CN108645336B - A kind of digital holographic camera without reference light and calibration method - Google Patents

Abstract

The invention discloses a reference light-free digital holographic camera and a calibration method. The camera includes: an image sensor, a beam splitter, a random phase plate and a diaphragm; a beam splitter, a random phase plate and a diaphragm are sequentially fixed in parallel in front of the image sensor; When the light beam is incident from the lower part of the beam splitter in the direction perpendicular to the lower surface of the beam splitter, the beam is reflected by the beam splitting surface of the beam splitter and then vertically incident on the receiving surface of the image sensor; the random phase plate is made of micro-nano processing technology, random The surface of the phase plate is engraved with a number of concave units with known depths and coordinates; the center of the diaphragm is provided with a light-passing hole. After the phase plate and beam splitter, it is perpendicular to the receiving surface of the image sensor. After the camera of the present invention is used and calibrated, the quality of digital holographic imaging without reference light can be improved.

Description

A kind of digital holographic camera without reference light and calibration method

technical field

The invention relates to the field of digital holographic imaging, in particular to a reference-free digital holographic camera and a calibration method.

Background technique

Digital holography is an imaging technology that can simultaneously acquire quantitative amplitude and phase information of object light waves. The general digital holographic imaging is realized based on the principle of light interference. It needs to use a known reference light wave to interfere with the unknown object light wave to form and record the hologram, and then use the known reference light wave information and hologram information. The reconstructed image of the object is obtained by the reconstruction method. The interference optical path structure is relatively complex, and the requirements for the environment are high. In practical applications, due to factors such as the environment, it is difficult to accurately know the exact information of the reference light wave. Therefore, it is limited in practical application.

Existing reference-free optical holographic imaging methods use optical frosted glass as the diffusing element, whose microstructure is unknown, and requires an accurate and complex calibration process to determine the transmission matrix of the holographic camera. Similar to sampling theory, the higher the resolution required for holographic imaging, the greater the number of transmission matrices that need to be measured. However, due to the limitation of the accuracy of the calibration system and the influence of environmental interference during the calibration process, the increase in the number of measurements may also increase the error of the transmission matrix, which makes the imaging results obtained by this method have low resolution and serious dispersion. speckle noise. Moreover, when the wavelengths are different, the transmission matrix of the holographic camera is also different. When imaging with different light waves, it needs to be re-calibrated, which is not conducive to the realization of color holographic imaging.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a reference light-free digital holographic camera and a calibration method, so as to improve the quality of reference light-free digital holographic imaging.

For achieving the above object, the present invention provides the following scheme:

A reference light-free digital holographic camera, the camera comprises: an image sensor, a beam splitter, a random phase plate, a diaphragm and a casing; the beam splitter, the random phase plate and the the diaphragm, the spectroscope, the random phase plate and the diaphragm are all placed in the casing;

The image sensor is an image sensor of a color digital camera; the beam splitting surface of the beam splitter and the receiving surface of the image sensor form an included angle of 45 degrees. When incident in the direction of the surface, the light beam is vertically incident on the receiving surface of the image sensor after being reflected by the beam splitting surface of the beam splitter;

The random phase plate is made by using micro-nano processing technology, and the surface of the random phase plate is engraved with a plurality of concave units with known depths and coordinates;

The center of the diaphragm is provided with a light-passing hole. When the light beam is incident from the light-transmitting hole of the diaphragm in a direction perpendicular to the surface of the diaphragm, the light beam passes through the random phase plate and the beam splitter in sequence and then becomes vertical. incident on the receiving surface of the image sensor.

Optionally, the camera further includes: a polarizer, the polarizer is fixed between the random phase plate and the beam splitter, and between the polarizer, the random phase plate and the beam splitter are all parallel.

Optionally, the thickness of the random phase plate is 1-2 mm, the depths of the recessed units are different, the depth values of all the recessed units satisfy the random distribution law, and the distribution range of the depth values is (0,1] μm. The size of the concave unit is 1-100 μm.

Optionally, the housing includes a light-transmitting window, and the light-transmitting window is located at the center of the lower surface of the beam splitter.

Optionally, the housing further includes a closing plate for closing the light-transmitting window.

A calibration method for a digital holographic camera without reference light, the calibration method comprising:

The plane illumination light is used to enter the diaphragm vertically; the plane illumination light is incident through the light aperture of the diaphragm, passes through the random phase plate and the beam splitter in sequence, and reaches the image sensor vertically;

Obtain the coaxial hologram recorded by the image sensor;

The plane reference light is used to enter the beam splitter vertically; the plane reference light is incident through the light-transmitting window of the housing, and after being reflected by the beam splitting surface of the beam splitter, it vertically reaches the image sensor;

obtaining an off-axis hologram recorded by the image sensor;

Guide high-resolution reconstruction of the on-axis hologram according to the low-resolution information of the off-axis hologram to obtain an intensity image and a phase image of the random phase plate;

The relative positional relationship between the random phase plate and the image sensor is determined according to the intensity image and the phase image of the random phase plate; the relative positional relationship includes the position of each concave unit on the random phase plate in the image The projection position on the sensor also includes the vertical distance between the random phase plate and the image sensor;

According to the parameters of the random phase plate and the relative positional relationship, the speckle responses of all input fundamental modes are obtained by numerical calculation, and the transmission matrices of the red, green and blue colors of the camera are obtained to complete the calibration; The parameters of the random phase plate include the thickness of the random phase plate and the positions, depths and sizes of all concave units.

Optionally, the plane illumination light and the plane reference light have coherence.

Optionally, obtaining the red, green, and blue transmission matrices of the camera to complete the calibration, and then further including:

Close the light-transmitting window on the housing.

According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

Since the microstructure of the random phase plate used as the diffusion element in the present invention is known, in the calibration process, it is only necessary to determine the relative position between the random phase plate and the image sensor through a simple process, and then obtain the The transmission matrix of the reference light digital holographic camera. The calibration process is simpler, and the error of the transmission matrix will not increase due to the increase of the input amount, and better imaging quality can be obtained. Moreover, after the relative position between the random phase plate and the image sensor is determined, the transmission matrix of the red, green and blue colors of the digital holographic camera without reference light can be easily obtained by numerical calculation. At the same time, the speckle image formed after the object light enters the reference light-free digital holographic camera is recorded by the color image sensor, and the color intensity image and phase image of the object can be reconstructed.

Description of drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings required in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some of the present invention. In the embodiments, for those of ordinary skill in the art, other drawings can also be obtained according to these drawings without creative labor.

FIG. 1 is a schematic structural diagram of a reference light-free digital holographic camera of the present invention;

2 is a schematic flowchart of a calibration method for a reference light-free digital holographic camera according to the present invention;

Fig. 3 is the first schematic diagram of using the digital holographic camera without reference light of the present invention;

FIG. 4 is a second schematic diagram of a reference light-free digital holographic camera of the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

In order to make the above objects, features and advantages of the present invention more clearly understood, the present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments.

FIG. 1 is a schematic structural diagram of a reference light-free digital holographic camera of the present invention. As shown in Figure 1, the camera includes:

Image sensor 1, spectroscope 2, polarizer 3, random phase plate 4, diaphragm 5 and housing 6; the image sensor 1 is an image sensor of a color digital camera; the front of the image sensor 1 of the color digital camera is sequentially equipped with a beam splitter Mirror 2, polarizer 3, random phase plate 4, diaphragm 5 constitute a digital holographic camera without reference light. The diaphragm 5, the random phase plate 4, the polarizer 3, and the beam splitter 2 are installed in the cylindrical casing 6 (the casing 6 in the figure is only half drawn), and the casing 6 is connected and fixed with the digital camera. The center of the diaphragm 5, the random phase plate 4, the polarizer 3, the beam splitter 2 and the image sensor 1 of the digital camera are on the axis of the cylindrical housing 6, and the diaphragm 5, the random phase plate 4, the polarizer 3 and the digital camera have The image sensors 1 are parallel.

The beam splitting surface 2-1 of the beam splitter 2 forms an angle of 45 degrees with the receiving surface of the image sensor 1. When the light beam is incident from the lower part of the beam splitter 2 in a direction perpendicular to the lower surface of the beam splitter 2, the beam passes through all the beams. The beam splitting surface 2 - 1 of the beam splitter 2 is reflected and vertically incident on the receiving surface of the image sensor 1 . The receiving surface of the image sensor 1 is the surface adjacent to the beam splitter.

The housing 6 includes a light-transmitting window, and the light-transmitting window is located at a portion below the beam splitting surface 2 - 1 and at the center of the lower surface of the beam splitter 2 . The housing 6 also includes a closing plate for closing the light passage window.

The random phase plate 4 is fabricated by using micro-nano processing technology. The random phase plate 4 is a circular transparent thin plate (with a thickness of about 1-2 mm), and many square or circular concave units (small pits) with known depth and coordinates are engraved on the surface by using the precision micro-nano processing technology. The size is about 1 to several tens of microns, such as 1 to 100 μm, or 1 to 50 μm, which is set according to actual needs. The size here refers to the size of the upper surface of the recessed unit. For example, when the upper surface of the recessed unit is circular, the size of the recessed unit refers to the diameter of the circle on the upper surface. When the upper surface of the recessed unit is rectangular, the size of the recessed unit refers to the length of the long side of the rectangle. When the upper surface of the recessed unit is irregular in shape, the size of the recessed unit refers to the longest diameter in the irregular shape of the upper surface. The depths of each concave unit are different, and the depth values of all concave units satisfy the random distribution law, and the distribution range is between 0 and 1 μm. The positions and depths of these recessed cells are randomly generated according to the computer and stored as known data, and these microstructural data will be used to calculate the transmission matrix of the reference-free optical holographic camera.

The center of the diaphragm 5 is provided with a light-passing hole, and the light-passing hole can be a square or a circle. When the light beam is incident from the aperture of the diaphragm 5 in a direction perpendicular to the surface of the diaphragm 5 , the light beam passes through the random phase plate 4 and the beam splitter 2 in sequence and then vertically enters the image sensor 4 . receiving surface.

The digital holographic camera without reference light needs to be calibrated before use to obtain its transmission matrix. FIG. 2 is a schematic flowchart of a calibration method for a reference light-free digital holographic camera according to the present invention. The calibration process consists of two parts:

First, the random phase plate is illuminated by plane light, an on-axis hologram is recorded when there is no reference light, an off-axis hologram is recorded after adding plane reference light, and the low-resolution information obtained from the off-axis hologram is used to guide the same The high-resolution reconstruction of the axial hologram can obtain the intensity image and phase image of the random phase plate, so as to determine the relative positional relationship between the random phase plate and the image sensor. As shown in the figure, including steps 100-600, the steps are as follows:

Step 100: Use the plane illumination light to enter the diaphragm vertically. The plane irradiation light is incident through the light-transmitting hole of the diaphragm, passes through the random phase plate and the beam splitter in sequence, and reaches the image sensor vertically. The plane illumination light is vertically irradiated on the random phase plate, and the transmitted light passes through the beam splitter and reaches the image sensor of the digital camera. This beam of light is called object light.

Step 200: Obtain the coaxial hologram recorded by the image sensor.

Step 300: Use the plane reference light to enter the beam splitter vertically. The plane reference light is incident through the light-transmitting window of the housing, and after being reflected by the light-splitting surface of the light-splitting mirror, vertically reaches the image sensor. The plane reference light is irradiated on the beam splitting surface of the beam splitter, and then reflected on the image sensor of the digital camera. This beam of light is called the plane reference light.

Step 400: Obtain an off-axis hologram recorded by an image sensor.

The plane irradiation light has coherence with the plane reference light, that is, the object light has coherence with the plane reference light, and there is a small angle (about tens of mrad) between the two beams when irradiated on the image sensor. The image formed by the two beams of light on the image sensor is an off-axis digital hologram. Block the plane reference light, only the image formed by the object light on the image sensor will get the coaxial digital hologram.

Step 500: Guide high-resolution reconstruction of the on-axis hologram according to the low-resolution information of the off-axis hologram to obtain the intensity image and phase image of the random phase plate. The specific process is as follows:

1: Determine the distribution of the first phase of the object light wave in the image sensor plane and the distribution of the second phase of the object light wave in the plane of the random phase plate according to the off-axis digital hologram;

确定图像传感器平面内的物光波的第一复振幅，其中，O'表示第一复振幅，I_in表示同轴数字全息图的强度，

表示物光波的第一相位，j表示虚数符号。2: Determine the first complex amplitude of the object light wave in the image sensor plane according to the intensity and the first phase of the coaxial digital hologram; specifically, according to the formula:

Determine the first complex amplitude of the object light wave in the plane of the image sensor, where O' represents the first complex amplitude, I _in represents the intensity of the coaxial digital hologram,

represents the first phase of the object light wave, and j represents the sign of the imaginary number.

3: Using the angular spectrum propagation method to reversely propagate the first complex amplitude into the plane of the random phase plate to obtain the second complex amplitude of the object light wave in the plane of the random phase plate, and transfer the second complex amplitude to the plane of the random phase plate. Among the complex amplitudes, the amplitude of the pixel corresponding to the amplitude greater than 1 is updated to 1, and the phase of the pixel corresponding to the amplitude greater than 1 is updated according to the phase of the corresponding pixel in the second phase distribution. Specifically, updating the phase of the pixel point corresponding to the amplitude greater than 1 according to the phase of the corresponding pixel point in the distribution of the second phase specifically includes: correspondingly updating the phase of the pixel point corresponding to the amplitude greater than 1 to the corresponding one in the distribution of the second phase. The phase of the pixel point.

4: Forward the second complex amplitude into the image sensor plane by using the angular spectrum propagation method to obtain the third complex amplitude of the object light wave in the image sensor plane, and calculate the third complex amplitude according to the coaxial digital The intensity of the hologram updates the amplitude in the third complex amplitude;

5: Determine whether the difference between the third complex amplitude and the first complex amplitude is less than a set threshold; if not, go to step 3; if so, determine the intensity image and phase image of the random phase plate according to the second complex amplitude.

Step 600: Determine the relative positional relationship between the random phase plate and the image sensor according to the intensity image and the phase image of the random phase plate. The relative positional relationship includes the projected position of each concave unit on the random phase plate on the image sensor, and also includes the vertical distance between the random phase plate and the image sensor.

(n表示随机相位板折射率，d表示不同点的凹陷单元的深度，λ表示光波波长。Specifically, the distance parameter between the image sensor plane and the random phase plate plane in the numerical calculation in step 500 is adjusted. When the clearest intensity image is obtained, the distance parameter in the numerical calculation is the distance between the image sensor and the random phase plate. vertical distance. Compare the intensity image and phase image with the known microstructure parameters of the random phase plate to find their corresponding relationship, so as to obtain the phase shift of each point in the plane of the random phase plate according to the microstructure parameters of the random phase plate

(n represents the refractive index of the random phase plate, d represents the depth of the concave unit at different points, and λ represents the wavelength of the light wave.

Second, according to the known microstructure data of the random phase plate and the measured relative positional relationship data between the random phase plate and the image sensor, the speckle responses of all input fundamental modes are obtained by numerical calculation, that is, the red , green and blue transmission matrix. Proceed as follows:

Step 700: According to the parameters of the random phase plate and the relative positional relationship, the speckle responses of all input fundamental modes are obtained by numerical calculation, and the transmission matrix of the red, green and blue colors of the camera is obtained, and the calibration is completed. The parameters of the random phase plate include the thickness of the random phase plate and the positions, thicknesses and sizes of all concave units. details as follows:

1. The software simulates the spherical light waves emitted by different point sources within the imaging observation range. The spherical light waves emitted by each point source are irradiated on the digital holographic camera without reference light to form an input fundamental mode. Determine the number and spacing of point sources according to the field of view and resolution requirements of the reference-light-free digital holographic camera. Assuming that the imaging observation range is a cuboid space, all point sources can be distributed in a 3-dimensional lattice, that is, L layers are arranged along the z-axis, and each layer is arranged in N rows and M columns, with a total of M×N×L point and number all point sources sequentially. Accordingly, M×N×L input fundamental modes can be generated.

2. Since the phase shift of each point in the plane of the random phase plate has been determined. Using the angular spectrum propagation algorithm (other algorithms can also be used) to calculate the complex amplitude distribution of the spherical light wave emitted by each point source after passing through the random phase plate in the image sensor plane, which is the speckle response of the input fundamental mode.

3. When the point source emits a red spherical wave, arrange the data of the speckle response of the first input fundamental mode into a row in a row-by-row, end-to-end form, that is, the data of the first row of the transmission matrix is obtained; the second input The data of the speckle response of the fundamental mode is arranged in a row in a row-by-row form, that is, the data of the second row of the transmission matrix is obtained; and so on, the speckle response of the last input fundamental mode is arranged in the last row of the transmission matrix . That is, the transmission matrix of red light is obtained.

4. Set the spherical wave emitted by the point source in step 3 as green and blue, and use the same method to obtain the transmission matrix of green light and the transmission matrix of blue light respectively.

At this point, the calibration process is completed. After calibration is complete, close the light window on the housing.

3 is a first schematic diagram of using the reference light-free digital holographic camera of the present invention. The transmission-type sample is placed in the imaging observation range in front of the camera, and a reference-light-free digital holographic image of the transmission type can be realized. 4 is a second schematic diagram of using the reference light-free digital holographic camera of the present invention. The reflective sample is placed in the imaging observation range in front of the camera, and a reflective-type reference light-free digital holographic image can be realized.

As shown in Figures 3 and 4, when using a reference-light-free digital holographic camera for imaging, the sample is illuminated by three colors of laser light, red, green, and blue, and the object light transmitted or reflected by the sample enters the reference-light-free digital holographic camera. After passing through the random phase plate, laser speckles of three colors of red, green and blue are formed, and the speckle images of three colors of red, green and blue are recorded by a color digital camera. Then, in the computer, the complex amplitude information of red, green and blue light waves is obtained from the transmission matrix of the calibrated digital holographic camera without reference light and the recorded speckle image through the reconstruction algorithm. Finally, the color intensity image and phase image of the sample are synthesized.

The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments, and the same and similar parts between the various embodiments can be referred to each other.

In this paper, specific examples are used to illustrate the principles and implementations of the present invention. The descriptions of the above embodiments are only used to help understand the methods and core ideas of the present invention; meanwhile, for those skilled in the art, according to the present invention There will be changes in the specific implementation and application scope. In conclusion, the contents of this specification should not be construed as limiting the present invention.

Claims (8)

1. A reference light-free digital holographic camera, characterized in that the camera comprises: an image sensor, a beam splitter, a random phase plate, a diaphragm and a housing; the random phase plate and the diaphragm, the beam splitter, the random phase plate and the diaphragm are all placed in the casing; The image sensor is an image sensor of a color digital camera; the beam splitting surface of the beam splitter and the receiving surface of the image sensor form an included angle of 45 degrees. When incident in the direction of the surface, the light beam is vertically incident on the receiving surface of the image sensor after being reflected by the beam splitting surface of the beam splitter; The random phase plate is made by micro-nano processing technology, and the surface of the random phase plate is engraved with a plurality of recessed units with known depths and coordinates; the depths of the recessed units are different, and the depth values of all the recessed units are The random distribution law is satisfied, and the distribution range of the depth value is between (0,1] μm; the microstructure data of the recessed unit is used to calculate the transmission matrix of the reference-free light holographic camera, and the microstructure data includes the recessed unit. location and depth; The center of the diaphragm is provided with a light-passing hole. When the light beam is incident from the light-transmitting hole of the diaphragm in a direction perpendicular to the surface of the diaphragm, the light beam passes through the random phase plate and the beam splitter in sequence and then becomes vertical. incident on the receiving surface of the image sensor. 2 . The camera according to claim 1 , wherein the camera further comprises: a polarizer, the polarizer is fixed between the random phase plate and the beam splitter, and the polarizer, the Both the random phase plate and the beam splitter are parallel. 3 . The camera according to claim 1 , wherein the random phase plate has a thickness of 1-2 mm; the size of the concave units is all 1-100 μm. 4 . 4 . The camera of claim 1 , wherein the housing comprises a light-transmitting window, and the light-transmitting window is located at the center of the lower surface of the beam splitter. 5 . 5 . The camera of claim 4 , wherein the housing further comprises a closing plate for closing the light-transmitting window. 6 . 6. A calibration method for a digital holographic camera without reference light, wherein the calibration method comprises: The plane illumination light is used to enter the diaphragm vertically; the plane illumination light is incident through the light aperture of the diaphragm, passes through the random phase plate and the beam splitter in sequence, and reaches the image sensor vertically; Obtain the coaxial hologram recorded by the image sensor; The plane reference light is used to enter the beam splitter vertically; the plane reference light is incident through the light-transmitting window of the housing, and after being reflected by the beam splitting surface of the beam splitter, it vertically reaches the image sensor; obtaining an off-axis hologram recorded by the image sensor; Guide high-resolution reconstruction of the on-axis hologram according to the low-resolution information of the off-axis hologram to obtain an intensity image and a phase image of the random phase plate; The relative positional relationship between the random phase plate and the image sensor is determined according to the intensity image and the phase image of the random phase plate; the relative positional relationship includes the position of each concave unit on the random phase plate in the image The projection position on the sensor also includes the vertical distance between the random phase plate and the image sensor; According to the parameters of the random phase plate and the relative positional relationship, the speckle responses of all input fundamental modes are obtained by numerical calculation, and the transmission matrices of the red, green and blue colors of the camera are obtained to complete the calibration; The parameters of the random phase plate include the thickness of the random phase plate and the positions, depths and sizes of all concave units. 7 . The calibration method according to claim 6 , wherein the plane illumination light and the plane reference light have coherence. 8 . 8. The calibration method according to claim 6, wherein the obtaining the transmission matrices of the three colors of red, green and blue of the camera, and completing the calibration, further comprising: Close the light-transmitting window on the housing.

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