A laser speckle testing device based on hyperspectral imaging

Abstract

The application discloses a kind of laser speckle testing device based on hyperspectral imaging, which realizes high-precision measurement of laser spot and speckle signal under the condition of not relying on expensive refrigeration system by optimizing optical structure and control strategy.The device includes imaging lens, lens electric control module, liquid crystal tunable filter module, ND filter wheel and its driving mechanism, two-dimensional image sensor, main control unit, three-axis acceleration sensor, PC host and the screen to be measured.The noise calibration piece pre-stored in the ND filter wheel is used to collect noise data in real time before each measurement, and noise subtraction is carried out before the filter switching to improve the timeliness and accuracy of data correction;At the same time, the integrated three-axis acceleration sensor is used to monitor vibration, and when vibration exceeding the preset threshold is detected, it is determined that the current collected data is invalid and automatically triggers re-sampling.The application has compact structure and low cost, and is suitable for laser display, projection calibration and high-brightness light source detection and other fields.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of laser display and optical detection, and particularly relates to a laser speckle testing device based on hyperspectral imaging (central wavelength 400nm-700nm adjustable, bandpass width 5-20nm), which is used for realizing high-precision and real-time detection and analysis of laser speckle. BACKGROUND

[0002] With the continuous popularization of laser display, projection and high-brightness light source applications, the requirement for accurate measurement of laser spot and speckle characteristics is increasingly improved. Speckle testing is very sensitive to the thermal noise of an imaging sensor. Traditional speckle testing devices mainly rely on cooling of the imaging sensor (such as a CCD or CMOS sensor) to obtain lower thermal noise. This technical solution undoubtedly makes the BOM cost of the equipment reach more than hundreds of thousands of yuan, and further makes the sales price of the equipment higher, so that the speckle testing device cannot be effectively promoted to the market. The vibration during switching of the filter and external vibration have a direct impact on the imaging quality, and also make the traditional equipment have deficiencies in data consistency and stability during measurement, which is difficult to meet the demand for high-precision detection.

[0003] Traditional speckle testing devices often rely on cooling of a CCD or CMOS sensor to reduce thermal noise. Although the imaging quality can be improved, the cooling system not only increases the volume and energy consumption of the equipment, but also greatly increases the overall BOM cost, so that the sales price of the equipment is high, which restricts the large-scale market application of the speckle testing device. SUMMARY

[0004] The purpose of the present application is to provide a laser speckle testing device based on hyperspectral imaging, which effectively reduces the sensitivity to thermal noise of the imaging sensor while maintaining high-precision measurement, and overcomes the adverse effects of filter switching and external vibration on imaging quality, thereby realizing low-cost and high-performance speckle testing.

[0005] The present application effectively reduces the dependence on sensor cooling by introducing active anti-noise design and data reacquisition strategy in the hardware structure and the main control unit, thereby realizing thermal noise control and high-quality imaging without significantly increasing the cost.

[0006] The specific technical solution to achieve the purpose of the present application is:

[0007] A laser speckle testing device based on hyperspectral imaging, characterized in that it comprises:

[0008] An imaging lens for collecting speckle images of a measured target;

[0009] A lens electric control module is connected with the imaging lens and used for adjusting imaging parameters of the imaging lens.

[0010] An ND filter wheel is arranged in an optical path behind the imaging lens, and the ND filter wheel comprises neutral gray filters with different optical densities and is used for adjusting laser incidence intensity.

[0011] An ND filter wheel driving mechanism is connected with a rotating shaft of the ND filter wheel and used for driving the ND filter wheel to rotate so as to switch the neutral gray filters with different optical densities.

[0012] A liquid crystal tunable filter module is arranged in an optical path behind the ND filter wheel and used for selectively transmitting optical signals with different wavelength ranges through electric control so as to realize high-precision chroma measurement.

[0013] A two-dimensional image sensor is arranged in an optical path behind the liquid crystal tunable filter module and used for converting the optical signals after the ND filter wheel and the liquid crystal tunable filter module into digital image signals.

[0014] A three-axis acceleration sensor is connected with the main control unit and used for monitoring vibration signals in real time.

[0015] The main control unit is connected with the lens electric control module, the ND filter wheel driving mechanism, the liquid crystal tunable filter module, the two-dimensional image sensor and the three-axis acceleration sensor respectively, used for sending an electric signal to the lens electric control module to start an image acquisition task, sending an electric signal to the ND filter wheel driving mechanism to control the rotation of the filter wheel, receiving the digital image signals output by the two-dimensional image sensor and receiving the vibration signals of the three-axis acceleration sensor.

[0016] A PC host is in communication with the main control unit and used for analyzing the digital image signals, calculating chroma and evaluating laser speckle characteristics.

[0017] Further, the central wavelength of the liquid crystal tunable filter module is adjustable between 400 nm and 700 nm, and the bandpass width is 5 nm to 20 nm, so as to meet the high-spectral sampling requirements of single-wavelength laser and color laser speckle.

[0018] Further, the ND filter wheel has a plurality of holes, one of which is empty and does not place anything, one of which is placed with a noise calibration sheet, and the remaining holes are respectively provided with neutral gray filters with different optical densities, which are used for fine adjustment of laser beam intensity in high-brightness or weak light environment to prevent the two-dimensional image sensor from being overexposed.

[0019] Further, the noise calibration sheet is made of pure black material with reflectivity lower than 1%, which is used to collect noise data before each color measurement and deduct the noise data from the subsequent measurement results to improve the measurement accuracy.

[0020] Further, the main control unit receives the monitoring results of the three-axis acceleration sensor on the vibration signal, judges the current vibration state, and when the amplitude of the vibration signal exceeds a preset threshold, the main control unit discards the current digital image signal and reacquires, improving the accuracy of speckle measurement.

[0021] Further, the main control unit drives the liquid crystal tunable filter module to sequentially switch the center wavelengths of a plurality of light signals allowed to pass through the liquid crystal tunable filter module, and synchronously acquires corresponding image sequences to construct a hyperspectral data cube of the speckle image, which is used for subsequent chroma fitting and spectral feature extraction.

[0022] In the present application, the imaging lens and the lens electric control module work together to realize dynamic adjustment of the optical imaging parameters; the ND filter wheel and the liquid crystal tunable filter (LCTF) module set in the optical path are used for light intensity adjustment and chroma filtering respectively, and the LCTF realizes adjustable narrow-band wavelength transmission through voltage control, which is suitable for accurate extraction of chroma information of color laser. Since the bandwidth of the laser light source itself is extremely narrow, the 5~20nm bandwidth of the LCTF can fully meet the transmission requirements of monochromatic laser; in color laser speckle measurement, the traditional filter is easy to cause chroma error due to too wide bandwidth or center wavelength drift, at this time the narrow-band characteristic of the LCTF can significantly improve the resolution and accuracy of chroma extraction. Compared with the traditional XYZ filter wheel, the LCTF avoids mechanical switching vibration, and has continuous wavelength selection capability, improving the flexibility and stability of imaging.

[0023] In order to solve the adverse effects of filter switching process and external environmental vibration on imaging quality, the present application introduces a three-axis acceleration sensor to monitor the motion and vibration of the device in real time. When a vibration signal exceeding a preset threshold is detected, the system actively determines that the currently acquired data is invalid, and notifies the main control unit to trigger image data reacquisition, ensuring that the data used for speckle measurement is obtained in a stable state, thereby ensuring the consistency and repeatability of the image data.

[0024] The invention also includes a noise calibration plate with a reflectivity of less than 1%, which is placed in the ND filter wheel as a black level reference. Before each colorimetric measurement, the system can switch to the position of this calibration plate to acquire an ambient noise image for dark level subtraction, thereby improving the overall measurement accuracy and dynamic response capability.

[0025] Furthermore, to further reduce the system's reliance on the cooling module, this invention employs a high-performance back-illuminated CMOS sensor. This sensor boasts advantages such as low readout noise and a wide dynamic range, which can mitigate the impact of thermal noise on speckle imaging to some extent. Combined with the data processing algorithms of the main control unit and the dedicated software platform in the PC host, the system further suppresses noise in the speckle image through methods such as bad pixel calibration and dark current compensation, ensuring the accuracy and repeatability of the measurement results.

[0026] In summary, the laser speckle testing device of this invention, by introducing a liquid crystal tunable filter, an ND filter wheel, a vibration detection module, and a high dynamic range image acquisition and processing mechanism, forms a comprehensive testing platform with high precision, high stability, and cost advantages. It is particularly suitable for hyperspectral colorimetric analysis and stability evaluation of speckle images of color laser systems, providing a low-cost, high-precision, stable, and reliable testing solution for fields such as laser display, projection calibration, and high-brightness light source detection. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the structure of Embodiment 1 of the present invention;

[0028] Among them: 1-imaging lens, 2-lens electronic control module, 3-ND filter wheel, 4-ND filter wheel drive mechanism, 5-liquid crystal tunable filter (LCTF) module, 6-two-dimensional image sensor, 7-main control unit, 8-three-axis accelerometer, 9-PC host, 10-screen under test. Detailed Implementation

[0029] The specific embodiments of the laser speckle testing device of the present invention will be described in detail below with reference to the accompanying drawings and examples. The following embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention.

[0030] Example 1: As Figure 1 As shown, the overall structure of this embodiment includes: an imaging lens 1, a lens electronic control module 2, an ND filter wheel 3, an ND filter wheel drive mechanism 4, a liquid crystal tunable filter (LCTF) module 5, a two-dimensional image sensor 6, a main control unit 7, a three-axis accelerometer 8, a PC host 9, and a test screen 10. The test screen 10 is a target object used to emit or display laser spot and speckle signals, and is positioned appropriately in front of the imaging lens 1.

[0031] In the present embodiment, the screen 10 to be measured is located in front of the imaging lens 1, which is used to collect the laser speckle image thereof and focus and transmit the light rays thereof to the lens electric control module 2. The lens electric control module 2 automatically adjusts the focal length and aperture according to the parameters set by the master control unit 7, so as to adapt to different laser brightness and target distances, and ensure clear and stable images. The focused light rays enter the ND filter wheel 3 and the liquid crystal tunable filter (LCTF) module 5 arranged in sequence in the optical path.

[0032] The ND filter wheel 3 is provided with a plurality of neutral gray filters with different optical densities, a hole, and a hole position provided with a noise calibration sheet. The laser incident intensity can be switched between multiple levels according to the actual situation through accurate control by the ND filter wheel driving mechanism 4, so as to prevent overexposure during imaging. The noise calibration sheet is composed of black material with a reflectivity of less than 1%. Before each colorimetric measurement starts, the master control unit 7 controls the ND filter wheel to switch to the noise calibration sheet position, and the two-dimensional image sensor 6 collects the background noise image, which serves as a reference benchmark for subsequent image data correction.

[0033] The liquid crystal tunable filter (LCTF) module is used for selective transmission of the wavelength components of the light rays. The LCTF can realize continuous adjustment of the wavelength from 400 nm to 700 nm through voltage control without changing the structure of the optical path, with a bandwidth of 5 nm to 20 nm. This structure greatly improves the separation ability of the device for different spectral component speckle images, and is especially suitable for colorimetric analysis of color laser speckle.

[0034] Specifically, the laser speckle test device of the present application supports two speckle measurement modes:

[0035] 1) Single-color speckle measurement mode: suitable for testing single-wavelength (such as red or green) laser source speckle scenarios. Since the single-color laser itself has a very narrow spectral width, the bandwidth of the LCTF can completely cover the target wavelength. In this mode, the system sets the LCTF to a fixed center wavelength, and performs single-band image acquisition on the laser spot, realizing intensity distribution analysis of the speckle pattern.

[0036] 2) Color speckle measurement mode: suitable for testing the color speckle generated by the composite wavelength or RGB laser combination projection system. In this mode, the system controls the LCTF to scan step by step according to the set wavelength sequence (for example, 450nm, 520nm, 630nm), and image data is collected at each wavelength band respectively. By reconstructing the chrominance space distribution under each wavelength window, the error caused by spectral overlap in the traditional broadband filtering scheme is effectively avoided, and the resolution and accuracy of chrominance calculation are improved. The three-axis acceleration sensor 8 is used to monitor the vibration state of the device in real time. When it is detected that the current is in the vibration condition, the main control unit 7 will determine that the current frame image is invalid and automatically start image reacquisition, ensuring that the image data used for speckle analysis is obtained under stable conditions.

[0037] The main control unit 7 controls the ND filter wheel driving mechanism 4 to rotate to the hole or the selected optical density filter position, and controls the liquid crystal tunable filter (LCTF) module 5 to adjust the center wavelength to the target value, so that the current chrominance band pass window is opened. The liquid crystal tunable filter (LCTF) module 5 realizes continuous narrowband filtering based on liquid crystal electric control polarization principle, and is suitable for multi-band frame imaging in the laser wavelength range.

[0038] During the chrominance image acquisition process, the device deducts the previously acquired noise image data from the current image to obtain the corrected laser speckle image. In order to ensure that the imaging process is not affected by external interference, the device of the application integrates a three-axis acceleration sensor 8 to monitor the device posture and vibration state in real time. When detecting vibration higher than the preset threshold, the main control unit 7 will actively determine that the frame image is invalid, and trigger the reacquisition mechanism until the data frame under stable state is acquired.

[0039] After the image acquisition is completed, the main control unit 7 transmits the image data to the PC host 9. The PC host 9 performs high dynamic range (HDR) synthesis, bad point correction, dark current compensation and chrominance fitting analysis on the image, and outputs the corresponding chrominance map, speckle intensity distribution map or hyperspectral data cube according to the measurement target. Finally, the results can be displayed through the graphical interface or output in data format for user analysis and comparison.

[0040] In actual operation, the test steps of monochromatic speckle can include the following processes:

[0041] Step 1: Position and install the measured laser light source or the measured screen 10 in front of the test platform, ensure that it is directly opposite the imaging lens 1, keep it stable and non-vibrating, and start and preheat the equipment;

[0042] Step 2: Clearly define the RGB wavelength information of the measured laser light source, or use a spectrometer to first record the RGB wavelength information of the measured laser light source;

[0043] Step 3: The measured laser light source projects a red field (R) image on the measured screen 10;

[0044] Step 4: The lens electric control module 2 automatically completes focusing and aperture adjustment according to the preset parameters, so that the laser speckle image is clearly imaged;

[0045] Step 5: Adjust the wavelength window of the LCTF to adapt to the wavelength of the red light R of the measured laser light source;

[0046] Step 6: The ND filter wheel driving mechanism 4 is controlled by the main control unit 7 to rotate the ND filter wheel 3 to the position provided with the noise calibration sheet, and the ambient background noise image is collected by the two-dimensional image sensor 6 (the subsequent steps can also refer to CN110987382A and CN214137350U);

[0047] Step 7: The ND filter wheel driving mechanism 4 is controlled by the main control unit 7 to rotate the ND filter wheel 3 to the appropriate density position, adapt to the current laser intensity, and prevent the two-dimensional image sensor 6 from being overexposed or underexposed;

[0048] Step 8: Use the two-dimensional image sensor 6 to collect the speckle image of the red field, and if the three-axis acceleration sensor 8 detects vibration during the collection process, restart this step;

[0049] Step 9: After the speckle image is deducted from the ambient background noise image, monochromatic speckle contrast calculation is performed (calculation method can be found in national standard GB / T 43590.504-2024);

[0050] Step 10: The main control unit 7 transmits all image data to the PC host 9, and displays the speckle measurement process image and monochromatic speckle contrast value:

[0051] Step 11: Repeat steps 3-10 to measure the monochromatic speckle contrast of the green field (G) and the blue field (B).

[0052] Step 12: On the PC host 9, the monochromatic speckle contrast value, chroma variance, and covariance value of the red field (R), green field (G), and blue field (B) are calculated based on the images obtained in steps 10-11 (calculation method can be found in national standard GB / T 43590.504-2024).

[0053] In summary, by placing the measured screen 10 in front of the imaging lens 1 and combining automatic focusing, ND filter adjustment, LCTF multi-wavelength switching, noise calibration, and three-axis vibration sensing mechanism, high-precision measurement of laser speckle is achieved under non-refrigeration conditions. The device has compact structure, high efficiency of function cooperation, good measurement stability and practicality, and is suitable for laser display debugging, projection equipment detection, and speckle analysis and chroma evaluation in laser lighting applications.

[0054] Embodiment 2: This embodiment is the control of liquid crystal tunable filter (LCTF) module and synchronous image acquisition.

[0055] In order to construct the hyperspectral data cube of speckle image, the control of liquid crystal tunable filter (LCTF) module 5 and synchronous image acquisition follows the following steps:

[0056] Step 1: The operator sets the scanning range (for example, from 400 nanometers to 700 nanometers) and wavelength step (for example, 5 nanometers or 10 nanometers) of the LCTF according to the spectral characteristics of the measured light source or the measurement requirements. These parameters are sent and stored in the master control unit 7;

[0057] Step 2: The master control unit 7 tunes the center wavelength of the liquid crystal tunable filter (LCTF) module 5 to the starting wavelength (for example, 400 nanometers) of the scanning range according to the received parameters;

[0058] Step 3: After confirming that the liquid crystal tunable filter (LCTF) module 5 is stable, the master control unit 7 sends a trigger signal to the two-dimensional image sensor 6 to collect a frame of speckle image at this wavelength;

[0059] Step 4: The collected single-frame image data together with its corresponding center wavelength information are temporarily stored by the master control unit 7. Then, the master control unit 7 sends an instruction to the liquid crystal tunable filter (LCTF) module 5 to increase the center wavelength by the set step (for example, tune to 405 nanometers);

[0060] Step 5: Steps 3 and 4 are repeated, that is, after the center wavelength at each new center wavelength is stable, a frame of image is synchronously collected, and the correspondence between the wavelength and the image is recorded. This process is repeated until the center wavelength of the LCTF reaches the end wavelength (for example, 700 nanometers) of the scanning range;

[0061] Step 6: After the image sequence of all wavelength points is collected, the master control unit 7 packages the entire image sequence and corresponding wavelength data and transmits them to the PC host 9. The PC host 9 is responsible for integrating this image sequence to construct a three-dimensional hyperspectral data cube. Two dimensions of the data cube represent the spatial pixel coordinates (X, Y) of the image, and the third dimension represents the spectral wavelength (λ). The value of each point (X, Y, λ) in the cube corresponds to the light intensity information of the spatial position at the specific wavelength.

[0062] Based on the constructed hyperspectral data cube, the PC host 9 can perform subsequent analysis and processing.

Claims

1. A laser speckle testing device based on hyperspectral imaging, characterized in that, The application relates to a laser speckle imaging device. The device comprises: an imaging lens for collecting a speckle image of a measured target; a lens electric control module connected with the imaging lens for adjusting imaging parameters of the imaging lens; an ND filter wheel arranged in a light path behind the imaging lens, the ND filter wheel comprising a plurality of neutral density filters with different optical densities for adjusting laser incident intensity; an ND filter wheel driving mechanism connected with a rotating shaft of the ND filter wheel for driving the ND filter wheel to rotate to realize switching of the neutral density filters with different optical densities; a liquid crystal tunable filter module arranged in a light path behind the ND filter wheel for selectively transmitting light signals of different wavelength ranges through electric control to realize high-precision chromaticity measurement; a two-dimensional image sensor arranged in a light path behind the liquid crystal tunable filter module for converting the light signals after the ND filter wheel and the liquid crystal tunable filter module into digital image signals; a three-axis acceleration sensor connected with a main control unit for monitoring vibration signals in real time; the main control unit is connected with the lens electric control module, the ND filter wheel driving mechanism, the liquid crystal tunable filter module, the two-dimensional image sensor and the three-axis acceleration sensor respectively, and is used for sending an electric signal to the lens electric control module to start an image collection task, sending an electric signal to the ND filter wheel driving mechanism to control the rotation of the filter wheel, receiving the digital image signals output by the two-dimensional image sensor and receiving the vibration signals of the three-axis acceleration sensor; 2. The laser speckle testing device of claim 1, wherein, a PC host computer in communication with the main control unit for analyzing the digital image signals, calculating chromaticity and evaluating laser speckle characteristics.

3. The laser speckle testing device of claim 1, wherein, The central wavelength of the liquid crystal tunable filter module is adjustable between 400-700 nm, and the bandpass width is 5-20 nm, so that the high-spectral sampling demand of single-wavelength laser and color laser speckle can be met.

4. The laser speckle testing device of claim 3, wherein, The ND filter wheel has a plurality of holes, one of which is vacant, one of which is provided with a noise calibration sheet, and the remaining holes are respectively provided with neutral density filters with different optical densities for finely adjusting the intensity of the laser beam in a high-brightness or weak-light environment to prevent the two-dimensional image sensor from being overexposed.

5. The laser speckle testing device of claim 1, wherein, The noise calibration sheet is made of pure black material with a reflectivity lower than 1%, and is used for collecting noise data before each color measurement, and deducting the noise data in subsequent measurement results to improve the measurement accuracy.

6. The laser speckle testing device of claim 1, wherein, The main control unit receives the monitoring results of the three-axis acceleration sensor on the vibration signals, judges the current vibration state, and when the amplitude of the vibration signal exceeds a preset threshold, the main control unit discards the current digital image signal and re-collects to improve the accuracy of speckle measurement. The main control unit drives the liquid crystal tunable filter module to sequentially switch the central wavelengths of a plurality of light signals allowed to pass through the liquid crystal tunable filter module, and synchronously collects corresponding image sequences to construct a hyperspectral data cube of the speckle image, which is used for subsequent chromaticity fitting and spectral feature extraction.

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