Polarization recycling method for light-pipe-based optical engine Qian Zhang, Zhenjie Liu, Wenzi Zhang, and Feihong Yu* State Key Laboratory of Modern Optical Instrumentation, Optical Engineering Department, Zhejiang University, Hangzhou 310027, China *Corresponding author: [email protected] Received 23 August 2013; revised 14 November 2013; accepted 15 November 2013; posted 26 November 2013 (Doc. ID 196323); published 17 December 2013
In this paper, a polarization recycling method is proposed for a light-pipe-based liquid crystal on silicon (LCoS) pico-optical engine. The method is based on making use of the virtual light sources array forming at the light pipe’s input surface. With traditional imaging optics, the virtual light sources array can be imaged to a plane after the light pipe, where the separated beams array can be obtained. By applying the polarization conversion system to the separated beams, the incoming unpolarized light can be converted to polarized light. The polarized light is then collected and transferred to the LCoS panel through the relay system. This new polarization recycling method can highly improve the light efficiency. A design example of a 0.29 in. (7.366 mm) color-filter LCoS pico-optical engine with 852 × 480 resolution is listed. High light efficiency of about 10.5 lm per LED Watt and high irradiance uniformity of about 95% has been achieved. The thickness of the optical engine is 8 mm. © 2013 Optical Society of America OCIS codes: (220.3620) Lens system design; (220.2945) Illumination design; (080.3685) Lightpipes; (230.3720) Liquid-crystal devices. http://dx.doi.org/10.1364/AO.52.008827
1. Introduction
In the pico-projection market, there are two main technologies. One is digital light processor (DLP) technology. The other is liquid crystal on silicon (LCoS) technology. Compared to DLP technology, the light efficiency for an LCoS optical engine is low, though its cost is also low. For the current LCoS pico-projection optical engine, brightness is the biggest problem restricting its application. So improving the light collection efficiency is the key project we are now working on. For an LCoS pico-projector, the light source emits natural and unpolarized light, so a polarization beam splitter (PBS) [1] is usually applied in LCoS-based optical engines to convert the natural light into polarized light. When passing through the PBS, the natural light from the light source will split into 1559-128X/13/368827-07$15.00/0 © 2013 Optical Society of America
two separated parts with different polarization states. Then the two parts of the polarized light will propagate in two different directions, respectively. Thus, only half-polarized light with the same polarization state can reach the target illumination area and modulated by the LCoS display. Obviously, almost half of the light flux was inevitably wasted. But if both of the two half-polarized light can be used in the LCoS pico-optical engine, things may be different. The key point of this research is to increase the light efficiency for the LCoS pico-optical engine by applying a polarization recycling [2] method. Within the previous research, several methods have already been proposed for polarization recycling in LCoS projection [3,4]. The general one is to use the polarization conversion system (PCS) element in a fly-eyes [1,5]-based LCoS optical engine. The main problem of the fly-eye-based system lies in the small cell sizes of fly eyes and the PCS element, which may be less than 1 mm for LCoS panels with diagonal size of less than 0.3 in. (7.62 mm). This 20 December 2013 / Vol. 52, No. 36 / APPLIED OPTICS
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leads to difficulties for the fly eyes’ molding and polishing, which will increase the cost of the optical engine. Besides, the overall length of the fly-eye system is much longer than the light-pipe-based system. In the fly-eye system, the collimation lens design is difficult, with the high-degree collimation demand of fly-eyes light splitting. As only part of the source rays can be collimated within the demanding fly-eye incident angle, inevitable energy loss is caused that is usually considerable. For the polarization recovery method based on birefringent crystal [4], one needs a birefringent crystal block, which is not easy to fabricate. As to other polarization methods based on reflective polarizers [6–8], the light conversion efficiency is low. In this paper, a polarization recycling method for an LCoS optical engine is proposed. By combining the nonimaging optics and traditional imaging optics, light from the LED can be reshaped. While passing through a light pipe and the splitting lens, the LED source ray will split into a separated beams array at a plane where PCS is applied. By passing through the PCS, all the reshaped beams then will be converted to polarized light with the same polarization state before transmission to PBS and will finally be collected and transferred to the target illumination area by the subsequent relay lens. The light-splitting processes work in different ways in both polarizationrecycling systems. The fly-eye-based system applies fly eyes to the split wave-front of the collimated illumination light to achieve light splitting, while the light-pipe-based system uses light pipe to modulate the incident light distribution of different ray angles to achieve the light reshaping. Within this light-pipe-based method, neither fly-eyes elements nor birefringent crystal block is used, and the high light conversion efficiency can be achieved without increasing the aperture of the whole illumination optics. The rest of the paper is organized as follows. In Section 2, the light-pipe-based polarization recycling method is introduced, which describes how the light beam was reshaped, collected, polarization recovered, and passed to the LCoS panel. A design example for a color-filter LCoS pico-optical engine with 852 × 480 resolution is described in Section 3, and the conclusions are drawn in Section 4.
Fig. 1. Schematic illustration of the mirror-imaging method.
As shown in the Fig. 1, this method of mirror imaging makes use of the fact that any source ray reflected by the mirror is geometrically equivalent to an undeviated ray from a virtual image of the light source. For example, the ray BC is equivalent to the ray S−2 C, where S1 is the reflection image of S through the top mirror face. Multiple reflections between the top and bottom faces generate a linear array of reflected images of the LED source, and multiple totally internal reflections between all four faces of the solid light-pipe will generate a two-dimensional array [9–12] of virtual source images, as shown in Fig. 2. With a proper selection of the LED chip size, light-pipe size, and material, a specific distribution of the two-dimensional virtual light sources array can be acquired at the light pipe’s entrance surface. B. Light Splitting and Integration
The proposed polarization recycling method is based on light reshaping with a combination of nonimaging optics and traditional imaging optics. A schematic of this optical system is shown in Fig. 3. Virtual sources at the input end of the light pipe are labeled as thick lines with spaces. The virtual sources are imaged to the PCS plane through the splitting lens, where the magnified and collimated images are formed. The exit surface of the light pipe is the entrance pupil of the splitting lens. The unpolarized magnified image is converted to polarized light through the PCS, which will be described in the sequential section. The polarization converted images array is then integrated and transferred to LCoS through the integration lens (relay lens) [13]. Thus uniform irradiance can be acquired
2. Light-Pipe-Based Polarization Recycling Method
In this section, a polarization recycling method will be introduced. It mainly includes three parts, which are the formation of virtual light sources array, light splitting and integration, and the polarization conversion. A.
Virtual Light Sources Array
Virtual light source images will form at the light pipe’s input end while the light from the source passes through a light pipe. For the rectangular hollow light pipe, it is straightforward to determine whether a ray comes from the source or image by applying the method of mirror imaging [9]. 8828
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Fig. 2. Schematic of the two-dimensional virtual sources array at the entrance of the light pipe.
Fig. 3. Schematic of the light-pipe-based splitting and integration system.
Fig. 4. Gaussian optical model corresponding to Fig. 3.
Fig. 5. Sketch of light source ray propagating within the optical system. (a) Ray from a perfect point source and (b) ray from a 1 mm × 1 mm LED chip. 20 December 2013 / Vol. 52, No. 36 / APPLIED OPTICS
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M1
Fig. 6. Polarization conversion system.
with the splitting and integration process. No fly eye is used in this polarization recycling system, which will not restrict the cost and size of the pico-optical engine. Figure 4 is a simplified Gaussian optical model corresponding to Fig. 3. For the splitting lens, the virtual sources plane is the object plane. The equivalent object distance is L∕n, where L is the length of the light pipe, and n is the refractive index of the light pipe. L1 is the distance between the light pipe’s exit end and the splitting lens. L01 is the distance between the splitting lens and the PCS plane. Plane 1 and plane 10 are the conjugate planes about the splitting lens. And the following equations can be acquired with the Gaussian optics. The position of the PCS plane can be obtained from Eq. (2), and the magnification M 1 between the virtual sources array plane and the PCS plane is described in Eq. (3): 1 1 1 0 ; − L L01 f 1 −L
(1)
f 01 Ln − L1 ; L01 L 0 − L − f 1 1 n
(2)
n
1
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(3)
f 01 is the effective focal length of the splitting lens. W LED and W PCS stand for the width of the LED chip and the width of every single PCS cell, respectively. Through the splitting lens, separated magnified virtual sources array can be acquired at the PCS plane. As shown in Fig. 4, with the combination of the integration lens and splitting lens, the light pipe’s exit end is imaged to the LCoS panel. Since the light is well homogenized in the light pipe, a uniform irradiance profile can be expected at the exit end of the light-pipe. So the irradiance achieved at the display device is also uniform. Plane 2 and plane 20 are the conjugate planes about the splitting lens combined with the relay lens. The following relationships can be obtained with Gaussian optics: f 0 −f
f 01 f 02 ; Δd
1 1 1 Δd − ; L02 − L0H L1 − LH f 0 f 01 f 02
M2
W 01 W 02 L02 − L0H ; W 1 W 2 L1 − LH
(4)
(5)
(6)
where f 0 is the combined effective focal length of the splitting and integration lens and f 02 is the effective focal length of the integrating lens. W 1 and W 2 stand for the X and Y cross section widths of the light pipe’s exit end, while W 1 and W 2 are the X and Y cross section widths of the LCoS display, respectively. M 2 is the magnification between the LCoS panel and the light pipe’s exit end.
Fig. 7. Layout of the optical engine. 8830
L01 W PCS : L −L1 n W LED
Fig. 8. Simulated irradiance distribution at the PCS plane.
The sketch of rays from a perfect point source and a 1 mm × 1 mm LED chip propagating within the light-pipe-based light splitting and integration optical system is shown in Figs. 5(a) and 5(b), respectively. It can be easily found how the light source ray splits into several separated parts while passing through the splitting lens and then integrated and transferred to the LCoS panel by the sequential relay lens.
The P polarized light is converted to S polarization with one piece of half-wave plate, while the S polarized light is reflected with its polarization state unchanged. Thus all the light is S polarized [1,3]. The size of the PCS is chosen according to the distribution of the separated two-dimensional images array [12] acquired at the PCS plane. 3. Design Example
C.
Polarization Conversion
A structure of the PCS is shown in Fig. 6. Its size is related to the distribution of the split beams array obtained at the PCS plane. The PCS consists of small PBS cells and half-wave plates. When the incoming light passes PCS, it is split to S and P polarized parts.
Fig. 9. Distribution of polarization degree for the light output from the PCS.
In this section, a color-filter LCoS-based optical engine [14] is designed with this light-pipe based polarization recycling method. A. System Layout
The layout of the designed optical engine is shown in Fig. 7. The width (Z direction) of this optical engine is 41 mm, while its height is 28.5 mm. By applying rectangular apertures for the relay and projection lens, the thickness of the optical engine can be decreased to 8 mm. The high-power white LED here used has a chip surface emitting size of 1.0 mm × 1.0 mm, and it can output light flux of 100 lm with a power consumption of 1 W. The LED chip has a Lambertian intensity distribution. Light output from LED is collected by the solid rectangular light pipe. The entrance size is 1.3 mm × 2.2 mm, and its length is 3 mm. The material of the light-pipe is ZF4 from CDGM glass of China. The size of primary PBS is 9.5 mm × 9.5 mm × 8 mm, and each small cell of the PCS has a size of 1.8 mm × 1.8 mm × 8 mm, which is not difficult for mass production. Both of them use the material of SF57-HHT from Schott glass. The color-filter LCoS here used has a diagonal of 0.29 in. (7.366 mm), and its resolution is 852 × 480. The projection lens has two pieces of glass lens and two pieces of plastic aspherical lens. The effective focal length (EFL) and F/# are 14 mm and 2, respectively. The redundant thickness of the projection 20 December 2013 / Vol. 52, No. 36 / APPLIED OPTICS
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Fig. 10. Simulated irradiance distribution on the LCoS panel.
lens aperture is also sliced to make sure that the thickness of the optical engine is less than 8 mm. B.
Simulation
By tracing 1 million rays from the LED source with the wavelength of 550 nm within TracePro [15], the system performance can be simulated. For the simplicity of optical analysis, perfect models for LCoS, half-wave plate, antireflecting coating, and PBS coating are used in the simulation. The simulated irradiance distribution at the PCS plane is shown in Fig. 8. The image of the virtual sources array at the PCS plane is matched to the PCS cell with half-wave plate, while the dark spaces between the virtual sources array image is matched to the rest PCS cells. About 85.6% of the LED light flux has reached this area. The distribution of polarization degree for the polarization recovered light is
shown in Fig. 9. It can be seen that the good polarization degree has been acquired. The polarization recovered light then passes through the relay lens and PBS and reaches the LCoS panel. Figure 10 shows the simulated irradiance distribution on the LCoS panel. About 67% of LED light reaches the active area of the LCoS panel. The LCoS panel is projected to a 12 in. (304.8 mm) screen at a projection distance of 0.5 m by the projection lens. The simulated irradiance distribution is shown in Fig. 11. About 63% of LED light finally reaches the screen. That is to say, only 4% of the light is lost while transferring from LCoS panel to the screen. The irradiance varies smoothly from the screen center to the screen edge. The irradiance uniformity, which is defined as the ratio of the minimal to average irradiances of the 1–9 ANSI center points on screen, is about 95% [16]. With the light efficiency
Fig. 11. Simulated irradiance distribution on screen. 8832
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Table 1.
Item
Light Efficiency Evaluation
Value (%)
Collection efficiency LCoS reflectivity
63 24
Antireflective coating
87.78
PCS efficiency PBS efficiency Total
92 86 10.5
Notes Simulation with perfect model Color-filter LCoS from HIMAX [17] 0.5% average reflectivity for each AR-coated surface PBS cells array F/1.9 MacNeille PBS 10.5 lm at 1 W
evaluation shown in Table 1, it can be found that the output flux of this pico-optical engine will be 10.5 lm per LED Watt, which is an almost 100% boost in light efficiency compared to pico-optical engine without polarization recycling. If the white LED is driven to 1000 mA (the LED power consumption is about 3 W), the light flux of LED is about 200 lm, and the optical engine will output light flux of 21 lm. This can provide a moderate irradiance on the 12 in. (304.8 mm) screen. 4. Conclusion
A polarization recycling method has been proposed for the LCoS pico-optical engine. By combining nonimaging optics and traditional imaging optics, the rays emitted from the LED can be reshaped by light pipe as well as the splitting lens, which will be polarization recovered after passing through the PCS. In this light-pipe-based polarization recycling optical system, complex fabrication process for fly eyes or birefringent crystals blocks will no longer be needed. One design example for LCoS based pico-optical engine with WVGA resolution is demonstrated by applying this new polarization recycling method. This pico-optical engine has an output flux of 10.5 lm with one watt LED power consumption, which is an almost 100% increase compared with LCoS pico-optical engine without polarization recovery. The light uniformity is about 95%, and the thickness of the designed optical engine is 8 mm. Applying
this method will not increase the thickness of the pico-optical engine. References 1. E. H. Stupp and M. S. Brennesholtz, Projection Displays (Wiley, 1998), Chap. 7. 2. S. Bierhuizen, “Single panel color sequential projectors with polarization recovery,” SID Int. Symp. Dig. Tech. Pap. 33, 1350–1353 (2002). 3. W. Zhang, B. Qu, and F. Yu, “Novel polarization recovery method for LCoS pico projection,” Opt. Eng. 51, 093001 (2012). 4. Q. X. Liu, W. Z. Zhang, H. F. Gao, and F. H. Yu, “A new multiplexing method for the micro LCoS projector optical system,” Proc. SPIE 7506, 75061A (2009). 5. M. Duelli and A. T. Taylor, “Novel polarization conversion and integration system for projection displays,” SID Int. Symp. Dig. Tech. Pap., 34, 766–769 (2003). 6. M. Duelli and T. McGettigan, “Integrator rod with polarization recycling functionality,” SID Int. Symp. Dig. Tech. Pap. 33, 1078–1080 (2002). 7. M. Duelli, T. McGettigan, and C. Pentico, “Polarization recovery system based on light pipe,” Proc. SPIE 4657, 9–16 (2002). 8. K. K. Li, S. Sillyman, and S. Inatsugu, “Dual paraboloid reflector and polarization recycling systems for projection displays,” Proc. SPIE 5002, 31 (2003). 9. B. A. Jacobson, R. D. Gengelbach, and J. M. Ferri, “Beamshape transforming devices in high-efficiency projection systems,” Proc. SPIE 3139, 141 (1997). 10. C.-M. Cheng and J.-L. Chern, “Illuminance formation and color difference of mixed-color light emitting diodes in a rectangular light pipe: an analytical approach,” Appl. Opt. 47, 431–441 (2008). 11. W. R. Powell, “Transmission characteristics of specularly reflecting light pipes uniformly irradiated by obliquely inclined rays,” Appl. Opt. 13, 952–954 (1974). 12. C. M. Cheng and J. L. Chern, “Optical transfer functions for specific-shaped apertures generated by illumination with a rectangular light pipe,” J. Opt. Soc. Am. A 23, 3123–3132 (2006). 13. Y. Meuret, B. Vangiel, F. Christiaens, and H. Thienpont, “Efficient illumination in LED-based projection systems using lenslet integrators,” Proc. SPIE 6196, 619605 (2006). 14. W. Z. Zhang, “LED illumination system for CF-LCoS based pico-projection optical engine,” Ph.D. thesis (Zhejiang University, 2010), Chap. 5. 15. TracePro, www.lambdares.com. 16. A. Csaszar, “Data projection equipment and large screen data displays, test, and performance measurements,” in SID 1991 Digest (1991), pp. 265–267. 17. HIMAX, http://www.himax.com.tw/en/home/index.asp.
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In this paper, a polarization recycling method is proposed for a light-pipe-based liquid crystal on silicon (LCoS) pico-optical engine. The method is based on making use of the virtual light sources array forming at the light pipe’s input surface. With traditional imaging optics, the virtual light sources array can be imaged to a plane after the light pipe, where the separated beams array can be obtained. By applying the polarization conversion system to the separated beams, the incoming unpolarized light can be converted to polarized light. The polarized light is then collected and transferred to the LCoS panel through the relay system. This new polarization recycling method can highly improve the light efficiency. A design example of a 0.29 in. (7.366 mm) color-filter LCoS pico-optical engine with 852 × 480 resolution is listed. High light efficiency of about 10.5 lm per LED Watt and high irradiance uniformity of about 95% has been achieved. The thickness of the optical engine is 8 mm. © 2013 Optical Society of America OCIS codes: (220.3620) Lens system design; (220.2945) Illumination design; (080.3685) Lightpipes; (230.3720) Liquid-crystal devices. http://dx.doi.org/10.1364/AO.52.008827
1. Introduction
In the pico-projection market, there are two main technologies. One is digital light processor (DLP) technology. The other is liquid crystal on silicon (LCoS) technology. Compared to DLP technology, the light efficiency for an LCoS optical engine is low, though its cost is also low. For the current LCoS pico-projection optical engine, brightness is the biggest problem restricting its application. So improving the light collection efficiency is the key project we are now working on. For an LCoS pico-projector, the light source emits natural and unpolarized light, so a polarization beam splitter (PBS) [1] is usually applied in LCoS-based optical engines to convert the natural light into polarized light. When passing through the PBS, the natural light from the light source will split into 1559-128X/13/368827-07$15.00/0 © 2013 Optical Society of America
two separated parts with different polarization states. Then the two parts of the polarized light will propagate in two different directions, respectively. Thus, only half-polarized light with the same polarization state can reach the target illumination area and modulated by the LCoS display. Obviously, almost half of the light flux was inevitably wasted. But if both of the two half-polarized light can be used in the LCoS pico-optical engine, things may be different. The key point of this research is to increase the light efficiency for the LCoS pico-optical engine by applying a polarization recycling [2] method. Within the previous research, several methods have already been proposed for polarization recycling in LCoS projection [3,4]. The general one is to use the polarization conversion system (PCS) element in a fly-eyes [1,5]-based LCoS optical engine. The main problem of the fly-eye-based system lies in the small cell sizes of fly eyes and the PCS element, which may be less than 1 mm for LCoS panels with diagonal size of less than 0.3 in. (7.62 mm). This 20 December 2013 / Vol. 52, No. 36 / APPLIED OPTICS
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leads to difficulties for the fly eyes’ molding and polishing, which will increase the cost of the optical engine. Besides, the overall length of the fly-eye system is much longer than the light-pipe-based system. In the fly-eye system, the collimation lens design is difficult, with the high-degree collimation demand of fly-eyes light splitting. As only part of the source rays can be collimated within the demanding fly-eye incident angle, inevitable energy loss is caused that is usually considerable. For the polarization recovery method based on birefringent crystal [4], one needs a birefringent crystal block, which is not easy to fabricate. As to other polarization methods based on reflective polarizers [6–8], the light conversion efficiency is low. In this paper, a polarization recycling method for an LCoS optical engine is proposed. By combining the nonimaging optics and traditional imaging optics, light from the LED can be reshaped. While passing through a light pipe and the splitting lens, the LED source ray will split into a separated beams array at a plane where PCS is applied. By passing through the PCS, all the reshaped beams then will be converted to polarized light with the same polarization state before transmission to PBS and will finally be collected and transferred to the target illumination area by the subsequent relay lens. The light-splitting processes work in different ways in both polarizationrecycling systems. The fly-eye-based system applies fly eyes to the split wave-front of the collimated illumination light to achieve light splitting, while the light-pipe-based system uses light pipe to modulate the incident light distribution of different ray angles to achieve the light reshaping. Within this light-pipe-based method, neither fly-eyes elements nor birefringent crystal block is used, and the high light conversion efficiency can be achieved without increasing the aperture of the whole illumination optics. The rest of the paper is organized as follows. In Section 2, the light-pipe-based polarization recycling method is introduced, which describes how the light beam was reshaped, collected, polarization recovered, and passed to the LCoS panel. A design example for a color-filter LCoS pico-optical engine with 852 × 480 resolution is described in Section 3, and the conclusions are drawn in Section 4.
Fig. 1. Schematic illustration of the mirror-imaging method.
As shown in the Fig. 1, this method of mirror imaging makes use of the fact that any source ray reflected by the mirror is geometrically equivalent to an undeviated ray from a virtual image of the light source. For example, the ray BC is equivalent to the ray S−2 C, where S1 is the reflection image of S through the top mirror face. Multiple reflections between the top and bottom faces generate a linear array of reflected images of the LED source, and multiple totally internal reflections between all four faces of the solid light-pipe will generate a two-dimensional array [9–12] of virtual source images, as shown in Fig. 2. With a proper selection of the LED chip size, light-pipe size, and material, a specific distribution of the two-dimensional virtual light sources array can be acquired at the light pipe’s entrance surface. B. Light Splitting and Integration
The proposed polarization recycling method is based on light reshaping with a combination of nonimaging optics and traditional imaging optics. A schematic of this optical system is shown in Fig. 3. Virtual sources at the input end of the light pipe are labeled as thick lines with spaces. The virtual sources are imaged to the PCS plane through the splitting lens, where the magnified and collimated images are formed. The exit surface of the light pipe is the entrance pupil of the splitting lens. The unpolarized magnified image is converted to polarized light through the PCS, which will be described in the sequential section. The polarization converted images array is then integrated and transferred to LCoS through the integration lens (relay lens) [13]. Thus uniform irradiance can be acquired
2. Light-Pipe-Based Polarization Recycling Method
In this section, a polarization recycling method will be introduced. It mainly includes three parts, which are the formation of virtual light sources array, light splitting and integration, and the polarization conversion. A.
Virtual Light Sources Array
Virtual light source images will form at the light pipe’s input end while the light from the source passes through a light pipe. For the rectangular hollow light pipe, it is straightforward to determine whether a ray comes from the source or image by applying the method of mirror imaging [9]. 8828
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Fig. 2. Schematic of the two-dimensional virtual sources array at the entrance of the light pipe.
Fig. 3. Schematic of the light-pipe-based splitting and integration system.
Fig. 4. Gaussian optical model corresponding to Fig. 3.
Fig. 5. Sketch of light source ray propagating within the optical system. (a) Ray from a perfect point source and (b) ray from a 1 mm × 1 mm LED chip. 20 December 2013 / Vol. 52, No. 36 / APPLIED OPTICS
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M1
Fig. 6. Polarization conversion system.
with the splitting and integration process. No fly eye is used in this polarization recycling system, which will not restrict the cost and size of the pico-optical engine. Figure 4 is a simplified Gaussian optical model corresponding to Fig. 3. For the splitting lens, the virtual sources plane is the object plane. The equivalent object distance is L∕n, where L is the length of the light pipe, and n is the refractive index of the light pipe. L1 is the distance between the light pipe’s exit end and the splitting lens. L01 is the distance between the splitting lens and the PCS plane. Plane 1 and plane 10 are the conjugate planes about the splitting lens. And the following equations can be acquired with the Gaussian optics. The position of the PCS plane can be obtained from Eq. (2), and the magnification M 1 between the virtual sources array plane and the PCS plane is described in Eq. (3): 1 1 1 0 ; − L L01 f 1 −L
(1)
f 01 Ln − L1 ; L01 L 0 − L − f 1 1 n
(2)
n
1
APPLIED OPTICS / Vol. 52, No. 36 / 20 December 2013
(3)
f 01 is the effective focal length of the splitting lens. W LED and W PCS stand for the width of the LED chip and the width of every single PCS cell, respectively. Through the splitting lens, separated magnified virtual sources array can be acquired at the PCS plane. As shown in Fig. 4, with the combination of the integration lens and splitting lens, the light pipe’s exit end is imaged to the LCoS panel. Since the light is well homogenized in the light pipe, a uniform irradiance profile can be expected at the exit end of the light-pipe. So the irradiance achieved at the display device is also uniform. Plane 2 and plane 20 are the conjugate planes about the splitting lens combined with the relay lens. The following relationships can be obtained with Gaussian optics: f 0 −f
f 01 f 02 ; Δd
1 1 1 Δd − ; L02 − L0H L1 − LH f 0 f 01 f 02
M2
W 01 W 02 L02 − L0H ; W 1 W 2 L1 − LH
(4)
(5)
(6)
where f 0 is the combined effective focal length of the splitting and integration lens and f 02 is the effective focal length of the integrating lens. W 1 and W 2 stand for the X and Y cross section widths of the light pipe’s exit end, while W 1 and W 2 are the X and Y cross section widths of the LCoS display, respectively. M 2 is the magnification between the LCoS panel and the light pipe’s exit end.
Fig. 7. Layout of the optical engine. 8830
L01 W PCS : L −L1 n W LED
Fig. 8. Simulated irradiance distribution at the PCS plane.
The sketch of rays from a perfect point source and a 1 mm × 1 mm LED chip propagating within the light-pipe-based light splitting and integration optical system is shown in Figs. 5(a) and 5(b), respectively. It can be easily found how the light source ray splits into several separated parts while passing through the splitting lens and then integrated and transferred to the LCoS panel by the sequential relay lens.
The P polarized light is converted to S polarization with one piece of half-wave plate, while the S polarized light is reflected with its polarization state unchanged. Thus all the light is S polarized [1,3]. The size of the PCS is chosen according to the distribution of the separated two-dimensional images array [12] acquired at the PCS plane. 3. Design Example
C.
Polarization Conversion
A structure of the PCS is shown in Fig. 6. Its size is related to the distribution of the split beams array obtained at the PCS plane. The PCS consists of small PBS cells and half-wave plates. When the incoming light passes PCS, it is split to S and P polarized parts.
Fig. 9. Distribution of polarization degree for the light output from the PCS.
In this section, a color-filter LCoS-based optical engine [14] is designed with this light-pipe based polarization recycling method. A. System Layout
The layout of the designed optical engine is shown in Fig. 7. The width (Z direction) of this optical engine is 41 mm, while its height is 28.5 mm. By applying rectangular apertures for the relay and projection lens, the thickness of the optical engine can be decreased to 8 mm. The high-power white LED here used has a chip surface emitting size of 1.0 mm × 1.0 mm, and it can output light flux of 100 lm with a power consumption of 1 W. The LED chip has a Lambertian intensity distribution. Light output from LED is collected by the solid rectangular light pipe. The entrance size is 1.3 mm × 2.2 mm, and its length is 3 mm. The material of the light-pipe is ZF4 from CDGM glass of China. The size of primary PBS is 9.5 mm × 9.5 mm × 8 mm, and each small cell of the PCS has a size of 1.8 mm × 1.8 mm × 8 mm, which is not difficult for mass production. Both of them use the material of SF57-HHT from Schott glass. The color-filter LCoS here used has a diagonal of 0.29 in. (7.366 mm), and its resolution is 852 × 480. The projection lens has two pieces of glass lens and two pieces of plastic aspherical lens. The effective focal length (EFL) and F/# are 14 mm and 2, respectively. The redundant thickness of the projection 20 December 2013 / Vol. 52, No. 36 / APPLIED OPTICS
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Fig. 10. Simulated irradiance distribution on the LCoS panel.
lens aperture is also sliced to make sure that the thickness of the optical engine is less than 8 mm. B.
Simulation
By tracing 1 million rays from the LED source with the wavelength of 550 nm within TracePro [15], the system performance can be simulated. For the simplicity of optical analysis, perfect models for LCoS, half-wave plate, antireflecting coating, and PBS coating are used in the simulation. The simulated irradiance distribution at the PCS plane is shown in Fig. 8. The image of the virtual sources array at the PCS plane is matched to the PCS cell with half-wave plate, while the dark spaces between the virtual sources array image is matched to the rest PCS cells. About 85.6% of the LED light flux has reached this area. The distribution of polarization degree for the polarization recovered light is
shown in Fig. 9. It can be seen that the good polarization degree has been acquired. The polarization recovered light then passes through the relay lens and PBS and reaches the LCoS panel. Figure 10 shows the simulated irradiance distribution on the LCoS panel. About 67% of LED light reaches the active area of the LCoS panel. The LCoS panel is projected to a 12 in. (304.8 mm) screen at a projection distance of 0.5 m by the projection lens. The simulated irradiance distribution is shown in Fig. 11. About 63% of LED light finally reaches the screen. That is to say, only 4% of the light is lost while transferring from LCoS panel to the screen. The irradiance varies smoothly from the screen center to the screen edge. The irradiance uniformity, which is defined as the ratio of the minimal to average irradiances of the 1–9 ANSI center points on screen, is about 95% [16]. With the light efficiency
Fig. 11. Simulated irradiance distribution on screen. 8832
APPLIED OPTICS / Vol. 52, No. 36 / 20 December 2013
Table 1.
Item
Light Efficiency Evaluation
Value (%)
Collection efficiency LCoS reflectivity
63 24
Antireflective coating
87.78
PCS efficiency PBS efficiency Total
92 86 10.5
Notes Simulation with perfect model Color-filter LCoS from HIMAX [17] 0.5% average reflectivity for each AR-coated surface PBS cells array F/1.9 MacNeille PBS 10.5 lm at 1 W
evaluation shown in Table 1, it can be found that the output flux of this pico-optical engine will be 10.5 lm per LED Watt, which is an almost 100% boost in light efficiency compared to pico-optical engine without polarization recycling. If the white LED is driven to 1000 mA (the LED power consumption is about 3 W), the light flux of LED is about 200 lm, and the optical engine will output light flux of 21 lm. This can provide a moderate irradiance on the 12 in. (304.8 mm) screen. 4. Conclusion
A polarization recycling method has been proposed for the LCoS pico-optical engine. By combining nonimaging optics and traditional imaging optics, the rays emitted from the LED can be reshaped by light pipe as well as the splitting lens, which will be polarization recovered after passing through the PCS. In this light-pipe-based polarization recycling optical system, complex fabrication process for fly eyes or birefringent crystals blocks will no longer be needed. One design example for LCoS based pico-optical engine with WVGA resolution is demonstrated by applying this new polarization recycling method. This pico-optical engine has an output flux of 10.5 lm with one watt LED power consumption, which is an almost 100% increase compared with LCoS pico-optical engine without polarization recovery. The light uniformity is about 95%, and the thickness of the designed optical engine is 8 mm. Applying
this method will not increase the thickness of the pico-optical engine. References 1. E. H. Stupp and M. S. Brennesholtz, Projection Displays (Wiley, 1998), Chap. 7. 2. S. Bierhuizen, “Single panel color sequential projectors with polarization recovery,” SID Int. Symp. Dig. Tech. Pap. 33, 1350–1353 (2002). 3. W. Zhang, B. Qu, and F. Yu, “Novel polarization recovery method for LCoS pico projection,” Opt. Eng. 51, 093001 (2012). 4. Q. X. Liu, W. Z. Zhang, H. F. Gao, and F. H. Yu, “A new multiplexing method for the micro LCoS projector optical system,” Proc. SPIE 7506, 75061A (2009). 5. M. Duelli and A. T. Taylor, “Novel polarization conversion and integration system for projection displays,” SID Int. Symp. Dig. Tech. Pap., 34, 766–769 (2003). 6. M. Duelli and T. McGettigan, “Integrator rod with polarization recycling functionality,” SID Int. Symp. Dig. Tech. Pap. 33, 1078–1080 (2002). 7. M. Duelli, T. McGettigan, and C. Pentico, “Polarization recovery system based on light pipe,” Proc. SPIE 4657, 9–16 (2002). 8. K. K. Li, S. Sillyman, and S. Inatsugu, “Dual paraboloid reflector and polarization recycling systems for projection displays,” Proc. SPIE 5002, 31 (2003). 9. B. A. Jacobson, R. D. Gengelbach, and J. M. Ferri, “Beamshape transforming devices in high-efficiency projection systems,” Proc. SPIE 3139, 141 (1997). 10. C.-M. Cheng and J.-L. Chern, “Illuminance formation and color difference of mixed-color light emitting diodes in a rectangular light pipe: an analytical approach,” Appl. Opt. 47, 431–441 (2008). 11. W. R. Powell, “Transmission characteristics of specularly reflecting light pipes uniformly irradiated by obliquely inclined rays,” Appl. Opt. 13, 952–954 (1974). 12. C. M. Cheng and J. L. Chern, “Optical transfer functions for specific-shaped apertures generated by illumination with a rectangular light pipe,” J. Opt. Soc. Am. A 23, 3123–3132 (2006). 13. Y. Meuret, B. Vangiel, F. Christiaens, and H. Thienpont, “Efficient illumination in LED-based projection systems using lenslet integrators,” Proc. SPIE 6196, 619605 (2006). 14. W. Z. Zhang, “LED illumination system for CF-LCoS based pico-projection optical engine,” Ph.D. thesis (Zhejiang University, 2010), Chap. 5. 15. TracePro, www.lambdares.com. 16. A. Csaszar, “Data projection equipment and large screen data displays, test, and performance measurements,” in SID 1991 Digest (1991), pp. 265–267. 17. HIMAX, http://www.himax.com.tw/en/home/index.asp.
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