Optical System Design

Optical design in illumination system of digital light processing projector using laser and gradient-index lens

[+] Author Affiliations
Dawei Rui

University of Electronic Science and Technology of China, School of Opto-electronic Information, Chengdu, Sichuan, China, 610054

Zulun Lin

University of Electronic Science and Technology of China, School of Opto-electronic Information, Chengdu, Sichuan, China, 610054

Kangcheng Qi

University of Electronic Science and Technology of China, School of Opto-electronic Information, Chengdu, Sichuan, China, 610054

Wenbin Chen

University of Electronic Science and Technology of China, School of Opto-electronic Information, Chengdu, Sichuan, China, 610054

Opt. Eng. 51(1), 013004 (Jan 20, 2012). doi:10.1117/1.OE.51.1.013004
History: Received June 13, 2011; Revised October 16, 2011; Accepted November 7, 2011
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Abstract.  A novel structure of illumination system based on laser sources and gradient-index (GRIN) lens arrays for single-plate digital light processing (DLP) projector is proposed. Under the evaluation of light collection efficiency and illumination uniformity, we designed two different architectures of GRIN lens arrays: the unit lens cross section being either square or circular. Also, we designed the one-to-one coupling mechanism between GRIN lens array and Gaussian beam array. By means of three-dimensional modeling and ray tracing, we simulated the optical characteristics of illumination system, and the optical parameters are evaluated. Results show that the total light collection efficiency is 37.9%, the illumination uniformity is 89.7% and the volume is only 12.4cm3. Thus we improved optical characteristics and reduced the physical dimension of DLP optical engine.

Figures in this Article

Recently, portable, high performance pico projectors have become readily available. The optical engine of a digital light processing (DLP) projector generally consists of light source, illumination system, spatial light modulator as digital micro-mirrors device (DMD), and projection lens. The function of illumination system is to realize condensation, uniformity, and color mixing. Given a vested luminous efficiency of light source, the illumination system is the main factor in deciding the light collection efficiency and the physical dimension of the optical engine. The conventional illumination systems failed to overcome the low collection efficiency, or to save large physical size. That is because the conventional light sources such as light emitting diodes (LED) obey the Lambert Law, and their light are very difficult to be collected or distributed. The structure of optical engine, and especially of the illumination system, must be improved in order to fulfill the projector’s high performance and small size.

Laser, with high directivity and monochromatic, possess potential dominance over LED. Laser diodes (LD) of visible bands have demonstrated tremendous advantages in output power and luminous efficiency. The most significant advantage in choosing LD as the projection light source is that it can produce efficient optical coupling on DMD, and minimize the étendue of illumination system by properly expanding and collimating the Gaussian beam without using complex condensing and uniformimg components. Thus, the key problem that needs to be solved is how to design the illumination system reasonably for laser. In this paper, a design of illumination system based on laser source and gradient-index (GRIN) lens for single-plate type DLP projector is proposed. This illumination system, which consists of only single stage lens, can successfully realize the expansion, collimation and uniformity of Gaussian beam emitted by LD, and provides a high efficiency lighting spot that can be directly utilized by DMD.

Collimation Mechanism of GRIN Lens

In optical fiber communication, a column GRIN lens is applied to make collimator, to collimate, or focus near-infrared Gaussian beams. The refractive index distribution in the meridian plane of the GRIN lens is given by an empirical formula as follows1Display Formula

n(r)=n0[112(αr)2](1)
where α is the gradient coefficient of the lens material, r is the radius variable of lens and n0 is the central refractive index. It can be deduced that the raytrajectory inside the GRIN lens is sinusoidal. The optical length for one circle of the sinusoid is called a pitch. A GRIN lens in an optical length of 1/4 pitch with a point light source on the center of one lens side can spontaneously emit collimated and symmetrical plane light wave from the other side, as shown in Fig. 1. The transmissivity of visible light in a 1/4 pitch GRIN lens can reach 98% with antireflective (AR) coating.

Graphic Jump LocationF1 :

Beam distribution of gradient-index (GRIN) lens simulated by Zemax.

Correction and Expansion of Laser Beam

The far field beam emitted by LD chip is elliptical whereas a GRIN lens requires a circular input. In order to make the value of the fast axis radius equal to that of the slow axis radius, a beam correcting lens must be mounted at the exit end of laser source.2 The correcting lens depicted in Fig. 2(a) contains the rectangular cross section and two mutually perpendicular cylindrical surfaces, one per side. The cylindrical surface with smaller curvature radius focuses the fast axis light whereas the other one with bigger curvature radius focuses the slow axis light. Thus, the light of the fast and slow axes are both corrected to the same radius and same focal length, forming a circular Gaussian beam.

Graphic Jump LocationF2 :

Schematic diagram of beam correction mechanism: (a) 3D model of correcting lens, and (b) beam expansion in orthogonal sections of GRIN lens.

Moreover, to ensure that the expanded beam cross section equals to the cross section of GRIN lens, the angle of the beam entering GRIN lens must be divergent, and equals to the maximal aperture angle θmax, which is deduced from the formula of central numerical aperture (NA) as follows3Display Formula

θmax=arcsinNAn0=arcsinαrmax1+(αrmax)2,(2)
where, rmax is the radius of lens. Given a typical value 0.339 to α, the beam angle would be approximately 30 deg. However, the far field angle of Gaussian beam, especially for the short axis,3 is usually less than 10 deg, thus, the beam cannot be naturally expanded to fully fill in the cross section of GRIN lens. Hence, the correcting lens must produce a light cone with a sizable divergent beam angle toward GRIN lens. One resolution to this problem is to use the principle of telescope,4 the correcting lens must have a short focal length, as shown in Fig. 2(b). θ0f and θ0s are the internal far field angle of fast axis and slow axis of LD, respectively. After propagating through the correcting lens with the focal length fc, the angle of image Gaussian beam changes into θ0, each angle is as follows4,5Display Formula
θ0f=limz2ω(z)/z=2λ/πω0f,θ0s=2λ/πω0s,θ0=2λ/πω0(3)
We take the fast axis plane for consideration, and its conclusions are also true for the slow axis. Then ω0θ0=ω0fθ0f. If ω0<ω0f, then θ0>θ0f. Judging from the image-forming principle, when fcl (where l is the distance between initial waist ω0f and the correcting lens), the object Gaussian beam is focused in the center of the front focal plane of the correcting lens. Synchronously, a minimum circular beam waist ω0 and a maximum far field angle θ0 are both obtained as below4,5Display Formula
ω0=λfc/πωf(l),θ0=ω(l)/fc,(4)
where ωf(l) is the beam radius on lens surface. As ω0 is fitly located in the rear focus of GRIN lens, not only would the Gaussian beam be well collimated with a tiny beam angle θ0, but it would also fully distribute in the entire cross section of GRIN lens in great uniformity. Following the above technique, the collimation and uniformity of illumination system are realized by single-stage GRIN lens.

Array of GRIN Lens and Laser Source

The diameter of GRIN lens is ϕ1mm or ϕ1.8mm under the existing manufacture process, so the beam expanding ability of GRIN lens is limited to the diameter of itself. Obviously, it is not possible for a single GRIN lens provides illumination for the entire surface of 0.3 (7.62 mm) DMD. If the GRIN lens gives light cone instead of collimated beam to cover the entire DMD, the uniformity of effective projection area would be broken, and the image aberration brought by the divergent light cone entering projection lens would be significant. So it is necessary to give DMD a collimated lighting spot. Also, in order to maximize the collection efficiency, the circular lighting spot needs to be adjusted to rectangle that fits the running surface of the DMD. Thus, a GRIN lens array is required.

Under the evaluation of light collection efficiency, illumination uniformity as well as spot shape, we designed two different architectures of GRIN lens arrays: the cross section of unit lens being square and circular. Each array consisted of GRIN lenses in same physical dimension, optical characteristics and mechanical accuracy. Also, we designed the one-to-one coupling between GRIN lens array and Gaussian beam array.

In order to produce rectangular lighting spot, every single lens was cut to the right square prism which is 1.4×1.4mm in cross section and 1/4 pitch in length. The 12 lenses were tightened in flank direction with each other, and form a 4×3 matrix, as shown in Fig. 3(a). Thus, the rectangular spot on illuminated area perfectly matches the optical surface of DMD. As the assembly mode and processing precision were considered, a gap between interfaces of random lenses was introduced to this model of GRIN lens array. The illuminating surfaces of lens were AR coated whereas the flank surfaces were coated with absorbing layer. The distribution of refractive index in cross section was still circular, so the gradient index was non-rotational symmetry in square cross section, and the uniformity of marginal rays transmitting in lens fringe was disturbed. To remedy this problem, we precisely adjusted the length and rear operating distance of GRIN lens to produce a tiny divergent beam angle to improve uniformity by certain overlapped illumination between adjacent lenses.

Graphic Jump LocationF3 :

Two types of architecture of gradient-index (GRIN) lens array: (a) array of right square prisms, and (b) array of columns.

To overcome the waste of marginal rays in lens fringe due to cutting, we improved the architecture of GRIN lens array. Twelve uncut column lenses of ϕ1.8mm were used to form a dense accumulated array, as shown in Fig. 3(b). The shape of lighting spot does not exactly match the DMD, leading to a certain loss of illuminating energy; however, the internal rays of column GRIN lens were totally emitted without being absorbed, thus, the luminous efficiency was greatly improved—synchronously the craft difficulties and the production costs were both reduced.

To match the above architectures of GRIN lens array, the light source adopts three groups of red-green-bleu (RGB) tricolor laser diodes, and each color has three same LDs. The central emission wavelength is 650, 532, and 470 nm, with average power of 1 watt per color, respectively. The specific power levels of LDs were adjusted for white balance according to the colorimetry. The drive mode of LDs for single plate DMD was time-sharing color mixing. As there were 12 GRIN lenses ranked in three rows, it was required that every single GRIN lens received a laser input at its rear focus, but ordinary LD can not guarantee the one-to-one coupling between LD and every unit lens of array due to LD size. Thus, we introduced the light path in ethernet link using fiber and couplers. The plan sketch in the left part of Fig. 4 depicts the light distribution of red LD for one row of GRIN array. Where coupler-1 halves its input laser power, couplers-2 and -3 share the equal power and go on halving. Thereafter two stages of power division, every unit GRIN lens in one row of the array gets the equal power of LD. Likewise, three red LDs were used, one per row for the GRIN lens array, and the same for the other two colors.

Graphic Jump LocationF4 :

Sketch of optical engine for single-plate DLP projector using laser sources and gradient-index (GRIN) lenses.

As explained above, the double-stage couplers and correcting lens were contained in laser source, but for fairness in comparing collection efficiencies of the novel architectures with that of LED based architectures, the efficiency of laser source structures (couplers and correcting lenses) should also be considered. The ordinary 50/50 fiber coupler products, according to resent survey, is usually of 2.4 to 3.0 dB insertion loss in each output ports, which means the collection efficiency for single stage coupler is approximately 70.7% to 75.8%, and double stage is squared to 50% to 57.5% for each LD. The collection efficiency of correcting lens for each LD is over 99.2% with AR coating simulation.

For laser diode, it is true that the coupling loss would be significant when the beam is coupled from free space optics to waveguide structures like fiber, but here we assumed the LD is encapsulated with fiber collimator, and LD’s output power from its fiber tap is the whole system’s original injected power we used to calculate efficiency. In comparison, the LED is also the source device encapsulated with a certain optic structure (like silica gel and condenser) rather than naked LED chip, so output power of LED source is regarded as the original injected power of LED based systems. Thus, we compare these two approaches in equal circumstances.

Structure of Optical Engine and Illumination system

The conventional illumination systems consist of complex elements. The incandescent source, such as the Ultra High Performance (UHP), emits infrared and UV light synchronously with visible light, leading to the addition of volume and costs.5 Further, on account of DLP projectors, spectral separator was demanded to be set up to split the incandescent light into RGB. Thus, the volume and costs were also added and the color gamut was down. Finally, the collection efficiency was decreased with the number increasing of optics components. Tricolor LEDs provide a better color gamut and do not need a spectral separator. However, due to LED’s Lambertian distribution, the mode of light collection is still the main restriction to the collection efficiency. The illumination system, matching the Lambert light source above, consists mainly of the condensing components (e.g., concentrator, lens, tapered light pipe), the uniforming components (e.g., rod, fly eyes), and the color mixing components (e.g., X-prism, dichroic mirror). Passing through the whole complex illumination system, the light finally reaching the DMD remains only 2030% of its original power.69

In comparison, this novel illumination system based on laser sources and GRIN lens arrays greatly simplifies the structure of optical engine. As shown in Fig. 4, the optical engine includes three groups of RGB light source, three GRIN lens arrays (one for each color), three dichroic mirrors (one for each array), one total reflection mirror, one DMD and one projection lens. Accordingly, the illumination system contains the GRIN lens arrays, the dichroic mirrors and the total reflection mirror. The light path is as follows: RGB sources inject laser to their corresponding GRIN lens arrays by averaging the powers in ethernet links, the arrays then produce three rectangular lighting spot of solid color respectively. Each dichroic mirror receives and selective transmits one solid color spot and then together accomplish spatial color mixing. After the reflection from the total reflection mirror, the DMD receives and modulates the lighting spot, so the image information is reflected to projection lens while the irrelevant light is dispelled.

Standards of Evaluation for Optical Parameters

The main optical parameters for an illumination system are collection efficiency, uniformity and étendue. The total collection efficiency is the product of the aspects of efficiency of coupling (correcting lens and double-stage couplers), unifying (GRIN lens), color mixing (dichroic mirrors and total reflection mirror), and projecting (effective illuminating area on DMD). The efficiency of each part is the ratio of its output power to input power. The total efficiency ηtotal is calculated as Display Formula

η=totalηcoupling·ηunifing·ηmixing·ηprojecting.(5)
To estimate the illumination uniformity, the effective illuminating area is divided into nine equal rectangle parts, and there are nine central points in each part. The uniformity is evaluated using ANSI/NAPM IT7.228-1997 standard,10,11 by which we choose those nine measurement points to find the minimum value Lm and calculate the average value La among L1L9, where L refers to the illuminance value. Then the evaluation goes to Display Formula
[1|LmLa|/La]×100%=N%,(6)
in which, the minimum value of N% is considered as the uniformity.

The étendue of illumination system can be calculated by the formula below6Display Formula

E=π·S·(NA)2,(7)
where S is the total area of the cross section of the single GRIN lens array, NA is the numerical aperture of the beam in single GRIN lens. As deduced above, the angle of beam emitted by GRIN lens is very small, so the étendue is conservative.

By means of 3-D modeling and ray tracing using the software of TracePro, we simulated the optical characteristics of illumination system based on these two architectures of GRIN lens arrays, and evaluated the optical parameters and the volume of elements. The optical parameters for the illumination system corresponding to both architectures of GRIN lens array are shown, respectively, in Table 1.

Figure 5 shows a simulation of the light path in which the red laser on the longest optical distance transmits through its corresponding GRIN array and three dichroic mirrors, and is then reflected by the total reflection mirror before reaching the DMD. It can be estimated from the illuminace map of illumination system using right square prism arrays in Fig. 6 that the lighting spot in the shape of 43 rectangle completely covered the region of 5.4×4.05mm restricted by 0.3 DMD and the projecting efficiency, which is the ratio of the effective illuminating area to lighting spot area, is 84.2%. The illumination uniformity in this area is evaluated to be 89.7%. Taken a typical value of double-stage coupler’s efficiency 55%, the total light collection efficiency of this illumination system reaches 35.4%.

Graphic Jump LocationF5 :

Ray tracing of GRIN lens array illumination system.

Graphic Jump LocationF6 :

Illuminace map of illumination system using right square prism GRIN lens arrays.

Table Grahic Jump Location
Table 1Optical performance of illumination system using GRIN lens array.

Figure 7 shows the illuminace map of the illumination system using column arrays. Although the lighting spot produced by 12 accumulated column arrays is not a regular rectangle, leading to a certain decrease in projecting efficiency, the total light collection efficiency of this illumination system is 37.9%, which is higher than that of right square prism because of the fully utilizing for fringe rays in every single column lens. Also, the illumination uniformity achieves 85.8%.

Graphic Jump LocationF7 :

Illuminance map of illumination system using column GRIN lens arrays.

Further, a comparison of collection efficiency between GRIN based and LED based DLP illumination system is estimated in Table 2610 So far the lumen efficiency of LD is no more than that of LED in visible band, especially in blue, thus, the GRIN based system’s lumen efficiency in white balance temporarily lacked advantages. However, the total light collection efficiency of this illumination system is 35.437.9%, which is superior to the value of 28.2% of LED based illumination system in X. Zhao’s research.8 The volume of optical elements (couplers, correcting lenses, GRIN arrays and mirrors) is only 12.4cm3, which is superior to the value of 18cm3 in J. Pan’s work.9 Furthermore, the illumination uniformity is 85.589.7%, close to the 93% in J. Pan’s results.9 Beside, the étendue is conserved between 0.027sr·mm2 to 0.037sr·mm2, performing better collimation than that of any LED based system.

Table Grahic Jump Location
Table 2Comparison chart of collection efficiency between GRIN based and LED based DLP illumination system.

In this paper, under the strict consideration of optical characteristics and mechanical accuracy, we designed and simulated the illumination system based on laser diodes and GRIN lens arrays. The GRIN lens array improves the light collection efficiency, but also achieves the illumination uniformity, restricts the étendue, and greatly simplifies the structure of optical engine as well. Additional work toward solving the speckle problem caused by coherent nature of laser is still in progress. Further experimentation is on the march and more research toward improving the coupling efficiency of laser source is required. Moreover, with the continuous rising of LD’s lumen efficiency, the illumination system based on laser source and GRIN lens is a significant choice full of potential, and it is predictable that the novel approach is a promising direction for pico projectors.

This research is partially supported by the Applied Basic Research Foundation of Sichuan Province of P. R. China. (No. 2009JY0054).

Chi  Z., Chen  W., Applied Optics and Elements of Optical Design. ,  Southeast University Press ,  Nanjing , pp. 402 –406 (2008).
Jutamulia  S., Zhai  H., Mu  G., “Beam correction optics for laser diodes,” Proc. SPIE. 6024, (01 ) (2005). 0277-786X CrossRef
Wilson  R. G., “Ball-lens coupling efficiency for laser-diode to single mode fiber: comparison of independent studies by distinct methods” Appl. Opt.. 37, (15 ), 3201 –3205 (1998). 0003-6935 CrossRef
Zhou  B. et al., Principle of Laser. ,  National Defense Industry Press ,  Beijing , pp. 79 –83 (2009).
Svelto  O., Hanna  D. C., Principles of Lasers. ,  Plenum Press ,  New York , pp. 463 –492 (1998).
Murat  H. et al., “Increased lumens per Etendue by combining pulsed LEDs,” Proc. SPIE. 5740, , 1 –12 (2005). 0277-786X CrossRef
Harbers  G., Paolini  S., Keuper  M., “Performance of high-power LED illuminators in projection displays,” in  SID Microdiaplay 2002, Digest of Papers , 22 –25 (2002). 1551-319X 
Zhao  X. et al., “Study on the optical engine of the mini-projector,” Acta Opt. Sin.. 27, (5 ), 913 –918 (2007). 0253-2239 CrossRef
Pan  J. et al., “High efficiency pocket-size projector with a compact projection lens and a light emitting diode-based light source system,” Appl. Opt.. 47, (19 ), 3406 –3414 (2008). 0003-6935 CrossRef
ANSI/NAPM IT 7.228-1997, “Electronic projection-fixed resolution projectors,” American National Standard for Audiovisual Systems
Zhen  Y., Ye  Z., Yu  F., “Ultrahigh-performance lamp illumination system with compound parabolic retroreflector for a single liquid-crystal-on-silicon panel display,” Opt. Eng.. 46, , 054001  (2007). 0091-3286 CrossRef

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Dawei Rui received his bachelor’s degree in 2007 from the University of Electronic Science and Technology of China (UESTC). During 2007 and 2008 he served as an optical communication product engineer in OPLINK Ltd. He is now studying in School of Optoelectronic Information of UESTC for his Master’s degree, majoring in optical engineering. His reach interests are mainly the design of optical engine for micro-projection display system. His awards include one authorized National Patent and four academic papers.

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Zulun Lin is a professor in School of Optoelectronic Information of UESTC, expert of State Department Special Allowance, member of optoelectronics and display technology committee of Sichuan province and Chengdu City. His research subjects are micro-display, field emission, electron and ion beam, electro-vacuum devices, optoelectronic sensors and solar energy. His achievements include more than 30 national and local projects and three technology conversions. His awards include eight national and provincial Scientific Progress Prizes. He has more than 40 academic papers published.

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Kangcheng Qi received his PhD degree from UESTC in 2008. During 2007 and 2008 he served as a visiting scholar at Nanyang Technology University in Singapore researching ZnO nanometer material. Now he is an assistant professor in School of Optoelectronic Information of UESTC, and his research interests are display technology, cathode materials and RF sputtering coating.

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Wenbin Chen received his PhD degree from UESTC in 2002. During 2005 and 2006 he served as a visiting scholar at the University of Washington. Now he is an associate professor in School of Optoelectronic Information. He has conducted significant research into projection display and OLED devices. Currently, his research interests include thin-film transistors and flexible displays.

© 2012 Society of Photo-Optical Instrumentation Engineers

Citation

Dawei Rui ; Zulun Lin ; Kangcheng Qi and Wenbin Chen
"Optical design in illumination system of digital light processing projector using laser and gradient-index lens", Opt. Eng. 51(1), 013004 (Jan 20, 2012). ; http://dx.doi.org/10.1117/1.OE.51.1.013004


Figures

Graphic Jump LocationF4 :

Sketch of optical engine for single-plate DLP projector using laser sources and gradient-index (GRIN) lenses.

Graphic Jump LocationF3 :

Two types of architecture of gradient-index (GRIN) lens array: (a) array of right square prisms, and (b) array of columns.

Graphic Jump LocationF6 :

Illuminace map of illumination system using right square prism GRIN lens arrays.

Graphic Jump LocationF2 :

Schematic diagram of beam correction mechanism: (a) 3D model of correcting lens, and (b) beam expansion in orthogonal sections of GRIN lens.

Graphic Jump LocationF1 :

Beam distribution of gradient-index (GRIN) lens simulated by Zemax.

Graphic Jump LocationF5 :

Ray tracing of GRIN lens array illumination system.

Graphic Jump LocationF7 :

Illuminance map of illumination system using column GRIN lens arrays.

Tables

Table Grahic Jump Location
Table 1Optical performance of illumination system using GRIN lens array.
Table Grahic Jump Location
Table 2Comparison chart of collection efficiency between GRIN based and LED based DLP illumination system.

References

Chi  Z., Chen  W., Applied Optics and Elements of Optical Design. ,  Southeast University Press ,  Nanjing , pp. 402 –406 (2008).
Jutamulia  S., Zhai  H., Mu  G., “Beam correction optics for laser diodes,” Proc. SPIE. 6024, (01 ) (2005). 0277-786X CrossRef
Wilson  R. G., “Ball-lens coupling efficiency for laser-diode to single mode fiber: comparison of independent studies by distinct methods” Appl. Opt.. 37, (15 ), 3201 –3205 (1998). 0003-6935 CrossRef
Zhou  B. et al., Principle of Laser. ,  National Defense Industry Press ,  Beijing , pp. 79 –83 (2009).
Svelto  O., Hanna  D. C., Principles of Lasers. ,  Plenum Press ,  New York , pp. 463 –492 (1998).
Murat  H. et al., “Increased lumens per Etendue by combining pulsed LEDs,” Proc. SPIE. 5740, , 1 –12 (2005). 0277-786X CrossRef
Harbers  G., Paolini  S., Keuper  M., “Performance of high-power LED illuminators in projection displays,” in  SID Microdiaplay 2002, Digest of Papers , 22 –25 (2002). 1551-319X 
Zhao  X. et al., “Study on the optical engine of the mini-projector,” Acta Opt. Sin.. 27, (5 ), 913 –918 (2007). 0253-2239 CrossRef
Pan  J. et al., “High efficiency pocket-size projector with a compact projection lens and a light emitting diode-based light source system,” Appl. Opt.. 47, (19 ), 3406 –3414 (2008). 0003-6935 CrossRef
ANSI/NAPM IT 7.228-1997, “Electronic projection-fixed resolution projectors,” American National Standard for Audiovisual Systems
Zhen  Y., Ye  Z., Yu  F., “Ultrahigh-performance lamp illumination system with compound parabolic retroreflector for a single liquid-crystal-on-silicon panel display,” Opt. Eng.. 46, , 054001  (2007). 0091-3286 CrossRef

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