Interference-free coded aperture correlation holographic spectral imaging method based on Bessel beam

Abstract

This invention proposes a method for improving the spectral resolution of a coded aperture correlation holographic imaging system, relating to the field of computational optical imaging. This method multiplexes Bessel beams and a lens phase mask to encode an object's spatial and spectral information into a hologram. This hologram is then reconstructed through cross-correlation with point spread holograms of corresponding wavelengths, achieving precise imaging under multi-wavelength conditions. By using Bessel beams, the present invention only requires recording a single object hologram at the same wavelength, eliminating the need for multiple recordings. Furthermore, the method exhibits high spectral sensitivity and resolution. This method can be widely applied in fields such as biomedical imaging and remote sensing, improving image quality and spectral analysis capabilities.

Description

Technical Field

[0001] The present invention relates to the field of spectral imaging technology, belonging to the field of computational optical imaging technology, and in particular to a coded aperture correlation holographic spectral imaging method based on Bessel beams. Background Art

[0002] Interferenceless Coded Aperture Correlation Holography (I-COACH) is a novel 3D imaging technique with the potential to extend to higher dimensions. Unlike traditional holography, I-COACH does not rely on laser sources or light wave interference. Instead, it encodes 3D scene information into a 2D hologram using a coded phase mask. This method facilitates information recording, storage, and transmission, while also offering advantages such as being speckle-free and suitable for passive imaging. It has demonstrated practical application in fields such as astronomical and medical imaging.

[0003] As an important optical analysis technology, spectral imaging is widely used in remote sensing monitoring, medical diagnosis, food safety, and industrial testing. Spectral imaging technology achieves qualitative and quantitative analysis of objects by acquiring their spectral information at different wavelengths. Traditional spectral imaging methods mainly rely on optical spectroscopic systems and spectroscopic elements to perform spatial separation of spectral information, or use light sources of different wavelengths to image objects. In contrast, the application of I-COACH technology in spectral imaging has the advantages of being free of speckle noise, having high spatial resolution, and being suitable for passive imaging, enabling it to demonstrate higher accuracy and efficiency in spectral analysis in complex scenes.

[0004] The present invention proposes a coded aperture correlation holographic spectral imaging method based on Bessel beams, combining I-COACH technology with Bessel beams to achieve the fusion of incoherent holographic imaging and spectral analysis. The use of Bessel beams can significantly improve the imaging quality and spectral resolution of the system. This method not only inherits the advantages of I-COACH in terms of speckle noise-free and efficient information acquisition, but also achieves more accurate spectral analysis through multi-wavelength imaging. Compared with traditional spectral imaging methods, this method significantly improves the spectral resolution and spatial resolution of the system while simplifying the complex optical path, providing a new solution for high-dimensional spectral analysis of complex scenes. Summary of the Invention

[0005] The present invention proposes a coded aperture correlation holographic spectral imaging method based on Bessel beams, which can effectively distinguish objects at different wavelengths and realize I-COACH spectral imaging.

[0006] The method includes the following three steps: S1, multiplexing the Bessel beam with the lens and taking its phase as the coded phase mask; S2, recording the point spread function and object hologram under red and green light illumination respectively; S3, superimposing the object hologram and then using the point spread hologram at the corresponding wavelength to reconstruct the object recorded at the corresponding wavelength.

[0007] For convenience, the following abbreviations are given: Light Emitting Diode LED; Coded Phase Mask CPM; Phase Spatial Light Modulator PSLM; Point Spread Hologram PSH; Object Hologram OH; Non-Linear Reconstruction NLR.

[0008] The coded aperture correlation holographic spectral imaging method based on Bessel beams has a specific optical system comprising: a monochromatic LED light source (1), a first lens (2), a target object (3), a second lens (4), a polarizer (5), an aperture stop (6), a beam splitter (7), a PSLM (8) and an image sensor (9).

[0009] S1: The Bessel beam is multiplexed with a lens and its phase is taken as the CPM.

[0010] The light field expression of Bessel beam in cylindrical coordinate system is as follows

[0011] E l (ρ,φ,z)=J l (k r ρ)exp(ilφ+ik z z)

[0012] Where: ρ is the radial coordinate, φ is the azimuthal coordinate, z is the axial coordinate, l is the order of the Bessel function, J l is the first kind l-order Bessel function, k r and k z are radial and longitudinal wave vectors, respectively. λ is the wavelength of the light wave.

[0013] Taking the Bessel beam as CPM, we only need to pay attention to the transverse (radial and angular) phase distribution and ignore the longitudinal wave number k z and the phase factor in the propagation direction z, the simplified Bessel beam light field expression is as follows

[0014] E l (ρ,φ)=J l(k r ρ)exp(ilφ)

[0015] The lens phase expression is as follows

[0016]

[0017] Where: f is the focal length of the lens, (x,y) represents the coordinates of the light field in the rectangular coordinate system on the cross section perpendicular to the propagation direction of the light beam.

[0018] The phase expression of the multiplexed Bessel beam and lens is as follows

[0019]

[0020] The phase of the above formula is used as the CPM of the I-COACH system

[0021]

[0022] Where: arg(J l (k r ρ)) is the value of J l (k r ρ) takes the phase angle, which can be 0 or π.

[0023] S2: Record the point spread function and object hologram under red and green light illumination, respectively.

[0024] The pixel coordinates of any object point on the object plane perpendicular to the propagation direction of the light beam are defined as (x s ,y s ), the pixel coordinates on the PSLM plane perpendicular to the beam propagation direction are (x1, y1), and the pixel coordinates on the image sensor plane perpendicular to the beam propagation direction are (x2, y2).

[0025] Assume that there is an object point on the object plane Its amplitude is The intensity distribution on the image sensor plane after CPM modulation is:

[0026]

[0027] in, represents the coordinates (x2, y2) of the image sensor, z s is the distance from the object plane to the CPM, f0 is the focal length of the lens in front of CPM, and the distance from the lens to CPM is approximately considered to be 0. is the coordinate (x1, y1) of the CPM plane, z his the distance from the CPM plane to the image sensor plane.

[0028] Considering the use of incoherent light illumination, the intensity distribution formed by the object can be regarded as the superposition of PSH intensities. The intensity distribution generated by any two-dimensional object can be expressed as

[0029]

[0030] Among them: a n is a constant related to the object point.

[0031] PSH recorded under red light is I PSH,1 , the OH of object 1 is I OH,1 ; PSH recorded under green light is 1 PSH,2 , the OH of object 2 is I PSH,2 .

[0032] S3: Superimpose the object holograms of the two wavelengths to obtain the total object hologram

[0033] I OH,sum =I OH,1 +I OH,2

[0034] The reconstruction process of the non-interference coded aperture correlation holographic system is actually an optical pattern recognition problem. The essence of the NLR algorithm is actually the conversion of the spatial domain cross-correlation reconstruction algorithm into the frequency domain for calculation. In the NLR algorithm, it is first necessary to convert I OH and I PSH Transformed into the frequency domain is expressed as follows

[0035]

[0036] in: represents the Fourier transform.

[0037] The PSH and the total object hologram under the corresponding wavelength are used to perform NLR respectively to obtain the object at this wavelength.

[0038] Using red light PSH and the total object hologram reconstruction, we can obtain the object 1 recorded under red light illumination.

[0039]

[0040] Using green light PSH and the total object hologram reconstruction, we can obtain the object 2 recorded under green light illumination.

[0041]

[0042] in: represents the inverse Fourier transform; o and r are the amplitude spectrum adjustment parameters of OH and PSH, respectively, and both range from -1 to +1.

[0043] The powers o and r can be selected to adjust the power spectrum and reconstruction function of the target, respectively. When o≠1, the effect on the size of the object spectrum makes the reconstruction process of multi-point targets nonlinear. This nonlinearity in the reconstruction process improves the signal-to-noise ratio of the reconstructed image without losing the shift invariance of linear correlation. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 Schematic diagram of the coded aperture correlation holographic spectral imaging system based on Bessel beam.

[0045] Figure 2 Flowchart for the implementation of coded aperture correlation holographic spectral imaging based on Bessel beam.

[0046] Figure 3 Phase diagrams of a Bessel beam, a lens, and a Bessel beam multiplexed with a lens: (a) Bessel beam, (b) lens, and (c) phase of a Bessel beam multiplexed with a lens.

[0047] Figure 4 Object 1 recorded under red light and object 2 recorded under green light: (a) Object 1, (b) Object 2.

[0048] Figure 5 Point spread holograms under red and green light: (a) I PSH,1 , (b)I PSH,2 .

[0049] Figure 6 Object hologram under red light, object hologram under green light and total object hologram: (a) I OH,1 , (b)I OH,2 , (c)I OH,sum .

[0050] Figure 7 The results of object reconstruction under red light and green light respectively: (a) I R , (b)I G .

[0051] Description of reference numerals:

[0052] 1. Monochromatic LED light source, 2. First lens, 3. Target object, 4. Second lens, 5. Polarizer, 6. Aperture stop, 7. Beam splitter, 8. PSLM, 9. Image sensor. DETAILED DESCRIPTION

[0053] 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.

[0054] Example

[0055] First, the Bessel beam is multiplexed with a lens and its phase is taken as the CPM.

[0056] The light field expression of Bessel beam in cylindrical coordinate system is as follows

[0057] E(ρ,φ,z)=J l (k r ρ)exp(ilφ+ik z z)

[0058] Where: ρ is the radial coordinate, φ is the azimuthal coordinate, z is the axial coordinate, l is the order of the Bessel function, J l is the first kind l-order Bessel function, k r and k z are radial and longitudinal wave vectors, respectively. λ is the wavelength of the light wave.

[0059] Taking the Bessel beam as CPM, we only need to pay attention to the transverse (radial and angular) phase distribution and ignore the longitudinal wave number k z And the phase factor in the propagation direction z, the simplified expression is as follows

[0060] E l (ρ,φ)=J l (k r ρ)exp(ilφ)

[0061] The lens phase expression is as follows

[0062]

[0063] Where: f is the focal length of the lens, (x,y) represents the coordinates of the light field in the rectangular coordinate system on the cross section perpendicular to the propagation direction of the light beam.

[0064] The phase expression of the multiplexed Bessel beam and lens is as follows

[0065]

[0066] The phase of the above formula is used as the CPM of the I-COACH system

[0067]

[0068] Where: arg(J l (k r ρ)) is the value of Jl (k r ρ) takes the phase angle, which can be 0 or π.

[0069] like Figure 1 As shown, the lighting system adopts critical lighting, the red light center wavelength of the monochromatic LED light source (1) is 660nm, and the green light center wavelength is 520nm. The distance from the monochromatic LED light source (1) to the first lens (2) is z1, and the distance from the first lens (2) to the target object (3) is z2. f1 is the focal length of the first lens, then z1 and z2 satisfy 1 / z1+1 / z2=1 / f1. The word "Bessel" is selected as the object under red light (i.e., object 1), and the word "I-COACH" is selected as the object under green light (i.e., object 2). The target object (3) first uses a 25μm pinhole, which is approximately regarded as an object point on the target plane. Its amplitude is Its complex amplitude in the plane passing through the second lens (4) can be expressed as

[0070]

[0071] in, The distance from the object point to the second lens (4) is z s , λ is the central wavelength of the light source, z s The size of is equal to the focal length f0 of the second lens (4). After passing through the second lens (4), the light beam carrying the information of the target object (3) is collimated, so the distance from the second lens (4) to the PSLM (8) can be approximately regarded as 0, z s It is also the distance from the object point to PSLM (8).

[0072] The light beam emitted by the second lens (4) passes through the polarizer (5), aperture stop (6) and beam splitter (7) and is incident on the PSLM (8). The modulation polarization direction of the polarizer (5) and the PSLM (8) are consistent. The aperture stop (6) plays a truncation role, which slightly improves the quality of the light beam so that it can be better represented by a Gaussian mathematical function. The CPM is loaded on the PSLM (8), and the plane intensity distribution on the image sensor after CPM modulation is

[0073]

[0074] in, represents the coordinates (x2, y2) of the image sensor, is the coordinate (x1, y1) of the CPM plane, z h is the distance from the CPM plane to the image sensor (9) plane.

[0075] At this time, what is recorded on the image sensor (9) is a PSH intensity map, which reflects the intensity distribution of an object point passing through the system.

[0076] At the same position of the target object (3) in the system, the pinhole is replaced with the target image and the object hologram is recorded. Considering the use of incoherent light illumination, the intensity distribution formed by the object can be regarded as the superposition of PSH intensities. The intensity distribution generated by any two-dimensional object can be expressed as

[0077]

[0078] Among them, a n is a constant related to the object point.

[0079] Place a red LED at the same position as the system's monochromatic LED light source (1), and place a pinhole at the same position as the system's target object (3). Record the PSH under red light as I PSH,1 ,like Figure 5 As shown in (a).

[0080] At the same position of the system target object (3), replace the pinhole with object 1 and record the OH of object 1 under red light as I OH,1 ,like Figure 6 As shown in (a).

[0081] Replace the red LED with a green LED at the same position of the system's monochromatic LED light source (1), and place a pinhole at the same position of the system's target object (3). Record the PSH under green light as I PSH,2 ,like Figure 5 (b)

[0082] At the same position of the system target object (3), replace the pinhole with object 2 and record the OH of object 2 under green light as I OH,2 ,like Figure 6 (b) shown.

[0083] The two wavelengths of OH are superimposed to obtain the total object hologram, such as Figure 6 (c)

[0084] I OH,sum =I OH,1 +I OH,2

[0085] The reconstruction process of the non-interference coded aperture correlation holographic system is actually an optical pattern recognition problem. The essence of the NLR reconstruction algorithm is actually the conversion of the spatial domain cross-correlation reconstruction algorithm into the frequency domain for calculation. In the NLR algorithm, it is first necessary to convert I OH and I PSH Transformed into the frequency domain is expressed as follows

[0086]

[0087] in: represents the Fourier transform.

[0088] The PSH and the total object hologram under the corresponding wavelength are used to perform NLR respectively to obtain the object under this wavelength.

[0089] Using red light PSH and the total object hologram reconstruction, we can obtain the object 1 recorded under red light illumination, as shown in Figure 7 (a)

[0090]

[0091] Using the green light PSF and the total object hologram reconstruction, we can obtain the object 2 recorded under green light illumination, as shown in Figure 7 (b)

[0092]

[0093] in: represents the inverse Fourier transform; o and r are the amplitude spectrum adjustment parameters of the object hologram and PSH, respectively, and both range from -1 to +1.

[0094] In the NLR reconstruction algorithm, As a reconstruction function, As the objective function, the parameters o and r are selected to adjust the target's power spectrum and reconstruction function, respectively. When o ≠ 1, the effect on the object's spectral size makes the reconstruction process of multi-point targets nonlinear. This nonlinearity in the reconstruction process improves the signal-to-noise ratio of the reconstructed image without losing the shift invariance of the linear correlation.

[0095] This method realizes interference-free coded aperture correlation holographic spectral imaging. By multiplexing the Bessel beam and the lens and taking the phase as CPM, it can effectively distinguish objects at different wavelengths.

[0096] 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. A Bessel-beam-based interference-free coded aperture correlation holographic spectral imaging method, characterized in that: The specific implementation device includes a monochromatic LED (Light Emitting Diode) light source, a first lens, a target object, a second lens, a polarizer, an aperture stop, a beam splitter prism, a phase spatial light modulator PSLM (Phase Spatial Light Modulator) and an image sensor, wherein: Light beams emitted by monochromatic LED light sources with wavelengths of 660nm and 520nm, respectively, are focused on a target object after passing through a first lens; the diffracted light of the target object is converted into collimated parallel light by a second lens, and the light beam is incident on a polarizer whose polarization angle is the same as the modulation angle of the PSLM. The light beam passing through the polarizer is modulated by the CPM (Coded Phase Mask) loaded on the PSLM; the modulated light beam is incident on the image sensor, which records object holograms OH (object hologram) illuminated by red light and green light, respectively; the recording method remains unchanged, but the target object is replaced by a pinhole, and the image sensor records point spread holograms PSH (point spread hologram) illuminated by red light and green light; the OH and PSH are reconstructed using a nonlinear reconstruction (NLR) algorithm. The method comprises the following three steps: S1: First, multiplex the Bessel beam with the lens and take its phase as CPM; the light field expression of the Bessel beam in the cylindrical coordinate system is as follows E l (ρ,φ,z)=J l (k r ρ)exp(ilφ+ik z z) Where: ρ is the radial coordinate, φ is the azimuthal coordinate, z is the axial coordinate, J l is the first kind l-order Bessel function, k r and k z are radial and longitudinal wave vectors, respectively. λ is the wavelength of the light wave; the simplified expression is as follows E l (ρ,φ)=J l (k r p)exp(ilφ) The lens phase expression is as follows Where: f is the focal length of the lens, (x,y) represents the coordinates of the light field in the rectangular coordinate system on the cross section perpendicular to the beam propagation direction; the phase expression of the multiplexed Bessel beam and the lens is as follows The phase of the above formula is used as the CPM of the I-COACH system Where: arg(J l (k r ρ)) is the value of J l (k r ρ) takes the phase angle, which can be 0 or π; S2: Record the point spread function and object hologram under red and green light illumination respectively; Assume that there is an object point on the object plane Its amplitude is After CPM modulation, the intensity distribution on the image sensor plane is: The pixel coordinates of any object point on the object plane, PSLM plane and image sensor plane that define the vertical beam propagation direction are (x s ,y s ), (x1,y1) and (x2,y2), represents the coordinates (x2, y2) of the image sensor, λ is the wavelength of the light wave, z s is the distance from the object plane to the CPM, f0 is the focal length of the lens in front of CPM, and the distance from the lens to CPM is approximately considered to be 0. is the coordinate (x1, y1) of the CPM plane, z h is the distance from the CPM plane to the image sensor plane; the intensity distribution generated by any two-dimensional object can be expressed as Among them: a n is a constant related to the object point; PSH recorded under red light is I PSH,1 OH is I OH,1 ; PSH recorded under green light is 1 PSH,2 OH is I OH,2 ; S3: I recorded under red light OH,1 and I recorded under green light OH,2 The total object hologram I is obtained by superposition OH,sum ; In the NLR algorithm, we first need to OH and I PSH Transformed into the frequency domain is expressed as follows in: represents Fourier transform; Using red light PSH and the total object hologram reconstruction, we can obtain the object 1 recorded under red light illumination. Using green light PSH and the total object hologram reconstruction, we can obtain the object 2 recorded under green light illumination. in: represents the inverse Fourier transform; o and r are the amplitude spectrum adjustment parameters of OH and PSH, respectively, and both range from -1 to 1.

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