeve 9 respectively through a sensor support 14. Since the axial restraint structure ensures that the inner and outer sleeves can only slide relative to each other axially, the deformation of the sample 17 in the tensile direction is directly converted into axial displacement between the two, which is captured and measured in real time by the high-precision LVDT sensor.

[0039] To overcome the interference of intense thermal radiation on optical measurements under high-temperature conditions and to acquire full-field deformation information of the sample 17 surface, this invention integrates a synchronous imaging module independent of the mechanical loading structure, namely a time-gated active imaging DIC system, such as... Figure 2 As shown. The core components of this module (pulse light source 19 and camera 18) are arranged outside the heating chamber, and an optical path is established between them and the sample 17 inside through a specially designed observation window 16 on the chamber. The observation window 16 is made of a high-temperature resistant optical material (such as quartz glass), and its function is to allow active illumination light of a specific wavelength to pass through to illuminate the sample while maintaining a high temperature and sealed environment inside the heating chamber, and to allow reflected light from the sample surface to pass through for the camera to collect.

[0040] The synchronous imaging module includes an active illumination unit, an image acquisition unit, and a synchronous control unit 20. The active illumination unit mainly consists of a pulsed light source 19 (such as a pulsed laser or a high-power LED) and a projection grating (or an element for generating speckle). It generates high-intensity, specific-wavelength transient structured light, which illuminates the surface feature pattern (such as high-temperature speckle) of the gauge length section of the specimen 17 through the observation window 16. The image acquisition unit mainly includes a camera 18 with fast exposure capability (such as a gated camera or a global shutter camera), and a narrow-bandpass filter 181 with a center wavelength strictly matched to the illumination source is installed in front of its lens to filter out broadband thermal radiation stray light from the specimen and furnace that passes through the observation window 16 to the maximum extent possible. The synchronous control unit 20 is the "nerve center" of this module. It receives load or displacement cycle signals from the fatigue testing machine (which provides tensile-torsional composite load tests to the specimen) and, according to a preset strategy, synchronously issues trigger commands at a specific phase of each load cycle (e.g., near the load peak, when thermal radiation interference may be relatively stable or weak). This command causes the pulsed light source 19 to emit a high-intensity light pulse for an extremely short duration (such as microseconds or nanoseconds), while simultaneously instructing the camera 18 to open the shutter for exposure during the duration of this light pulse.

[0041] In practice, sample 17 is installed inside a high-temperature furnace, and its surface is pre-formed with high-temperature resistant speckle patterns. After the device starts working, the mechanical loading measurement module placed inside the furnace applies a set tensile-torsional combined fatigue load to sample 17. The LVDT sensor 15 and encoder circuit board 13 continuously output axial displacement and torsional angle signals for contact point measurement. At the same time, the synchronous imaging module located outside the furnace is activated. The synchronous control unit 20 synchronously triggers pulse illumination and camera 18 exposure at predetermined times in each cycle according to the load frequency. Within the extremely short exposure time window, the intense active illumination light is reflected by the speckle pattern on the surface of sample 17, and the reflected light is again received by camera 18 through observation window 16. Although the broadband thermal radiation light emitted by the sample and furnace also passes through the observation window, its energy is very low during the nanosecond-level exposure time, and most of it is blocked by narrowband filter 181, thus significantly suppressing its interference with imaging. The camera 18 ultimately captures a clear speckle image with high contrast, dominated by active illumination light. By performing digital image correlation analysis on the time-series images acquired in this manner throughout the fatigue process, the evolution history of the full-field, continuous two-dimensional or three-dimensional displacement and strain fields on the surface of specimen 17 can be obtained, especially the axial strain field and the shear strain field reflecting torsional deformation. This full-field data not only visually displays the spatial distribution and inhomogeneity of deformation and identifies potential damage initiation locations, but also allows for cross-verification and supplementation with local point measurement results obtained by LVDT and encoders, greatly improving the reliability and information richness of material deformation measurement results under high-temperature and complex loads.

[0042] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that the present invention includes, but is not limited to, the contents described in the accompanying drawings and the specific embodiments above. Any modifications that do not depart from the functional and structural principles of the present invention will be included within the scope of the claims.

Claims

1. A high-temperature tensile-torsional composite fatigue testing device with integrated time-gated DIC, characterized in that, Includes a mechanical loading measurement module and an imaging module that works in step with it; The mechanical loading measurement module is equipped with a clamping assembly and a transmission assembly for clamping the specimen and transmitting tensile and torsional combined loads, as well as a tensile testing assembly and a torsional testing assembly for measuring the axial deformation and torsional deformation of the specimen, respectively. The imaging module includes an active illumination unit, an image acquisition unit, and a synchronization control unit. The synchronization control unit is configured to synchronously trigger the active illumination unit and the image acquisition unit at a predetermined phase based on the fatigue load cycle signal, so as to acquire a temporal structured light image of the sample surface.

2. The high-temperature tensile-torsional composite fatigue testing device with integrated time-gated DIC according to claim 1, characterized in that, The clamping assembly includes a clamping handle (1), a clamping spring (2), and a cutting edge (3); the cutting edge (3) is mounted on the front end of the clamping handle (1), the clamping spring (2) is coaxially sleeved on the clamping handle (1), and the cutting edge (3) clamps the sample through the clamping spring (2).

3. The high-temperature tensile-torsional composite fatigue testing device with integrated time-gated DIC according to claim 2, characterized in that, The clamping assembly further includes an upper clamping sleeve (5) and a lower clamping sleeve (4); the upper clamping sleeve (5) is connected to the clamping handle (1), and the lower clamping sleeve (4) is used to receive and position the transmission assembly.

4. The high-temperature tensile-torsional composite fatigue testing device with integrated time-gated DIC according to claim 3, characterized in that, The transmission assembly includes an inner heat insulation sleeve (8), an outer heat insulation sleeve (9), a rolling ball (6), and a retainer (7); the inner heat insulation sleeve (8) and the outer heat insulation sleeve (9) are coaxially sleeved, and the rolling ball (6) is disposed between the two through the retainer (7); an axial limiting structure is also provided between the inner heat insulation sleeve (8) and the outer heat insulation sleeve (9).

5. The high-temperature tensile-torsional composite fatigue testing device with integrated time-gated DIC according to claim 4, characterized in that, The axial limiting structure includes an axial limiting block (10) disposed on the inner heat insulation sleeve (8) and an axial limiting hole (11) disposed on the outer heat insulation sleeve (9); the axial limiting block (10) passes through the axial limiting hole (11) and can move axially within the axial limiting hole (11).

6. The high-temperature tensile-torsional composite fatigue testing device with integrated time-gated DIC according to claim 4, characterized in that, The rolling ball (6) is embedded in the slide rail provided between the inner heat insulation sleeve (8) and the upper clamp sleeve (5), and the inner heat insulation sleeve (8) rotates relative to the upper clamp sleeve (5) through the rolling ball (6).

7. The high-temperature tensile-torsional composite fatigue testing device with integrated time-gated DIC according to claim 4, characterized in that, The torsion test assembly includes a code disk (12) and an encoder circuit board (13); the code disk (12) is mounted on the inner heat insulation sleeve (8), and the encoder circuit board (13) is mounted at the port connected to the clamping assembly, for detecting the relative rotation angle between the code disk (12) and the encoder circuit board (13) caused by the torsion deformation of the sample.

8. The high-temperature tensile-torsional composite fatigue testing device with integrated time-gated DIC according to claim 4, characterized in that, The tensile testing assembly includes an LVDT linear displacement sensor (15); the LVDT linear displacement sensor (15) is mounted on the inner heat insulation sleeve (8) and the outer heat insulation sleeve (9) via a sensor support (14) and is used to measure the axial relative displacement between the two.

9. The high-temperature tensile-torsional composite fatigue testing device with integrated time-gated DIC according to claim 1, characterized in that, The active illumination unit includes a pulsed light source and a projection grating, used to project a high-frequency pulsed structured light pattern onto the surface of a sample under high-temperature conditions.

10. A high-temperature tensile-torsional composite fatigue testing device with integrated time-gated DIC according to claim 1, characterized in that, The image acquisition unit includes a camera and a narrow bandpass filter disposed in front of the camera lens; the center wavelength of the narrow bandpass filter matches the illumination wavelength of the active illumination unit.

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