Design of a symmetric blazed grating sheet embedded
in an autostereoscopic display
Chien-Yue Chen,1,* Qing-Long Deng,2and Hui-Hsiung Lin3
1Department of Electronic Engineering, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan 2Institute of Photonic Systems, National Chiao Tung University, Tainan City 71150, Taiwan
3Industrial Technology Research Institute of Taiwan, Chutung, Hsinchu County 31040, Taiwan
*Corresponding author: chencyue@yuntech.edu.tw Received July 14, 2011; accepted August 8, 2011;
posted August 10, 2011 (Doc. ID 151062); published August 29, 2011
This study proposes a diffractive autostereoscopic display technology that utilizes blazed grating embedded in the liquid crystal panel to deliver a stereo image pair to both eyes. Having the diffractive red green blue beams as the color source of the panel, color filters are no longer required in this system. From the simulation analyses, not only could the brightness achieve 77.90%, but no serious chromatic aberration or cross talk appeared. © 2011 Optical Society of America
OCIS codes: 120.2040, 120.4570, 230.1950.
Autostereoscopic visual images are generated from the left-viewing image and the right-viewing image being directly delivered to the left and right retinas without wearing any devices so that the brain generates the stereopsis and determines the depth of the object through the binocular disparity of the two images [1]. Nonetheless, present technologies are not problem-free and common problems include insufficient brightness, moiré, and cross talk, so that users cannot watch for a long period of time [2,3]. Chen et al. first proposed dif-fractive optical elements (DOEs) to replace the present splitters of shaded and lenticular stereo image pairs, where DOEs were directly attached to the color filter of the panel so that the symmetric design separately gen-eratedþ1 order diffraction, which was the binocular po-sition to view stereoscopic images [4]. Su et al. further proposed producing the splitters of stereoscopic displays with holographic optical elements and proved the feasi-bility of diffractive stereoscopic splitting [5]. Direct attachment on panels was considered convenient; how-ever, abrasion resistance was required for the substrate of the material. Besides, the design of DOEs was based on specific wavelengths of red green blue (RGB) gener-ated by laser light, further complicating the process.
For this reason, this study proposed to embed blazed grating in the panel so that it could be protected from external damage, such as press, scratches, and spikes, to prolong the time of use. A single-size grating cycle could benefit from the simplification of the implementa-tion and the high efficiency of the yield. Moreover, the diffractive RGB beams being directly treated as the color sources of the panel allowed for the deletion of the color filter on the panel so that the panel could be made more lightweight, reducing production costs and resulting in easier mass production [6]. The entire struc-ture is shown as Fig.1. An 8:9 in: panel was utilized as the
reference basis. In order to achieve the disparity angle of binocular stereoscopic images, this study designed the highest diffractive intensity withþ1 order diffraction as the entrance pupil direction to generate the positions of left and right fields of view (FOVs) with a symmetric arrangement, such as the left and right zones shown in Fig.2.
The backlight applied the mixed light of three domi-nant wavelengths, namely R 625 nm, G 525 nm, and B 465 nm. With the calculation of Eq. (1), the relations between the diffraction angle and the cycle of blazed grating could be defined.
n sin θm¼mλp ; ð1Þ whereθmrepresents the diffraction angle,m ¼ 1 the dif-fraction order,T the horizontal size of the panel, E the binocular distance,n ¼ 1:5 the material of blazed grating, λ the wavelength, and P the blazed grating cycle.
Nevertheless, a planoconvex lens array sheet needed to be attached on the back of the blazed grating in order to accurately deliver the diffractive RGB beams to the RGB subpixel in the liquid crystal panel. Based on the acceptable light range of pupils, a prism sheet attached under the glass panel allowing the emergent light to enter
Fig. 1. (Color online) Structure diagram of diffractive stereo-scopic system and blazed grating.
T θ θ θ R L ER EL E Left zone Right zone Distance θm A
Fig. 2. (Color online) Optical path diagram of diffractive stereoscopic system.
3422 OPTICS LETTERS / Vol. 36, No. 17 / September 1, 2011
the allowable area of pupils could prevent it from chro-matic aberration. Referring to the panel specifications and the element structure in Fig. 3, the focal length of the planoconvex lens was 3:3 mm, the pitch being the panel pixel size 189 μm, the thickness 50 μm, and R1 and R2 being 863 μm and infinity, respectively. Figure3also shows the relations between the emergence angle of prism sheetθ4and the vertex angleα and the incidence angle θ1. The emergence angle θ4 was further defined as the relevant parameter of the prism sheet obtained from Eq. (2). θ4¼ sin−1 n sin sin−1 sinðθ1þ αÞ n − α : ð2Þ Having completed the design of the blazed grating, the lens array sheet, and the prism sheet, the optical simula-tion software LightTool was utilized to be embedded in the diffractive autostereoscopic display system, as shown in Fig.4. In the figure, when the backlight is pas-sing the blazed grating,þ1 order diffraction could indeed be generated and divided into three light sources. After passing the lens array sheet, RGB could accurately enter the subpixel areas of the panel, as shown in the arrange-ment on the right.
With rigorous coupled-wave analysis, when a blazed grating cycle was less than 1=10 wavelength, the change of the wavelength could affect the change of diffraction efficiency [7,8]. In this case, when 625 nm (λR), 525 nm
(λG), and 465 nm (λB) were passing the 4 μm blazed grating, the diffraction efficiency presented 72.52%, 84.08%, and 85.41%, respectively. With simulations and the calculation of Eq. (3), the overall system efficiency showed 77.90%, as shown in Fig.5. In comparison with traditional shaded stereoscopic displays, which have dark and bright alternate vertical lines attached on the panel, the brightness performance dropped down to 22.42%. The chrome-plated reflection grating proposed by Chen et al. in 2008, which could absorb the shaded grating or recycle the diffused light, could merely in-crease up to 43.2% [2], which was less than half of the original brightness. Consequently, the proposed splitting Fig. 3. (Color online) System structure and light path diagram.
Fig. 4. (Color online) Simulated light path of the diffractive stereoscopic system panel.
Fig. 5. (Color online) Brightness performance of diffractive autostereoscopic display system.
with DOEs could solve the problem of insufficient brightness.
Total efficiency¼PWoutput PWinput
× 100%: ð3Þ Aiming at the verification of chromatic aberration, color-matching was utilized. Having a set of magenta and cyan alternate images being simulated, the pair of images could be actually delivered to both the left and right FOV, as shown in Fig.6. Generally speaking, traditional stereoscopic displays were the stereoscopic images based on pixels, but shaded or cylindrical lenticular stereoscopic images were likely to suffer a chromatic aberration [2]. Having RGB diffraction as the color source of the panel, it was easier to control the move-ment of light; besides, with symmetric blazed grating to accurately separate left and right color lights, overlaps would not appear. Moreover, cross talk in the traditional autostereoscopic technology was about 2–10% [9]. From the simulation results, as shown in Fig. 7, the left FOV completely received the light, while the right FOV did not.
According to the above simulations, the proposed DOEs not only could successfully separate the required left and right images from the stereoscopic image, but the diffractive RGB beams could also replace color filters. Furthermore, the simulated results verified that the effi-ciency could reach 77.90%, which did not merely improve the problem of brightness reduction with traditional shaded technology, but also solved the problem of
chromatic aberration. Moreover, the symmetric splitting allowed the left and right images to be delivered to accu-rate positions that it did not cause cross talk in compar-ison with traditional atuotereoscopic displays.
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Fig. 7. (Color online) Simulation results of cross talk. 3424 OPTICS LETTERS / Vol. 36, No. 17 / September 1, 2011
