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Abstract
A novel catadioptric retroreflector floating device (CRA) used in the 3D floating light-field system is proposed. The floating light-field image constructed by the CRA is aberration-suppressed. The luminance and the contrast of the image are substantially improved in a 90-degree viewing angle. The CRA is constituted of the designed catadioptric retroreflector (CR). The CR consists of three lenses, the first and the second lens is to refract the light, and the rear surface of the third lens is coated with reflective coating in order to reflect the incident light. The CRA is processable and the fabrication process using UV embossing is also described. A spectrophotometer is utilized to measure the retroreflective efficiency of the CRA. The average retroreflective efficiency of the CRA is 80.1%. A beam quality analyzer is utilized to measure the beam spot quality of the CRA, and the image quality can satisfy the requirements of human eye observation. In the experiment, compared to the floating light-field image constructed by the micro-beads type retroreflector floating device (MRA), the image quality of the floating light-field image constructed by the CRA is significantly enhanced. In the quantitative computer simulation, the PSNR values of the images are increased from 23.0185 to 32.1958.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The floating 3D light-filed display is a new kind of 3D display technology that integrates floating 3D images with real-world scenes. The floating light-field display system is combined both floating display system and light-field display system. The light-field display system can provide smooth motion parallax 3D images with quasi-continuous viewpoints. The floating display system can realize the extended depth of field, and therefore the generated images and the real sense can be merged together. The apparent characteristic of floating light-filed display is the optical see-through, which permits the overlaying of reconstructed 3D images on viewers’ real-world view, so that the 3D images are seamlessly blended into the real-world scene. Because of its stunning visual effects and broad application prospects, the floating light-filed display technology can find many augmented reality applications in military, medical, education, entertainment, and other fields [1–10].
Figure 1 shows the principle operation of the conventional floating light-field display system [7–10]. There are three stages in the formation process of a 3D light-field floating image. In the first stage, the light-field image is constructed by the light-field display system. The light-field image acts as an objective image for the whole system. In the second stage, the light-field image is transmitted to the floating device by the beam splitter, and the tilt angle is $\theta $. Then the light-field image is modulated to the floating light-field image by the floating device. In the third stage, the generated floating light-field image is captured by the observers. The floating distance (take the $B^{\prime}$ as an example) is defined as the distance from $B^{\prime}$ to the beam splitter (${L_4}$). The viewing angle ($FOV$) of the floating light-field system can be divided into two parts. One is the viewing angle ($FOV1$) of the light-field image, and the other is the viewing angle ($FOV2$) of the floating device. The FOV can be expressed as:
The pivotal component to convert the light-field image to the floating light-field image is the floating device. Nonetheless, many types of research have focused on improving the property of the light-field image, rather than the floating device. The retroreflector floating device is an optical device that reflects light in reversely toward the incident direction, and the retroreflected rays reproduce a real image as a floating image of the source image. The retroreflector does not require electricity and an arbitrary floating image can be displayed by merely placing the retroreflector with a source image. Furthermore, the retroreflector can show parallax images continuously when the line-of-sight is changed. The retroreflector floating device used in the floating system is very common [11–18]. H. Bando et al. fabricated a crossed-mirror retroreflector floating device for LED lamps in order to realize an aerial image of a large LED panel [15]. Miyazaki et al. proposed an aerial image system based on a dihedral corner retroreflector floating device, which brings a compact system configuration and volumetric real image generation with very low distortion [18]. Nevertheless, the viewing angle of those type (prism type) retroreflector floating device is insufficient.
The micro-beads type retroreflector floating device (MRA) is the another frequently used retroreflector floating device because of its large viewing angle [19–22]. Norikazu Kawagishi et al. proposed that the viewing angle of the MRA is much more extensive than the prism type [22]. The element of MRA is two hemispheres as shown in Fig. 2(a). According to the geometrical relation shown in Fig. 2(a), the Eq. (2) can be obtained:
It shows that the two hemispheres become a spherically symmetric structure (spherical lens, ${R_1} = {R_2}$) when the material with refractive index of 2 is used (${n_{ball}} = 2$). For the convenience of analysis, the following mentioned MRAs are all spherical lens arrays. Figure 2(d) shows that the light rays incident in the single unit (spherical lens) of MRA from different angles. According to the principle of optical imaging, the focal length of the spherical lens can be calculated as Eq. (3):
Fig. 2. The schematic diagram of the geometrical relation and the optical ray tracing in MRA. (a) The geometrical relation in the single unit of MRA. (b) The optical ray tracing in the single unit (spherical lens) of ideal MRA. (c) The optical ray tracing in the single unit (spherical lens) of practical MRA. (d) The optical ray tracing in the single unit (spherical lens) of MRA in different angles. (e) The sketch map about the viewing angle of the MRA.
Because the single unit of the MRA is the spherical lens, ${F_{ball}} = {R_{ball}}$ can be obtained. For the spherical lens, the reflecting surface is also the focal plane. Furthermore, the spherical lens belongs to the spherically symmetric structure, so the incident light rays can be retroreflected in the approximately 180-degree viewing angle range. Figure 2(e) shows that the light rays incident in the MRA from different angles. The effects between the adjacent single unit (spherical lens) are in consideration, based on the geometrical relationship. The viewing angle of the MRA ($FO{V_{MRA}}$) is over 90 degrees (approximate to 150-degree).
Therefore, the MRA equipped in the floating system has attracted much attention [23–24]. Toru Iwane et al. presented aerial light-field imaging based on retroreflective sheeting. The full parallax image in mid-air was successfully reconstructed [23]. Yamamoto et al. presented an LED floating aerial image based on micro-beads type retroreflective sheeting, which acquired approximately 90-degree visual angle [24]. However, the aberration (only spherical aberration $\delta L^{\prime}$) of the MRA is unacceptable. Different from the ideal retroreflected rays emergent in the original path (shown in Fig. 2(b)), there is a divergence angle $\Delta ^{\prime}$ between the incident rays and the practical retroreflected rays because of the spherical aberration (shown in Fig. 2(c)). It can be seen that the value of $\Delta ^{\prime}$ increases with the spherical aberration. Therefore, the floating light-field image is hazy The luminance and the contrast of the image are declined.
For MRA, the method to suppress the spherical aberration is to decrease the aperture of the spherical lens based on Eq. (4). Nevertheless, the miniature dimension will lead to a complicated fabrication process and high fabrication cost [25–26]. Here an example fitted curve about the single spherical lens radius and the RMS radius of the spot diagram as a reference are given in Fig. 3., If the RMS radius of the spot diagram is around 10 um, the radius of the single spherical lens is less than 0.05 mm. If the radius of the single spherical lens is around 0.5 mm, the RMS radius of the spot diagram is more massive than 130.0 um.
Here, a 90-degree viewing angle catadioptric retroreflector floating device (CRA) is designed. The reasonable optical design process and detailed aberration evaluation of the CRA are demonstrated. The aberrations of the CRA are well suppressed in the 90-degree viewing angle. Two optical testing experiments are proposed in order to illustrate the essential optical characteristics of the CRA. The average retroreflective efficiency of the CRA is 80.1%. The beam spot quality of the CRA is adequate. The CRA is processable and the fabrication process using UV embossing is also described.
Furthermore, the floating light-field display system based on the CRA is set up. The displayed floating light-field image can be observed in a 90-degree viewing angle. The floating distance (depth of field) is 226.8 mm. In comparison with the floating light-field image constructed by the MRA, the floating light-field image constructed by the CRA is aberration-suppressed. The luminance and the contrast of the image are also dramatic improvements. Meanwhile, in the quantitative computer simulation, the PSNR values of images are increased from 23.0185 to 32.1958. The floating light-field image of magic cube mixed with real magic cube is also provided. The floating light-field display performance can be applied in many augmented reality domains, such as medical simulation, cultural relic demonstration, and commercial exhibition.
2. Optical design, evaluation and fabrication of the 90-degree viewing angle CRA
2.1 Optical design of the CRA
The proposed CRA is an array structure. The element of the CRA is the designed catadioptric retroreflector (CR). The CR is comprised of three lenses, as shown in Fig. 4(a). The first lens and the second lens form a concentric lens assembly, and the optical axis at any angle passes through the center of curvature. The spherically symmetric structure can help to improve the viewing angle of the CRA. The third lens has an aspheric reflecting surface in order to reflect the incident rays and realize the retroreflective modulation. As shown in Fig. 4(b), ${n_{air}}$ is used for the refractive indices of the air. ${n_1}$, ${n_2}$, ${n_3}$ is used for the refractive index of the first lens, second lens, and third lens; ${r_1}$ and ${r_2}$ are for the radius of curvature of the first lens and the second lens; ${r_3}$, ${k_3}$ and ${4_{nd}}$ are for the parameters of the aspheric reflecting surface.
Before calculating the corresponding structure parameters, two essential rules should be in consideration, the field weights and the color weights.
In order to evaluate the spot diagrams in different viewing angles reasonably, the Radau integration method is used. The Radau integration [27] takes the light rays from the whole viewing angle, especially the central ray in the pupil (principal ray) into account. The relative weight of the principal ray is 1 because the principal ray has the highest priority compared to the other rays. The Radau integration method is used to calculate the corresponding normalized field weights (${w_n}(h)$) (shown in Fig. 5(a)), which can acquire a higher precision. The floating light-field display system belongs to the visual instrument. The detector of the visual instrument is the human eyes. In order to evaluate the chromatic aberrations more reasonably, the chromatic response of the eye (shown in Fig. 5(b)) is taken into account. The F(486.1 nm), d(587.5 nm) and C(656.2 nm) lights are chosen as the characteristic wavelengths. The Gaussian integration [27] method is adopted to calculate the proper weights (${w_i}(\lambda )$) of those three characteristic wavelengths (F, d, C). The Gaussian integration method can take the whole standard human eye sensitivity curve function into account, which helps to improve the computational accuracy. After the calculation, the proper weights of F, d and C lights are 0.06963, 0.54553 and 0.38483, respectively.
Fig. 5. (a) The relative weight versus the normalized object height. (b) Sensitivity curve for the standard human eye.
In order to obtain an optimized structure, the aberration theory is considered. The necessary optical ray-tracing schematic diagram of the designed CR is demonstrated in Fig. 6.
Based on the optical ray-tracing in the CR, the function relationship (${f_{1 - 7}}( \cdots )$) between the variables (${h_{1 - 4}},{\alpha _{1 - 5}},{r_{1 - 3}},{n_{1 - 3}},{d_{1 - 2}},{k_3},{4_{nd}}$) and the constants (${S_1} - {S_7}$) is as follows.
After the aberrations are balanced, an optimized structure and corresponding structure parameters are achieved, as shown in Table 1.
Table 1. The structure parameters of the designed catadioptric retroreflector.
The aspheric surface formula is given in Eq. (7), and the corresponding aspheric reflecting surface parameters are also achieved, as shown in Table 2.
Table 2. The parameters of the aspheric reflecting surface.
Meanwhile, the RMS radius spot diagram merit function in different viewing angle is given.
2.2 Optical evaluation of the CRA
2.2.1 Retroreflective efficiency measurement of the CRA
Two experimental setups for retroreflector optical testing are proposed in order to illustrate the essential optical characteristics of the CRA. The retroreflective efficiency is a significant characteristic of the retroreflector. It can be defined as the ratio of the retroreflected radiant or luminous flux to the incident flux within the narrow confines of the incident and retroreflected geometrical conditions. The proposed CRA should reflect as much light as possible within a confined cone of light (high retroreflective efficiency) so as to generate less stray light and improve the luminance. Figure 9 is an experimental setup to measure the retroreflective efficiency of the CRA.
In accordance with Fig. 9, (a) 632.8 nm He-Ne laser is used as the light source. By rotating the CRA, the beam enters the CRA at different incident angle ${0^0} - {45^0}$ after passing through the half-mirror (50R/50 T). Since the CRA enables light to travel back along its original path, the luminance of retroreflected light can be observed by the spectrophotometer. Meanwhile, a dielectric mirror (DM) is used instead of the CRA and measured the luminance in the same manner. In the experiment, the retroreflective efficiency ${R_r}$ is expressed by
where ${R_d}$ is the specific reflectance of the DM, ${I_r}$ and ${I_d}$ are the luminance in the cases of the CRA and the DM, respectively.Figure 10 gives the measurement results of retroreflective efficiency in different incident angles. The reflectivity of the DM is also shown in Fig. 10 as a reference. Meanwhile, in order to make a reasonable contrast with CRA, the retroreflective efficiency of the MRA with the same aperture is also measured in the experiment. The measured MRA is composed of the spherical lens array. The aperture of the single spherical lens is 1 mm, the curvature radius is 0.5 mm and the refractive index is 2.
The retroreflective efficiency detailed measurement data of CRA and MRA at different viewing angles are also shown in Table 3.
Table 3. The retroreflective efficiency measurement data of CRA and MRA at different viewing angles.
From the measure results, the average retroreflective efficiency of the CRA is 80.1% and the average retroreflective efficiency of the MRA is 40.5%. Compared to the MRA, the retroreflective efficiency is significantly enhanced. Therefore, the CRA is almost no loss of the luminance of the light-field image. From Table 3, the retroreflective efficiency population standard deviation (PSD) of the CRA is 2.29, the retroreflective efficiency population standard deviation of the MRA is 4.61. The standard deviation calculation results demonstrate that the retroreflective efficiency of the CRA is more stable in the 90-degree viewing angle.
2.2.2 Beam spot quality measurement of the CRA
In order to evaluate the impact of the proposed CRA on the floating image quality, an experimental setup to measure the beam spot quality of the CRA is given in Fig. 11. Here two 632.8 nm He-Ne lasers equipped with iris diaphragm are used as the light source. The beam diameter is $0.6 \pm 0.05$mm and the beam divergence is $1.2 \pm 0.2$mrad. By rotating the CRA, two beams (equal intensity) enter the CRA at normal incidence and oblique incidence (${10^ \circ }$, $\textrm{2}{\textrm{0}^ \circ }$, $\textrm{3}{\textrm{0}^ \circ }$, $\textrm{4}{\textrm{0}^ \circ }$, ${45^ \circ }$) after passing through the half-mirror (50R/50 T). Since the CRA enables light to travel back along its original path, the retroreflected beam spot can be analyzed by the beam quality analyzer. Besides, the space between two laser-spots is set as a pixel pitch of the light-field image.
Two beam spots normalized intensity curves from different incident angles are given in Fig. 12. The resultant intensity curve is also given in the corresponding diagram. The oscillating tail of the measurement curves is the diffraction fringes caused by the optical aperture of the CRA.
Fig. 12. The normalized intensity distribution measurement results of the retroreflected beam spots: (a) For the 0-degree incidence on the CRA. (b) For the 10-degree incidence on the CRA. (c) For the 20-degree incidence on the CRA. (d) For the 30-degree incidence on the CRA. (e) For the 40-degree incidence on the CRA. (f) For the 45-degree incidence on the CRA.
The floating light-field display system is a visual optical system based on incoherent illumination. According to the analysis of the intensity distribution curves in Fig. 12, it can be seen that the minimum values of resultant intensity from different viewing angles are all less than 0.735. According to the Rayleigh criterion, such an image quality could satisfy the requirements of human eye observation. Meanwhile, in order to make a reasonable contrast with CRA, the beam spot quality of the MRA with the same aperture is also measured in the experiment. The measured MRA is composed of the spherical lens array. The aperture of the single spherical lens is 1 mm, the curvature radius is 0.5 mm and the refractive index is 2. The same experimental method and facilities are used. The measurement results are shown in Fig. 13.
Since the amount of light intensity is the same with and without aberrations, the irradiance at the center of the beam spot has to decrease when the beam spot size increases. The maximum values of two beam spots are less than the maximum values of the resultant intensity curve because of the severe aberration. Comprehensive analysis of the intensity distribution curve shown in Fig. 13, it can be seen that the maximum values of two beam spots are less than 0.6. According to the Strehl ratio, such an image quality is unappeasable.
The beam spot grey-scale map shown in Fig. 14 can verify such an experimental conclusion more directly. Meanwhile, the presence of a halo around the central beam spot can be observed in Fig. 14(b). This phenomenon is also due to the severe aberration of the MRA. Furthermore, the floating light-field image contrast will decrease. Therefore, compared to the MRA, the CRA has little impact on the resolution and the contrast of the light-field image.
Fig. 14. The grey-scale map of the retroreflected beam spot: (a) The grey-scale map result of the CRA. (b) The grey-scale map result of the MRA.
2.3 Fabrication of the CRA
This section describes the UV embossing technique is applied in the fabrication of CRA. The UV embossing process, using a UV light for curing at low temperature and low pressure, is much easier for manufacturing the array structure. Additionally, refractive indices of UV-curable polymers can be easily controlled within the range between 1.4 and 1.61. The two major stages of the process are to fabricate the compounded unit lens array and to align the compounded unit lens array to form the CRA.
2.3.1 Fabricating the compounded unit lens array
The third lens with an aspheric reflecting surface is the first manufactured element. Figure 15 illustrates the overall processing steps of the UV-embossing for machining the third lens of the CRA. A metal mold created by the diamond turning is used in step-(1) to duplicate multiple lenses, resulting in a master template for the next steps. The master is then transferred to a soft (photopolymer) mold at step-(3) after special surface treatment with an anti-sticking layer at step-(2). In general, an anti-sticking layer is applied on the photopolymer surface to improve release characteristics (step-(4)) by decreasing the surface energy of the polymer. Then an extra Ag coating step only for the third lens is performed to obtain the aspheric reflecting surface (step-(5)). The silver film is deposited by the e-beam evaporation process, and its thickness is 220 nm. No damage to the third lens by the deposition process is observed, and the measured profile after the silver deposition still satisfies the designed aspheric shape. Afterward, the photopolymer materials (refractive index ${n_d} = {n_3}$) is filled and solidified. Meanwhile, in order to machine the cavities of half of the second lens, the soft mold is employed at step-(7).
Fig. 15. UV-embossing process: (1) Machining master template. (2) Anti-sticking of the master template surface. (3) Machining soft mold. (4) Separating the master and the soft mold (release process). (5) Ag coating (for the aspheric reflecting surface only). (6) The manufactured aspheric reflecting surface. (7) Filling and solidifying the photopolymer (refractive index ${n_d} = {n_3}$). (8) Final optical third lens array.
On the foundation of the manufactured optical third lens array in Fig. 15 (step-(8)), the photopolymer materials (refractive index ${n_d} = {n_2}$) are filled and solidified to process half of the second lens array in Fig. 16 (step-(1)). In the meantime, the soft mold is employed again to machine the cavities of half of the first lens. Similar methods are repeated to fabricate half of the first lens array at Fig. 16 (step-(3)). After the Fig. 16 (step-(3)), the compounded unit lens array one is manufactured.
Fig. 16. (1) Filling and solidifying the photopolymer (refractive index ${n_d} = {n_2}$). (2) The manufactured second lens array. (3) Filling and solidifying the photopolymer (refractive index ${n_d} = {n_1}$). (4) The manufactured third lens array with half of the second and first lens array (the compounded unit lens array 1).
The similar steps are repeated to fabricate another half of the first and the second lens array (the compounded unit lens array 2), as shown in Fig. 17.
Fig. 17. UV-embossing process for fabricating another half of the first and the second lens array (the compounded unit lens array 2). (1) Machining master template. (2) Anti-sticking of the master template surface. (3) Machining soft mold. (4) Anti-sticking of the soft mold. (5) Machining the half of the second lens array. (6) Machining the half of the first lens array.
2.3.2 Alignment of two compounded unit lens arrays
The ultimate process of manufacturing the retroreflector is to align two compounded unit lens arrays, as shown in Fig. 18. The absorbed layer is coated at step-(1) aim to suppress the stray light. The Laser calibration technique is used to assemble two compounded unit lens arrays, and the manufactured CRA is demonstrated in Fig. 19.
Fig. 18. (1) Absorbed layer coating. (2) The manufactured unit lens arrays. (3) Two units bonded together by Laser calibration. (4) The manufactured CRA.
Fig. 19. (a) The top view of the proposed CRA from a microscope. (b) The back view of the proposed CRA from DSLR.
3. Experimental facility and the results
The floating light-field display system based on the CRA is set up for experimental validation in the section of experimental research. Figure 20 exhibits the configuration of the experimental system.
Fig. 20. (a) The configuration of the experimental floating light-field display system. (b) The side elevation of the experimental floating light-field display system.
For the convenience of description, the experimental floating light-field display system is divided into two parts. The light-field image is constructed by the light-field display system, which was proposed by Xunbo Yu et al. [28–29]. This system consists of three projectors, lenticular lens array, and holographic functional screen (HFS). This system can generate a 90-degree light-field image. In order to observe the floating 3D image, the light-field image is projected to the CRA by the beam splitter. In the experimental prototype, the beam splitter has 50% reflectivity and 50% transmittance. The corresponding experimental dimension parameters of the floating light-field display system are listed in Table 4.
Table 4. The dimension parameters of the floating light-field display system.
Eventually, when the coded elemental image array is loaded on the projectors, the correct floating light-field image can be presented to the observers. The example of a floating light-field image (several geometric models) is captured from five different directions by SLR camera. The actual display result shows that the whole floating light-field image can be viewed within the viewing angle of 90°. The floating distance (depth of field) is 226.8 mm. The image quality comparison between the floating light-field display using MRA and the experimental floating light-field display is shown in Fig. 21.
Fig. 21. The comparison of floating light-field display image for the floating light-field display based on CRA (see Visualization 1) and the floating light-field display based on MRA (several geometric models from different angles).
From Fig. 21, we can see that the floating light-field image based on the CRA is aberration-suppressed, and the luminance and the contrast of the image are rapidly progressed. In order to evaluate the improvement of the floating light-field image quality based on the CRA quantitatively, the display process of a floating light-field image is simulated to confirm the validity of the proposed CRA. The original image is shown in Fig. 22(a). Figure 22(b) and 22(c) show the display performance simulation using the CRA and the MRA, respectively. Comparing Fig. 22(b) with Fig. 22(c), we can see that the image quality is noticeably improved. The image quality measured in PSNRs for the images are shown in Fig. 22 (bottom). The PSNR values of images are increased (from 23.0185 to 32.1958), which indicates that the floating light-field image quality is enhanced with the proposed CRA.
Fig. 22. The computer image processing PSNR simulation: (a) Original images. (b) Floating light-field image based on proposed CRA. (c) Floating light-field image based on MRA.
What’s more, a Spectroradiometer is used to measure the luminance of the floating light-field images. In the floating light-field display based on MRA, the Spectroradiometer shows that the average luminance is 81.2nit. As for floating light-field display based on CRA, the Spectroradiometer shows that the average luminance is 132.6nit. The luminance in the whole viewing angle is uniform.
Such an effect floating light-field display system can find many augmented reality applications, such as medical simulation, cultural relic demonstration, and commercial exhibition. Here, the floating light-field image results of magic cube mixed with real magic cube are shown in Fig. 23.
Fig. 23. Floating light-field image results of magic cube mixed with real magic cube in different viewing angle (see Visualization 2).
As the experimental results are shown in Fig. 23, the two constructed magic cube models can be clearly seen in the different angles of views among which there is apparent horizontal parallax. When viewed from the left angle (${22.5^ \circ }$, ${45^ \circ }$), the geometries are close together, but they are separated by a certain distance when viewed from the right angle ($\textrm{ - }{22.5^ \circ }$, $\textrm{ - 4}{\textrm{5}^ \circ }$). At the same time, the real object magic cube can be observed clearly.
4. Conclusion
A novel 90-degree viewing angle CRA with aberration-suppressed is proposed. Based on the reasonable optical ray-tracing, the optimized structure and corresponding structure parameters of the CRA are achieved. The practical optical performance of the CRA is measured. Compared to the MRA, the retroreflective efficiency of the CRA is enhanced (80.1%). According to the normalized intensity distribution curve, the CRA has little impact on the resolution and the contrast of the light-field image. The CRA is processable and the UV embossing technique is applied in the fabrication of CRA. In the experimental research, the floating light-field image constructed by the CRA can be observed in a 90-degree viewing area. The floating distance (depth of field) is 226.8 mm. Compared with the floating light-field image established by the MRA, the display performance of the floating light-field display system with the CRA is significantly enhanced. The generated floating light-field image is aberration-suppressed, the luminance and the contrast of the image are notable improvements. In the quantitative computer simulation, the PSNR values of images are increased from 23.0185 to 32.1958. The floating light-field image results of magic cube mixed with real magic cube are also provided. Achieve such effect floating light-field display performance can be applied in many augmented reality domains, such as medical simulation, cultural relic demonstration, and commercial exhibition.
Funding
Fundamental Research Funds for the Central Universities (2016ZX01, 2018PTB-00-01, 2019PTB-018, 2019RC13, No. 248201913); National Natural Science Foundation of China (61575025, 61705014); National Key Research and Development Program of China (2017YFB1002900).
Disclosures
The authors declare no conflicts of interest. This work is original and has not been published elsewhere.
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