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Abstract
Large-scale three-dimensional (3D) display can evoke a great sense of true presence and immersion. Nowadays, most of the large-scale autostereoscopic displays are based on parallax barrier with view zone jumping, which also sacrifices much brightness and leads to uneven illumination. With a 3840 × 2160 LED panel, a large-scale horizontal light field display based on aspheric lens array (ALA) and holographic functional screen (HFS) is demonstrated, which can display high quality 3D image. The HFS recomposes the light distribution, while the ALA improves the quantity of perspective information in a horizontal direction by using vertical pixels and it can suppress the aberration that is mainly caused by marginal light rays. The 162-inch horizontal light field display can reconstruct 3D images with the depth range of 1.5 m within the viewing angle of . The feasibility of the proposed display method is verified by the experimental results.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the development of science and technology, the demand of three-dimensional (3D) display is increasing and numerous researches are performed to improve the 3D display quality [1–8] for the past few years. The most mature commercialized 3D display technologies are autostereoscopic displays based on parallax barrier or lenticular sheet [9–16], which can direct different parallax images to the left and right eyes and then provide the viewer with binocular disparity and 3D depth impression. The parallax barrier based 3D displays suffer from low brightness since the barrier blocks most of the light rays from the 2D display panel. Although lenticular sheet based 3D displays have high transmittance, inherent convergence–accommodation conflict for binocular disparity based 3D displays often leads to visual fatigue. Light field display can reproduce the light field distribution of the real 3D scene, which is considered as the most promising 3D display technology [17–22]. Unlike the binocular disparity based 3D displays, the light field display can reconstruct real 3D light field distribution, which is capable of providing audience with real and natural 3D perception. Our previous research introduced an interactive full-parallax light field display, which provide viewer with 9216 viewpoints within the viewing angle of [23].
Large-scale 3D displays can give audience an amazing 3D experience. The most popular large-scale autostereoscopic displays are based on parallax barrier, which suffer from convergence–accommodation conflict, low resolution and uneven brightness. The large-scale light field displays based on projector array and diffuse screen have been studied by many researchers [24,25,28]. The biggest advantage is that the spatial resolution does not decrease as the viewpoint increases. M. Kawakita, et al. introduced a 200-inch 3D display system based on projector array, diffuser film and Fresnel lens, which can create an immersive feeling for the audience [30]. However, these display methods require large longitudinal volume and it is difficult to improve the angular resolution since it depends on the number of projectors. LED panel is a common 2D display device for large-scale 3D displays due to the advantages of high brightness and expansibility. For traditional integral imaging based light-field display, the number of viewpoints depends on the pixel number covered by a pitch of the micro lens array. If the LED panel is adopted, the traditional integral imaging based light-field display cannot provide viewer with enough viewpoints since the pixel unit of LED is too large. Yizhou Huang, et al. proposed a 3D pixel mapping method for LED holoscopic 3D display [26], the resolution and display size were increased. However, there are few pixels covered by a single pitch, which leads to relative low viewpoint number and narrow viewing angle. To improve viewpoint number, our previous work demonstrated a large-size horizontal light-field display, which employs a micro-pinhole unit array to improve the number of horizontal viewpoints [27]. However, it sacrifices much brightness.
Here, a large-scale horizontal light field display based on ALA and HFS [28,29] is presented. To cover more pixels and improve the viewing angle, the pitch of ALA is increased to 15mm. To verify the feasibility of the proposed display method, a 162-inch light field display system is constructed, where the LED panel with the resolution of 3840 × 2160 is used to display the encoded elemental image array. The reconstructed high quality 3D images with the depth of 1.5 m can be perceived in the viewing angle of .
2. Principle of the proposed method
Figure 1(a) is the schematic of the full-parallax 3D light-field display that we demonstrated earlier [23]. Light rays from all pixels in the LCD panel illuminate every point on HFS after passed through the lens-array. Each point on the HFS emits multiple light rays in different directions, as if they are emitting from the real 3D object. The light field display system can provide full-parallax perspective information, but the resolution of the reconstructed 3D image is decreased. In our previous work, the 2D display device is a 5K LCD panel, and the demonstrated lateral resolution of 3D image is about 650. Different from the previous work, the large-scale horizontal-parallax-only light field display given as shown in Fig. 1(b), here a LED panel is used to display encoded elemental image array and an ALA is used to improve the lateral viewpoint number and resolution. The HFS is utilized to recompose the light distribution from the ALA to approximate the light-field distribution of the real 3D scene. The mask is placed between the LED and the ALA to prevent crosstalk of each lens.
Fig. 1 (a) Previous full-parallax light field display. (b) the proposed large-scale horizontal-parallax-only light field display.
Because of low pixel density of LED and inefficient utilization ratio of vertical parallax in large-scale 3D display, the pixels in the vertical direction is used to improve the horizontal viewpoint number and resolution. To simplify the analysis, the operation principle of ALA is given by three aspheric lenses, as shown in Fig. 2. The elemental images with different color at different height emit light bundle through the corresponding lens and image at different height on the HFS plane. The HFS processes the necessary optical transformation on these images. As shown in the front view of Fig. 2, the m th, m + 1 th and m + 2 th images are spread in horizontal and vertical directions. These elemental images can be composed together to reconstruct the 3D light field. The schematic of pixel arrangement is shown in the top view of Fig. 2. The viewpoints generated by different elemental images are cross arrangement. When the observers look at the ALA, they can obtain different perspective information from different lens. Although there is overlap between the adjacent viewpoints, the influence is slight due to there are dense viewpoints and the parallax of the adjacent viewpoints is small. The HFS is holographically printed with speckle patterns exposed on proper sensitive material and the diffusion angle is determined by the shape and size of speckles [17,28,29]. The diffusion property of HFS enables the light distribution from ALA be recombined. The diffusion angle of HFS should ensure that the light beams can be recomposed together. As shown in Fig. 3, the horizontal diffusion angle of HFS can be expressed as
where p denotes the horizontal pitch and D denotes the distance between the LED and HFS. The vertical diffusion angle should be as large as possible to ensure that the viewer can view the whole 3D image in the entire vertical direction.As shown Fig. 3, the display unit is composed of lenses, and , where thedenotes the m th row and n th column lens in ALA. Light rays emit from the m th, m + 1 th and m + 2 th row elements image through the corresponding lenses all contribute to the reconstructed 3D image. The viewpoint number and resolution are increased because pixels of LED panel in the vertical direction are used to produce the 3D light field in the horizontal direction. Assuming that the ALA consists of lenses and one lens covers r pixels, the biggest horizontal viewpoint number can be increased to r. The viewing angle is enlarged, which can be expressed as
where g denotes the distance between the LED and ALA. Different from the traditional integral imaging display, the lens here is not a micro lens. The pitch p is designed as large as possible to cover more pixels to improve the horizontal information capability. However, with the size of pitch p increasing, the field angle of the light rays passing through lens increases, which will aggravate the effect of aberration, especially for the marginal light rays. As shown in the top view of Fig. 3, the half field angle of the lens is increased towhere b denotes the vertical and horizontal distance between the nearest lens element. To improve the quality of the reconstructed 3D image, an aspheric lens is designed and fabricated to suppress the aberration. The aspheric model uses the base radius of curvature and the conic constant. The surface sag is given bywhere c is the vertex curvature, r is the radial coordinate, k is the conic constant and are the aspheric coefficients. The damped least-squares method is used to optimize the primary aberrations and other higher order optical aberrations. The optimized structure and corresponding parameters are shown in Fig. 4(a). Figure. 4(b) shows that the spot diagram of the aspheric lens is obviously improved, compared with the standard lens with the same focal length and the same diameter. The biggest root mean square (RMS) radius of the half field angle () of the designed aspheric lens is only 32.268 and the pixel pitch of the adopted LED panel is 0.9 mm. In this way, the high quality 3D image can be achieved. Ideally, because the different scattering characteristics of the HFS depend on the horizontal and vertical directions, the lens element of ALA should be designed to be anamorphic. However, considering the production difficulty and cost, rotationally symmetric aspheric surface is used. The proposed system displays 3D image only with horizontal parallax, thus the inconsistency of the distribution of the HFS and the lens element in vertical direction can be tolerated, and the experimental results verified it.Fig. 4 (a) The designed structure of the aspheric lens. (b) spot diagrams for the designed aspheric lens and the standard lens.
3. Experimental results
In order to verify the effectiveness of the introduced large-scale horizontal display method, a 162-inch 3D display system is constructed, where the 2D display device is a 162-inch LED panel. An ALA is placed in front of the LED panel, which consists of 240 × 146 lenses. A HFS is placed 800 mm away from the ALA. The light rays from all pixels in the LCD panel are modulated after passed through the ALA and HFS. Each point on the HFS emits multiple light rays in different directions, as if they are emitting from the real 3D scene. The main parameters of the introduced light field display are listed in Table 1. To preserve the density of horizontal viewpoints and the spatial resolution, 120 viewpoints is generated by combining with vertical pixels, and the 3D images with smooth horizontal motion parallax can be perceived. Here, the horizontal pitch of the ALA is 15 mm and the pixel pitch of LED panel is 0.9 mm. For the traditional light field display method, one lens covers 16 pixels, and only 16 horizontal viewpoints can be used. The backward ray tracing technique is used to generate elemental image array. The following experimental results are captured with Canon 60D camera at the viewing distance of 5000 mm.
Table 1. Parameters of the experiment.
The reconstructed 3D image (dinosaur fossil) with the light field display is captured from different directions, as shown in Fig. 5. The actual display result shows that the whole 3D image can be viewed within the viewing angle of without any jumping phenomenon. The depth of field of the 3D images is about 1.5 m, and the distances out of the screen and inside of the screen are about 1 m and 0.5 m, respectively. Because the light distribution from ALA is recomposed by HFS, the motion parallax is smooth and the luminance is uniform. Large-scale 3D light-field displays can find many applications, such as military exercise, cultural relic demonstration and commercial exhibition. Here, 3D display results of urban terrain and dinosaur fossil with the proposed 3D light field display system are shown in Fig. 6. We can clearly see the details of the 3D images and the relative depth position relation of different portion. The ALA is spliced with three parts for easy handling, which results in the two vertical bright lines of 3D images. The blinking block on the right side is caused by the broken pixels. In order to compare with the conventional large-scale 3D display, an autostereoscopic display system based on parallax barrier is assembled. As shown in Fig. 7(a), the experimental results shows that the luminance is low and nonuniform and the strip of parallax barrier is obvious. Besides, the same 3D scene (dinosaur fossil) is displayed by our previous system [27], which is based on a 54 inch LED panel with resolution of 1280720, as shown in Fig. 7(b). The result shows that the brightness is low. Specifications of the 54 inch LED panel are the same as those of the 162 inch LED panel. A luminance detector is used to measure the brightness of different display system. The luminance of the proposed system, the autostereoscopic display based on parallax barrier and our previous system are 223, 105 and 98 nits, respectively. The results show that brightness of 3D images is noticeably improved for the demonstrated light-field display system.
Fig. 6 Video of the experimental results (a) urban terrain (see Visualization 1) and (b) dinosaur fossil (see Visualization 2).
Fig. 7 Experimental results of (a) the autostereoscopic display based on parallax barrier and (b) our previous system (using pinhole array).
4. Conclusion
The traditional large-scale autostereoscopic displays based on parallax barrier can hardly reproduce the natural 3D image with smooth motion parallax in high resolution. Here, a 162-inch horizontal light-field display with the depth range of 1.5 m is demonstrated for viewing angle. The proposed ALA can increase the horizontal viewpoint number and resolution by combining with the vertical pixels. The vivid 3D image with uniform luminance is perceived since the HFS recomposes the light distribution. The proposed large-scale light field display can find many applications, such as military, cultural relic demonstration, education, biomedical and commercial exhibition.
Funding
National Key Research and Development Program (2017YFB1002900), National Natural Science Foundation of China (61575025), Fundamental Research Funds for the Central Universities (2018PTB-00-01).
Acknowledgment
This research is supported by the State Key Laboratory of Information Photonics and Optical Communications.
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