Underwater polarization imaging device and method based on interference-free coded aperture correlation holography

Abstract

The invention discloses an underwater polarization imaging device and method based on interference-free coded aperture correlation holography, and belongs to the field of computational optical imaging. According to the technical scheme, on the basis of an interference-free coded aperture correlation holographic system, two light paths are utilized, a target object hologram and a point spread function are synchronously collected through cross-polarization double light paths (0 degree and 90 degrees), after target information light is separated out through an underwater polarization imaging model, low-frequency background noise is filtered out through a nonlinear filtering reconstruction algorithm, and the target object hologram and the point spread function are obtained. And the phase-coded high-frequency object structure information is reserved, and the global estimation precision is optimized. According to the method, a complex optical path of traditional polarization imaging is simplified, the influence of local noise on polarization parameters is avoided, the image definition and the signal-to-noise ratio in a turbid underwater environment can be effectively improved, and the method is suitable for high-scattering scenes such as marine resource exploration, underwater robot navigation and military underwater target detection.

Description

Technical Field

[0001] The present invention relates to interference-free coded aperture correlation holography and an underwater polarization imaging method, and belongs to the field of computational optical imaging, in particular to an underwater polarization imaging device and method based on interference-free coded aperture correlation holography. Background Art

[0002] Interferenceless Coded Aperture Correlation Holography (I-COACH) has demonstrated significant advantages in the field of three-dimensional imaging and has the potential to extend to higher-dimensional information reconstruction. The core of this technology is to phase-modulate the information of the three-dimensional light field through a coded phase mask (CPM) to generate a two-dimensional hologram, and then use computational reconstruction algorithms to resolve the three-dimensional spatial information of the object. This technology avoids the complexity of traditional interferometric optical paths and has significant advantages in dynamic scenes and low-light environments. It can also be further extended to high-order information reconstruction by fusing multi-dimensional physical quantities (such as polarization and spectrum).

[0003] Polarization imaging technology, due to its unique polarization resolution capability, has demonstrated significant advantages in the field of computational imaging: by capturing multi-dimensional features such as polarization state, spectral response, and spatial morphology generated by the interaction between light waves and targets, it breaks through the information dimension limitations of traditional light intensity imaging. This technology not only serves as a key supplement to light intensity imaging, but also plays an irreplaceable role in scenarios such as remote sensing, biological tissue pathology analysis, and target identification in complex environments, achieving in-depth analysis of the target's intrinsic properties and a comprehensive improvement in scene adaptability. However, due to the strong scattering and wavelength-selective absorption of light in underwater environments, traditional optical imaging faces serious degradation problems, manifested as blurred images, low contrast, color distortion, and loss of detail.

[0004] This paper proposes an underwater polarization imaging device and method based on interference-free coded aperture correlation holography. Using polarization holograms recorded by an I-COACH system, the device reconstructs them through nonlinear filtering, effectively filtering out low-frequency background components while preserving the object's structural information. This method optimizes global estimation, avoids the influence of local noise on polarization parameters, and significantly improves the image's structural similarity index. Summary of the Invention

[0005] The present invention proposes an underwater polarization imaging device and method based on interference-free coded aperture correlation holography. The optical system specifically implemented by the method includes: a monochromatic LED (Light Emitting Diode) (1), a monochromatic LED (2), a first polarizer (3), a first lens (4), a second polarizer (5), a second lens (6), a water tank containing 5 ml of a mixture of milk and water (the volume of the water tank is 100 mm×80 mm×110 mm) (7), a target (8), a pinhole (9), a first beam splitter prism (10), a third lens (11), a half-wave plate (12), a second beam splitter prism (13), a phase spatial light modulator (PSLM) (14) and an image sensor (15). The optical system adopts a dual-channel optical path with critical illumination: the light source of path 1 is a monochromatic LED light source (1) with a wavelength of 528 nm. The distance between the monochromatic LED light source (1) and the first lens (4) is d1, the distance between the first lens (4) and the target (8) in the turbid medium is d2, f1 is the focal length of the first lens, then d1 and d2 satisfy 1 / d1+1 / d2=1 / f1. The distance between the target (8) and the third lens (11) is the object distance z s , that is, the focal length f2 of the third lens (11). The third lens (11) plays a collimating role. The fourth element of the third group of the resolution plate of the target object (8) is selected as the entire object. The collimated light beam passes through the half-wave plate (12), and the half-wave plate (12) is rotated to 0° and 45° in turn, satisfying the polarization modulation angle of the PSLM (10). The light source of path2 is a monochromatic LED light source (2) with a wavelength of 528nm. The distance from the monochromatic LED light source (2) to the second lens (6) is d1, and the distance from the second lens (6) to the pinhole (9) is d2. f1 is the focal length of the second lens, then d1 and d2 satisfy 1 / d1+1 / d2=1 / f1. The distance from the pinhole (9) to the third lens (11) is the object distance z s , i.e., the focal length f2 of the third lens (11). The third lens (11) plays a collimating role. The collimated light beam passes through the half-wave plate (12), and the half-wave plate (12) is rotated to 0° and 45° in sequence, satisfying the polarization modulation angle of the PSLM (14).

[0006] This method includes the following three steps:

[0007] S1: hologram and point spread function acquisition;

[0008] The horizontal direction of the cross section perpendicular to the propagation direction of the optical path is defined as 0°, and the counterclockwise direction is the positive direction.

[0009] Loading a coded phase mask on the PSLM, measuring the object hologram corresponding to the turbid scene at orthogonal polarization angles of 0° and 90° (OH,θ) And the system's point spread function PSF (Point Spread Function);

[0010] A monochromatic LED (1) is turned on, and the incoherent light it emits passes through a first polarizer (3), and the incident light becomes 0° or 90° linearly polarized light. A first lens (4) focuses the light beam emitted from the first polarizer (3) onto a target object (8) in a turbid medium, and the diffracted light emitted from the target object (8) is collimated by a third lens (11).

[0011] The polarization angle of the first polarizer (3) is rotated to 0° and 90° in sequence. The collimated light beam passes through the half-wave plate (12), and then rotates to 45° and 90° respectively. Without changing the light intensity, the half-wave plate is used to change the polarization angle of the light beam to 90°. At this time, the polarization angle of the light beam meets the polarization modulation angle of the PSLM and is changed to 90°. A sparse point type coded phase mask is loaded on the PSLM (14). The modulated light beam passes through the second beam splitter prism (12) and enters the image sensor (15) after reflection. The object intensity I corresponding to the polarization angle of the first polarizer (3) is recorded in sequence on the image sensor (14). (OH,0°) and I (OH,90°) ;

[0012] The monochromatic LED (1) is turned off and the monochromatic LED (2) is turned on. The incoherent light emitted by the monochromatic LED (1) is converted into 0° or 90° linear polarized light by the second polarizer (5). The second lens (6) focuses the light beam emitted from the second polarizer (5) onto the pinhole (9). The diffracted light emitted from the pinhole (9) is collimated by the third lens (11).

[0013] The polarization angle of the second polarizer (5) is rotated to 0° and 90° in sequence. The collimated light beam is rotated to 45° and 90° respectively after passing through the half-wave plate (12). The light beam is reflected by the beam splitter prism (13) and enters the image sensor (15). The point spread functions corresponding to the polarization angle of the second polarizer (3) are recorded in sequence on the image sensor (15). (PSF,0°) and I (PSF,90°) ;

[0014] S2, target information optical calculation;

[0015] Computational Object Hologram I (OH,θ) Target information hologram I POH , which is expressed as follows: I (x,y) =I (OH,90°) +I (OH,0°) Among them, I (OH,0°) and I (OH,90°) They are two orthogonal polarization sub-images obtained by adjusting the different positions of the polarizer, one of which contains the maximum backscattered light intensity and is recorded as I (OH,90°) , and the image orthogonal to it contains the smallest backscattered light, denoted as I (OH,0°) .

[0016] In the I-COACH underwater polarization imaging system, the recorded I (OH,θ) It can be expressed as: Among them, "·" represents multiplication, "*" represents convolution, represents the complex amplitude after passing through the lens, C1 is a complex constant, Q is a binary phase function, and the expression is L is the linear phase function, expressed as L(r0 / z s )=exp[j2π(x0x+y0y) / (λz s )],z s and z h are the object distance and image distance respectively, r0=(x0,y0) is the complex amplitude of the object plane coding phase mask, which is expressed as exp(iφ), φ is the CPM phase modulation term, generated by the Gerchberg-Saxton (GS) algorithm. This algorithm constrains the frequency domain amplitude to be distributed as multiple isolated points, so that the random phase has a sparse point response characteristic.

[0017] In the process of underwater polarization imaging, the target information light I POH It can be expressed as: Among them, the transmittance of the target through the medium is t (x,y) , A ∞ is the background scattered light at infinity, and (x, y) is the pixel position of the target object.

[0018] The medium transmittance can be expressed as: Among them, A ∞ It is the background scattered light at infinity underwater, and it generally selects the light in the underwater scene without the target area.

[0019] The polarization degree of background scattered light can be expressed as: Where Ω represents the selected background area.

[0020] Calculate the background scattered light at infinity as follows:

[0021] The point spread function of the system can be expressed as follows: I PSF =I (PSF,90°) +I (PSF,0°)

[0022] S3, nonlinear filtering reconstruction;

[0023] The essence of nonlinear filtering reconstruction algorithm is to convert the spatial domain cross-correlation reconstruction algorithm into the frequency domain for calculation. In the nonlinear filtering reconstruction algorithm, it is necessary to first convert I POH and I PSH Transformed into the frequency domain, it can be expressed as follows: in, represents the Fourier transform.

[0024] Using the calculated I POH and the point spread function of the system I PSF Perform nonlinear filtering to reconstruct the restored image I OBJ : in, represents the inverse Fourier transform, and the reconstruction index (α, β) is and The amplitude spectrum adjustment parameters are (α,β)∈[-1,1]; i is an imaginary unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of an underwater polarization imaging system based on interference-free coded aperture correlation holography.

[0026] Figure 2 Flowchart for implementing an underwater polarization imaging method based on interference-free coded aperture correlation holography.

[0027] Figure 3 Object intensity map recorded during lens phase: I LENS .

[0028] Figure 4 Intensity images of objects at 0° and 90° recorded using a sparse point-coded phase mask: (a) I (OH,0°) (b)I (OH,90°) .

[0029] Figure 5 Systematic point spread holograms at 0° and 90° recorded using a sparse point coding phase mask: (a) I (PSF,0°) (b)I (PSF,90°) .

[0030] Figure 6 Calculate the target information hologram reconstruction results at 0° and 90°: I POH .

[0031] Figure 7 Target information optical hologram I POH and the point spread function of the system I PSF The restored image obtained by nonlinear filtering reconstruction: I OBJ .

[0032] Figure 8 Comparison of lens imaging and restored image: (a) I LENS (b)I OBJ

[0033] Description of reference numerals:

[0034] 1 and 2, monochrome LEDs, 3 and 4, first lens, 5, second polarizer, 6, second lens, 7, water tank containing 5 ml of a mixture of milk and water (the volume of the water tank is 100 mm × 80 mm × 110 mm), 8, target object, 9, pinhole, 10, first beam splitter prism, 11, third lens, 12, half-wave plate, 13, second beam splitter prism, 14, phase-type spatial light modulator (PSLM), 15, image sensor. DETAILED DESCRIPTION

[0035] In order to better explain the implementation process of the present invention, the present invention will be further described in detail with reference to an embodiment below, but the present invention is not limited to this embodiment.

[0036] Example

[0037] like Figure 1 The figure shows a schematic diagram of an underwater polarization imaging device and method system based on interference-free coded aperture correlation holography of the present invention. The optical system adopts a dual-channel optical path with critical illumination: the light source of path 1 is a monochromatic LED light source (1) with a wavelength of 528nm. The distance from the monochromatic LED light source (1) to the first lens (4) is d1, and the distance from the first lens (4) to the target (8) in the turbid medium is d2. f1 is the focal length of the first lens, then d1 and d2 satisfy 1 / d1+1 / d2=1 / f1. The distance from the target (8) to the third lens (11) is the object distance z s, that is, the focal length f2 of the third lens (11). The third lens (11) plays a collimating role. The fourth element of the third group of the resolution plate of the target object (8) is selected as the entire object. The collimated light beam passes through the half-wave plate (12), and the half-wave plate (12) is rotated to 0 and 45 degrees respectively, satisfying the polarization modulation angle of the PSLM (10). The light source of path2 is a monochromatic LED light source (2) with a wavelength of 528nm. The distance from the monochromatic LED light source (2) to the second lens (6) is d1, and the distance from the second lens (6) to the pinhole (9) is d2. The focal length of the second lens is f1, then d1 and d2 satisfy 1 / d1+1 / d2=1 / f1. The distance from the pinhole (9) to the third lens (11) is the object distance z s , i.e., the focal length f2 of the third lens (11). The third lens (11) plays a collimating role. The collimated light beam passes through the half-wave plate (12), and the half-wave plate (12) is rotated to 0° and 45° in sequence, satisfying the polarization modulation angle of the PSLM (14).

[0038] The half-wave plate (12) adjusts the polarization state of the light beam without affecting the light intensity, and the light beam satisfies the polarization modulation angle of the PSLM (14).

[0039] In the I-COACH imaging mode, the lens phase is loaded on the PSLM (14). The light beam modulated by the lens phase through the PSLM (14) is reflected by the second beam splitter prism (13) and enters the image sensor (15). The recorded object intensity image I LENS ,like Figure 3 shown.

[0040] In the I-COACH imaging mode, the PSLM (14) is loaded with a sparse point phase mask. According to the rotation of the first polarizer (3) and the half-wave plate (12), the image sensor (15) records the reference object intensity at 0° and 90°, which are I (OH,0°) and I (OH,90°) ,like Figure 4 (a) and (b); then path1 is closed and path2 is opened. PSLM (14) is equipped with a sparse point-coded phase mask. The system point spread holograms at 0° and 90° polarization directions recorded through the pinhole reflect the intensity distribution of an object point passing through the system, which are I (PSF,0°) and I (PSF,90°) like Figure 5 As shown in (a) and (b).

[0041] The target information hologram reconstruction results of the 0° and 90° object holograms are calculated using the underwater polarization imaging model, as shown in Figure 6 shown.

[0042] I POHThe target information hologram reconstruction formula is expressed as:

[0043] Target information optical hologram I POH and the point spread function of the system I PSF Perform nonlinear filtering reconstruction to obtain the restored image I OBJ ,like Figure 7 shown.

[0044] The nonlinear filtering reconstruction formula can be expressed as: Here, α=0 and β=1 are selected for reconstruction.

[0045] Figure 8 is the lens imaging and restored image, the restored image I OBJ Imaging with Lens I LENS The structural similarity index is 0.8816, which is 15% higher than that of traditional underwater active polarization imaging.

[0046] The above description is only a preferred embodiment of the present invention and does not represent the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the principles of the present invention are included in the scope of protection of the present invention.

Claims

1. An underwater polarization imaging device and method based on interference-free coded aperture correlation holography, wherein the optical system comprises: A monochromatic LED (Light Emitting Diode) (1), a monochromatic LED (2), a first polarizer (3), a first lens (4), a second polarizer (5), a second lens (6), a water tank containing 5 ml of a mixture of milk and water (the volume of the water tank is 100 mm×80 mm×110 mm) (7), a target (8), a pinhole (9), a first beam splitter (10), a third lens (11), a half-wave plate (12), a second beam splitter (13), a phase spatial light modulator (PSLM) (14) and an image sensor (15); the optical system is a dual-channel optical path using critical illumination: the light source of path 1 is a monochromatic LED light source (1) with a wavelength of 528 nm; the distance from the monochromatic LED light source (1) to the first lens (4) is d2, f1 is the focal length of the first lens, and d1 and d2 satisfy 1 / d1+1 / d2=1 / f1; the distance from the target (8) to the third lens (11) is the object distance z s , i.e., the focal length f2 of the third lens (11); the third lens (11) plays a collimating role, and the fourth element of the third group of the resolution plate of the target object (8) is selected as the entire object. The collimated light beam passes through the half-wave plate (12), and the half-wave plate (12) is rotated to 0° and 45° in sequence, satisfying the polarization modulation angle of the PSLM (10); the light source of path2 is a monochromatic LED light source (2) with a wavelength of 528nm, the distance from the monochromatic LED light source (2) to the second lens (6) is d1, the distance from the second lens (6) to the pinhole (9) is d2, f1 is the focal length of the second lens, d1 and d2 satisfy 1 / d1+1 / d2=1 / f1; the distance from the pinhole (9) to the third lens (11) is the object distance z s , i.e., the focal length f2 of the third lens (11); the third lens (11) plays a collimating role, and the collimated light beam passes through the half-wave plate (12), and the half-wave plate (12) is rotated to 0° and 45° in sequence, thereby satisfying the polarization modulation angle of the PSLM (14); The method comprises the following three steps: The horizontal direction of the section perpendicular to the propagation direction of the light path is defined as 0°, and the counterclockwise direction is the positive direction; S1: hologram and point spread function acquisition; A sparse point-coded phase mask is mounted on a phase-type spatial light modulator (PSLM) to measure the object hologram at orthogonal polarization angles of 0° and 90°. (OH,θ) The invention relates to a point spread function (PSF) of the system; firstly, a monochromatic LED (1) is turned on, and the incoherent light emitted by the monochromatic LED (1) is converted into 0° or 90° linear polarized light after passing through a first polarizer (3); a first lens (4) focuses the light beam emitted from the first polarizer (3) onto a target (8) in a turbid medium; and a diffracted light emitted from the target (8) is collimated by a third lens (11); the polarization angle of the first polarizer (3) is rotated to 0° and 90° in sequence, and the collimated light beam is rotated to 45° and 90° respectively after passing through a half-wave plate (12); and the polarization angle of the light beam is changed to 90° by using the half-wave plate without changing the light intensity. At this time, the polarization angle of the light beam satisfies the polarization modulation angle of the PSLM, which is changed to 90°; a coding phase mask is mounted on the PSLM (14); the modulated light beam is reflected by a beam splitter (13) and enters an image sensor (15), and the object intensity corresponding to the polarization angle of the first polarizer (3) is recorded on the image sensor (15) in sequence as I (OH,0) and I (OH,90) The monochromatic LED (1) is turned off and the monochromatic LED (2) is turned on. The incoherent light emitted by the monochromatic LED (1) is converted into 0° or 90° linear polarized light by the second polarizer (5). The second lens (6) focuses the light beam emitted from the second polarizer (5) onto the pinhole (9). The diffracted light emitted from the pinhole (9) is collimated by the third lens (11). The collimated light beam is rotated to 45° and 90° respectively by the half-wave plate (12). The light beam is reflected by the dichroic prism (13) and enters the image sensor (15). The point spread functions corresponding to the polarization angle of the second polarizer (3) are recorded on the image sensor (15) in sequence. (PSF,0°) and I (PSF,90°) ; S2: target information optical calculation; Computational Object Hologram I (OH,θ) Target information hologram I POH , the expression is as follows: Among them, "·" represents multiplication, "*" represents convolution, represents the complex amplitude after passing through the lens, C1 is a complex constant, Q is a binary phase function, and the expression is L is the linear phase function, expressed as L(r0 / z s )=exp[j2π(x0x+y0y) / (λz s )],z s and zh are the object distance and image distance respectively, r0=(x0,y0) is the complex amplitude of the object plane coding phase mask, which is expressed as exp(iφ), and φ is the CPM phase modulation term, generated by the Gerchberg-Saxton (GS) algorithm. This algorithm constrains the frequency domain amplitude to be distributed as multiple isolated points, so that the random phase has a sparse point response characteristic; In the process of underwater polarization imaging, the target information light I POH It can be expressed as: Among them, the transmittance of the target through the medium is t (x,y) , A ∞ is the background scattered light at infinity, (x, y) is the pixel position of the target object; S3: nonlinear filtering reconstruction; Using the calculated I POH and the point spread function of the system I PSF Perform nonlinear filtering to reconstruct the restored image I OBJ : in, represents the inverse Fourier transform, and the reconstruction index (α, β) is and The amplitude spectrum adjustment parameters are (α,β)∈[-1,1], i is an imaginary unit.

Images