1. Introduction

Three dimensional (3D) display is the device that enables a user to perceive depth of an image for a given object. Holography is an ideal 3D display technology that can reconstitute both the intensity and wave-front information of an object. But the recording media are too slow to provide real-time perceive for viewers [1, 2]. Alternatively, electronic holography is considered to be a promising 3D display method [3–6]. Limited by the refreshable data volume, however, the state of the art SLM can hardly offer satisfactory 3D video. To suppress the processing the data volume, multiview 3D displays use a finite number of viewing zones to approximate a 3D scene. Nevertheless, an illusion of continuous parallax can be created by providing enough viewing zones [7, 8]. Multiview 3D displays based on geometrical optics, such as parallax barrier, lenticular, or micro lens [9–14], are generally criticized for providing limited resolution, ghost images and visual fatigue. Multiple projectors have also been employed to generate multiple viewing zones with high resolution [15–17], but it is difficult to implement the system into a portable device due to the complex system setup and the high cost.

By using a diffractive element to separate the phase information from the refreshable information, multiview 3D display based on diffractive optics provides wide viewing angle and small volume of processing data [18]. David Fattal et al. used an directional backlight to generate wide-angle full parallax view [19]. The technique attracted lots of attention, because it may be applied to mobile devices for its low-profile volume. In the technique, the directional backlight, as the most essential component in the design, re-directs the collimated incident light to several viewing zones. However, crosstalk between viewing zones can hardly be avoided due to the fixed spatial frequency and orientation of the diffractive gratings for each voxel. Furthermore, the size of the directional backlight, as well as the display screen, was limited by E-beam writing lithography. To solve the two problems, we proposed a lithography system to fabricate nano-gratings at a high throughput [20]. The period of the diffractive gratings can be tuned at a step as small as 1 nm. We have achieved a 64-view 3D static images with 50 degrees of field of view (FOV) by using a binary mask to provide the amplitude information of a light field.

In this paper, we propose a holography based multiview 3D display by combining a phase plate and a liquid crystal display (LCD). A 5.5 inch TFT-LCD with a pixel size of 95 μm is used to modulate the amplitude information at a rate of 20 frames per second. We fabricate a 5.5 inch 4-view phase plate with a full coverage of pixelated nano-gratings. The period and orientation of the nano-grating in each pixel are carefully calculated so that the transmitted light beam converge to four viewing points. As a result, the angular divergence of the converged light beam is confined to 1.02 degrees (FWHM) by average, slightly larger than the diffraction limit of 0.94 degrees. By refreshing the LCD panel, a series of animated sequential 3D images are dynamically presented at 4 viewing points. The resolution of each view is 640 × 360. Images for each viewing point are well separated and no ghost images are observed.

2. Multiview 3D holographic display

Figure 1 shows the schematic of the holography based multiview 3D display. The phase plate, as the most important optical element in the system, reconstructs the phase information of lightfield. A collimated incident beam is modulated by the phase plate, on which nano-gratings with various grating vector re-directs the emergent beam to the viewing points. To be specific, assume a voxel in the phase plate contains N nano-grating pixels such that N viewing points can be formed. Further assume the phase plate contains M voxels. Then there will be M × N pixelated nano-gratings in total. The period and orientation of each nano-grating are carefully calculated so that the transmitted light beam of the phase plate converge to the pre-defined viewing points. Well aligned with the phase plate, a LCD provide the refreshable amplitude information of lightfield. Finally, a multiview 3D images is formed and each view will be consisted of M pixels (Fig. 1 shows a voxel is consist of four nano-grating pixels for 4 viewing points).

 

Fig. 1 Schematic of the holography based multiview 3D display

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As shown in Fig. 1, The grating vector of the phase plate can be calculated by the holographic recording and readout theory [21]. The relationship between the incident beam and the emergent beam can be written as:

The orientation angle of the grating vector φ from the y axis can be calculated by:

Therefore, the propagation direction of the emergent beam is determined by the grating vector of the nano-grating in the phase plate. In other words, to converge the emergent light beam to the viewing points, the grating vector for each nano-grating should be calculated according to the position of the pixel in the phase plate. Assuming the wavelength of the incident beam is 532 nm, and the adjacent viewing points are separated by 3 degree, the increment for the period of nano-grating needs to be as small as 2 nm. The slight variation of the grating vector between each nano-grating brings up large difficulties in fabrication, especially when the size of the phase plate need to be larger than 4-inch.

3. Experiments and results

3.1 Fabrication of the phase plate

As shown in Fig. 2(a), a 5.5 inch TFT-LCD with a pixel size of 95 μm was used to modulate the image information at a rate of 20 frames per second. We designed a 4-view phase plate with an angle separation of 4 degree. The best viewing distance was set to be 200 mm from the LCD screen. To fabricate a 5.5 inch 4-view phase plate, as shown in Fig. 2(b), we pre-cleaned a glass plate and covered it with positive photoresist (RJZ-390, RUIHONG Electronics Chemicals). Then the glass plate was patterned with nanostructures by a homemade lithography system (NANOCRYSTAL200, SVG Optronics) at a speed of 20 mm²/mins [20]. The phase plate was consisted of nano-gratings with a period varied from 550 nm to 1000 nm. The size of each pixel in the phase plate was 50 μm × 65 μm, slightly smaller than the pixel size of the LCD, providing an alignment tolerance between the phase plate and the LCD. The SEM photo of the nano-grating array was shown in Fig. 2(c)-2(d). The total number of the pixels was 1280 × 720 for the 4-view phase plate, so the resolution of each view was 640 × 360. Each image loaded in the LCD panel was a combination of four parallax sub-images, designed by 3D computer graphic.

 

Fig. 2 (a) Photograph of the LCD. (b) Photograph of the 5.5 inch 4-view phase plate. (c) The SEM photo of the 5.5 inch phase plate with nano-gratings. (d) The blow-up SEM photo of nano-grating.

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3.2 Transmitted light intensity distribution

The phase plate was conglutinated at the back of the LCD to form a multiview 3D display system. To avoid crosstalk between adjacent pixels, each nano-grating in the phase plate was well aligned with the corresponding pixel in the LCD by an aligner. The optical properties of the system were measured under the illumination of a collimated laser beam with a wavelength of 532 nm. As shown in Fig. 3(a), the emergent light was focused to 4 well separated light spot at the distance of 200 mm from the LCD screen. By modulating the amplitude information on LCD, we lit up the viewing points one by one while kept the other three viewing points completely dark. Then we measured the intensity distribution of the transmitted light of all viewing points, as shown in Fig. 3(b). The viewing angle spanned from −6 degree to + 6 degree with an angle separation of 4 degree, in consistency with the calculation. The illumination of the each viewing point exhibited Gaussian distributions. The angular divergence was 1.02 degrees (FWHM) by average, slightly larger than the diffraction limit (0.94 degrees). The well confined illumination of each viewing angle was benefit from the continuously changed spatial frequency and orientation of nano-gratings in the phase plate. Moreover, no crosstalk was observed between the viewing points.

 

Fig. 3 (a) The emergent light was focused to 4 well separated light spot at a distance of 200 mm. (b) The transmitted light intensity distribution at each viewing point.

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3.3 4-view animated sequence 3D images

To produce stereoscopic videos, a series of animated parallax images were designed by 3D computer graphic. Four images having horizontal parallax were combined to one hybrid image according to the design of nano-gratings in the phase plate. The hybrid image displayed in the LCD panel was then separated into four images by the modulation of phase plate. A green light-emitting diode with 550 nm center wavelength was used as the light source to reduce laser speckle noise.

Figure 4 showed the independence of the light beam between different views. Firstly, a letter “A” to a letter “D” was projected to the viewing points of −6 degree to 6 degree, respectively. Then, the images projected to these four viewing points were replace by images of letter “E”, letter “F”, letter “G” and letter “H”, respectively. Next, letter “E” was changed to “I” until letter “H” was changed to letter “L”. Finally, letter “M” to letter “P” were shown at the four viewing points. As shown in Fig. 4, these four groups of letters repeated themselves and formed four groups of independent photos at time sequence. Ghost images could be ignored at the four viewing points.

 

Fig. 4 The images of four groups of letters taken at view1 to view 4 at time sequence.

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Although 4 parallax viewing points were designed in the 3D display prototype, we adopted only two of them to form stereoscopic images and videos, namely, view 1 and view 3 with a viewing angle of −6 degree and 2 degree, respectively. Figure 5 showed stereoscopic images by presenting images at view 1 and view 3. Letter “S” was projected to a shorter distance while letter “G” was shown at a larger distance from the observer. In Visualization 1 and Visualization 2, the letter “S” and the letter “G” rotated around the letter “V” by refreshing the LCD panel at a rate of 20 frames per second. Again, no ghost 3D images was observed in the proposed 3D display system.

 

Fig. 5 Images of scrolling letters with two parallax (see Visualization 1 and Visualization 2).

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We further loaded two more complicated parallax images on the LCD. Analogously, the parallax images could be reproduced to the pre-defined viewing points. Figure 6(a) showed a rotating automobile and Fig. 6(b) showed a running car through the trees. The image quality of the proposed 3D dynamic display based on holography can be further improved by using another SLM with better performance.

 

Fig. 6 (a) The 3D images of a rotating automobile (see Visualization 3 and Visualization 4). (b) The 3D images of a running car through the trees (see Visualization 5 and Visualization 6).

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4. Discussion and conclusions

In this paper, we have proposed a multiview 3D dynamic display by combining a phase plate and a LCD panel to provide the phase information and refreshable amplitude information of 3D images, respectively. We fabricated a 5.5 inch 4-view phase plate with a full coverage of nano-grating pixels using the lithography system (NANOCRYSTAL200, SVG Optronics) in the prior study at a speed of 20 mm²/mins. The viewing angle spanned from −6 degree to + 6 degree with an angular separation of 4 degree. A 5.5 inch TFT-LCD was used to modulate the image information at a rate of 20 frames per second. The proposed 3D display prototype was illuminated by a collimated laser beam with a wavelength of 532 nm. The angular divergence was 1.02 degrees (FWHM) averagely, slightly larger than the diffraction limit of 0.94 degrees. The well confined illumination of each viewing angle was benefit from the continuously changed spatial frequency and orientation of nano-gratings in the phase plate. The small discrepancy between theory and experiments might be attributed to the finite aperture and angular distribution of the incident light.

To prove the concept of the multiview 3D display system, we designed a series of animated parallax images using 3D computer graphic. Two images having horizontal parallax were combined to generate a hybrid image that was displayed on the LCD. The emergent light beams from the phase plate separated the hybrid image into two images, which were then project to the viewing points of −6 degree and 2 degree. The parallax images at these two views formed a stereoscopic image. The resolution of the image at each view was 640 × 360. The proposed multiview 3D display prototype have provided real-time 3D video without ghost image or crosstalk. The speckle noises in Fig. 5 and Fig. 6 were mostly caused by the coherence of the incident beam. We have reduced the speckle noise by using a light-emitting diode as the illuminating light source. The speckle noise can be further reduced by a rotating diffuser. The resolution of the stereoscopic images and the refreshing rate in the 3D dynamic display can be easily improved by using another SLM with better performance.

The prototype hold the promise to be used in the mobile electronics. For future work, collimated illumination through waveguide can be applied to the prototype. Moreover, chromatic 3D images can be realized by the integration of RGB color filter.