Full-Color See-Through Three-Dimensional Display Method Based on Volume Holography

sensors

Article Full-Color See-Through Three-Dimensional Display Method Based on Volume Holography

Taihui Wu 1,2 , Jianshe Ma 2, Chengchen Wang 1,2, Haibei Wang 2 and Ping Su 2,*

1 Department of Precision Instrument, Tsinghua University, Beijing 100084, China; [email protected] (T.W.); [email protected] (C.W.) 2 Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China; [email protected] (J.M.); [email protected] (H.W.) * Correspondence: [email protected]

Abstract: We propose a full-color see-through three-dimensional (3D) display method based on volume holography. This method is based on real object interference, avoiding the device limitation of spatial light modulator (SLM). The volume holography has a slim and compact structure, which realizes 3D display through one single layer of photopolymer. We analyzed the recording mechanism of volume holographic gratings, diffraction characteristics, and influencing factors of refractive index modulation through Kogelnik’s coupled-wave theory and the monomer diffusion model of photopolymer. We built a multiplexing full-color reflective volume holographic recording optical system and conducted simultaneous exposure experiment. Under the illumination of white light, full-color 3D image can be reconstructed. Experimental results show that the average diffraction efficiency is about 53%, and the grating fringe pitch is less than 0.3 µm. The reconstructed image of volume holography has high diffraction efficiency, high resolution, strong stereo perception, and large observing angle, which provides a technical reference for augmented reality.

Citation: Wu, T.; Ma, J.; Wang, C.; Keywords: holographic display; volume holography; photopolymer; augmented reality Wang, H.; Su, P. Full-Color See-Through Three-Dimensional Display Method Based on Volume Holography. Sensors 2021, 21, 2698. 1. Introduction https://doi.org/10.3390/s21082698 When the movie Avatar was released in 2009, people were deeply impressed by the 3D display. With the vigorous development of augmented reality (AR), new application Academic Editor: Gregory P. Nordin scenarios have been brought to 3D display. Augmented reality has been widely used in medical, education, industry, entertainment, and construction ﬁelds [1,2]. Head-mounted Received: 16 March 2021 displays (HMDs) are common AR devices. However, many HMDs are based on the Accepted: 9 April 2021 Published: 11 April 2021 stereoscopic display technology of binocular parallax, which belongs to pseudo 3D display and cannot completely reproduce the continuous wavefront information of the object.

Publisher’s Note: MDPI stays neutral In addition, binocular parallax causes the problem of convergence-accommodation conflicts, with regard to jurisdictional claims in which can easily cause visual fatigue [3,4]. Among various 3D display technologies, published maps and institutional affil- holography can reconstruct the amplitude and phase information required by the human iations. eye with high quality, which has become an important research hotspot in AR display field. The holographic 3D display records the wavefront information of the object through the interference, and realizes the reproduction of the object by illuminating the hologram. This technology can retain the depth information of the object and is considered to be one of the most promising 3D display technologies. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Currently, holographic 3D displays have some typical problems, such as speckle noise, This article is an open access article resolution and spatial bandwidth product limitations, and so on. In addition, due to the distributed under the terms and small diffraction angle and the number of pixels of the spatial light modulator (SLM), conditions of the Creative Commons the observing angle and image size of the holographic 3D display are limited. Large Attribution (CC BY) license (https:// perspective and full-color display are important technical difficulties for current holo- creativecommons.org/licenses/by/ graphic display. In response to these problems, many researchers have proposed different 4.0/). 3D holographic display solutions. Juan Liu et al. [5] proposed a full-color transparent

Sensors 2021, 21, 2698. https://doi.org/10.3390/s21082698 https://www.mdpi.com/journal/sensors Sensors 2021, 21, 2698 2 of 13

near-eye holographic display, which has an 80◦ field of view and an extended eye box. The system has a large field of view and eliminates crosstalk. Hongyue Gao et al. [6] proposed a holographic 3D virtual reality and augmented reality display method based on a 4K SLM. The viewing angle of the display was expanded from 3.8◦ to 16.4◦ through SLM splicing. Gang Li et al. [7] used holographic optical elements (HOE) to realize a see- through augmented reality holographic display, which has the functions of a mirror and a lens at the same time. Koki Wakunami et al. [8] proposed a projection-type see-through holographic 3D display, HOE produced by this method has a wide range of application potential in smart glasses, HMD, and vehicle displays. Seung-Ho Shin et al. [9] proposed a coherent 3D display method through optical refraction volume holographic storage and integral imaging, which effectively reduces speckle noise. Takeshi Yamaguchi et al. [10] designed a volume holographic printer to record 3D object images, segment 3D objects through multiple SLMs, and use 4f optical system to improve image reconstruction quality. Keehoon Hong et al. [11] proposed a hogel overlay method to enhance the lateral resolution of holographic stereograms. Jinyoung Roh et al. [12] designed a full-color holographic projection display system using an achromatic Fourier filter. Zhenxiang Zeng et al. [13] designed a full-color holographic display that uses a layered Fresnel diffraction algorithm to generate RGB (Red, Green, Blue) phase-only holograms. Mei-lan Piao et al. [14] proposed a full-color holographic diffuser with time-sharing and iterative exposure. This method is based on the wavelength multiplexing characteristics of volume holographic gratings. The three-wavelength hologram is recorded on a holographic dry plate to achieve a specific color balance of the multicolor holographic diffuser. In this research, we propose a full-color see-through 3D display method based on volume holography. Since the diffraction angle and the number of pixels of the SLM are small, we use real object for interference recording. This approach avoids the device limitations caused by SLM, such as the small diffraction angle and the number of pixels. We use the simplified holographic grating diffusion model to analyze the influencing factors of refractive index modulation, including exposure intensity, exposure energy and diffusion time constant. Subsequently, the transmission spectrum and diffraction efficiency of the hologram were analyzed through experiments. Finally, the RGB multiplexing reflective volume holography recording optical system was built. Experimental results show that the RGB multiplexing optical setup can record full-color 3D image by only one single layer of photopolymer. The thickness of the holographic display film is only 16 µm, which can be applied to the next generation of integrated displays. The reconstructed image of volume holography has high resolution, strong stereoscopic effect and large observing angle, and realizes full-color 3D display. The paper is arranged as follows: In Section2, related theories are introduced, including Bragg’s law, Kogelnik’s coupled wave theory and the monomer diffusion model. In Section3, the transmission spectrum and diffraction efficiency of the photopolymer were tested through experiments. In the Section4, the RGB multiplexing full-color reflective volume holographic recording optical setup is built, and the simultaneous exposure experiment were carried out. Section5 is a summary of the full text.

2. Design Theory and Method 2.1. Coupled Wave Theory Volume holographic gratings can reproduce the wavefront information of real 3D object. The principle of volume holographic 3D display is based on the interference, and the diffraction phenomenon of volume holography occurs under the irradiation of reproduced light. When the grating constant, the Bragg angle and the wavelength of the reproduced light satisfy Bragg’s law, the volume holographic grating exhibits the strongest diffraction efﬁciency. The equation of Bragg’s law is as follows [15]:

2nΛ sin θ0 = λ, (1) Sensors 2021, 21, 2698 3 of 13

1 SF = , (2) Λ

where Λ represents the grating constant, and θ0 represents the Bragg angle. λ is the wavelength of reproduced beam in vacuum. n represents the refractive index of the medium. SF represents the grating spatial frequency. It can be known from Bragg’s law that when the Bragg angle is 90◦, the grating spatial frequency is the largest. In this case, the angle between the reference light and the object light is 180◦, and the plane of the recording medium bisects the reference light and the object light vertically. Kogelnik’s coupled wave theory reveals the influence of factors on the diffraction efficiency in volume holography. The diffraction efficiency of non-absorbent reflective volume holographic gratings can be calculated according to the coupled wave theory [16]: p sh2 ν2 − ξ2 η = , (3) R p 2 sh2 2 − 2 + ( − ξ ) ν ξ 1 ν2 πd∆n ν = √ , (4) λ cos θr cos θs 2 ξ = kd∆θ sin(φ − θ0)/2 cos θs − k d∆λ/8πn cos θs, (5)

where ηR represents the diffraction efficiency of the non-absorption reflective volume holographic grating. n is the refractive index of the medium. λ is the wavelength of incident light. v represents the coupling strength of the volume holographic grating. ξ represents the Bragg mismatch. d is the thickness of the recording medium. ∆n represents the refractive index modulation. θr is the angle between the reproduced light wave and the Z axis in the dielectric material. θs is the angle between the diffracted light wave and the Z axis in the dielectric material. ∆θ is the angular offset. ∆λ is the wavelength shift. k = 2π/Λ, which represents the size of the grating vector. φ is the tilt angle of the grating. Sorting out the above equations, the refractive index modulation of the volume holographic grating when the Bragg condition is satisfied can be calculated by the diffraction efficiency: p √ λ arctanh(η ) cos θr cos θs ∆n = R , (6) πd Setting the angle between the reference light and the object light is 180◦ and the thickness of the recording medium is 16 µm, we simulated the relationship between the refractive index modulation and the diffraction efficiency in the reflector holographic grating. As shown in Figure1, the diffraction efficiency increases with the increase in refractive index modulation, and the growth rate gradually tends to be flat. When the refractive index modulation is greater than 0.01, the diffraction efficiency of each wavelength exceeds 70%. When the refractive index modulation is greater than 0.03, the diffraction efficiency of each wavelength is close to 100%. In order to improve the diffraction efficiency of volume holographic gratings, it is necessary to increase the refractive index modulation as much as possible. Therefore, it is necessary to analyze the influencing factors of the refractive index modulation of the volume holographic grating. Next, we will analyze the refractive index modulation through the monomer diffusion model of the holographic grating.

which reduces the monomer concentration in the recording area. Since the monomer polymerization speed in the bright stripe area is faster, the monomer concentration in the bright stripe area is lower than that in the dark stripe area. A concentration gradient is formed between the dark area and the bright area. Under the effect of the distribution gradient, the monomer molecules continuously diffuse from the dark stripe area to the light stripe area. The above process leads to changes in the refractive index of different regions in the photopolymer, resulting in refractive index modulation, and ﬁnally forming a volume holographic diffraction phenomenon. In order to describe the above process, in 1994, Zhao proposed a classic photopolymer grating diffusion model [18]:

∂u(x, t) ∂ ∂u(x, t) = D(x, t) − F(x, t)u(x, t), (7) ∂t ∂x ∂x

wherein u(x, t) represents the monomer concentration, F(x, t) represents the polymerization rate, and D(x, t) represents the diffusion coefficient. Under the illumination of interference of the object and reference waves in holographic exposure, the monomer and polymer present a periodic distribution in the material. Therefore, the monomer concentration can be expressed by Fourier series. The change of the diffusion coefficient is determined by the distribution of the polymer in the medium, so the diffusion coefficient can also be expressed by a Fourier series. The expressions of diffusion coefficient and monomer concentration by Fourier series are

∞ D(x, t) = ∑ Di(t) cos(iKx), (8) i=0

∞ u(x, t) = ∑ ui(t) cos(iKx), (9) i=0 Sensors 2021, 21, x FOR PEER REVIEW 4 of 14 According to Equations (7)–(9), a series of differential equations can be obtained by separating the variables, and Zhao conducted numerical simulations based on this. The simulation results show that during the formation of volume holographic gratings, the diffusion coefficient is mainly determined by the zero-order term in the Fourier series refractiveexpansion, index and the modulation monomer concentration through the is mainlymonomer determined diffusion by themodel first twoof the terms holographic grating.of the Fourier series expansion. Therefore, the diffusion coefficient can be approximately expressed as a constant.

FigureFigure 1. 1. ReflectiveReﬂective volumevolume holographic holographic grating: grating: the relationship the relationship between between refractive indexrefractive modula- index modu- lationtion andand diffraction diffraction efﬁciency. efficiency.

2.2. Monomer Diffusion Model Photopolymer does not require any chemical or heat treatment [17], and can perform RGB full-color and depth recording, with low detuning, small shrinkage, high resolution, and high diffraction efficiency. These excellent properties have made photopolymers re- ceive more and more attentions. In this study, photopolymer was used as the recording medium of volume holographic grating. The photopolymer is a type of free radical polymerization, and during interference exposure, a photochemical reaction occurs inside it. Holographic exposure results in the formation of light and dark interference fringes in the recording area. The monomer molecules are polymerized to form polymer chains, which reduces the monomer concentration in the recording area. Since the monomer polymerization speed in the bright stripe area is faster, the monomer concentration in the bright stripe area is lower than that in the dark stripe area. A concentration gradient is formed between the dark area and the bright area. Under the effect of the distribution gradient, the monomer molecules continuously diffuse from the dark stripe area to the light stripe area. The above process leads to changes in the refractive index of different regions in the photopolymer, resulting in refractive index modulation, and finally forming a volume holographic diffraction phenomenon. In order to describe the above process, in 1994, Zhao proposed a classic photopolymer grating diffusion model [18]: ∂∂∂ uxt(,)=− uxt (,) Dxt(,) Fxtuxt (,)(,), (7) ∂∂tx ∂ x

wherein (, ) represents the monomer concentration, (, ) represents the polymerization rate, and (, ) represents the diffusion coefficient. Under the illumination of interference of the object and reference waves in holographic exposure, the monomer and polymer present a periodic distribution in the material. Therefore, the monomer concentration can be expressed by Fourier series. The change of the diffusion coefficient is determined by the distribution of the polymer in the medium, so the diffusion coefficient can also be expressed by a Fourier series. The expressions of diffusion coefficient and monomer concentration by Fourier series are

Sensors 2021, 21, 2698 5 of 13

However, Zhao finally did not give an analytical solution to the monomer diffusion model. Subsequently, Pengfei Liu et al. [19] established a simplified diffusion model of the formation process of holographic gratings based on the Zhao diffusion model. This model comprehensively considers the influence of monomer polymerization and diffusion on the refractive index of the dielectric material, and finally obtains the refractive index modulation of the volume holographic grating [19]:

 δ +  δ (κI0 τ 1)t  1 + κI0 τ exp − τ   δ  ∆n(t) = mCnU −δ exp −I0 κt + δ , (10)  1 + κIδτ   0 

wherein Cn is the proportional coefficient. m is the degree of modulation of interference fringes. U is the initial monomer concentration. δ is the order of light response. I0 is the exposure intensity. τ = 1/Dk2 represents the diffusion time constant. D is the diffusion coefficient and κ is the polymerization coefficient, which are constants related to the material. When t → ∞ , that is, when the recording time is long enough, the above equation is simplified, and the saturation refractive index modulation of the material can be obtained:

mC U = n ∆nSAT δ , (11) δ(κI0 τ + 1) According to the theory of radical polymerization [20,21], we set δ as 1/2. It can be seen from this equation that the main factors affecting the saturation refractive index modulation are exposure light intensity and diffusion time constant. Through the numerical simulation of the equations, the qualitative inﬂuence of the two parameters on the refractive index modulation is analyzed. Since mCnU is a constant in the equation, ∆n(t)/(CnmU) is selected as the ordinate, and the exposure energy is the abscissa. In the numerical simulation, we set the polymerization coefﬁcient κ = 0.15 s−1mW−1/2cm, the diffusion time constant τ = 0.2 s, and the exposure energy change range is 0~100 mW/cm2. It can be seen Sensors 2021, 21, x FOR PEER REVIEW from the Figure2 that as the exposure energy increases, the refractive index modulation 6 of 14

gradually increases. With the recording light intensity increases, the saturation refractive index modulation gradually decreases. The greater the light intensity, the faster the refractive index modulation increases over time; that is, the exposure sensitivity increases. U) n Δn(t)/(mC

(a) (b)

FigureFigure 2. Analysis 2. Analysis of offactors factors affecting affecting th thee refractive refractive indexindex modulation: modulation: (a) ( exposurea) exposure time; time; (b) exposure (b) exposure energy. energy.

ItIt can can be be seen seen from from the the Figure Figure 33 thatthat asas thethe diffusiondiffusion time time constant constant increases, increases, the the sat- saturation refractive index modulation gradually decreases. Since the diffusion time uration refractive index modulation gradually decreases. Since the diffusion time constant =1/, =2∙, and D is a constant, the diffusion time constant is determined by the spatial frequency. The greater the spatial frequency, the smaller the diffusion time constant, which ultimately leads to an increase in the saturation refractive index modulation. As the spatial frequency increases, the fringe spacing decreases, and the distance of monomer diffusion becomes shorter, which makes the monomer polymerization reaction in the bright fringe area faster, and the refractive index modulation increases rapidly. Therefore, in order to increase the saturation refractive index modulation, a larger grating spatial frequency is required.

0.5

0.45

0.4

0.35

0.3

0.25 =1 s 0.2 =2 s =3 s 0.15 =4 s

0.1

0.05

0 0 102030405060 2 Exposure energy (mJ/cm ) Figure 3. Analysis of factors affecting refractive index modulation: diffusion time constant.

3. Tests of Volume Holography In Section 2, we analyzed the relationship between diffraction efficiency and refractive index modulation of reflective volume holograms. In order to improve the diffraction efficiency, we conducted a qualitative analysis of its influencing factors, including exposure time, exposure intensity, and diffusion time constant. We optimized the exposure parameters through the above theoretical analysis. In order to analyze the transmission spectrum and diffraction characteristics of the photopolymer, the following section will

Sensors 2021, 21, x FOR PEER REVIEW 6 of 14

U) n Δn(t)/(mC

(a) (b) Figure 2. Analysis of factors affecting the refractive index modulation: (a) exposure time; (b) exposure energy.

Sensors 2021, 21, 2698 6 of 13 It can be seen from the Figure 3 that as the diffusion time constant increases, the saturation refractive index modulation gradually decreases. Since the diffusion time constant =1/, =2∙, and D is a constant, the diffusion time constant is determined by theconstant spatialτ frequency.= 1/Dk2 ,Thek = greater2π·SF ,the and spatiaD isl afrequency, constant, the the smaller diffusion the time diffusion constant time is constant,determined which by ultimately the spatial leads frequency. to an increase The greater in the the saturation spatial frequency, refractive index the smaller modula- the tion.diffusion As the time spatial constant, frequency which increases, ultimately the leads fringe to spacing an increase decreases, in the saturationand the distance refractive of monomerindex modulation. diffusion Asbecomes the spatial shorter, frequency which increases,makes the the monomer fringe spacing polymerization decreases, reaction and the indistance the bright of monomer fringe area diffusion faster, becomes and the shorter, refractive which index makes modulation the monomer increases polymerization rapidly. Therefore,reaction in in theorder bright to increase fringe areathe saturation faster, and refractive the refractive index modulation, index modulation a larger increases grating spatialrapidly. frequency Therefore, is inrequired. order to increase the saturation refractive index modulation, a larger grating spatial frequency is required.

0.5

0.45

0.4

0.35

0.3

0.25 =1 s 0.2 =2 s =3 s 0.15 =4 s

0.1

0.05

0 0 102030405060 2 Exposure energy (mJ/cm ) FigureFigure 3. 3. AnalysisAnalysis of of factors factors affecting affecting refractive refractive index index modulation: modulation: diffusion diffusion time time constant. constant.

3.3. Tests Tests of of Volume Volume Holography Holography InIn Section Section 22,, wewe analyzedanalyzed thethe relationship relationship between between diffraction diffraction efficiency efficiency and and refractive refrac- tiveindex index modulation modulation of of reflective reflective volume volume holograms. holograms. In In orderorder toto improve the the diffraction diffraction efficiency,efficiency, we conducted aa qualitativequalitative analysis analysis of of its its influencing influencing factors, factors, including including exposure expo- suretime, time, exposure exposure intensity, intensity, and diffusion and diffusio timen constant. time constant. We optimized We optimized the exposure the exposure parame- parametersters through through the above the theoreticalabove theoretical analysis. anal In orderysis. In to order analyze to theanalyze transmission the transmission spectrum spectrumand diffraction and diffraction characteristics characteristics of the photopolymer, of the photopolymer, the following the sectionfollowing will section conduct will the test experiment of the reflective volume holograms on a photopolymer substrate, focusing on the analysis of diffraction efficiency and the recording effect of 3D image. We use CRT20 holographic film made by Litiholo, which is a photopolymer compos- ite material. The holographic film consists of two layers, the upper layer is a cellulose triacetate film (TAC) protective layer with a thickness of about 60 µm; the lower layer is a photopolymer with a thickness of about 16 µm. The material has a relatively wide sensitive band, and is sensitive to the visible spectral wavelength within 440–680 nm. The material can be used to record reflective volume transmission and reflection volume holograms, with simple operation, and no need for subsequent heat treatment or wet treatment. The spectral diffraction efficiency can be greater than 95%, the spectral bandwidth is greater than 15 nm, and the spectral shift is about 8 nm [22]. The material can achieve greater diffraction efficiency through reflection holographic recording. The diffraction efficiency is calculated by measuring the transmittance with a spectrophotometer. The calculation equation is as follows [23]: T − T η = max min × 100%, (12) Tmax

where in Tmin represents the minimum value of transmittance in the test band, and Tmax represents the sum of the transmitted light intensity and the diffracted light intensity of the material. (Tmax − Tmin) represents the diffracted light intensity of the material. Tmin is the trough of the transmission spectrum curve, and the wavelength shift occurs due to the shrinkage of the holographic recording material and the change in refractive index [23]. Sensors 2021, 21, x FOR PEER REVIEW 7 of 14

conduct the test experiment of the reflective volume holograms on a photopolymer substrate, focusing on the analysis of diffraction efficiency and the recording effect of 3D image. We use CRT20 holographic film made by Litiholo, which is a photopolymer compo- site material. The holographic film consists of two layers, the upper layer is a cellulose triacetate film (TAC) protective layer with a thickness of about 60 μm; the lower layer is a photopolymer with a thickness of about 16 μm. The material has a relatively wide sensitive band, and is sensitive to the visible spectral wavelength within 440–680 nm. The material can be used to record reflective volume transmission and reflection volume holograms, with simple operation, and no need for subsequent heat treatment or wet treatment. The spectral diffraction efficiency can be greater than 95%, the spectral bandwidth is greater than 15 nm, and the spectral shift is about 8 nm [22]. The material can achieve greater diffraction efficiency through reflection holographic recording. The diffraction efficiency is calculated by measuring the transmittance with a spectrophotometer. The calculation equation is as follows [23]: − TTmax min η=× 100% , (12) Tmax

where in represents the minimum value of transmittance in the test band, and represents the sum of the transmitted light intensity and the diffracted light intensity of the material. ( ) represents the diffracted light intensity of the material. is the trough of the transmission spectrum curve, and the wavelength shift occurs due to Sensors 2021, 21, 2698 7 of 13 the shrinkage of the holographic recording material and the change in refractive index [23]. Next, in order to test the diffraction efficiency of the material, as shown in Figure 4, a verticalNext, reflective in order optical to test thesetup diffraction was used efficiency for interference of the material, processing. as shown A laser in is Figure used4 for, a interferencevertical reflective (532 nm), optical the laser setup is was irradiated used for vertically interference on the processing. surface of the A laser photopolymer is used for asinterference the reference. (532 Then, nm), thethe laser laser is beam irradiated that pa verticallysses through on the the surface material of the is photopolymerirradiated verti- as callythe reference. on the mirror, Then, theand laser the light beam reflected that passes bythrough the mirror the materialis irradiated is irradiated vertically vertically on the surfaceon the mirror,of the material and the as light the reflected object. The by an thegle mirror between is irradiated the object vertically light and on the the reference surface lightof the is material180°, and as the the two object. beams The interfere angle between in the material. the object The light used and exposure the reference light lightinten- is ◦ sity180 is,, and the theexposure two beams time interfereis 4 min, in the the S material.polarized The light used is used exposure for interference, light intensity the is,dark the reactionexposure is time4 min is after 4 min, the the exposure, S polarized and lightthe mercury is used forlamp interference, is irradiated the for dark 2 min reaction for cur- is ing.4 min after the exposure, and the mercury lamp is irradiated for 2 min for curing.

Figure 4. Vertical reﬂection type holographic recording optical setup. Figure 4. Vertical reflection type holographic recording optical setup. In order to measure the absorption characteristics of the photopolymer, the trans- missionIn order spectrum to measure of the the material absorption was measured characteristics with aof spectrophotometer. the photopolymer, the The transmis- model of sionthe spectrophotometerspectrum of the material is U-4100 was Spectrophotometer measured with a spectrophotometer. (Liquid), the scanning The rangemodel is of from the spectrophotometer400 to 800 nm, and is the U-4100 scanning Spectrophotometer speed is 300 nm/min. (Liquid), Thethe scanning cured material range wasis from placed 400 nmin ato spectrophotometer 800 nm, and the scanning to measure speed the is 30 transmission0 nm/min. The spectrum cured material of the material, was placed and in the a spectrophotometerexperimental results to are measure shown the in Figuretransmission5. As shown spectrum in Figure of the5, thematerial, baseline and in the the experi- ﬁgure Sensors 2021, 21, x FOR PEER REVIEWmental results are shown in Figure 5. As shown in Figure 5, the baseline in the figure8 of 14is is used to approximate the transmittance of the bleached sample without holographic used to approximate the transmittance of the bleached sample without holographic re- recording. Tmax is expressed by the ordinate of the intersection of Tmin and the baseline. cording. is expressed by the ordinate of the intersection of and the baseline.

FigureFigure 5. 5. TheThe transmission transmission spectrum spectrum of of the the Litiholo Litiholo holographic holographic material material after after interference. interference. It can be seen from Figure5 that the transmittance of the volume holographic grating It can be seen from Figure 5 that the transmittance of the volume holographic grating near the 532 nm position is the smallest, at the trough position of the entire curve, and near the 532 nm position is the smallest, at the trough position of the entire curve, and the the transmittance at this time is 4.8%. The light transmittance of laser light deviating transmittance at this time is 4.8%. The light transmittance of laser light deviating from 532 from 532 nm in the material will increase sharply, and the diffraction efficiency will drop nm in the material will increase sharply, and the diffraction efficiency will drop sharply, sharply, which shows the wavelength selectivity of volume holographic gratings. In the which shows the wavelength selectivity of volume holographic gratings. In the entire entire curve, the maximum light transmittance is about 61%. According to Equation (12), curve, the maximum light transmittance is about 61%. According to Equation (12), the the diffraction efficiency is 92%. This shows that volume holographic gratings have high diffraction efficiency is 92%. This shows that volume holographic gratings have high dif- diffraction efficiency. fraction efficiency. As shown in Figure6, in order to test the 3D reconstruction effect of the volume As shown in Figure 6, in order to test the 3D reconstruction effect of the volume ho- holography, we built a monochromatic reflective volume holographic recording optical lography, we built a monochromatic reflective volume holographic recording optical setup. A 3D physical object (piggy, 27 mm × 27 mm × 34 mm) is used for interference recording, and a green longitudinal mode laser (532 nm) is used as the coherent light source. The experimental procedure consists of three stages, which are coherent light interference recording, dark reaction, and mercury lamp irradiation curing.

Figure 6. Monochromatic reflective volume holographic recording optical setup.

The reference light is irradiated on the surface of the object through the photopolymer, and the object light is generated by reflected diffusion. The angle between the reference light and the object light is 180°. The exposure intensity of the reference light is 1 mW/cm2, the exposure time is 4 min, the dark reaction time is 4 min, and the mercury lamp curing time is 2 min. The recontruction results of the recorded hologram under white light illumination are shown in Figure 7.

Sensors 2021, 21, x FOR PEER REVIEW 8 of 14

Figure 5. The transmission spectrum of the Litiholo holographic material after interference.

It can be seen from Figure 5 that the transmittance of the volume holographic grating near the 532 nm position is the smallest, at the trough position of the entire curve, and the transmittance at this time is 4.8%. The light transmittance of laser light deviating from 532 nm in the material will increase sharply, and the diffraction efficiency will drop sharply, which shows the wavelength selectivity of volume holographic gratings. In the entire curve, the maximum light transmittance is about 61%. According to Equation (12), the Sensors 2021, 21, 2698 diffraction efficiency is 92%. This shows that volume holographic gratings have high8 dif- of 13 fraction efficiency. As shown in Figure 6, in order to test the 3D reconstruction effect of the volume holography, we built a monochromatic reflective volume holographic recording optical setup.setup. A A 3D 3D physical physical object object (piggy, (piggy, 27 27 mm mm × 2727 mm mm ×× 3434 mm) mm) is is used used for for interference interference recording,recording, and and aa greengreen longitudinal longitudinal mode mode laser laser (532 (532 nm) nm) is used is used as the as coherent the coherent light source. light source.The experimental The experimental procedure procedure consists consists of three of stages, three whichstages, are which coherent are coherent light interference light in- terferencerecording, recording, dark reaction, dark andreaction, mercury and lamp mercury irradiation lamp irradiation curing. curing.

Figure 6. Monochromatic reﬂective volume holographic recording optical setup. Figure 6. Monochromatic reflective volume holographic recording optical setup. The reference light is irradiated on the surface of the object through the photopolymer, andThe the objectreference light light is generated is irradiated by reﬂectedon the su diffusion.rface of the The object angle through between the the photopoly- reference mer,light and and the the object object light light is generated 180◦. The exposureby reflected intensity diffusion. of the The reference angle between light is 1 the mW/cm refer-2, encethe exposurelight and timethe object is 4 min, light the is dark 180°. reaction The ex timeposure is 4intensity min, and of the the mercury reference lamp light curing is 1 Sensors 2021, 21, x FOR PEER REVIEW 2 9 of 14 mW/cmtime is, 2 the min. exposure The recontruction time is 4 min, results the dark of the reaction recorded time hologram is 4 min, underand the white mercury light lampillumination curing time are shownis 2 min. in Th Figuree recontruction7. results of the recorded hologram under white light illumination are shown in Figure 7.

FigureFigure 7.7. VolumeVolume holographyholography reproducesreproduces monochromatic monochromatic 3D 3D objects objects under under white white light. light.

TheThe recordedrecorded volume volume hologram hologram can can clearly clearly reproduce reproduce the the 3D piggy3D piggy under under the illumina- the illu- tionmination of white of light.white Observedlight. Observed from three from different three different angles on angles the left, on middle, the left, and middle, right, and the reconstructedright, the reconstructed images all haveimages obvious all have depth obvious information. depth information. It can be observed It can thatbe observed the text information,that the text “Hi,information, Lucas!”, in“Hi, the Lucas!”, reconstructed in the image,reconstructed which has image, high which resolution has high and goodreso- 3Dlution display and good effect. 3D As display the volume effect. hologram As the volume has wavelength hologram selectivity,has wavelength the illuminated selectivity, lightthe illuminated with a wavelength light with band a wavelength that deviates band from that the deviates recording from wavelength the recording will wavelength experience awill sharp experience drop in diffractiona sharp drop efficiency. in diffraction Therefore, efficiency. the virtual Therefore, image the observed virtual in image the pho- ob- topolymerserved in the is green.photopolymer In addition, is green. due In to additi the angularon, due selectivity to the angular of the selectivity volume hologram,of the vol- itume needs hologram, to be illuminated it needs to at be a specificilluminated angle at (thea specific direction angle of (the the originaldirection reference of the original light). Thereference angle deviationlight). The will angle cause deviation a sharp will decrease cause of a thesharp diffraction decrease efficiency. of the diffraction Measured effi- by aciency. spectrophotometer, Measured by a the spectrophotometer, diffraction efficiency the isdiffraction 80%, indicating efficiency that is the80%, monochromatic indicating that reflectivethe monochromatic volume hologram reflective display volume has hologram a high diffraction display has efficiency. a high diffraction efficiency. 4. Experimental Results and Discussion 4. Experimental Results and Discussion The above experiments prove that the photopolymer has good volume holographic The above experiments prove that the photopolymer has good volume holographic diffraction performance. According to the principle of three primary colors, most monochro- diffraction performance. According to the principle of three primary colors, most mono- matic lights can be synthesized in different proportions with the three primary colors of chromatic lights can be synthesized in different proportions with the three primary colors RGB. In order to achieve full-color 3D recording, we designed the RGB multiplexing optical of RGB. In order to achieve full-color 3D recording, we designed the RGB multiplexing setup. Figure8 shows the schematic diagram and physical diagram of the optical setup. optical setup. Figure 8 shows the schematic diagram and physical diagram of the optical setup.

Laser Laser Laser 532 nm 639 nm 473 nm

Attenuator Attenuator Attenuator Electronic Electronic Electronic shutter shutter shutter Mirror

Mirror Dichroic Dichroic Mirror 1 Mirror 2 Photopolymer Diaphragm HWP Full color 3D object Spatial filter PBS

Collimator

(a) (b)

Figure 8. Construction of the RGB multiplexing optical setup: (a) schematic diagram of the interference optical setup; (b) physical diagram of the interference optical setup.

The lasers used in this research are the single longitudinal mode lasers of Changchun New Industry Optoelectronics Technology Co., Ltd., which are blue light (473 nm), green light (532 nm), and red light (639 nm). The coherence length of the three lasers is greater than 10 m, the spectral line width is less than 0.00004 nm, the beam divergence angle (full angle) is less than 3 mrad, with single longitudinal mode and continuous wave (CW).

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Figure 7. Volume holography reproduces monochromatic 3D objects under white light.

The recorded volume hologram can clearly reproduce the 3D piggy under the illumination of white light. Observed from three different angles on the left, middle, and right, the reconstructed images all have obvious depth information. It can be observed that the text information, “Hi, Lucas!”, in the reconstructed image, which has high resolution and good 3D display effect. As the volume hologram has wavelength selectivity, the illuminated light with a wavelength band that deviates from the recording wavelength will experience a sharp drop in diffraction efficiency. Therefore, the virtual image observed in the photopolymer is green. In addition, due to the angular selectivity of the volume hologram, it needs to be illuminated at a specific angle (the direction of the original reference light). The angle deviation will cause a sharp decrease of the diffraction efficiency. Measured by a spectrophotometer, the diffraction efficiency is 80%, indicating that the monochromatic reflective volume hologram display has a high diffraction efficiency.

4. Experimental Results and Discussion The above experiments prove that the photopolymer has good volume holographic diffraction performance. According to the principle of three primary colors, most monochromatic lights can be synthesized in different proportions with the three primary colors Sensors 2021, 21, 2698 of RGB. In order to achieve full-color 3D recording, we designed the RGB multiplexing9 of 13 optical setup. Figure 8 shows the schematic diagram and physical diagram of the optical setup.

Laser Laser Laser 532 nm 639 nm 473 nm

Attenuator Attenuator Attenuator Electronic Electronic Electronic shutter shutter shutter Mirror

Mirror Dichroic Dichroic Mirror 1 Mirror 2 Photopolymer Diaphragm HWP Full color 3D object Spatial filter PBS

Collimator

(a) (b)

FigureFigure 8. 8. ConstructionConstruction of ofthe the RGB RGB multiplexing multiplexing optical optical setup: setup: (a) schematic (a) schematic diagram diagram of the of interference the interference optical optical setup; ( setup;b) Sensors 2021physical, 21, xdiagram FOR PEER of the REVIEW interference optical setup. 10 of 14 (b) physical diagram of the interference optical setup. TheThe lasers lasers used used in inthis this research research are are the the single single longitudinal longitudinal mode mode lasers lasers of Changchun of Changchun NewNew Industry Industry Optoelectronics Optoelectronics Technology Technology Co., Co., Ltd., Ltd., which which are are blue blue light light (473 (473 nm), nm), green green lightlightAfter (532 (532 the nm), nm), alignment and and red red light inspection light (639 (639 nm). nm). of The the The coherence RGB coherence mult lengthiplexing length of ofthe theoptical three three lasers setup, lasers is greater isit greateris found through thanthanexperiments 10 10 m, m, the the spectral spectralthat when line line width the width exposure is isless less than than intensity 0.00004 0.00004 nm, is nm, 300the the beamμW/cm beam divergence divergence for red angle anglelight, (full (full1800 μW/cm angle)angle)for green is isless less light,than than 3 and 3mrad, mrad, 900 with withμW/cm single single longitudinal for longitudinal blue light, mode mode we and andcan continuous continuoussynthesize wave wave better (CW). (CW). quality white Afterlight. the alignment inspection of the RGB multiplexing optical setup, it is found through µ 2 µ 2 experimentsWe conduct that when full-color the exposure 3D recording intensity is experiments 300 W/cm for through red light, RGB 1800 simultaneousW/cm expo- for green light, and 900 µW/cm2 for blue light, we can synthesize better quality white light.

sure.We The conduct dice full-color(single size 3D recording is 12 mm experiments × 12 mm through× 12 mm) RGB are simultaneous used as the exposure. actual object for Theinterference dice (single recording. size is 12 Three mm × laser12 mm beams× 12 are mm) controlled are usedas to the be actualturned object on and for off at the interferencesame time recording.through Threean electronic laser beams shutter. are controlled The exposure to be turned is 4 on min, and offand at thethe sametotal exposure timeenergy through is 360 an mJ/cm electronic2.The shutter. polymer The exposure is placed is in 4 min, darkness and the for total 4 min, exposure and energy finally is cured with 360a mercury mJ/cm2.The lamp polymer for 2 ismin. placed It can in darkness be seen for from 4 min, Figure and ﬁnally 9 that cured simultaneous with a mercury exposure can lampachieve for full-color 2 min. It can3D bedisplay. seen from The Figurephotopolymer9 that simultaneous can clearly exposure reproduce can achievefour dice of differ- full-color 3D display. The photopolymer can clearly reproduce four dice of different colors ent colors under the irradiation of white light. We observe from three different angles on under the irradiation of white light. We observe from three different angles on the left, middle,the left, and middle, right, and differentright, and sides different of the dice side cans of be the observed dice can from be different observed angles. from different Theangles. observing The observing angle of volume angle holography of volume isholography about 90◦. The is about colors 90°. of the The four colors dice in of the the four dice picturein the arepicture consistent are consistent with the actual with ones. the actual ones.

FigureFigure 9. 9.Volume Volume holography-based holography-based RGB multiplexingRGB multiplexing full-color fu 3Dll-color display 3D experimental display experimental results. results.

SinceSince the the object object light light is generated is generated by the by diffuse the diffuse reflection reflection of the surface, of the the surface, light the light intensity of the object light will change with the various of position, resulting in different intensity of the object light will change with the various of position, resulting in different diffraction efficiency of the photopolymer at different positions. In order to calculate thediffraction diffraction efficiency efficiency of more the accurately,photopolymer we replaced at different the dice position with as. mirrorIn order to maketo calculate the thediffraction object light efficiency more uniform, more andaccurately, ultimately we ensure replaced that thethe diffractiondice with efficiencya mirror ofto themake the ob- photopolymerject light more surface uniform, is equal and everywhere. ultimately ensure Then, we that use the the diffraction RGB multiplexed efficiency optical of the photo- setuppolymer in Figure surface8 for is interference equal everywhere. recording. Then, During we the use experiment, the RGB multiplexed the real object optical was setup in Figure 8 for interference recording. During the experiment, the real object was replaced with a mirror. The optical setup, experiment parameters and operation remained the same, and full-color volume holography was obtained through simultaneous exposure. After the volume holographic recording is completed, the transmission spectrum of the photopolymer hologram is measured with a spectrophotometer, as shown in Figure 10.

Figure 10. Transmission spectrum of full-color volume holography.

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After the alignment inspection of the RGB multiplexing optical setup, it is found through experiments that when the exposure intensity is 300 μW/cm for red light, 1800 μW/cm for green light, and 900 μW/cm for blue light, we can synthesize better quality white light. We conduct full-color 3D recording experiments through RGB simultaneous exposure. The dice (single size is 12 mm × 12 mm × 12 mm) are used as the actual object for interference recording. Three laser beams are controlled to be turned on and off at the same time through an electronic shutter. The exposure is 4 min, and the total exposure energy is 360 mJ/cm2.The polymer is placed in darkness for 4 min, and finally cured with a mercury lamp for 2 min. It can be seen from Figure 9 that simultaneous exposure can achieve full-color 3D display. The photopolymer can clearly reproduce four dice of different colors under the irradiation of white light. We observe from three different angles on the left, middle, and right, and different sides of the dice can be observed from different angles. The observing angle of volume holography is about 90°. The colors of the four dice in the picture are consistent with the actual ones.

Figure 9. Volume holography-based RGB multiplexing full-color 3D display experimental results.

Since the object light is generated by the diffuse reflection of the surface, the light intensity of the object light will change with the various of position, resulting in different diffraction efficiency of the photopolymer at different positions. In order to calculate the diffraction efficiency more accurately, we replaced the dice with a mirror to make the ob- Sensors 2021, 21, 2698 ject light more uniform, and ultimately ensure that the diffraction efficiency of10 ofthe 13 photopolymer surface is equal everywhere. Then, we use the RGB multiplexed optical setup in Figure 8 for interference recording. During the experiment, the real object was replaced with a mirror. The optical setup, experiment parameters and operation remained the replaced with a mirror. The optical setup, experiment parameters and operation remained same,the same, and and full-color full-color volume volume holography holography was was obtained obtained through through simultaneous simultaneous exposure. exposure. AfterAfter the the volume holographicholographic recording recording is completed,is completed, the transmissionthe transmission spectrum spectrum of the of the photopolymerphotopolymer hologramhologram is is measured measured with with a spectrophotometer, a spectrophotometer, as shown as shown in Figure in 10Figure. 10.

FigureFigure 10. 10. Transmission spectrum spectrum of of full-color full-color volume volume holography. holography. It can be seen from Figure 10 that the simultaneous exposure can achieve higher diffraction efficiency. After calculation by Equation (12), diffraction efficiency of 66.81% for recording wavelength in 473 nm, 58.34% for 532 nm and 34.08% for 639 nm were achieved. Blue light has the highest efficiency, followed by green light, and red light has the lowest diffraction efficiency. The diffraction efficiency for both blue and green light exceeds 50%, and good results have been achieved. The diffraction efficiency of red light is relatively low. This is due to the low intensity of red light in this experiment. The exposure intensity of red light is the smallest, and the exposure energy of red light is the smallest, so red light has the lowest diffraction efficiency. It can be seen from the transmission spectrum that there is a slight wavelength shift (approximately 8 nm) at the position of Tmin, which is caused by the shrinkage and refractive index change of the photopolymer in the holographic recording. The volume holographic grating generated by simultaneous exposure recording has a high diffraction efficiency, with an average diffraction efficiency of 53.08%. In addition to the simultaneous exposure scheme, there is another way of sequential exposure. Although sequential exposure can reduce crosstalk [14], the simultaneous exposure scheme can shorten the overall exposure time and speed up the production efficiency of holograms. In addition, in the actual process of making holograms, for the scheme of sequential exposure, the overall exposure time becomes longer, and the optical setup is easily affected by the vibration of the platform, which will reduce the diffraction efficiency. Due to the limitation of equipment, the power of the blue laser used in this experiment is relatively small. After shaping by an attenuator, a dichroic mirror and a diaphragm, the maximum beam intensity irradiated on the surface of the photopolymer is only about 900 µW/cm2. This limits the maximum beam intensity of the RGB three-color laser. In this experiment, the exposure intensity is 300 µW/cm2 for red light, 1800 µW/cm2 for green light, 900 µW/cm2 for blue light, and the total exposure energy is 360 mJ/cm2. If high-power lasers are employed and the intensity of each laser is increased by 10 times, the recording time will be shortened by 10 times, and the exposure time can be reduced to 12 s. Therefore, the light intensity can be increased according to the requirement to shorten the exposure time. Since this experiment is to obtain higher diffraction efficiency, dark reaction and curing operations are augmented. In most practical applications, these two steps can Sensors 2021, 21, 2698 11 of 13

also be omitted. In summary, we can greatly reduce the exposure time by increasing the light intensity. According to Bragg’s law, the spatial frequency SF of the grating formed by blue, green and red light at a specific angle can be calculated. For reflective volume holography, when the Bragg angle is 90◦, the volume holographic grating can obtain the maximum spatial frequency, SF473nm = 6342 lp/mm, SF532nm = 5639 lp/mm, SF639nm = 4695 lp/mm. The largest spatial frequency means that the diffraction efficiency and refractive index modulation of volume holography can be improved. According to Kogelnik’s coupled wave theory, we can calculate the relevant parameters of the 3D volume holography in the simultaneous exposure scheme. It can be seen from Table1 that the average diffraction efficiency is 53.08%, and the average refractive index modulation is 0.01. The smallest spatial frequency is 4695 lp/mm, indicating that the grating fringe pitch is less than 0.3 µm, which improves the resolution and diffraction efficiency of the volume holography.

Table 1. 3D volume holography related parameters in the simultaneous exposure scheme.

Wavelength Exposure Energy (mJ/cm2) SF (lp/mm) η (%) ∆n B (473 nm) 108 6342 66.81 0.0108 G (532 nm) 216 5639 58.34 0.0106 R (639 nm) 36 4695 34.08 0.0085

Compared with the previous work [23–25], it can be seen from Table2 that the structure proposed in this paper has a higher average diffraction efficiency. The experimental results show that the full-color 3D image can be recorded by using the RGB multiplexed laser optical setup, and with only a single layer of photopolymer film. The reproduced image has high resolution (grating pitch less than 0.3 µm), strong three-dimensionality (depth of field greater than 10 mm), and large observing angle (about 90◦). The volume holography has a compact structure and light weight, which realizes full-color 3D display through one single layer of photopolymer.

Table 2. Diffraction efﬁciency of different structures.

Average Structure Blue (%) Green (%) Red (%) Diffraction Efﬁciency (%) by Hong et al. [24] 21.3 21.2 18.1 20.20 by Vázquez-Martín et al. [25] 22.5 14.5 24.7 20.57 by Bruder et al. [23] 22.2 24.7 27.2 24.70 the proposed structure 66.81 58.34 34.08 53.08

Figure 11 shows the recording and reconstruction procedure of volume holography. As shown in Figure 11a, we can generate object light by diffuse reﬂection. Subsequently, the object light interferes with the reference light to record the wavefront information in the photopolymer. Using physical interference recording volume holograms avoids the limitations of SLM’s spatial bandwidth product, the number of pixels and diffraction angles, therefore it can reconstruct high-resolution 3D image. As shown in Figure 11b, under the illumination of white light, the observer can observe the scene where the real object and the virtual object are superimposed through the photopolymer. As the volume hologram has angle and wavelength selectivity, the reconstruction light in the real environment does not meet the Bragg condition and passes through the photopolymer directly. In Figure 11c, the image in the solid frame is the real scene, and the image in the dashed frame is the virtual scene, which shows that the combination of virtual and real is realized by a volume hologram. The proposed structure realized a full-color, high resolution, and large perspective 3D volume holographic display, which has good application prospects in augmented reality. Sensors 2021, 21, x FOR PEER REVIEW 12 of 14

laser optical setup, and with only a single layer of photopolymer film. The reproduced image has high resolution (grating pitch less than 0.3 μm), strong three-dimensionality (depth of field greater than 10 mm), and large observing angle (about 90°). The volume holography has a compact structure and light weight, which realizes full-color 3D display through one single layer of photopolymer.

Table 2. Diffraction efficiency of different structures.

Structure Blue (%) Green (%) Red (%) Average Diffraction Efficiency (%) by Hong et al. [24] 21.3 21.2 18.1 20.20 by Vázquez-Martín et al. [25] 22.5 14.5 24.7 20.57 by Bruder et al. [23] 22.2 24.7 27.2 24.70 the proposed structure 66.81 58.34 34.08 53.08 Figure 11 shows the recording and reconstruction procedure of volume holography. As shown in Figure 11a, we can generate object light by diffuse reflection. Subsequently, the object light interferes with the reference light to record the wavefront information in the photopolymer. Using physical interference recording volume holograms avoids the limitations of SLM’s spatial bandwidth product, the number of pixels and diffraction angles, therefore it can reconstruct high-resolution 3D image. As shown in Figure 11b, under the illumination of white light, the observer can observe the scene where the real object and the virtual object are superimposed through the photopolymer. As the volume hologram has angle and wavelength selectivity, the reconstruction light in the real environment does not meet the Bragg condition and passes through the photopolymer directly. In Figure 11c, the image in the solid frame is the real scene, and the image in the dashed frame is the virtual scene, which shows that the combination of virtual and real is realized Sensors 2021, 21, 2698 by a volume hologram. The proposed structure realized a full-color, high resolution,12 and of 13 large perspective 3D volume holographic display, which has good application prospects in augmented reality.

(a) (b) (c)

Figure 11. RecordingRecording and reconstruction proc procedureedure of volume holography: ( a)) volume volume holographic holographic recording; recording; ( b) volume holographic reconstruction; (c) virtualvirtual and reality superposition.superposition.

5.5. Conclusions Conclusions InIn this this research, research, we we propose a full-color see-through 3D display method based on volumevolume holography. This method is based on real object interference, avoiding the device limitationlimitation of of spatial spatial light light modulator modulator (SLM). (SLM). The The volume volume holography holography has has a a light structure andand weight, weight, which which realizes realizes 3D 3D display display throug throughh one one single single film film of ofphotopolymer. photopolymer. For Forre- flectivereflective volume volume holography, holography, when when the the Bragg Bragg angle angle is is 90°, 90◦ ,the the volume volume holographic holographic grating grating cancan obtain obtain the the maximum maximum spatial spatial frequency, frequency, which means that the diffraction efficiency efficiency andand refractive index modulation of volume holography can be improved.improved. Experimental resultsresults show show that that the the simultaneous simultaneous exposure exposure can achieve achieve color color holographic holographic recording. The ◦ observingobserving angle angle of of the the reconstructed reconstructed image image is about about 90°, 90 , the the average average diffraction diffraction efficiency efficiency isis 53.08%, 53.08%, and and the the grating fringe fringe pitch is less than 0.3 µμm.m. The reconstructed image of volume holography has high resolution, strong stereo perception, and large observing angle. Holographic 3D display can reproduce true 3D scenes and is currently one of the most promising 3D display technologies. The volume holographic full-color 3D display method proposed in this paper provides a basic technical reference and potential method

for the practical application of augmented reality.

Author Contributions: Conceptualization, T.W., C.W., H.W., and J.M.; Data curation, T.W.; Funding acquisition, P.S. and J.M.; Investigation, J.M. and P.S.; Project administration, P.S. and J.M.; Software, T.W.; Supervision, J.M. and P.S.; Validation, T.W.; Writing–original draft, T.W.; Writing–review & editing, T.W., C.W., H.W., and P.S. All authors have read and agreed to the published version of the manuscript. Funding: This research is supported by Key R&D Project of Hunan Province (2018WK2101) and International Collaboration Project of Shenzhen (GJHZ20190821164405428). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors acknowledge Jinjie Zhang at Tsinghua University for providing the piggy model to record the volume hologram. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Rabbi, I.; Ullah, S. A survey on augmented reality challenges and tracking. Acta Graph. Znan. Cas.ˇ Tiskarstvo Graf. Komun. 2013, 24, 29–46. 2. Carmigniani, J.; Furht, B.; Anisetti, M.; Ceravolo, P.; Damiani, E.; Ivkovic, M. Augmented reality technologies, systems and applications. Multimed. Tools Appl. 2011, 51, 341–377. [CrossRef] 3. Ma, Q.; Cao, L.; He, Z.; Zhang, S. Progress of three-dimensional light-ﬁeld display. Chin. Opt. Lett. 2019, 17, 111001. [CrossRef] 4. Wick, B.; Currie, D. Convergence accommodation: Laboratory and clinical evaluation. Optom. Vis. Sci. Off. Publ. Am. Acad. Optom. 1991, 68, 226–231. [CrossRef] Sensors 2021, 21, 2698 13 of 13

5. Duan, X.; Liu, J.; Shi, X.; Zhang, Z.; Xiao, J. Full-color see-through near-eye holographic display with 80◦ field of view and an expanded eye-box. Opt. Express 2020, 28, 31316–31329. [CrossRef][PubMed] 6. Gao, H.; Xu, F.; Liu, J.; Dai, Z.; Zhou, W.; Li, S.; Yu, Y.; Zheng, H. Holographic three-dimensional virtual reality and augmented reality display based on 4k-spatial light modulators. Appl. Sci. 2019, 9, 1182. [CrossRef] 7. Li, G.; Lee, D.; Jeong, Y.; Cho, J.; Lee, B. Holographic display for see-through augmented reality using mirror-lens holographic optical element. Opt. Lett. 2016, 41, 2486–2489. [CrossRef][PubMed] 8. Wakunami, K.; Hsieh, P.; Oi, R.; Senoh, T.; Sasaki, H.; Ichihashi, Y.; Okui, M.; Huang, Y.; Yamamoto, K. Projection-type see-through holographic three-dimensional display. Nat. Commun. 2016, 7, 1–7. [CrossRef][PubMed] 9. Shin, S.; Javidi, B. Speckle-reduced three-dimensional volume holographic display by use of integral imaging. Appl. Opt. 2002, 41, 2644–2649. [CrossRef] 10. Yamaguchi, T.; Miyamoto, O.; Yoshikawa, H. Volume hologram printer to record the wavefront of three-dimensional objects. Opt. Eng. 2012, 51, 075802. [CrossRef] 11. Hong, K.; Park, S.; Yeom, J.; Kim, J.; Chen, N.; Pyun, K.; Choi, C.; Kim, S.; An, J.; Lee, H. Resolution enhancement of holographic printer using a hogel overlapping method. Opt. Express 2013, 21, 14047–14055. [CrossRef][PubMed] 12. Roh, J.; Kim, K.; Moon, E.; Kim, S.; Yang, B.; Hahn, J.; Kim, H. Full-color holographic projection display system featuring an achromatic Fourier filter. Opt. Express 2017, 25, 14774–14782. [CrossRef][PubMed] 13. Zeng, Z.; Zheng, H.; Yu, Y.; Asundi, A.K.; Valyukh, S. Full-color holographic display with increased-viewing-angle. Appl. Opt. 2017, 56, F112–F120. [CrossRef][PubMed] 14. Piao, M.; Kwon, K.; Kang, H.; Lee, K.; Kim, N. Full-color holographic diffuser using time-scheduled iterative exposure. Appl. Opt. 2015, 54, 5252–5259. [CrossRef][PubMed] 15. Collier, R. Optical Holography; Elsevier: Amsterdam, The Netherlands, 2013. 16. Kogelnik, H. Coupled wave theory for thick hologram gratings. Bell Syst. Tech. J. 1969, 48, 2909–2947. [CrossRef] 17. Bruder, F.; Fäcke, T.; Rölle, T. The chemistry and physics of Bayfol® HX film holographic photopolymer. Polymers 2017, 9, 472. [CrossRef][PubMed] 18. Zhao, G.; Mouroulis, P. Diffusion model of hologram formation in dry photopolymer materials. J. Mod. Opt. 1994, 41, 1929–1939. [CrossRef] 19. Liu, P.; Zhang, T.; Liu, Y.; Zhai, Q.; Tao, S.; Wan, X.; Wu, F. The Investigation of the Effect of Recording Conditions on Grating Formation in a Novel Holographic Photopolymer. In Holography and Diffractive Optics III, 2008; International Society for Optics and Photonics: Bellingham, WA, USA, 2008; p. 68320Y. 20. Matyjaszewski, K.; Davis, T.P. Handbook of Radical Polymerization; John Wiley & Sons: Hoboken, NJ, USA, 2003. 21. Moad, G.; Solomon, D.H. The Chemistry of Radical Polymerization; Elsevier: Amsterdam, The Netherlands, 2006. 22. AG, C. Bayfol® HX200 Description and Application Information. Available online: https://solutions.covestro.com/en/products/ bayfol/bayfol-hx200_86194384-20033146?SelectedCountry=CN (accessed on 16 March 2021). 23. Bruder, F.; Hansen, S.; Manecke, C.; Orselli, E.; Rewitz, C.; Rölle, T.; Wewer, B. Wavelength Multiplexing Recording of vHOEs in Bayfol HX Photopolymer film. In Digital Optics for Immersive Displays, 2018; International Society for Optics and Photonics: Bellingham, WA, USA, 2018; p. 106760H. 24. Hong, K.; Yeom, J.; Jang, C.; Hong, J.; Lee, B. Full-color lens-array holographic optical element for three-dimensional optical see-through augmented reality. Opt. Lett. 2014, 39, 127–130. [CrossRef][PubMed] 25. Vázquez-Martín, I.; Gómez-Climente, M.; Marín-Sáez, J.; Collados, M.V.; Atencia, J. True colour Denisyuk-type hologram recording in Bayfol HX self-developing photopolymer. In Holography: Advances and Modern Trends V, 2017; International Society for Optics and Photonics: Bellingham, WA, USA, 2017; p. 102331U.