Single projector multiview displays: directional illumination compared to beam steering

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1 Single projector multiview displays: directional illumination compared to beam steering Lawrence Bogaert a, Youri Meuret a, Stijn Roelandt a, Aykut Avci b, Herbert De Smet b,c and Hugo Thienpont a a Vrije Universiteit Brussel, Department of Applied Physics and Photonics, Pleinlaan 2, 1050 Brussels, Belgium; b Ghent University, Center for Microsystems Technology, Technologiepark 914, 9052 Ghent, Belgium; c Interuniversity MicroElectronics Center, Information Technology Division, Technologiepark 914, 9052 Ghent, Belgium ABSTRACT We present two multiview rear projection concepts that use only one projector with a digital micromirror device light modulator. The first concept is based on time sequentially illuminating the light modulator from different directions. Each illumination direction reflects on the light modulator toward a different viewing zone. We designed an illumination system that generates all distinct illumination beams and a lens system integrated into the projection screen to enlarge the viewing angles. The latter is crucial since the viewing extent of the viewing zones decreases inversely proportional to the size of the projected image. A second concept is based on a specific projection screen architecture that steers images into different horizontal directions. In this way, the entire acceptance étendue of the projection system can be used for every image. This is achieved by moving a double-sided lenticular sheet horizontally with respect to a sheet of microlenses with a square footprint. Both concepts are investigated with advanced optical simulations. Keywords: Three-dimensional display, multiview, projection display, digital micromirror device, time multiplexing, optical design 1. INTRODUCTION Multiview three-dimensional (3-D) displays show different views of an object or a scene in different directions. 1 At any viewing position, two different views are seen by both eyes stimulating depth perception. For this, the viewer does not have to wear any special eyeglasses as is the case with stereoscopic displays currently implemented in theaters and recently also in the home using flat panel displays. Because more than two different views are used, the viewer can see a different 3-D view of the image depending on the viewing position. This enables look around capability what increases the realism of the 3-D sensation. Such displays are currently available with a limited number of views (5 up to 9) and small spatial resolution. These displays are implemented using a flat panel display in combination with a lenticular or parallax barrier. 2 They are among others being commercialized by companies like Alioscopy, Tridelity and formerly also by Philips. Because of their high cost, they are currently strictly sold to businesses. The cost of the technology will have to decrease significantly to reach the consumer market. In lenticular-based systems, each cylindrical lens is aligned with a specific number of sub pixels of the display. The light emitted by each sub pixel is imaged toward a different direction. To obtain different images that propagate toward different directions, the image information of all views is spatially multiplexed over the display surface area. When the lenses are oriented vertically, the spatial resolution of the display is decreased in the perpendicular direction with a factor equal to the number of viewing zones the display is capable of showing. For this reason, the lenses are slanted to spread the resolution loss in both the horizontal and vertical direction. For a system with 9 views, this implicates a horizontal and Further author information: (Send correspondence to Lawrence Bogaert) Lawrence Bogaert: lbogaert@tona.vub.ac.be, Telephone: +32 (0) Stereoscopic Displays and Applications XXI, edited by Andrew J. Woods, Nicolas S. Holliman, Neil A. Dodgson, Proc. of SPIE-IS&T Electronic Imaging, SPIE Vol. 7524, 75241R 2010 SPIE-IS&T CCC code: X/10/$18 doi: / SPIE-IS&T/ Vol R-1

2 vertical resolution loss with a factor 3. So, a full HD screen (1920x1080 pixels) shows 3-D images with a resolution of 640x360. Parallax barriers have the same resolution loss issues. Furthermore, the brightness of the display is lower because part of the light is blocked to generate the viewing zones. Another approach uses the projection of images to show all viewing zones. For this, the same number of light modulators as viewing zones and a common imaging system can be used. 3 The resolution of the light modulators is maintained and no stringent requirements are imposed on the modulation speed. For this reason, high resolution liquid crystal on silicon (LCOS) light modulators can be considered. Another approach uses the same number of projectors as viewing zones. 4 Both methods have the advantage that they can show a large number of views. This can solve the accommodation convergence conflict that leads to eye fatigue when viewing 3-D images with negative parallax (images appear in front of the screen). On the other hand, the systems are complex, large and expensive. A system with only one projector can provide a solution for this. This is possible by placing a configurable slit aperture in the projection lens that only transmits one viewing direction. 5 7 With increasing number of views, more light is blocked by the aperture and therefore the emitted light flux of the system decreases. Again, the resolution of the light modulator is maintained. The light modulator has to be sufficiently fast to time sequentially modulate all the images such that moving images can be seen in all viewing zones. This is possible with digital light processing technology of Texas Instruments, 8 in which a digital micromirror device (DMD) is used to modulate the images. The pixels on the screen are formed with micromirrors that have a very high switching speed. If this is not sufficient to modulate all images, the required modulation speed can still be obtained by decreasing the number of gray levels of the images. In this paper, we focus on projection-type methods using a single projector because these systems can enable many viewing zones without resolution loss of the imaging device at a reasonable system cost. We introduce and discuss two multiview concepts that do not exhibit the limitation of low light throughput of other single projector approaches, and investigate both concepts with advanced optical simulations. 9 A first concept is based on illuminating the DMD light modulator from different directions within the angular acceptance cone in combination with a viewing angle enlarging projection screen. A second concept does not require a specific illumination of the DMD light modulator. It is based on a projection screen architecture with a movable component that steers images into different directions. Both concepts apply to rear projection systems. The paper is organized as follows. In Section 2, we discuss the design of a projector that generates images that propagate toward different viewing zones by illuminating a DMD light modulator from different directions. This includes the optical design of the illumination system and the viewing angle enlarging projection screen. In Section 3, we discuss the design of an alternative system that uses a specific projection screen architecture to steer images toward different horizontal viewing zones. This is implemented for two sizes of the projected image. Finally, we draw conclusions in Section GENERATING VIEWING ZONES BY ILLUMINATING A DMD LIGHT MODULATOR FROM DIFFERENT DIRECTIONS 2.1 Basic principle A DMD light modulator consists of an array of square-shaped micromirrors that can be individually switched between two orientation states: an on and off state. These are obtained by rotating the micromirrors 12 about a hinged diagonal axis. This is shown in Fig. 1. When a micromirror is in the on state, light is reflected to the projection lens and a bright pixel is seen on the projection screen. When a micromirror is in the off state, light is reflected to a light absorber and the pixel is dark. Each micromirror has a half-acceptance angle of 12 in the tilt direction. In the perpendicular direction, this angle can be larger as it will not influence the contrast ratio. 10 In this way, the system étendue of a DMD projection system can be enlarged if the f-number of the projection lens is decreased accordingly. A projector with one DMD light modulator can generate images that propagate toward different viewing zones by time sequentially illuminating the DMD light modulator with a light beam that has different angular distributions. For this, we divide the angular acceptance cone of the DMD light modulator into different areas that correspond to the different viewing zones. To guide light into a certain direction, the DMD light modulator is illuminated with the corresponding angular distribution. During this illumination period, the image for that SPIE-IS&T/ Vol R-2

3 Off state 1 DMD mirror On state Light absorber Off-state light cone On-state light cone Projection lens Illumination light cone Toward projection screen (a) (b) Figure 1. DMD functionality. The light steering action of a single micromirror is illustrated in (a) and a compact optical architecture for DMD projection systems in (b). Air gap DMD Illumination light ray Projection lens TIR prism Toward projection screen viewing zone is modulated by the DMD light modulator. The modulated image is then focused onto the rear of a dedicated projection screen. It consists of a Fresnel lens and a double-sided lenticular sheet that collimate and enlarge the viewing zones respectively. 2.2 Subdivision of the DMD s angular acceptance distribution into viewing zones To obtain horizontal viewing zones, we divide the DMD s angular acceptance cone along the direction that corresponds with projection in the horizontal plane. This direction is oriented 45 from the micromirrors tilt direction because of the specific orientation of the torsion hinges. Vertical viewing zones are obtained by an additional subdivision in the direction perpendicular to that for horizontal viewing zones. This is shown in Fig. 2 for five horizontal and three vertical viewing zones. We choose the same angular surface area for each light beam. By doing so, all modulated images will have the same viewing extent. This technique can be extended to implement any number of viewing zones. We remark that due to the directions for horizontal and vertical viewing zones, it is not possible to realize symmetrical viewing zones using the extended angular acceptance distribution of the micromirrors. 2.3 Illumination system for creating many distinct light beams The DMD light modulator has to be uniformly illuminated by each individual light beam. This is not practical with an equal number of separate illumination systems. Therefore, we have designed one optical system that is capable of creating all necessary light distributions at the DMD light modulator. The angular specifications are obtained with a lens system that transforms a spatially circular-shaped light distribution to an angularly Micromirror D MD On state 24 Rotation axis Micromirror s tilt direction Figure 2. Subdivision of the DMD s angular acceptance cone in fifteen areas that correspond with five horizontal and three vertical viewing zones. SPIE-IS&T/ Vol R-3

4 circular-shaped light distribution. 11 Uniform illumination of the DMD light modulator is realized by placing a lenslet integrator at the input of this optical system. 12 We simulated this for a multiview display with five horizontal viewing zones. This is shown in Fig. 3. The simulations are performed using the nonsequential raytracing program ASAP from Breault Research Organization. We remark that nonoverlapping spatial zones at the input of the illumination system have to be transformed to nonoverlapping angular zones at the DMD light modulator. This is taken into account when optimizing the lens parameters since angular overlap will result in crosstalk between adjacent viewing zones. The lens system uses three bi-convex lenses of which the second and third are identical and placed against each other. These can be replaced by a single lens with a higher optical power at the cost of increasing the angular overlap of adjacent viewing zones. This is shown in Fig. 4. We simulate an optical efficiency of 85% from the input of the illumination system to the DMD light modulator (at 550 nm). All optical interfaces are antireflection coated. We used a DMD light modulator with a 0.7 inch diagonal, 4:3 aspect ratio and a pixel pitch of μm (1024x768 pixels). The input light distribution for the illumination system can be created using a transparent liquid crystal light modulator that is uniformly illuminated. The light modulator is electronically controlled to generate different patterns that correspond with the spatial distributions for different viewing zones. This is similar to placing a configurable slit aperture in the projection lens and has the same drawback that more light needs to be blocked when the number of viewing zones is increased. Therefore, the optical throughput of the system will be low. It is more efficient to use light sources with small étendue that can be individually addressed. Light emitting diodes can be used, but their luminance is limited. Laser light sources, on the other hand, combine a small étendue with a high luminance. For that reason, they will provide the brightest images. Laser light sources however require Modulated light distribution Projection lens D MD DMD Illumination light beam TIR prism Lens system Enlarged projected image Input light distribution Figure 3. Ray-tracing model of the illumination system with indication of the light distributions at different positions. The DMD light modulator is uniformly illuminated by all five light beams. (a) (b) Figure 4. Reflected viewing zones on the DMD with all micromirrors in the on state for an illumination system using two lenses (a) and three lenses (b). SPIE-IS&T/ Vol R-4

5 mechanisms to reduce speckle in the projected images Rear projection screen for enlarging the viewing angles In a rear projection system, images are focused onto the rear of the projection screen with a certain magnification M. The viewer and projector are located on opposite sides of the screen. In this configuration, the viewing zones created by illuminating the DMD light modulator from different directions, will be distributed in different directions depending on the position in the image. This is shown in Fig. 5a. To correct this, we place a large Fresnel lens at the position of the projection screen that directs all viewing zones in the same direction over the entire screen surface. This is shown in Fig. 5b. We designed an aspheric Fresnel lens with a sawtooth-shaped profile to optimize this collimation. Its focal length is equal to the optical path between the projector and the projection screen. The resulting collimated light distribution is shown in Fig. 6. We remark that this nonimaging component induces some crosstalk between adjacent viewing zones. The viewing angle of the collimated viewing zones is M times smaller than the modulated angular distribution at the DMD light modulator. This means that the total viewing angle of the system will be extremely small. If we place a diffusing projection screen after the Fresnel lens, all viewing zones will be scrambled leading to unacceptable crosstalk of the viewing zone information. Thus, a specific viewing angle increasing mechanism is required. We designed a double-sided lenticular sheet to enlarge the viewing angles after the Fresnel lens. It consists of an array of convex cylindrical lenses followed by an array of concave cylindrical lenses with focal length f a en f b respectively. The cylindrical lenses are oriented vertically and their footprint corresponds to the size of the pixels at the projection screen. The optical system increases the horizontal propagation angle with a factor f a /f b and maintains the propagation direction. This is shown in Fig. 7. We simulate that the angular magnification is the same for all viewing angles. The enlarged viewing zones are shown in Fig. 8. The crosstalk that is induced by the Fresnel lens is also magnified by the optical system. We further remark that the pixel size at the output of the projection screen decreases in the horizontal direction with a value equal to the angular magnification. In the vertical direction, the propagation angle nor pixel size are changed. The vertical propagation angle can be enlarged by implementing a vertical diffuser at the projection screen. 3 Projection lens Projection lens Light rays Light rays Viewing zones Projected image f Fresnel lens Fresnel lens Viewing zones (a) (b) Figure 5. Rear projection system. In (a) the viewing zones propagate in different directions dependent on the position in the projected image. In (b) the viewing zones are directed in the same direction by a Fresnel lens. 12 / (5M) 12 / M Figure 6. Angular distribution of the collimated viewing zones over the entire screen surface for a magnification M of 10 times. SPIE-IS&T/ Vol R-5

6 f a f b Lens a Lens b Figure 7. Optical system to enlarge the horizontal viewing angles. Both lenses are part of a double-sided lenticular sheet. 12 / (5M) 12 / M x f a / f b Figure 8. Angular distribution of the horizontally enlarged viewing zones over the entire screen surface with a factor f a/f b equal to 10 (M=10). 2.5 Discussion Recently a prototype multiview projection display was presented that makes use of directional illumination to generate viewing zones. 14 It consists of a DMD light modulator that is time sequentially illuminated by 15 LEDs with the same color. In this way, 15 monochrome images are created that propagate toward 15 horizontal viewing zones. This system is part of four identical display modules of which the output is combined to show 60 viewing zones by a common Fresnel lens. The images are shown on a vertical diffuser. This is necessary because the four modules and the LEDs are arranged with a vertical offset because of space limitations. No angle enlarging system is provided at the screen. Because of this, the image sizes that can be obtained with this system are small since the angular extent of all viewing zones is decreased with the same factor as the image magnification. 3. GENERATING VIEWING ZONES BY BEAM STEERING AT THE PROJECTION SCREEN 3.1 Basic principle When viewing zones are created by illuminating the DMD light modulator from different directions, only a limited part of the system étendue is used for each image which limits the possible light output of the system. Therefore, we have investigated an approach that does not pose this problem. This is possible by creating viewing zones with a specific projection screen architecture that sequentially steers images toward different directions. In this way, the entire system étendue can be used to modulate each image. The projection screen consists of two parts. First, a large Fresnel lens is used to collimate the light rays. This has already been discussed in Section 2.4. The imaging characteristics of this component are not relevant here because the viewing zones have not yet been defined when passing through it. Next, an optical system with a movable component is used to control the propagation direction of the images. 15 This movable component has to be synchronized with the image information and it has to be periodically moved by an accurate mechanical translation system. 3.2 Optical system to create a steerable light beam We have investigated an optical system that uses three lenses to generate a light beam with parallel light rays of which the horizontal propagation angle can be changed. This is shown in Fig. 9. These lenses are part of three lens arrays with the same size as the projected image that enable steering of the entire image toward a specific viewing zone. The footprint of the lenses correspond to the size of the pixels in the projected image. SPIE-IS&T/ Vol R-6

7 δ φ f α d (a) (b) Figure 9. Optical system to create a steerable light beam in the horizontal direction. Illustrated in the horizontal (a) and vertical (b) plane. First, a lens with a square footprint is used to focus the collimated light by the Fresnel lens in both the horizontal and vertical direction. This is followed by two cylindrical lenses that simultaneously move in the plane perpendicular to the optical axis. Both movable lenses have the same focal length and are separated by adistanced equal to their focal length. The second lens is placed at the back focal plane of the first lens with focal length f α. Light rays can be steered in different horizontal directions by controlling the horizontal position δ of the movable lenses with respect to the first lens. The movable lenses do not affect the vertical propagation direction of the output beam because they do not focus light in this direction. The horizontal steering angle ϕ can be calculated by: ( ) δ ϕ = atan (1) d We introduce a parameter r that expresses the ratio of the size of a pixel at the output of the steering projection screen compared to the input of the projection screen. The latter cannot be larger than one because otherwise adjacent pixels at the output of the projection screen will overlap. The distance d can be expressed as the product of f α and r. This is valid if we assume that the lenses are surrounded by air. Eq. (1) can be reformulated as: ( ) δ ϕ = atan R α (n 1) r, where we have replaced f α with the lens-maker s formula in paraxial approximation. The maximal horizontal steering angle ϕ max is obtained for the value of δ equal to half the footprint of the lenses and for the smallest value of the radius of curvature of the first lens R α.thisis 2 times larger than half the footprint of the lenses because the first lens has a square footprint. If we bring this into account in Eq. (2), we find that ϕ max is dependent of the refractive index n and the pixel size ratio r: ( ) (n 1) ϕ max = atan (3) 2r If the lenses are implemented in a transparent plastic such as PMMA with a refractive index n of 1.48, we calculate a maximal steering angle, in both horizontal directions, of 18.7 for r equal to 1 and 34.2 for r equal to Divergence of the output beam We have implemented the movable lens arrays as one component consisting of a PMMA sheet with cylindrical lenses on both sides. The thickness of the sheet is n pmma times larger than the required distance in air. This is because light rays, at the air-plastic interface, refract toward the optical axis and necessitate a longer distance to realize a certain r. The first lens array is also implemented in PMMA. We have chosen to simulate the optical system with the following parameter values: r equals 0.8, the maximal δ equals a quarter of the footprint of the lenses and f α is 50% larger than its smallest value. For these parameter values, we calculate a maximal steering angle of 8 using Eq. (2). We simulate that the divergence of the output beam is minimal when the first lens focuses light at the vertex of the second lens. To minimize spherical aberrations that increase the spot size, the lenses of the first lens (2) SPIE-IS&T/ Vol R-7

8 array have an aspheric profile. When the value of δ is increased, the focused light spot moves away from the vertex position. Because of this, the divergence of the output beam increases. We have optimized the thickness of the movable lens sheet to realize the same divergence in all steering directions and to minimize its overall value. We implemented this for two multiview projection systems: one that steers images that are enlarged 10 times by the projection lens and one that steers images that are enlarged 50 times. For this we use the same DMD light modulator as in Section 2.3. The parameters of the lens sheets are shown in Fig. 10a and Fig. 10b respectively. We simulate a half-angle divergence of 1.7 in all steering directions for the 10-times enlarged image and 0.35 for the 50-times enlarged image. We remark that the divergence is smaller for the system that steers the larger image. This is due to the angular extent of the collimated light rays by the Fresnel lens that is smaller 100 µm 302 µm 300 µm PMMA PMMA R 1 = -145 µm R 2 = 116 µm R 3 = -116 µm K 1 = -(n pmma) µm µm (a) 100 µm 1511 µm 1470 µm PMMA PMMA R 1 = -725 µm K 1 = -(n pmma) 2 R 2 = 580 µm R 3 = -580 µm 684 µm µm (b) Figure 10. Horizontal cross-section of the optimized steering system for a 10-times (a) and 50-times (b) enlarged image. Shown for zero and maximal considered shift (footprint/4) of the movable lens sheet. SPIE-IS&T/ Vol R-8

9 for the larger image. Therefore, the focused light spot at the vertex position of the second lens is also smaller and induces less divergence of the output beam. We also simulate a horizontal steering angle of 8.7 and 8.9 for the 10-times and 50-times enlarged image respectively. This corresponds to the estimated value in paraxial approximation for a horizontal shift of a quarter of the footprint of the lenses. 4. CONCLUSION We have investigated two concepts that enable potentially low-cost high-resolution multiview 3-D projection displays using one projector with a DMD light modulator that time sequentially modulates images in a rear projection setup. The first concept makes use of time sequential illumination of the DMD light modulator with light beams with different angular distributions. Because these light beams have different propagation directions, they reflect on the DMD light modulator toward different viewing zones. The extent of the viewing zones decreases inversely proportional to the size of the projected image. Therefore, this approach is combined with a dedicated projection screen that increases the viewing angles. For this, we have designed a double-sided lenticular sheet. By dividing the angular acceptance distribution of the DMD light modulator, several horizontal and vertical viewing zones can be created. This subdivision however decreases the étendue of the individual illumination light beams which limits the possible light output of the system. The second concept is no longer subject to this étendue limitation. For this, we have designed a projection screen that time sequentially steers images into different directions using the entire acceptance étendue of the projection system. The projection screen consists of two lens sheets of which the second can move. By changing its horizontal position, the images are steered toward different horizontal viewing zones. We optimized the system parameters to obtain a parallel output beam with the same divergence in all steering directions. We conclude that the second approach is a viable technique to create multiview displays with high light output using existing projection displays. ACKNOWLEDGMENTS This work is supported by the Research Foundation - Flanders (FWO-Vlaanderen). The project is titled Compact LCOS projection displays for high-quality 3D images with high spatial and angular resolution. Lawrence Bogaert is indebted to FWO-Vlaanderen for an Aspirant grant. REFERENCES [1] Son, J. and Javidi, B., Three-dimensional imaging methods based on multiview images, J. Display Technol. 1(1), (2005). [2] Willemsen, O., de Zwart, S., Hiddink, M., de Boer, D., and Krijn, M., Multi-view 3D displays, SID Symposium Digest 38, (2007). [3] Takaki, Y., High-density directional display for generating natural three-dimensional images, Proc. IEEE 94(3), (2006). [4] Balogh, T., The HoloVizio system, in Stereoscopic Displays and Virtual Reality Systems XIII, Woods, A., Dodgson, N., Merritt, J., Bolas, M., andmcdowall, I., eds., Proc. SPIE 6055, 60550U (2006). [5] Kupiec, S., Markov, V., Hopper, D., and Saini, G., Multiview multiperspective time multiplexed autostereoscopic display, in Stereoscopic Displays and Applications XIX, Woods, A., Holliman, N., and Merritt, J., eds., Proc. SPIE 6803, 68030N (2008). [6] Cossairt, O., Moller, C., Travis, A., and Benton, S., Novel view sequential display based on DMD technology, in Stereoscopic Displays and Virtual Reality Systems XI, Woods, A., Merritt, J., Benton, S., and Bolas, M., eds., Proc. SPIE 5291, (2004). [7] Son, J., Komar, V., Chun, Y., Sabo, S., Mayorov, V., Balasny, L., Belyaev, S., Semin, M., Krutik, M., and Jeon, H., A multiview 3-D imaging system with full color capabilities, in Stereoscopic Displays and Virtual Reality Systems V, Bolas, M., Fisher, S., and Merritt, J., eds., Proc. SPIE 3295, (1998). [8] Hornbeck, L., Digital light processing for high-brightness high-resolution applications, in Projection Displays III, Wu, M., ed., Proc. SPIE 3013, (1997). SPIE-IS&T/ Vol R-9

10 [9] Bogaert, L., Meuret, Y., and Thienpont, H., Multiview three-dimensional displays using a single projector: analysis of two novel concepts, J. Display Technol. (2010). Accepted for publication. [10] Cheng, C. and Chern, J., Design of a dual-f-number illumination system and its application to DMD projection displays, J. Soc. Inf. Display 14(9), (2006). [11] Bogaert, L., Meuret, Y., Van Giel, B., De Smet, H., and Thienpont, H., Design of a compact projection display for the visualization of 3-D images using polarization sensitive eyeglasses, J. Soc. Inf. Display 17(7), (2009). [12] Van Giel, B., Meuret, Y., and Thienpont, H., Using a fly s eye integrator in efficient illumination engines with multiple light-emitting diode light sources, Opt. Eng. 46(4), (2007). [13] Shin, S., Yoo, S., Lee, S., Park, C., Park, S., Kwon, J., and Lee, S., Removal of hot spot speckle on laser projection screen using both the running screen and the rotating diffuser, Displays 27(3), (2006). [14] Kanebako, T. and Takaki, Y., Time-multiplexing display module for high-density directional display, in Stereoscopic Displays and Applications XIX, Woods, A., Holliman, N., and Merritt, J., eds., Proc. SPIE 6803, 68030P (2008). [15] Bogaert, L., Meuret, Y., and Thienpont, H., Lens system for three-dimensional display applications, (2009). Patent pending. SPIE-IS&T/ Vol R-10