Self-interference digital holographic imaging system based on birefringent crystal lens

Abstract

The present application relates to a kind of self-interference digital holographic imaging systems based on birefringent crystal lens.By using incoherent light source illumination to remove the influence of laser speckle on image quality caused by laser illumination, the imaging quality is improved. By the combination of the polarization characteristics of alpha-BBO birefringent crystal lens, quarter-wave plate and polarization camera, single-shot imaging and fast imaging are realized. The present application overcomes the problem of image degradation caused by laser speckle and the safety hazard caused by using laser illumination for human face and animal imaging, solves the problem of optical path difference compensation in coaxial optical path under incoherent illumination, and realizes the need for large field of view color 3D imaging of daily objects.

Description

Technical Field

[0001] The present invention belongs to the technical field of self-interference digital holography, and in particular relates to a self-interference digital holographic imaging system based on a birefringent crystal lens. Background Art

[0002] Digital holography, a method for recording and reconstructing wavefront information about an object, typically requires the use of a coherent light source, such as a laser. Self-interference digital holography overcomes this limitation, allowing the use of incoherent light sources, such as white light or LEDs, to provide high-quality images of objects with three-dimensional information. In coherent holography, the characteristics of coherent light sources limit its widespread practical application. However, by using incoherent light sources, self-interference digital holography breaks the reliance on coherent light sources for holographic imaging. Its principle is to illuminate an object with an incoherent light source, record the resulting interference pattern, and then use digital image processing techniques to reconstruct the object's topography and three-dimensional information. A significant advantage of this method is that it can use common light sources such as white light or LEDs, reducing the complexity and cost of the imaging system and enabling real-time imaging of humans and animals. The development of self-interference digital holography has had a profound impact on multiple fields. In medicine, it is used in biomedical imaging applications such as cytology, medical diagnosis, and pathology. Its excellent performance enables medical imaging to achieve higher resolution and more accurate information. In addition, in the industrial field, it has important applications in three-dimensional morphology measurement and industrial inspection, especially in the detection of tiny devices and surface defects. In virtual reality and augmented reality, self-interference digital holography provides new possibilities for achieving more realistic and vivid images and videos. Although self-interference digital holography has many advantages, it also always faces some challenges. Due to the characteristics of the incoherent light source itself, the resolution and contrast of the obtained interference image are limited and need further improvement and optimization. In general, as an emerging imaging technology, incoherent digital holography has attracted much attention for its wide application and overcoming the limitations of traditional coherent holography, but further research and development are still needed to realize its maximum potential in different fields. Summary of the Invention

[0003] The present invention aims to provide a self-interference digital holographic imaging system based on a birefringent crystal lens. This system overcomes the effects of speckle noise on image quality in conventional coherent holographic imaging, overcomes the uneven response of passive wavefront segmentation elements to different wavelengths, and improves the low quality of current incoherent white-light interferometric holograms. By combining the polarization properties of an α-BBO birefringent crystal lens with a quarter-wave plate and a polarization camera, single-shot imaging is achieved. This system is of great significance for capturing large-field 3D images of real-world objects under incoherent illumination.

[0004] To achieve the above-mentioned purpose, the technical solution of the present invention is: a self-interference digital holographic imaging system based on a birefringent crystal lens, comprising an incoherent light source, an optical path difference compensation optical path with a built-in wavefront splitting device, an image acquisition system and an image reconstruction module (the image reconstruction module is a computer or built into a computer or other equipment); the incoherent light source irradiates the imaging target to generate a light signal which passes through a semi-transparent and semi-reflective mirror and an optical path difference compensation optical path, is received by the image acquisition system, and is transmitted to the image reconstruction module for image reconstruction.

[0005] In one embodiment of the present invention, the incoherent light source is an incoherent light source with autocorrelation characteristics including an LED light source and a bioluminescent light source.

[0006] In one embodiment of the present invention, the wavefront splitting device is an α-BBO birefringent crystal lens, which is made of α-BBO birefringent crystal lens and ground into a plano-convex lens. The crystal optical axis is parallel to the plane of the lens, and both sides are coated with 400-700nm anti-reflection film.

[0007] In one embodiment of the present invention, the optical path difference compensation optical path compensates the optical path difference through a wavefront splitting device, namely, an α-BBO birefringent crystal lens itself.

[0008] In one embodiment of the present invention, the optical path difference compensation optical path includes an α-BBO birefringent crystal lens, a broadband quarter-wave plate in the visible light band, and a reflective silver mirror; the α-BBO birefringent crystal lens, the broadband quarter-wave plate in the visible light band, and the reflective silver mirror are coaxially arranged at the same height, and the optical axis direction is 45° to the plane of the semi-transparent and semi-reflective mirror.

[0009] In one embodiment of the present invention, the image acquisition system includes a broadband quarter-wave plate of the second visible light band and a polarization camera, which are coaxially placed at the same height as the optical path difference compensation optical path.

[0010] In one embodiment of the present invention, each pixel of the polarization camera is inlaid with a micropolarizer grid that can independently collect light of different polarization states. The micropolarizer grid has four equidistant polarization directions (0, π / 2, π, 3π / 2) and is precisely arranged on the sensor in a repeating square pattern.

[0011] In one embodiment of the present invention, the image reconstruction module has a built-in image reconstruction algorithm, which first reads the four collected holograms with precise phase shifts, calculates the complex-valued hologram through a four-step phase shift method, and then performs Fresnel diffraction corresponding to the reconstruction distance to obtain a reconstructed image.

[0012] In one embodiment of the present invention, the system implements the following steps for image reconstruction:

[0013] Step S1, build the optical path difference compensation optical path, that is, place the alpha-BBO birefringent crystal lens, a broadband quarter-wave plate in the visible light wave band and a reflecting silver mirror in high co-axial, rotate the broadband quarter-wave plate in the visible light wave band to make the broadband quarter-wave plate optical axis direction of the visible light wave band and the optical axis direction of the alpha-BBO birefringent crystal lens form 45°, and the main optical axis direction and the half-transmission half-reflection mirror plane form 45°;

[0014] Step S2, place the imaging target object on the object table, place a linear polarizer between the imaging target object and the half-transmission half-reflection mirror and adjust the height and the optical path difference compensation optical path to be in the same height, use a non-coherent light source reflection or transmission illumination, the light incidence direction and the half-transmission half-reflection mirror plane form 45°, and the optical path difference compensation optical path main optical axis is perpendicular;

[0015] Step S3, build the image acquisition system, that is, place the second broadband quarter-wave plate in the visible light wave band in front of the polarization camera and in the same height and co-axial with the optical path difference compensation optical path, the output end of the polarization camera is connected to the image reconstruction module, and four holograms containing different phase shift values of the imaging target object are obtained by shooting;

[0016] Step S4, build the image reconstruction algorithm in the image reconstruction module, input the collected imaging target object image into the image reconstruction algorithm to obtain a reconstructed image.

[0017] In an embodiment of the present application, the image reconstruction algorithm first reads four holograms with accurate phase shift, calculates the complex-valued hologram through the four-step phase shift method, and then performs the Fresnel diffraction corresponding to the reconstruction distance to obtain the reconstructed image, wherein the method for obtaining the complex-valued hologram and the reconstruction distance is as follows:

[0018] The alpha-BBO birefringent crystal lens utilizes its birefringence characteristic, so that when the polarization direction of the incident linearly polarized light is inconsistent with the optical axis direction of the alpha-BBO birefringent crystal lens, the incident light can be decomposed into two lights with certain phase delay and mutually orthogonal polarization states; the two lights with orthogonal polarization directions are separated after passing through the alpha-BBO birefringent crystal lens and the optical path difference compensation optical path, and are converted into left-handed and right-handed lights after passing through the broadband quarter-wave plate in the visible light wave band to form an interference hologram on the imaging plane of the polarization camera; when the phase delay of o light and e light is 0°, 90°, 180° and 270°, the synthesized linearly polarized light is respectively in the direction of 0°, 45°, 90° and 135°, and then the linearly polarized camera is used for collection; assuming E o and E e are the electric fields of o light and e light, then the light intensity collected by the polarization camera in each polarization direction is:

[0019] I0=|E e +E0| 2

[0020] I 45 =|E e +jE0| 2

[0021] I 90 =|E e -E0| 2

[0022] I 135 =|E e -jE0| 2

[0023] Therefore, the complex amplitude light field can be calculated from the light intensity:

[0024]

[0025] Where j is an imaginary number. The polarization camera integrates linear polarization gratings in four directions, 0°, 45°, 90°, and 135°, at adjacent pixels. It can simultaneously capture images in these polarization directions through single-shot imaging.

[0026] Under the paraxial approximation, the beam coincidence condition is selected based on the trade-off between the required resolution and the field of view, and the polarization camera is placed at the position where the corresponding beam coincidence condition is generated; assuming that the focal lengths of the o-light and e-light generated by the α-BBO birefringent crystal lens are f e With f o , then for the object at distance z s The imaging target object is obtained by the geometric optical imaging formula to obtain the image distance v i With magnification M i They are:

[0027]

[0028]

[0029] Where i = e, o, assuming that the distance between the photosensitive surface of the polarization camera and the α-BBO birefringent crystal lens is z h , and the two beams of light can also be described as two waves emitted from the image point position, then the distance from the o-light and e-light image points to the polarization camera's photosensitive surface is:

[0030]

[0031] The light emitted from the image point propagates through the Fresnel to reach the camera's photosensitive surface, and the light field on the polarization camera is expressed as:

[0032]

[0033] Among them E iθIndicates the light intensity at the image point, j is the imaginary number symbol, λ is the wavelength of light, k is the wave number, x and y are the coordinates on the photosensitive interface of the polarization camera, x s 、y s represents the coordinates of the luminous point on the plane where the light source is located; because the polarization camera plane will be in front of the image point, the propagation direction is opposite, and the phase that is unrelated to the x and y coordinates is ignored. Finally, the light intensity on the photosensitive surface of the polarization camera is the coherent superposition of the o light and the e light; the complex amplitude light field can be calculated through the measurement of the polarization camera as:

[0034]

[0035] in:

[0036]

[0037]

[0038] Therefore, as long as the positions of the o-light and e-light image points are known, the final image reproduction distance can be calculated. When the imaging target is placed at the physical depth z s When in the r Reconstruct the image at z r With z s The difference also shows that the depth obtained is not the real physical depth but the converted depth of the real scene; on this basis, the lateral magnification M′ can also be calculated.

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

[0040] 1. The present invention uses incoherent light source illumination to eliminate the impact of speckle noise on image quality, achieving safe and widely applicable imaging conditions;

[0041] 2. The optical element of the present invention responds uniformly to all wavelengths in the visible light band. Combined with the method of using the crystal lens itself to compensate for the optical path difference, it can meet the imaging needs under incoherent white light illumination.

[0042] 3. The present invention realizes single-shot imaging by combining the polarization characteristics of the α-BBO birefringent crystal lens with a quarter-wave plate and a polarization camera inlaid with a micro-polarization array, and can quickly and in real time capture three-dimensional images of dynamic objects. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagram of the structure of the imaging optical path device according to an embodiment of the present invention.

[0044] In the figure: 1-target object; 2-linear polarizer; 3-reflective silver mirror; 4-quarter wave plate; 5-α-BBO birefringent crystal lens; 6-semi-transparent and semi-reflective mirror; 7-quarter wave plate; 8-polarization camera; 9-computer. DETAILED DESCRIPTION

[0045] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings.

[0046] Please refer to Figure 1 This embodiment provides a self-interference digital holographic imaging system based on a birefringent crystal lens, comprising an incoherent light source, an optical path difference compensation optical path with a built-in wavefront splitting device, an image acquisition system, and an image reconstruction module (the image reconstruction module is a computer or built into a computer or other device); the incoherent light source irradiates the imaging target to generate a light signal which passes through a semi-transparent and semi-reflective mirror and the optical path difference compensation optical path, is received by the image acquisition system, and is transmitted to the image reconstruction module for image reconstruction.

[0047] The steps of image reconstruction implemented by the system of the present invention are as follows:

[0048] Step S1, constructing an optical path difference compensation optical path, placing an α-BBO birefringent crystal lens 5, a broadband quarter-wave plate 4 for visible light, and a reflective silver mirror 3 at equal heights and coaxially, rotating the quarter-wave plate 4 so that the optical axis of the wave plate is at 45° to the optical axis of the crystal lens, and the main optical axis is at 45° to the plane of the semi-transparent and semi-reflective mirror 6;

[0049] Step S2: Place the imaging target 1 on the stage and place a linear polarizer 2 between the target and the semi-transparent mirror 6, adjusting the height to be equal to the optical path difference compensation optical path. Use an incoherent light source for reflection or transmission illumination, with the incident light direction at a 45° angle to the plane of the semi-transparent mirror and perpendicular to the main optical axis of the optical path difference compensation optical path.

[0050] Step S3: placing a broadband quarter-wave plate 7 of the second visible light band in front of a polarization camera 8 and coaxially with the optical path difference compensation optical path at the same height, connecting the output end of the polarization camera 8 to a computer 9, and capturing four holograms of the target object 1 with different phase shift values;

[0051] Step S4: construct an image reconstruction algorithm of the image reconstruction module, input the collected target object image into the algorithm, and obtain a reconstructed image.

[0052] In this embodiment, a white LED light is used as the light source, and a commercial polarization imaging camera (Lucid, VP-PHX050S-PC) is used as the image sensor. The pixel size of the polarization camera is 2440*2048.

[0053] In this embodiment, the α-BBO birefringent crystal lens used is a plano-convex lens with a diameter of 12.5 mm and a thickness of 2-3 mm. The curvature of the surface is 100 mm, and the refractive indices of the e-light and the o-light are n=1 and n=2, respectively. e =1.55345,n o =1.67755, the focal lengths of light e and light o are f e =180.7mm, f o =147.6mm, the crystal optical axis is parallel to the lens plane, and both sides are coated with 400-700nm anti-reflection film.

[0054] In this embodiment, the computer processing system with a built-in image reconstruction module performs four-step phase shifts on four holograms with different polarizations to obtain a complex-valued hologram under the Matlab software platform, and after calculating the corresponding diffraction distance, performs a Fresnel diffraction operation on the complex-valued hologram at the distance.

[0055] Example 1:

[0056] Taking a white LED bulb 1 with an 8mm diameter as an example, the distances between the reflector 3, quarter-wave plate 4, α-BBO birefringent crystal lens 5, semi-transparent mirror 6, and polarization camera are adjusted to position the polarization camera's image sensor plane for optimal beam magnification and appropriate beam overlap. After viewing a high-quality hologram on computer 9, the quarter-wave plate 7 is rotated to maximize the hologram's clarity. Four holograms with different phase shift values ​​are captured in a single exposure. The resulting image is subjected to a four-step phase shift to obtain a complex-valued hologram: The polarization camera 5, which is integrated with a micro-polarization array, can capture four holograms with different phase shift values ​​in a single exposure. The complex-valued hologram Hi is obtained using the following formula:

[0057]

[0058] in:

[0059]

[0060]

[0061] Using the calculated corresponding diffraction distance zr, the complex-valued hologram is subjected to a Fresnel diffraction operation at that distance to ultimately obtain a reconstructed image.

[0062] In summary, the holograms obtained by this method are not affected by the monochromaticity of the light source, effectively eliminating the influence of laser speckle and improving the imaging quality. The four-step phase shift method combined with a polarization camera also successfully achieved single-shot imaging.

[0063] The above are preferred embodiments of the present invention. Any changes made according to the technical solution of the present invention, as long as the resulting functions and effects do not exceed the scope of the technical solution of the present invention, shall fall within the scope of protection of the present invention.

Claims

1. A self-interference digital holographic imaging system based on a birefringent crystal lens, characterized in that: The system comprises an incoherent light source, an optical path difference compensation optical path with a built-in wavefront splitting device, an image acquisition system, and an image reconstruction module; the incoherent light source irradiates the imaging target to generate a light signal which is received by the image acquisition system through a semi-transparent and semi-reflective mirror and the optical path difference compensation optical path, and is then transmitted to the image reconstruction module for image reconstruction; The specific steps of image reconstruction in this system are as follows: Step S1, constructing an optical path difference compensation optical path, namely placing an α-BBO birefringent crystal lens, a broadband quarter-wave plate for visible light, and a reflective silver mirror at equal heights and coaxially, rotating the broadband quarter-wave plate for visible light so that the optical axis of the broadband quarter-wave plate for visible light is at 45° to the optical axis of the α-BBO birefringent crystal lens, and the main optical axis is at 45° to the plane of the semi-transparent and semi-reflective mirror; Step S2: Place the imaging target on the stage, place a linear polarizer between the imaging target and the semi-transparent mirror and adjust its height to be the same as the optical path difference compensation optical path, use an incoherent light source for reflection or transmission illumination, and make the incident light direction at a 45° angle to the plane of the semi-transparent mirror and perpendicular to the main optical axis of the optical path difference compensation optical path; Step S3: constructing an image acquisition system, that is, placing a broadband quarter-wave plate of the second visible light band in front of a polarization camera and coaxially with the optical path difference compensation optical path at the same height, connecting the output end of the polarization camera to an image reconstruction module, and capturing four holograms of the imaging target object with different phase shift values; Step S4: constructing an image reconstruction algorithm in the image reconstruction module, inputting the acquired imaging target image into the image reconstruction algorithm to obtain a reconstructed image; The image reconstruction algorithm first reads and collects four holograms with precise phase shifts, calculates a complex-valued hologram using a four-step phase shift method, and then performs Fresnel diffraction corresponding to the reconstruction distance to obtain a reconstructed image. The method for obtaining the complex-valued hologram and the reconstruction distance is as follows: The α-BBO birefringent crystal lens utilizes its birefringence property, so that when the polarization direction of the incident linear polarized light is inconsistent with the optical axis direction of the α-BBO birefringent crystal lens, it can decompose the incident light into two beams with a certain phase delay and mutually orthogonal polarization states; the two beams of light with orthogonal polarization directions are separated after passing through the α-BBO birefringent crystal lens and the optical path difference compensation optical path, and are converted into left-right rotated light after passing through a broadband quarter-wave plate in the visible light band, and then combined on the imaging surface of the polarization camera to form an interference hologram; when the phase delay difference between the o light and the e light is 0°, 90°, 180° and 270°, the synthesized linear polarized light is at 0°, 45°, 90° and 135° respectively, and then collected by the linear polarization camera; assuming E o and E e is the electric field of o light and e light, then the light intensity collected by the polarization camera in each polarization direction is: I0=|E e +E0| 2 I 45 =|E e +jE0| 2 I 90 =|E s -E0| 2 I 135 =|E e -jE0| 2 Therefore, the complex amplitude light field can be calculated from the light intensity: Where j is the imaginary symbol. The polarization camera integrates linear polarization gratings in four directions of 0°, 45°, 90°, and 135° on adjacent pixels, and can simultaneously capture images in these polarization directions through single-shot imaging. Under the paraxial approximation, the beam coincidence condition is selected based on the trade-off between the required resolution and the field of view, and the polarization camera is placed at the position where the corresponding beam coincidence condition is generated; assuming that the focal lengths of the o-light and e-light generated by the α-BBO birefringent crystal lens are f e With f o , then for the object at distance z s The imaging target object is obtained by the geometric optical imaging formula to obtain the image distance v i With magnification M i They are: Where i = e, o, assuming that the distance between the photosensitive surface of the polarization camera and the α-BBO birefringent crystal lens is z h , and the two beams of light can also be described as two waves emitted from the image point position, then the distance from the o-light and e-light image points to the polarization camera's photosensitive surface is: The light emitted from the image point propagates through the Fresnel to reach the camera's photosensitive surface, and the light field on the polarization camera is expressed as: Among them E i0 Indicates the light intensity at the image point, j is the imaginary number symbol, λ is the wavelength of light, k is the wave number, x and y are the coordinates on the photosensitive interface of the polarization camera, x s 、y s represents the coordinates of the luminous point on the plane where the light source is located; because the polarization camera plane will be in front of the image point, the propagation direction is opposite, and the phase that is unrelated to the x and y coordinates is ignored. Finally, the light intensity on the photosensitive surface of the polarization camera is the coherent superposition of the o light and the e light; the complex amplitude light field can be calculated through the measurement of the polarization camera as: in:

2. The self-interference digital holographic imaging system based on a birefringent crystal lens according to claim 1, characterized in that: The incoherent light source is an incoherent light source with autocorrelation characteristics, including an LED light source and a bioluminescent light source.

3. The self-interference digital holographic imaging system based on a birefringent crystal lens according to claim 1, characterized in that: The wavefront splitting device is an α-BBO birefringent crystal lens, which is made of α-BBO birefringent crystal and ground into a plano-convex lens. The crystal optical axis is parallel to the plane of the lens, and both sides are coated with 400-700nm anti-reflection film.

4. The self-interference digital holographic imaging system based on a birefringent crystal lens according to claim 1, characterized in that: The optical path difference compensation optical path compensates the optical path difference through the wavefront splitting device, namely the α-BBO birefringent crystal lens itself.

5. The self-interference digital holographic imaging system based on a birefringent crystal lens according to claim 1, characterized in that: The optical path difference compensation optical path includes an α-BBO birefringent crystal lens, a broadband quarter-wave plate in the visible light band, and a reflective silver mirror; the α-BBO birefringent crystal lens, the broadband quarter-wave plate in the visible light band, and the reflective silver mirror are coaxially arranged at equal heights, and the optical axis direction forms a 45° angle with the semi-transparent and semi-reflective mirror plane.

6. The self-interference digital holographic imaging system based on a birefringent crystal lens according to claim 1, characterized in that: The image acquisition system includes a broadband quarter-wave plate of the second visible light band and a polarization camera, which are coaxially placed at the same height as the optical path difference compensation optical path.

7. The self-interference digital holographic imaging system based on a birefringent crystal lens according to claim 6, characterized in that: Each pixel of the polarization camera is inlaid with a micropolarizer grid that can independently collect light of different polarization states. The micropolarizer grid has four equidistant polarization directions of 0, π / 2, π, and 3π / 2, which are precisely arranged on the sensor in a repeated square pattern.

8. The self-interference digital holographic imaging system based on a birefringent crystal lens according to claim 1, characterized in that: The image reconstruction module has a built-in image reconstruction algorithm. It first reads the four collected holograms with precise phase shifts, calculates the complex-valued hologram through a four-step phase shift method, and then performs Fresnel diffraction at the corresponding reconstruction distance to obtain a reconstructed image.

Images