阅读说明：本技术 一种基于光束偏转的全息投影方法 (Holographic projection method based on light beam deflection ) 是由 杨光临 谢涛敏 于 2020-04-30 设计创作，主要内容包括：本发明提供一种基于光束偏转的全息投影方法,属于全息投影三维图像显示领域。该方法采用层析法制作多角度计算全息图,利用液晶空间光调制器施加不同相位因子,基于时分复用方法,按时序投影得到分离的再现光束,最后基于正偏心透镜组的光束偏转原理,对光路进行偏转控制,汇聚各视角光线得到再现图像,实现全息投影三维图像显示视场角的扩大。(The invention provides a holographic projection method based on beam deflection, and belongs to the field of holographic projection three-dimensional image display. The method adopts a chromatography method to manufacture a multi-angle calculation hologram, utilizes a liquid crystal spatial light modulator to apply different phase factors, obtains separated reappearance light beams by time sequence projection based on a time division multiplexing method, finally carries out deflection control on an optical path based on a light beam deflection principle of a positive eccentric lens group, converges light rays at various visual angles to obtain reappearance images, and realizes the expansion of the display field angle of the holographic projection three-dimensional image.)

1. a holographic projection method based on beam deflection specifically comprises the following steps:

1) making a multi-angle calculation hologram by using a chromatography method;

2) the liquid crystal spatial light modulator adds a corresponding phase factor to the different calculation holograms, time sequence projection is carried out on the calculation holograms at different angles by using a time division multiplexing method, and light rays of different time sequence projection imaging are reflected and separated into emergent light rays which are horizontally separated;

3) and building a positive eccentric lens group, wherein the positive eccentric lens group consists of three positive lenses with equal focal lengths, emergent rays are respectively converged through the first lens, the second lens is a field lens for reducing ray loss, the first lens and the third lens form a beam control system, and finally the emergent rays are converged at the third lens to form a reproduced image with a wide visual angle.

2. The beam-deflection-based holographic projection method of claim 1, wherein step 1) specifically comprises: modeling and point-cloud processing are carried out on an original three-dimensional object, the original three-dimensional object is divided into a plurality of two-dimensional slice layer data images according to the depth of the original three-dimensional object, Fresnel diffraction light field distribution of each layer on a holographic surface is independently calculated, two-dimensional fast Fourier transform is utilized to create a diffraction pattern for each layer, the patterns are multiplied by holographic lenses with corresponding focal lengths of the depth of the layer, then three-dimensional calculation holograms and original three-dimensional object models with different visual angles are obtained through addition, and a multi-angle calculation hologram is generated.

3. The beam deflection based holographic projection method of claim 1, wherein the step 1) further comprises: and the shielded redundant data is eliminated, and the calculated data is not listed, so that the required calculated amount is simplified.

4. The beam-deflection-based holographic projection method of claim 3, wherein the simplified holograms produced before and after the simplification are compared to confirm the imaging effect after the simplification.

5. The beam deflection based holographic projection method of claim 1, wherein the step 2) further comprises: the reflecting mirror is used for controlling each light beam to form a horizontal light path direction.

6. The method for holographic projection based on beam deflection of claim 1, wherein the first lens in step 3) is two or more lenses for achieving angular focusing.

Technical Field

The invention provides a holographic projection method based on beam deflection, and particularly relates to the field of holographic projection three-dimensional image display.

Technical Field

At present, the three-dimensional display technology is rapidly developed, the technology becomes an ideal medium in multiple aspects such as virtual reality, multimedia display and the like, and the technology is more and more closely related to all aspects of our lives. The three-dimensional display technology has the advantages that the two-dimensional display technology cannot be compared, not only can an object be vividly and stereoscopically reproduced, but also vivid scene simulation can be carried out, and the three-dimensional display technology has great application value in various fields such as military, industry, medical treatment, daily life and the like. Among three-dimensional display technologies, the holographic projection technology is considered to be the most ideal way to achieve three-dimensional display, because it can completely record the amplitude and phase information of a three-dimensional object, and reproduce a three-dimensional image identical to the original object under certain conditions. The holographic projection technology can not only provide a real viewing effect, but also does not need to wear other equipment, so that a viewer has a convenient, real and comfortable experience. Therefore, the ideal true reconstruction technology, namely the holographic projection technology, is always a research hotspot of the international naked-eye true three-dimensional display technology. The Spatial Light Modulator (SLM) is a key device in holographic imaging, but is limited by performance parameters such as pixel size and array size, so that the viewing angle of a reproduced image displayed in three dimensions is narrow. Therefore, holographic projection three-dimensional display technology faces research challenges to expand imaging viewing angles.

Hahn^[1]Et al, use multiple spatial light modulators to obtain successive images of different viewing angles to expand the final field of view of the reproduced image. Multiple tilted SLMs are loaded with different holograms. The holograms loaded by the different SLMs contain information of a certain viewing angle of the three-dimensional object. Each SLM is treated as a window to expand the viewing angle. The optical system has a complicated structure and the adjustment of the gaps between the plurality of SLMs is difficult. Yaras^[2]And others have developed curved holographic video display systems. The system still uses the multi-SLM to be spliced in an inclined mode, and the semi-transparent semi-reflecting mirror is used for eliminating gaps among the SLMs, so that the field angle of the reproduced three-dimensional image is enlarged, and meanwhile, the system structure is simplified. However, the imaging system with multiple SLMs is high in cost, complex in structure and high in building difficulty. Takaki^[3]And Yusuke^[4]Etc. all achieve a 360 deg. field of view projection system using a single spatial light modulator. Takaki et al use a digital micromirror device DMD to load a hologram, using a rotating off-axis fresnel mirror surface as a display screen in an optical system; the wavefront modulated by the DMD of Yusuke et al enters a rotating mirror that is tilted vertically downward. The rotating mirror and the hologram displayed on the DMD can be synchronized with the wavefront reconstruction propagating in all horizontal directions. The display system allows multiple viewers to view simultaneously in close proximity, but the mechanical rotation speed of the mirror needs to be consistent with the refresh rate of the DMD. In addition, the mechanical scanning method using time division multiplexing has high requirements for mechanical control of the scanning system. At present, many research methods for expanding the field angle have respective advantages, but the system simplification and the field angle expansion effect need to be further optimized, and the imaging effect and operability need to be providedHigh.

Disclosure of Invention

The invention provides a holographic projection method based on beam deflection, which can effectively enlarge the field angle of a holographic projection three-dimensional display reproduction image.

The technical scheme provided by the invention is as follows:

the holographic projection method based on beam deflection comprises the following specific steps:

1) utilizing a chromatography method to manufacture a multi-angle calculation hologram, namely modeling original three-dimensional objects with different visual angles, layering the three-dimensional objects along the depth direction, independently calculating the Fresnel diffraction light field distribution of each layer on a holographic surface, and then completely superposing to manufacture the hologram of the three-dimensional object; and generating the multi-angle calculation hologram based on different visual angles. Then, simplifying the required data calculation amount by adopting a method of redundant elimination of the occlusion data;

2) aiming at each calculated hologram in the step 1), adding a corresponding phase factor, performing time sequence projection on the holograms at different angles by using a time division multiplexing method through a liquid crystal spatial light modulator, and reflecting reflected light rays projected and imaged at different moments to separate from each other and reflect the reflected light rays into emergent light rays separated horizontally;

3) and (3) building a positive eccentric lens group, wherein the positive eccentric lens group consists of three positive lenses with equal focal lengths, emergent rays separated in the step 2) are respectively converged by the first lens, the second lens is a field lens for reducing ray loss, the first lens and the third lens group form a light beam control system, and finally the emergent rays are converged by the third lens to form a reproduced image with a wide visual angle.

Further, the step 1) specifically comprises the following steps:

11) modeling and point-cloud processing are carried out on the original three-dimensional objects with different visual angles, so that the subsequent data volume is simplified conveniently, and the three-dimensional objects are divided into a plurality of two-dimensional slice layer data images according to the depth of the two-dimensional slice layer data images;

12) the fresnel diffraction field distribution of the layers on the holographic surface is calculated separately, creating a diffraction pattern for each layer using two-dimensional fast fourier transforms. And multiplying the pattern by the holographic lens of the focal length corresponding to the depth of the layer and then adding to create the final hologram as shown in the following equation:

in the formula 2DFFT_x,yIs a two-dimensional fast Fourier transform; lens (z)_l) A lens function for generating depth information; l is the number of depth layers. Finally, all the three-dimensional objects are overlapped to form a hologram of the whole three-dimensional object;

13) in order to further simplify the data, the shielded redundant data is eliminated, the calculated data is not listed, the required calculated amount is simplified, the hologram reproduction images manufactured before and after simplification are compared, and the imaging effect after simplification is not influenced.

Further, the step 2) specifically comprises the following steps:

21) applying different phase factors on the basis of calculating the hologram at each visual angle, wherein the different phase factors correspond to different projection directions; the phase factor has a phase modulating effect of the lens and can be expressed as:

in the off-axis case it can be written as:

wherein (x)₀,y₀) Is the position of the optical axis. Loading different phase factors on the calculation holograms at different visual angles;

22) and (3) realizing the phase modulation of the formula (3) by using a liquid crystal spatial light modulator according to the phase factor modulation in the step 21), and projecting each visual angle light beam to different directions in time sequence. So as to provide a separation path of each angle light beam for realizing angle deflection in the subsequent projection process;

23) controlling each light beam by using a reflector according to the separated light beams projected in the step 22) to form a horizontal light path direction.

Further, the step 3) specifically comprises the following steps:

a positive decentration lens group is built, wherein two or more lenses for realizing angle focusing are collectively called as a first lens. And a second lens, i.e. a field lens, is built to reduce light losses. And then a third lens is set up to converge all the light beams, and the field angle of the reproduced image is expanded by deflecting the light beams.

The scan range of the beam depends on the lens focal length f and the displacement Δ, which will result in a deflection angle θ, which can be expressed as follows.

Wherein, Δ is the distance of the light from the center position, and f is the focal length of the lens. The light beam passing through the lens group is deflected, and the angular deflection of the light beam reaches theta. The light beams can be controlled to realize large-angle scanning convergence to obtain a final reproduced image.

According to the invention, different holograms are loaded in a time sequence mode by using a liquid crystal spatial light modulator through a time division multiplexing method, optical paths of all visual angles are projected and separated, and a reproduced image with an expanded visual angle is obtained through a beam deflection system.

Compared with the prior art, the invention has the beneficial effects that:

the expansion of the three-dimensional image field angle is realized by using a single spatial light modulator. Compared with a multi-spatial light modulator visual angle expanding system, the system has the advantages of few expensive devices such as the spatial light modulator and the like, low cost and better application and popularization values.

And (II) mechanical scanning is not required. The light path is adjusted and controlled by utilizing the reflector and the lens group, so that the deflection of the light beam is realized. Compared with other single spatial light modulator mechanical scanning systems, the scheme reduces the complexity of the system, eliminates the problems of complex system construction, time synchronization and the like caused by a mechanical system, and is high in system stability.

And (III) the visual angle expansion principle of the spatial light modulator is combined with the multi-lens beam deflection principle, so that the system is simple in structure and strong in operability, and more importantly, the display visual angle of the holographic projection three-dimensional image can be effectively enlarged.

Drawings

FIG. 1 is a block flow diagram of a holographic projection method based on beam deflection according to the present invention;

FIG. 2 is a schematic representation of a chromatographic computed hologram;

FIG. 3 is a comparison graph of reconstruction results before and after optimization of a computed hologram;

FIG. 4 is a multi-view computed hologram generation diagram;

FIG. 5 is a multi-view computed hologram, phase map and simulated reconstruction map;

FIG. 6 is a beam deflection multi-lens group;

FIG. 7 is a schematic view of SLM multi-view deflection;

FIG. 8 is a tiling scheme of multiple SLMs, where (a) plane tiling (b) curved surface tiling;

FIG. 9 is a schematic diagram of diffraction tilt;

fig. 10 is a schematic diagram of a holographic projection display field angle expanding system.

Detailed Description

The invention will be further described by way of examples of implementation in connection with the accompanying drawings, without in any way limiting the scope of the invention.

The flow chart of the holographic projection display field angle expanding system based on beam deflection provided by the invention is shown in figure 1. In the embodiment of the present invention, the method provided by the present invention specifically includes the following steps:

1) preparing a computed hologram by using a chromatography method;

the chromatography is to layer the data and then perform Fresnel diffraction calculation. Due to visual occlusion, a lot of occluded data exists between layers, and the data cannot be observed visually in holographic projection. Therefore, based on the visual shielding effect, the redundant data are eliminated, so that the data amount required to be calculated is reduced during holographic calculation, and the aim of improving the calculation speed is fulfilled.

The invention disperses the three-dimensional object into a plurality of object planes parallel to each other along the z-axis direction (depth direction), and each layer of object plane x_j-y_jAt a distance z from holographic surface ξ - η_jAnd diffracting each layer of object surface to the holographic surface for superposition, and adding reference light for calculation to obtain a calculated hologram. Finally, the reference light or the conjugate light of the reference light is added for reproduction.

Complex amplitude of object light field in Fresnel diffraction zone, holographic surfaceExpressed as:

wherein N is the total layer number of the three-dimensional object, lambda is the wavelength, k is the wave number, k is 2 pi/lambda, ξ and η are holographic surface coordinates, and x and y are object surface coordinates;is the complex amplitude of the object plane of the ith layer,is the reference light complex amplitude.

Starting from the S-FFT algorithm of Fresnel diffraction, the object light field distribution on the holographic surface is as follows:

wherein FFT {. cndot } represents a Fourier transform. Let the pixel numbers of the holographic surface and the object surface be s × t, Δ x, Δ y be the sampling interval of the object surface, and Δ ξ, Δ η be the sampling interval of the holographic surface. At the diffraction limit of the object field, the relationship between these parameters is:

and layering the three-dimensional object along the depth direction for imaging, independently calculating the Fresnel diffraction light field distribution of each layer on the holographic surface, then completely superposing, introducing reference light for interference, and encoding to prepare the hologram of the whole three-dimensional object. The method of layered processing is relatively simple, and the method is suitable for calculating the three-dimensional object hologram with a simple shape; the introduction of the fast Fourier transform makes the Fresnel diffraction of each layer higher in calculation efficiency. The layering-based method steps are shown in figure 2. A three-dimensional object is divided into two-dimensional slice images according to a depth map of the three-dimensional object. The 2DFFT is used to create a diffraction pattern for each layer that is multiplied by the holographic lens for the focal length corresponding to the depth of the layer and then summed to create the final hologram as shown below:

in the formula 2DFFT_x,yIs a two-dimensional fast Fourier transform; lens (z)_l) A lens function for generating depth information; l is the number of depth layers.

The experimental results of the chromatographic calculation hologram are shown in figure 3. Experimental results show that the reproduction result after the optimization algorithm is adopted is not worse than the effect before optimization.

2) Making a multi-view computed hologram by using angle compensation;

the single computer hologram projected by the liquid crystal spatial light modulator can only form narrow-view-angle imaging. The angle compensation is to enlarge the 3D image field angle using a plurality of views. The total view angle is composed of a plurality of sub-views, each sub-view only occupies a narrow view angle, and a wide view angle imaging graph is composed of a plurality of sub-views.

In order to enlarge the field angle, a corresponding deflection angle is added on the surface of each calculation hologram, and the liquid crystal spatial light modulator performs time sequence projection on the holograms with different angles by using a time division multiplexing method so as to achieve the angle deflection in the projection process.

The liquid crystal spatial light modulator has a function of realizing phase modulation on light beams, and can generate the same phase modulation effect as that of the optical lens on the light beams passing through the liquid crystal spatial light modulator. For the imaging process of a point light source on an optical axis passing through a convergent lens, the distance between the point light source and a plane close to the front of the lens is set as l_oDistance between imaging point behind lens and plane next to lensIs set to be₁. The complex amplitude distribution of the divergent spherical waves emitted by the monochromatic point light sources on the front plane of the lens is as follows:

wherein k is 2 pi/λ.

After the divergent spherical wave passes through the convergent lens, the complex amplitude distribution generated on the rear plane of the lens is as follows:

in the (formula 8) and (formula 9), the phase factor exp (jkl)_o) And exp (-jkl)₁) Only constant phase transitions are represented, without affecting the relative spatial distribution of the phases on the plane. The complex amplitude transmittance function of the lens is:

according to the gaussian imaging formula:

wherein f is the focal length of the lens. So (equation 11) can be simplified as:

the phase modulation effect, i.e., the phase factor, of the lens is shown in (equation 2). In the off-axis case (equation 2) can be written as:

wherein (x)₀,y₀) Is the position of the optical axis. If the phase modulation of (equation 3) is implemented using a liquid crystal light modulator, it is equivalent to one focal length f, the axis being off-axis at the (x0, y0) positionA Fresnel lens. Different phase factors are loaded on the calculation holograms with different visual angles, the time sequence is loaded on the SLM, and the angle deflection is realized in the subsequent projection process.

The method for making the multi-view hologram loaded by the liquid crystal spatial light modulator is shown in figure 4, and a phase diagram with proper phase factors is applied to each angle. Taking three visual angles as an example, the experiment is carried out to generate the left, middle and right view point calculation holograms. That is, holograms with different angles are generated by using a computed tomography hologram algorithm, which are shown in fig. 5 (a), (b), and (c), respectively. The computed holograms at the three viewing angles are applied with different phase patterns corresponding to different phase factors, respectively, as shown in fig. 5(d) (e) (f). The generated simulated reproduction images are respectively shown in fig. 5(g) (h) (i). The generated multi-view computed hologram is projected by a liquid crystal spatial light modulator and then passes through a deflection system, and a multi-view synthesized wide-view imaging result is obtained through reconstruction.

3) Building a multi-lens beam deflection system;

in order to realize the light beam deflection function, a positive eccentricity lens set is built. The positive decentration lens group consists of three positive lenses with equal focal lengths, and the two-dimensional scanning of the light beams can be realized by moving the first positive lens in a plane perpendicular to the optical axis. Fig. 6 shows the beam deflection of a fully convex lens. The first lens and the third lens constitute a simple beam control system. The second lens is a field lens which reduces the loss of system light due to vignetting when the beam is off axis.

The deflection lens group shows the working principle of maximally deflecting the light beam. The algorithm for beam deflection can be understood by moving the first single positive lens, as shown in fig. 6. The scan range of the beam depends on the lens focal length f and the displacement Δ, which will result in a deflection angle θ, which can be expressed as follows.

Wherein, Δ is the distance of the light from the center position, and f is the focal length of the lens. The light beams are deflected by the three lenses, and the angular deflection of the light beams reaches theta.

And according to the light beam deflection system, a multi-lens system is built as required. As shown in fig. 6, the first lens is two or more lenses for realizing angular focusing, the second lens is a field lens for multi-view beams, and the third lens is a field angle expanding lens for converging and deflecting the beams. The liquid crystal spatial light modulator is matched with the light beam deflection lens group, so that the light beam can be controlled to realize large-angle scanning. The deflection system can enlarge the viewing angle of the three-dimensional image to a certain extent.

4) Holographic projection for three-dimensional reconstruction

The optical imaging part, as shown in fig. 7, uses phase deflection light to make SLM non-mechanical scanning and adjusting reconstruction beam according to time division multiplexing principle for holographic projection three-dimensional display. The liquid crystal spatial light modulator is loaded with holograms at different angles in a circulating mode, the holograms at different angles correspond to phase diagrams at different positions, and the phase diagrams are projected to corresponding directions in a time sequence mode. Taking three angles as an example, each angle corresponds to the hologram and the corresponding phase diagram of each angle, so that the light is reflected by the SLM to three paths, and then the No. 1 deflected light is reflected by the Mirro1 to obtain the horizontal projection path 1. The middle is the undeflected projection path # 2. The No. 3 deflected light is reflected by Mirro2, resulting in horizontal projection path 3. The light rays with different angles projected by the SLM pass through the reflector to deflect the light rays with different angles to the horizontal direction, the projection paths of different visual angles are distinguished, and corresponding wide projection paths are provided for a following light beam deflection system.

Is usually defined as the maximum diffraction angle theta of a single SLM_dmaxWhen the light illuminates the SLM, its viewing angle is θ:

θ＝2θ_dmax2arcsin (λ/2p) (formula 12)

Where λ is the wavelength of the incident light and p is the pixel pitch of the SLM. For a fixed wavelength, a larger field of view requires a smaller pixel pitch, but this is already limited by the device parameters of the SLM. The single SLM display angle theta is limited and multiple SLMs can increase the display information. However, the planar structure of the SLM array shown in fig. 8 cannot expand the viewing angle because the viewing angle is determined by the pixel pitch.

The curved SLM array can enlarge the field angle, as shown in the curved surface splicing mode shown in figure 8, if N SLMs construct an arc array and form an inclination alpha (theta), the total field angle can reach

Theta + (N-1) alpha (formula 13)

The total field angle is subject to the intrinsic diffraction angle θ, the number of SLMs N, and the angle of deflection α. However, maintaining proper illumination of each SLM requires precise deflection configurations of the SLM and beam. This increases the difficulty of system construction. In practice, the essence of a curved SLM array is to tilt the outgoing light from each SLM relative to each other. If multiple SLMs are configured in the same plane, but the diffraction is oriented in a tilted manner, the effect of enlarging the field angle is equivalent to a curved SLM array.

Such as the oblique illumination of the one-dimensional grating of fig. 9. From the raster equation, one can derive

Where λ is the wavelength and 2p is the grating period. Theta_inAnd theta_outIncident angle and exit angle. For practical situations, a small angle approximation is true. So sin theta and lambda/2 p are approximately equal to theta_dmaxI.e. the diffraction angle as normal illumination. Thus, the viewing angle of the SLM is the same in both cases. However, the image position of the oblique illumination may have an additional rotation angle θ_inIt is equivalent to rotating the SLM θ_in. In fact, oblique illumination corresponds to the addition of a θ to the SLM surface_inThe phase of (c). If we can apply a suitable phase factor to the SLM, it can achieve the same field angle magnification effect as the curved SLM array, while reducing the number of SLMs required.

Combining the SLM multi-view deflection section and the multi-lens beam deflection system as shown in FIG. 10, different angle holograms are loaded by the SLM corresponding to different phase factor phase patterns. Therefore, by the time division multiplexing method, the reflected light rays of the projection imaging at different moments are reflected and separated, the reflected light rays at two sides pass through the reflectors at all sides, and the inclined projection light rays are reflected into emergent light rays which are horizontally separated. The first Lens1 is a single Lens or multiple lenses, and the separated light rays corresponding to the holograms at each angle are respectively converged by the first Lens1 according to the requirements of the emergent light rays. The second Lens2 is a field Lens for reducing light loss, and finally, the light is gathered in the third Lens3 to form a wide-angle imaging effect. A large field angle reproduced image is presented.

For a liquid crystal spatial light modulator with a pixel pitch of 8 microns, the field angle is only 3.8 if illuminated with 532nm green light. By adopting the method, if the focal lengths of the lenses in the Lens group are all 100mm, and the offset of each Lens in Lens1 from the center position is 9mm, the angle of view can be enlarged to 10.3 ° according to (equation 4). The viewing angle expansion range can be further increased based on different system parameters. Verification shows that the three-dimensional reproduction image viewing angle can be effectively enlarged by the method.

It is noted that the disclosed embodiments are intended to aid in further understanding of the invention, but those skilled in the art will appreciate that: various substitutions and modifications are possible without departing from the spirit and scope of the invention and appended claims. Therefore, the invention should not be limited to the embodiments disclosed, but the scope of the invention is defined by the appended claims.

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