CN105184295B - A kind of holoscan space length extracting method based on wavelet transformation and connected domain - Google Patents

Abstract

The present invention proposes a kind of holoscan space length extracting method based on wavelet transformation and connected domain, belongs to optical scanner and image reconstruction field, mainly solves the problems, such as the axial space position of Automatic-searching object under test.The present invention extracts the marginal information of reconstruction image by wavelet transformation, and remove defocus noise edge using connected domain algorithm, to improve the accuracy of space length extraction in traditional optical scanner holophotal system.The present invention can effectively realize space length extraction, and improve the accuracy of space length extraction, and this method is suitable for multiple fields.

Description

A Method for Extracting Spatial Distance of Holographic Scanning Based on Wavelet Transform and Connected Domain

technical field

The present invention relates to the field of optical scanning holography and image reconstruction, in particular to a method for extracting spatial distance of holographic scanning based on wavelet transform and connected domain.

Background technique

Optical scanning holography technology, referred to as OSH, is a digital holography technology. It was first proposed by Poon and Korpel. OSH can store the information of 3-dimensional objects as 2-dimensional holograms. Since the technology was proposed, it has been widely used in the fields of scanning holographic microscope, 3D image recognition and 3D optical remote sensing.

Spatial distance extraction technology is an important technology in OSH. It can extract the axial spatial information of the object to be measured, and the axial spatial information of the object to be measured is an important parameter for image reconstruction. Therefore, the spatial distance extraction technology also has important research value. The research proves that in the iterative operation, the maximum value of the edge of the reconstructed image can be found through the wavelet decomposition and the connected domain algorithm, and the edge of the out-of-focus image can be suppressed. When the maximum value of the edge is obtained, the axial direction of the object to be measured can be inferred Spatial information, so as to realize spatial distance extraction.

The document "Autofocusing in optical scanning holography" proposes the Wigner distribution to extract the axial spatial information of the object to be measured, but this method has many limitations and low accuracy.

The document "Blind sectional image reconstruction for optical scanningholography" proposes to use the Prewitt operator to extract the edges of the reconstructed image. This method has good practicability, but due to the existence of defocus noise edges, the accuracy of the calculation results is affected.

Contents of the invention

The purpose of the present invention is to automatically focus on optical holographic scanning, and propose a method for extracting space distance of holographic scanning through wavelet transform and connected domain. The method extracts image high-frequency components and removes low-frequency components by using wavelet transform, thereby extracting image edges, and at the same time , using the connected domain algorithm to suppress the edge of the out-of-focus image and retain the complete edge of the in-focus image.

The technical scheme adopted in the present invention is:

A method for extracting distance from a holographic scanning space realized by wavelet transform and connected domain, the process of which is shown in Figure 1, including the following steps:

Step 1. Divide the laser light into two beam paths through the first polarizing beam splitter, one of which passes through an AOM and a beam expander in sequence, and the other beam passes through another AOM, a beam expander and a convex lens in sequence Finally, the two beams are condensed by the second polarizing beam splitter and then interfered to form Fresnel interference fringes;

Step 2. Scan the object to be measured through the Fresnel interference fringes, and use the photodetector to receive the scanned light information, so as to obtain a hologram of the object to be measured;

Step 3. set the initial value and upper limit value of the distance parameter, set the scan step length of the distance parameter, and use the initial value as the current value of the distance parameter;

Step 4. After performing Fourier transform on the obtained hologram, after multiplying with the conjugate of the frequency domain expression of the traditional optical transfer function with the distance parameter, and then through inverse Fourier transform, you can get A reconstructed image of the object to be tested;

Step 5. Perform wavelet decomposition on the reconstructed image, that is, perform a defocused image suppression operation on the image to remove low-frequency component information, retain high-frequency component information, and obtain an edge image of the object to be measured containing defocus noise;

Step 6. Use the connected domain algorithm to remove the defocus noise of the edge image of the object to be measured, and finally obtain an edge image without defocus noise; calculate and record the edge length of the edge image without defocus noise and its corresponding distance parameter value;

Step 7. Add a scan step to the current value of the distance parameter as the latest value of the distance parameter, and repeat steps 4 to 6 until the current value of the distance parameter has reached the set upper limit, thus Obtaining a plurality of edge lengths; extracting distance parameter values corresponding to the two largest values of the edge lengths, the two distance parameter values being the position interval of the object to be measured in the axial space.

The beneficial effects of the present invention are:

(1) The present invention uses optical scanning holography technology, and realizes the extraction of axial spatial information of the object to be measured through wavelet decomposition and connected domain algorithm. The application scope of holographic scanning space distance extraction technology includes biological cell observation, mechanical control, optical focusing, etc. Therefore, the present invention is applicable to various fields and has a very wide range of applications;

(2) The present invention utilizes wavelet decomposition to realize the suppression of the out-of-focus image and extracts the edge of the focus image, and further processes the image through the connected domain algorithm, thereby improving the accuracy of spatial distance extraction;

(3) The present invention only needs to adjust the detail coefficient threshold and connected domain threshold of wavelet decomposition to realize effective suppression of out-of-focus images and accurately extract the axial spatial information of the object to be measured;

(4) The present invention is not only simple in implementation and easy to operate, but also has strong practicability and is suitable for popularization and use.

Description of drawings

Fig. 1 is the schematic flow chart of the method provided by the present invention;

Fig. 2 is the basic structural diagram of the embodiment of the present invention;

3 is a schematic diagram of an object to be measured according to an embodiment of the present invention;

FIG. 4 is a hologram of an object to be measured according to an embodiment of the present invention;

Fig. 5 is the reconstruction image of the embodiment of the present invention;

Fig. 6 is an effect diagram of wavelet decomposition and connected domain algorithm of the focused image according to the embodiment of the present invention;

Fig. 7 is a defocused image wavelet decomposition and connected domain algorithm effect diagram according to an embodiment of the present invention;

Fig. 8 is a diagram of a bimodal function of edge length according to an embodiment of the present invention.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

Example

The process flow of the embodiment of the present invention is shown in Figure 1, and the basic structure adopted is shown in Figure 2, wherein the wavelength λ=632.8nm of the He-Ne laser Laser, and the conversion frequency of the acousto-optic modulator (AOFS1, AOFS2) is Ω/ 2π=79.98MHz and (Ω+ΔΩ)/2π=40MHz, the focal length of the convex lens L1 is 500mm, the object to be measured is shown in Figure 3, the XOY plane size is 1.5cm×1.5cm, the matrix size is 500×500, and its axis The positions in the directions are z ₁ =870mm, z ₂ =1070mm, and the scanning process can be realized according to the following steps:

Step 1. Divide the laser light into two beam paths through the first polarizing beam splitter, one of which passes through an AOM and a beam expander in sequence, and the other beam passes through another AOM, a beam expander and a convex lens in sequence Finally, the two beams are condensed by the second polarizing beam splitter and then interfered to form Fresnel interference fringes;

The system structure used in this embodiment is shown in Figure 2. The light with angular frequency ω emitted by the same light source is divided into two beams by the first polarizing beam splitter BS1, one of which passes through the acousto-optic modulator AOFS1 and the convex lens L1 to form Spherical wave; the other beam passes through the acousto-optic modulator AOFS2 to form a plane wave; the two beams are aggregated by the second polarizing beam splitter and interfere in front of the object to be measured to form Fresnel interference fringes, which are in the axial direction of the object to be measured The spatial expression is as follows:

Among them, the object to be measured is divided into n layers in the axial direction, z _i represents the axial position of the i-th layer of the object to be measured, NA represents the numerical aperture, λ represents the laser wavelength, k _x and _ky represent frequency domain coordinates;

Step 2. Scan the object to be measured through the Fresnel interference fringes, and use the photodetector to receive the scanned light information, so as to obtain a hologram of the object to be measured;

In this embodiment, Fresnel interference fringes are used to scan the object, and the formed hologram is stored in the computer, as shown in Figure 4, as follows:

Among them, g(x, y) represents the hologram, F ^-1 and F represent inverse Fourier transform and Fourier transform respectively, i represents the i-th layer of the object to be measured in the axial direction, I(x, y; z _i ) represents the complex amplitude function of the object to be measured;

Step 3. Set the initial value of the distance parameter and its upper limit value, set the scan step length of the distance parameter, and use the initial value as the current value of the distance parameter; in this embodiment, set the initial value of the distance parameter to z _o = 854.6mm, the upper limit is z _t = 1239.6mm;

Step 4. After performing Fourier transform on the obtained hologram, after multiplying with the conjugate of the frequency domain expression of the traditional optical transfer function with the distance parameter, and then through inverse Fourier transform, you can get The reconstructed image of the object to be measured, as shown in Figure 5;

In this embodiment, the specific expression of image reconstruction through the optical transfer function is as follows:

Among them, I _j (x, y) represents the reconstructed image information of the jth layer of the object under test, z _j represents the axial position of the jth layer of the reconstructed image of the object under test, and F ^-1 and F respectively represent the inverse Fourier transform and Fourier transform, * means conjugate, Indicates convolution;

Step 5. Perform wavelet decomposition on the reconstructed image, that is, perform a defocused image suppression operation on the image to remove low-frequency component information, retain high-frequency component information, and obtain an edge image of the object to be measured containing defocus noise;

In this embodiment, the specific expression for processing the reconstructed image through wavelet decomposition is as follows:

in Represents the detail coefficient, M×N represents the image size, k ₁ and k ₂ represent the conversion ratio of x and y axes; d=H, V, D, respectively represent the horizontal direction, vertical direction, and diagonal direction; The expression of is as follows:

in, represents the wavelet function, and ψ represents the scaling function, as follows:

in, represents the wavelet function coefficient, h _ψ represents the scaling function coefficient, and Using Haar wavelet, its expression is as follows:

Perform preliminary out-of-focus image suppression on the image, the specific operation is as follows:

in, Indicates the maximum value of all detail coefficients; the effect of edge extraction of in-focus images is shown in Figure 6(b); the effect of edge extraction of out-of-focus images is shown in Figure 7(b);

The image edge extraction expression is as follows:

Step 6. Adopt the connected domain algorithm to remove the defocus noise of the edge image of the object to be measured, and finally obtain the edge image without defocus noise, as shown in Fig. 6 (c) and 7 (c); calculate and record The edge length of the edge image without defocus noise and its corresponding distance parameter value;

Step 7. Add a scan step to the current value of the distance parameter as the latest value of the distance parameter, and repeat steps 4 to 6 until the current value of the distance parameter has reached the set upper limit, thus Obtain multiple edge lengths; extract the distance parameter values corresponding to the two maximum edge length values, and the range formed by the two distance parameter values is the position of the object to be measured in the axial space, as shown in Figure 8 , the distance corresponding to the two local maximum peaks is the axial distance of the object to be measured. It can be seen that the obtained axial spatial position of the object to be measured is consistent with its actual spatial position.

Claims (3)

1. A holographic scanning space distance extraction method based on wavelet transform and connected domain, comprising the following steps: Step 1. Divide the laser light into two optical paths through the first polarizing beam splitter, one of which passes through an AOM and a beam expander in sequence, and the other beam passes through another AOM, a beam expander and a convex lens in sequence Finally, the two beams are condensed by the second polarizing beam splitter and then interfered to form Fresnel interference fringes; Step 2. Use the Fresnel interference fringes to scan the object to be measured, and receive the scanned light information through the photodetector, so as to obtain a hologram of the object to be measured; Step 3. set the initial value and upper limit value of the distance parameter, set the scan step length of the distance parameter, and use the initial value as the current value of the distance parameter; Step 4. After performing Fourier transform on the obtained hologram, after multiplying with the conjugate of the frequency domain expression of the traditional optical transfer function with the distance parameter, and then through inverse Fourier transform, you can get A reconstructed image of the object to be tested; The frequency domain expression of the conventional optical transfer function with the stated distance parameter is as follows: Among them, the object to be measured is divided into n layers in the axial direction, z _i represents the axial position of the i-th layer of the object to be measured, i represents the i-th layer of the object to be measured in the axial direction, NA represents the numerical aperture, λ Indicates the laser wavelength, k _x and _ky represent frequency domain coordinates; Step 5. Decompose the reconstructed image by wavelet, that is, perform a defocus image suppression operation on the image to obtain an edge image of the object to be measured that contains defocus noise; Step 6. Use the connected domain algorithm to remove the defocus noise of the edge image of the object to be measured, and finally obtain an edge image without defocus noise; calculate and record the edge length and corresponding distance of the edge image without defocus noise parameter value; Step 7. Add a scan step to the current value of the distance parameter as the latest value of the distance parameter, and repeat steps 4 to 6 until the current value of the distance parameter has reached the set upper limit, thus Obtaining a plurality of edge lengths; extracting distance parameter values corresponding to the two largest values of the edge lengths, the two distance parameter values being the position interval of the object to be measured in the axial space. 2. the holographic scanning space distance extraction method based on wavelet transform and connected domain according to claim 1, is characterized in that, the hologram information described in step 2 is specifically as follows: Wherein, g(x,y) represents a hologram, F ^-1 and F represent inverse Fourier transform and Fourier transform respectively, and I(x,y; z _i ) represents a complex amplitude function of the object to be measured. 3. the holographic scanning space distance extraction method based on wavelet transform and connected domain according to claim 1, is characterized in that, the concrete expression of the reconstruction image described in step 4 is as follows: Among them, I _m (x, y) represents the reconstructed image information of the mth layer of the object under test, g(x, y) represents the hologram, z _m represents the axial position of the mth layer of the reconstructed image of the object under test, and F ^{− 1} , F represent inverse Fourier transform and Fourier transform respectively, * represents conjugate, represents convolution, and I(x,y; _zi ) represents the complex amplitude function of the object to be measured.