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Homogeneous free-form directional backlight for 3D display
Optics Communications 397 (2017) 112–117
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Homogeneous free-form directional backlight for 3D display a,b
a,b
c
a,b
a,b,⁎
Peter Krebs , Haowen Liang , Hang Fan , Aiqin Zhang , Yangui Zhou Kunyang Lia,b, Jianying Zhoua,b a b c
MARK a,b
, Jiayi Chen ,
State Key Laboratory of Optoelectronic Materials and Technology, Sun Yat-sen University, 510275 Guangzhou, China School of Physics, Sun Yat-Sen University, 510275 Guangzhou, China Mid-Technology, Inc. Co., 510275 Guangzhou, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Illumination design Displays Optical engineering
Realization of a near perfect homogeneous secondary emission source for 3D display is proposed and demonstrated. The light source takes advantage of an array of free-form emission surface with a specially tailored light guiding structure, a light diffuser and Fresnel lens. A seamless and homogeneous directional emission is experimentally obtained which is essential for a high quality naked-eye 3D display.
1. Introduction Optical illumination is a key issue in various branches of the optical science and engineering because of its widespread applications in solar energy harvesting, lighting, display and in scientific instrument. The illumination scheme, most frequently categorized as non-imaging optics [1,2], is in contrast to imaging optics where various optical aberration and distortion need to be taken into account. On the other hand, the requirement of the illumination for display is more stringent than with other non-imaging light illumination. As for conventional flat-panel 2D display, wide viewing angle and homogeneous illumination over the entire display surface are sufficient for most application purposes [3–7], while naked-eye 3D display, such as autostereoscopic display, relies on the use of functional optical elements, such as barrier and lens array, to achieve well-separated viewing channels for each eye [8–11]. Directional backlight illumination is now widely adopted to achieve high-quality autostereoscopic display [12–17] as well as energy-efficient display [18,19]. For conventional solar and LED lighting illumination, certain defect in the illumination area is tolerable. For display illumination, on the other hand, due to sensitive human visual system to the illuminance variation, the requirement on the illuminance homogeneity is much higher. Furthermore, visual system may even amplify defects, and this amplification is very pronounced with the so-called Mach bands [20]. Moreover, with a display system, viewers tend to move around during the viewing process, so that the added requirement should include the illuminance homogeneity insensitive to each viewing position. In this paper, a uniform backlight module consisting of free-form emission surface is proposed and experimentally demonstrated. By
⁎
introducing a specially tailored light-guide structure in conjunctions of volume scattering diffuser, seamless luminance distribution was obtained. Two major objectives are achieved with our directional backlight modular: highly uniform illumination on display screen and insensitive viewing position for the observed homogeneity, which are essential for naked eye 3D displays. 2. Design theory 2.1. Directional backlight 3D principle For typical directional backlight 3D (DB3D) displays [12], light emerging from one backlight unit (LED / light bar) illuminates one lens unit then converges into a viewing zone. The liquid crystal display (LCD) is set in front of the lens for refreshing left and right image sequentially at a refreshing rate of 120 Hz. As shown in Fig. 1, left and right light bars are located behind lens array and LCD screen. The flux emitting from left and right light bars is refracted by lens unit and then illuminates LCD panel before finally converging into left and right viewing zones. Usually, the viewing zones of an autostereoscopic display are prismatic shape in top view. If the human eyes locate at the preset viewing zones, when the LCD is refreshing left image, the corresponding left LED bar is turned on while right LED bar is off and vice versa in the next frame. The on and off left and right LED bars are synchronized with the refreshing left and right images on LCD. Therefore, when the LCD refreshing rate is operating at 120 Hz, the eyes at viewing zones can perceive 3D vision because of visual relaxation.
Corresponding author at: State Key Laboratory of Optoelectronic Materials and Technology, Sun Yat-sen University, 510275 Guangzhou, China. E-mail address: [email protected] (Y. Zhou).
http://dx.doi.org/10.1016/j.optcom.2017.04.002 Received 19 January 2017; Received in revised form 30 March 2017; Accepted 1 April 2017 0030-4018/ © 2017 Elsevier B.V. All rights reserved.
Optics Communications 397 (2017) 112–117
P. Krebs et al.
Fig. 1. Optical structure of DB3D display. Fig. 3. Luminance distribution scheme for different viewing points.
Fig. 4. Structure design of uninterrupted intelligent free-form surface backlight module.
Fig. 2. Backlight viewing zones arrangement of BD3D display.
2.2. Free-form surface backlight design As shown in Fig. 2, once the LED bars are well arranged behind the lens array and LCD panel, multi-viewing zones are formed at the optimum viewing distance. By increasing the number of LED bars, it should have provided a larger viewing angle and more viewing freedom in horizontal direction for DB3D display. However, two main problems arise. Firstly, an off-axis point source is unable to perfectly focus because of the inevitable aberration; therefore, the light bar with certain width will form an image on the viewing plane with a distorted distribution. As the off-axis angle θ becomes larger, the off-axis aberrations will result in dramatically degrade of uniformity. Therefore, the aberrations of lens needs to be corrected. Secondly, there are usually gaps between two light source units. Because the magnitude ratio from light bar region to viewing zone is relatively large, any small gap of light bar units might cause remarkable dark zone in the viewing space. In most cases, it is usually difficult to correct the spherical aberration and coma with a single lens [21]. Our previous work [16] shows that the light source positions arranged along a free-form curve could decrease the
Fig. 5. (a) The diagram of the backlight system for a 23 in. DB3D display; (b) the partial detailed structure for the middle backlight group.
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Table 1 The other designed parameters of the DB3D system. System parameters
Values
Focus length of Fresnel lens Width of Fresnel lens unit Number of backlight module Number of Fresnel lens array Number of LED bars in each unit Resolution of LCD Optimum viewing distance
121.2 mm 80 mm 5 5 22 1920*1080 90 cm
Fig. 8. Luminance distribution on different screen points for one viewing zone formed by light bars 1 & 2.
2.3. Uniformity map The evaluation of the uniformity in DB3D display is different from the conventional 2D displays. For 2D displays, luminance emitting from backlight unit near a 2π solid angle in the viewing space. However, for DB3D display shown in Fig. 2, the screen luminance perceived by eyes is viewing position dependent. Furthermore, luminance at different screen points perceived by eyes at the same viewing zone may not maintain the same value. Uniformity can be retrieved with traditional statistical approach tools [22–28]. Principally, the evaluation is based on minimummaximum ratio of luminance between the edge point and the middle point on the screen. The detailed mathematical explanation was presented in our previous published article [16]. Here we introduce a simple evaluation method to describe the uniformity index of DB3D display. As shown in Fig. 3, we take three extreme points on the screen, labeled as 1 (middle), 2 (left), and 3 (right); if one eye is at y1 on the
Fig. 6. Luminance distribution in viewing zone with an uninterrupted intelligent freeform surface backlight module.
off-axis aberrations introduced by a single lens. Based on the free-form calculation, we can find out the optimal LED bars arrangement to minimize the aberration on optimum viewing plane with one single lens.
Fig. 7. (a)Luminance distribution of group A; (b) crosstalk distribution in the viewing zone of group A; (c)Luminance distribution of group B; (d) crosstalk distribution of group B.
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Fig. 9. The luminance distribution at different position of the screen for all the viewing zones; from right to left, the curve is corresponding to light bars 1 & 2, 2 & 3,…10 & 11 (a) Luminance distribution for the point measured at the left edge, (b) at the middle, and (c) at the right edge of screen for different viewing zones. (d) The finally obtained distribution of the middle point, where the dash lines denote the switching positions governed by corresponding light bars.
To verify our design, we build up a directional backlight prototype with the free-form backlight modules for experimental measurement. The Fresnel lens is adhered on an LCD panel; the backlight module is set behind the Fresnel lens with 140 mm distance; and a spectroradiometer (Photo Research, Inc., PR655) is placed at the designed viewing distance of 90 cm to measure the luminance of the screen, as located in the viewing position as shown in Figs. 2 and 3. First of all, 4 adjacent LED bars in the module are labeled as 1, 2, 3, and 4 respectively (in Fig. 5). Every 2 adjacent LED bars are combined as one group. Thus, there are 3 Group denoted by 1 & 2, 2 & 3, and 3 & 4. Each group will be turned on separately when measuring the luminance. For these three groups, the luminance distributions at the viewing zone are shown in Fig. 6. As shown in Fig. 6, when the luminance of one group drops to less than 10% from its peak, the adjacent group will activate to assure the uniform illumination over a certain viewing area. Since the acceptable amount of luminance change is 20% in refer to EBU report [30], the luminance variation is not visible for an eye moving from one viewing zone to its adjacent one. When a viewer moves along the horizontal direction and is about to leave the viewing zone formed by LED bar 1 & 2, bar 1 will be turned off and bar 3 will be turned on, forming a new viewing zone by 2 & 3. Thus, dark-zone is no longer visible. We choose two groups of LED bars to test the performance of backlight module in different viewing angles. Group A consists of 5 LED bars in the middle of the module, which is labeled as LED bar 1, LED bar 2, LED bar 3, LED bar 4 and LED bar 5; while group B is formed by 5 LED bars at the edge of the module, which are labeled as LED bar 7, LED bar 8, LED bar 9, LED bar 10, and LED bar 11. Group A and B correspond to the center and edge viewing zones, respectively. Furthermore, crosstalk is an important standard of 3D display. As shown in Fig. 7(a) and (c), the peak luminance of group A and group B can be controlled at the same level of about 200 cd/m2; therefore, both the crosstalk in viewing area of group A and that of group B is around
viewing plane, the luminance of three main screen points perceived by eye at y1 are labeled as L11, L21, L31; if the eye move to y2 position, the three point luminance can be labeled as L12, L22, L32. Suppose the position on the screen is i, and the position of viewing point is j, the perceived luminance of point i viewing from point j can be label as L (i, j). The uniformity index U [10,29] can be denoted by:
U (%) =
L (i , j )min × 100% L (i , j )max
(1)
According to EBU report [30], the acceptable value of U is 80%. 3. Experiment Based on the calculation of free-form surface and the separation of different light source units, an uninterrupted backlight module is designed. As shown in Fig. 4, a Lambertian diffuser is arranged along the free-form surface, and the LEDs are arranged behind the diffuser with distance of 10 mm. Reflective films are inserted between two light source units to recycle stray light. As well, small concave grooves between each light source unit on the diffuser plane ensure the uninterrupted illumination, as shown in the right side in Fig. 4. To verify the design theory, a DB3D system is built up by 5 groups of freeform backlight modules arranged behind a piece of Fresnel lens array and a LCD panel. For each lens unit, there is a corresponding freeform curve in top view. The system arrangement schematic diagram is shown in Fig. 5(a) and the detail parameters of system design are shown in Table 1. The experimental backlight module is made with LEDs and a strong volume-scattering diffuser plate as shown in Fig. 5(b). The LEDs are arranged on the PCB boards. The diffuser plate is slotted at the joints between each light bar unit with about 1 mm groove depth; and then is bent along the calculated free-form surface forming the front emitting surface. Each light bar can be controlled to turn on or off independently by an electronic control processor. 115
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Fig. 10. A pair of 3D image generated by the DB3D display system. The upper are the left and right image while the below are the enlarged image of the crosstalk parts.
of the design. The measured results confirm that the free-form optical arrangement of backlight modules is able to eliminate the dark zones, giving rise to a global low crosstalk at the level of 3% over the available viewing area. Hence, this design provides an effective 3D display design scheme for satisfactory uniform illumination in a broad viewing range.
3%. As 5% crosstalk would cause uncomfortable feeling to the viewers [31], our design provides a feasible and effective scheme for comfortable 3D experience. However, because the off-axis sources (group B) lead to large aberration, the range with low crosstalk would be narrower. Finally, to evaluate the luminance uniformity of our design, the luminance on three points in the left, right and middle of the screen are measured. Fig. 8 shows the luminance distribution of three points on screen within a viewing zone formed by light bars 1 & 2. As shown in Fig. 8, the three luminance curves overlap with each other on the top part very well while deviation occurs at the edges. The curve of middle point is taken as standard, the other two curves are analyzed with the standard one to evaluate the uniformity by luminance deviation according to Eq. (3). Fig. 8 shows that the range from x=310 mm to 370 mm constitutes an effective viewing area. That denotes that the backlight design can provide about 60 mm width effective viewing zone, which is suitable for DB3D display application. The luminance distribution for each group in both three points is listed in Fig. 9(a)–(c). Fig. 9(d) shows the luminance profile in the available range formed by our backlight modules. The luminance keeps uniform and stable over a large range (from 100 to 400 mm) covered by the backlight modules. This experimental result confirms that the high luminance uniformity can be achieved at the same level for the whole screen. Finally, when the 3D images are displayed on the DB3D display, the display quality is shown in Fig. 10. The resolution maintains 1080P in both channels with high uniformity. The crosstalk remains nearly invisible throughout the entire screen, except for some overlapping in certain parts (also be magnified Fig. 10). Therefore, our proposed technique is able to provide high 3D display quality comparable to ever existing high quality 2D display.
Acknowledgment This work is supported by National Natural Science Foundation of China (NSFC) (11534017), and China Postdoctoral Science Foundation (2016M592572). References [1] R. Winston, J.C. Miñano, P.G. Benitez, Nonimaging Optics, Springer Berlin Heidelberg, 2001 (Chapter 1). [2] S.C. Shen, S.J. Chang, C.Y. Yeh, P.C. Teng, Design and testing of a uniformly solar energy TIR-R concentration lenses for HCPV systems, Opt. Express 21 (S6) (2013) A942–A952. [3] J.W. Pan, Y.W. Hu, Design of a hybrid light guiding plate with high luminance for backlight system application, J. Disp. Technol. 9 (12) (2013) 965–971. [4] J.W. Pan, C.W. Fan, High luminance hybrid light guide plate for backlight module application, Opt. Express 19 (21) (2011) 20079–20087. [5] K. Imai, I. Fujieda, Illumination uniformity of an edge-lit backlight with emission angle control, Opt. Express 16 (16) (2008) 11969–11974. [6] J.W. Pan, Y.W. Hu, Light-guide plate using periodical and single-sized microstructures to create a uniform backlight system, Opt. Lett. 37 (17) (2012) 3726–3728. [7] Y.H. Ju, J. Park, J.H. Lee, J. Lee, K. Nahm, J. Ko, Study on the simulation model for the optimization of optical structures of edge-lit backlight for LCD applications, J. Opt. Soc. Korea 12 (1) (2008) 25–30. [8] L. Kong, G. Jin, T. Wang, Analysis of Moiré minimization in autostereoscopic parallax displays, Opt. Express 21 (22) (2013) 26068–26079. [9] H. Kakeya, S. Sawada, Y. Ueda, T. Kurokawa, Integral volumetric imaging with dual layer fly-eye lenses, Opt. Express 20 (3) (2012) 1963–1968. [10] K.C. Huang, Y.H. Chou, L.C. Lin, H.Y. Lin, F.H. Chen, C.C. Liao, Y.H. Chen, K. Lee, W.H. Hsu, Investigation of designated eye position and viewing zone for a two-view autostereoscopic display, Opt. Express 22 (4) (2014) 4751–4767. [11] K.H. Yoon, H. Ju, I. Park, S.K. Kim, Determination of the optimum viewing distance for a multi-view auto-stereoscopic 3D display, Opt. Express 22 (19) (2014) 22616–22631. [12] J. Wang, H. Liang, H. Fan, Y. Zhou, P. Krebs, J. Su, Y. Deng, J. Zhou, High-quality autostereoscopic display with spatial and sequential hybrid control, Appl. Opt. 52 (35) (2013) 8549–8553. [13] H. Liang, S. An, J. Wang, Y. Zhou, H. Fan, P. Krebs, J. Zhou, Optimizing timemultiplexing auto-stereoscopic displays with a genetic algorithm, J. Disp. Technol. 10 (8) (2014) 695–699.
4. Conclusion An uninterrupted free-from backlight for directional 3D display homogeneous illumination is proposed and demonstrated in this paper. The free-form surface and the determination of light source size can effectively generate homogeneous viewing zones for 3D displays. A 3D system is built in our experiment to prove the validity 116
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lightpipes, Opt. Lett. 33 (11) (2008) 1165. [23] C.C. Sun, I. Moreno, S.H. Chung, W.T. Chien, C.T. Hsieh, T.H. Yang, Brightness management in a direct LED backlight for LCD TVs, J. Soc. Inf. Disp. 16 (4) (2012) 519–526. [24] D. Lim, K.K. Chu, J. Mertz, Wide-field fluorescence sectioning with hybrid speckle and uniform-illumination microscopy, Opt. Lett. 33 (16) (2008) 1819. [25] IESNA, The Lighting Handbook, 9th ed, Illuminating Engineering Society of North America, 2000. [26] Z. Qin, K. Wang, F. Chen, X. Luo, S. Liu, Analysis of condition for uniform lighting generated by array of light emitting diodes with large view angle, Opt. Express 18 (16) (2010) 17460–17476. [27] H. Yang, J.W.M. Bergmans, T.C.W. Schenk, J.P.M.G. Linnartz, R. Rietman, Uniform illumination rendering using an array of LEDs: a signal processing perspective, IEEE Trans. Signal Process. 57 (3) (2009) 1044–1057. [28] I. Moreno, Illumination uniformity assessment based on human vision, Opt. Lett. 35 (23) (2010) 4030–4032. [29] International Committee for Display Metrology, Information Display Measurements Standard, SID, 2012, Chap17. [30] EBU–TECH 3320, User requirements for Video Monitors in Television Production, 〈https://tech.ebu.ch/docs/tech/tech3320.pdf〉. [31] P.C. Wang, System cross-dimensional talk and three cue issues in autostereoscopic displays, J. Electron. Imaging 22 (1) (2013) 13–32.
[14] Y. Zhou, P. Krebs, H. Fan, H. Liang, J. Su, J. Wang, J. Zhou, Quantitative measurement and control of optical moiré pattern in an autostereoscopic liquid crystal display system, Appl. Opt. 54 (6) (2015) 1521–1527. [15] Y. Zhou, H. Fan, S. An, J. Li, J. Wang, J. Zhou, Y. Liu, Pseudo-random arranged color filter array for controlling moiré patterns in display, Opt. Express 23 (23) (2015) 29390–29398. [16] H. Fan, Y. Zhou, J. Wang, H. Liang, P. Krebs, J. Su, D. Lin, K. Li, J. Zhou, Full resolution, low crosstalk and wide viewing angle auto-stereoscopic display with a backlight unit on free-form surface, J. Disp. Technol. 11 (7) (2015) 620–624. [17] H. Yoon, S. Oh, D. Kang, J. Park, S. Choi, K.Y. Suh, K. Char, H. Lee, Arrays of Lucius micro prisms for directional allocation of light and autostereoscopic threedimensional displays, Nat. Commun. 2 (2011) 455. [18] S. Hsia, M. Sheu, J. C. C, S. Wang, High-performance local dimming algorithm and its hardware implementation for LCD backlight, J. Disp. Technol. 9 (7) (2013) 527–535. [19] S. Cha, T. Choi, H. Lee, S. Sull, An optimized backlight local dimming algorithm for edge-lit LED backlight LCDs, J. Disp. Technol. 11 (4) (2015) 378–385. [20] B.G. Von, Brightness distribution across the Mach bands measured with flicker photometry, and the linearity of sensory nervous interaction, J. Opt. Soc. Am. 58 (1) (1968) 1–8. [21] W.J. Smith, Modern Optical Engineering, SPIE PRESS, 2008 (Chapter 5). [22] F. Fournier, W.J. Cassarly, J.P. Rolland, Method to improve spatial uniformity with
117
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Homogeneous free-form directional backlight for 3D display a,b
a,b
c
a,b
a,b,⁎
Peter Krebs , Haowen Liang , Hang Fan , Aiqin Zhang , Yangui Zhou Kunyang Lia,b, Jianying Zhoua,b a b c
MARK a,b
, Jiayi Chen ,
State Key Laboratory of Optoelectronic Materials and Technology, Sun Yat-sen University, 510275 Guangzhou, China School of Physics, Sun Yat-Sen University, 510275 Guangzhou, China Mid-Technology, Inc. Co., 510275 Guangzhou, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Illumination design Displays Optical engineering
Realization of a near perfect homogeneous secondary emission source for 3D display is proposed and demonstrated. The light source takes advantage of an array of free-form emission surface with a specially tailored light guiding structure, a light diffuser and Fresnel lens. A seamless and homogeneous directional emission is experimentally obtained which is essential for a high quality naked-eye 3D display.
1. Introduction Optical illumination is a key issue in various branches of the optical science and engineering because of its widespread applications in solar energy harvesting, lighting, display and in scientific instrument. The illumination scheme, most frequently categorized as non-imaging optics [1,2], is in contrast to imaging optics where various optical aberration and distortion need to be taken into account. On the other hand, the requirement of the illumination for display is more stringent than with other non-imaging light illumination. As for conventional flat-panel 2D display, wide viewing angle and homogeneous illumination over the entire display surface are sufficient for most application purposes [3–7], while naked-eye 3D display, such as autostereoscopic display, relies on the use of functional optical elements, such as barrier and lens array, to achieve well-separated viewing channels for each eye [8–11]. Directional backlight illumination is now widely adopted to achieve high-quality autostereoscopic display [12–17] as well as energy-efficient display [18,19]. For conventional solar and LED lighting illumination, certain defect in the illumination area is tolerable. For display illumination, on the other hand, due to sensitive human visual system to the illuminance variation, the requirement on the illuminance homogeneity is much higher. Furthermore, visual system may even amplify defects, and this amplification is very pronounced with the so-called Mach bands [20]. Moreover, with a display system, viewers tend to move around during the viewing process, so that the added requirement should include the illuminance homogeneity insensitive to each viewing position. In this paper, a uniform backlight module consisting of free-form emission surface is proposed and experimentally demonstrated. By
⁎
introducing a specially tailored light-guide structure in conjunctions of volume scattering diffuser, seamless luminance distribution was obtained. Two major objectives are achieved with our directional backlight modular: highly uniform illumination on display screen and insensitive viewing position for the observed homogeneity, which are essential for naked eye 3D displays. 2. Design theory 2.1. Directional backlight 3D principle For typical directional backlight 3D (DB3D) displays [12], light emerging from one backlight unit (LED / light bar) illuminates one lens unit then converges into a viewing zone. The liquid crystal display (LCD) is set in front of the lens for refreshing left and right image sequentially at a refreshing rate of 120 Hz. As shown in Fig. 1, left and right light bars are located behind lens array and LCD screen. The flux emitting from left and right light bars is refracted by lens unit and then illuminates LCD panel before finally converging into left and right viewing zones. Usually, the viewing zones of an autostereoscopic display are prismatic shape in top view. If the human eyes locate at the preset viewing zones, when the LCD is refreshing left image, the corresponding left LED bar is turned on while right LED bar is off and vice versa in the next frame. The on and off left and right LED bars are synchronized with the refreshing left and right images on LCD. Therefore, when the LCD refreshing rate is operating at 120 Hz, the eyes at viewing zones can perceive 3D vision because of visual relaxation.
Corresponding author at: State Key Laboratory of Optoelectronic Materials and Technology, Sun Yat-sen University, 510275 Guangzhou, China. E-mail address: [email protected] (Y. Zhou).
http://dx.doi.org/10.1016/j.optcom.2017.04.002 Received 19 January 2017; Received in revised form 30 March 2017; Accepted 1 April 2017 0030-4018/ © 2017 Elsevier B.V. All rights reserved.
Optics Communications 397 (2017) 112–117
P. Krebs et al.
Fig. 1. Optical structure of DB3D display. Fig. 3. Luminance distribution scheme for different viewing points.
Fig. 4. Structure design of uninterrupted intelligent free-form surface backlight module.
Fig. 2. Backlight viewing zones arrangement of BD3D display.
2.2. Free-form surface backlight design As shown in Fig. 2, once the LED bars are well arranged behind the lens array and LCD panel, multi-viewing zones are formed at the optimum viewing distance. By increasing the number of LED bars, it should have provided a larger viewing angle and more viewing freedom in horizontal direction for DB3D display. However, two main problems arise. Firstly, an off-axis point source is unable to perfectly focus because of the inevitable aberration; therefore, the light bar with certain width will form an image on the viewing plane with a distorted distribution. As the off-axis angle θ becomes larger, the off-axis aberrations will result in dramatically degrade of uniformity. Therefore, the aberrations of lens needs to be corrected. Secondly, there are usually gaps between two light source units. Because the magnitude ratio from light bar region to viewing zone is relatively large, any small gap of light bar units might cause remarkable dark zone in the viewing space. In most cases, it is usually difficult to correct the spherical aberration and coma with a single lens [21]. Our previous work [16] shows that the light source positions arranged along a free-form curve could decrease the
Fig. 5. (a) The diagram of the backlight system for a 23 in. DB3D display; (b) the partial detailed structure for the middle backlight group.
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Table 1 The other designed parameters of the DB3D system. System parameters
Values
Focus length of Fresnel lens Width of Fresnel lens unit Number of backlight module Number of Fresnel lens array Number of LED bars in each unit Resolution of LCD Optimum viewing distance
121.2 mm 80 mm 5 5 22 1920*1080 90 cm
Fig. 8. Luminance distribution on different screen points for one viewing zone formed by light bars 1 & 2.
2.3. Uniformity map The evaluation of the uniformity in DB3D display is different from the conventional 2D displays. For 2D displays, luminance emitting from backlight unit near a 2π solid angle in the viewing space. However, for DB3D display shown in Fig. 2, the screen luminance perceived by eyes is viewing position dependent. Furthermore, luminance at different screen points perceived by eyes at the same viewing zone may not maintain the same value. Uniformity can be retrieved with traditional statistical approach tools [22–28]. Principally, the evaluation is based on minimummaximum ratio of luminance between the edge point and the middle point on the screen. The detailed mathematical explanation was presented in our previous published article [16]. Here we introduce a simple evaluation method to describe the uniformity index of DB3D display. As shown in Fig. 3, we take three extreme points on the screen, labeled as 1 (middle), 2 (left), and 3 (right); if one eye is at y1 on the
Fig. 6. Luminance distribution in viewing zone with an uninterrupted intelligent freeform surface backlight module.
off-axis aberrations introduced by a single lens. Based on the free-form calculation, we can find out the optimal LED bars arrangement to minimize the aberration on optimum viewing plane with one single lens.
Fig. 7. (a)Luminance distribution of group A; (b) crosstalk distribution in the viewing zone of group A; (c)Luminance distribution of group B; (d) crosstalk distribution of group B.
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Fig. 9. The luminance distribution at different position of the screen for all the viewing zones; from right to left, the curve is corresponding to light bars 1 & 2, 2 & 3,…10 & 11 (a) Luminance distribution for the point measured at the left edge, (b) at the middle, and (c) at the right edge of screen for different viewing zones. (d) The finally obtained distribution of the middle point, where the dash lines denote the switching positions governed by corresponding light bars.
To verify our design, we build up a directional backlight prototype with the free-form backlight modules for experimental measurement. The Fresnel lens is adhered on an LCD panel; the backlight module is set behind the Fresnel lens with 140 mm distance; and a spectroradiometer (Photo Research, Inc., PR655) is placed at the designed viewing distance of 90 cm to measure the luminance of the screen, as located in the viewing position as shown in Figs. 2 and 3. First of all, 4 adjacent LED bars in the module are labeled as 1, 2, 3, and 4 respectively (in Fig. 5). Every 2 adjacent LED bars are combined as one group. Thus, there are 3 Group denoted by 1 & 2, 2 & 3, and 3 & 4. Each group will be turned on separately when measuring the luminance. For these three groups, the luminance distributions at the viewing zone are shown in Fig. 6. As shown in Fig. 6, when the luminance of one group drops to less than 10% from its peak, the adjacent group will activate to assure the uniform illumination over a certain viewing area. Since the acceptable amount of luminance change is 20% in refer to EBU report [30], the luminance variation is not visible for an eye moving from one viewing zone to its adjacent one. When a viewer moves along the horizontal direction and is about to leave the viewing zone formed by LED bar 1 & 2, bar 1 will be turned off and bar 3 will be turned on, forming a new viewing zone by 2 & 3. Thus, dark-zone is no longer visible. We choose two groups of LED bars to test the performance of backlight module in different viewing angles. Group A consists of 5 LED bars in the middle of the module, which is labeled as LED bar 1, LED bar 2, LED bar 3, LED bar 4 and LED bar 5; while group B is formed by 5 LED bars at the edge of the module, which are labeled as LED bar 7, LED bar 8, LED bar 9, LED bar 10, and LED bar 11. Group A and B correspond to the center and edge viewing zones, respectively. Furthermore, crosstalk is an important standard of 3D display. As shown in Fig. 7(a) and (c), the peak luminance of group A and group B can be controlled at the same level of about 200 cd/m2; therefore, both the crosstalk in viewing area of group A and that of group B is around
viewing plane, the luminance of three main screen points perceived by eye at y1 are labeled as L11, L21, L31; if the eye move to y2 position, the three point luminance can be labeled as L12, L22, L32. Suppose the position on the screen is i, and the position of viewing point is j, the perceived luminance of point i viewing from point j can be label as L (i, j). The uniformity index U [10,29] can be denoted by:
U (%) =
L (i , j )min × 100% L (i , j )max
(1)
According to EBU report [30], the acceptable value of U is 80%. 3. Experiment Based on the calculation of free-form surface and the separation of different light source units, an uninterrupted backlight module is designed. As shown in Fig. 4, a Lambertian diffuser is arranged along the free-form surface, and the LEDs are arranged behind the diffuser with distance of 10 mm. Reflective films are inserted between two light source units to recycle stray light. As well, small concave grooves between each light source unit on the diffuser plane ensure the uninterrupted illumination, as shown in the right side in Fig. 4. To verify the design theory, a DB3D system is built up by 5 groups of freeform backlight modules arranged behind a piece of Fresnel lens array and a LCD panel. For each lens unit, there is a corresponding freeform curve in top view. The system arrangement schematic diagram is shown in Fig. 5(a) and the detail parameters of system design are shown in Table 1. The experimental backlight module is made with LEDs and a strong volume-scattering diffuser plate as shown in Fig. 5(b). The LEDs are arranged on the PCB boards. The diffuser plate is slotted at the joints between each light bar unit with about 1 mm groove depth; and then is bent along the calculated free-form surface forming the front emitting surface. Each light bar can be controlled to turn on or off independently by an electronic control processor. 115
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Fig. 10. A pair of 3D image generated by the DB3D display system. The upper are the left and right image while the below are the enlarged image of the crosstalk parts.
of the design. The measured results confirm that the free-form optical arrangement of backlight modules is able to eliminate the dark zones, giving rise to a global low crosstalk at the level of 3% over the available viewing area. Hence, this design provides an effective 3D display design scheme for satisfactory uniform illumination in a broad viewing range.
3%. As 5% crosstalk would cause uncomfortable feeling to the viewers [31], our design provides a feasible and effective scheme for comfortable 3D experience. However, because the off-axis sources (group B) lead to large aberration, the range with low crosstalk would be narrower. Finally, to evaluate the luminance uniformity of our design, the luminance on three points in the left, right and middle of the screen are measured. Fig. 8 shows the luminance distribution of three points on screen within a viewing zone formed by light bars 1 & 2. As shown in Fig. 8, the three luminance curves overlap with each other on the top part very well while deviation occurs at the edges. The curve of middle point is taken as standard, the other two curves are analyzed with the standard one to evaluate the uniformity by luminance deviation according to Eq. (3). Fig. 8 shows that the range from x=310 mm to 370 mm constitutes an effective viewing area. That denotes that the backlight design can provide about 60 mm width effective viewing zone, which is suitable for DB3D display application. The luminance distribution for each group in both three points is listed in Fig. 9(a)–(c). Fig. 9(d) shows the luminance profile in the available range formed by our backlight modules. The luminance keeps uniform and stable over a large range (from 100 to 400 mm) covered by the backlight modules. This experimental result confirms that the high luminance uniformity can be achieved at the same level for the whole screen. Finally, when the 3D images are displayed on the DB3D display, the display quality is shown in Fig. 10. The resolution maintains 1080P in both channels with high uniformity. The crosstalk remains nearly invisible throughout the entire screen, except for some overlapping in certain parts (also be magnified Fig. 10). Therefore, our proposed technique is able to provide high 3D display quality comparable to ever existing high quality 2D display.
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4. Conclusion An uninterrupted free-from backlight for directional 3D display homogeneous illumination is proposed and demonstrated in this paper. The free-form surface and the determination of light source size can effectively generate homogeneous viewing zones for 3D displays. A 3D system is built in our experiment to prove the validity 116
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