JOURNAL OF DISPLAY TECHNOLOGY, VOL. 3, NO.3, SEPTEMBER 2007
`
`295
`
`LED-Based Projection Systems
`
`Xing-Jie Yu, Y. L. Ho, L. Tan, Ho-Chi Huang, and Hoi-Sing Kwok, Fellow, IEEE
`
`Abstract-A novel design methodology for LED-array-based
`projection displays has been developed. By combining etendue
`limitation, system intensity, and efficiency requirements, a novel
`parameter space is proposed. Using this parameter space, LED
`lens-array and compound parabolic concentrator (CPC)-array
`illumination systems have been designed. A 1000-Im LED light
`source is built. Based on these lens-array and CPC-array illu(cid:173)
`minators, several LED-based liquid crystal on silicon (LCOS)
`projection systems are suggested. Among them, a one-panel LCOS
`projection system is proposed and tested. The method discussed
`here should be useful in the design of LED-array illumination
`systems for projectors in general.
`
`Index Terms-LED arrays, projectors.
`
`I. INTRODUCTION
`
`T HE system performance of a light-valve projector is very
`
`much dependent on the characteristics of the illumination
`system, which includes the projection lamp and its associated
`optics (mainly the light collection system). A number of the
`illumination system characteristics can have significant effects
`on the performance of the projection system. Among these are:
`lamp power, lamp efficacy, lamp spectrum, and illumination
`system etendue (or F/#). Other factors, such as lamp lifetime and
`warm-up time, can also influence the quality of the projector.
`Currently, there are two major types of projection lamps used
`for light-valve projection systems: tungsten-halogen lamps and
`high-intensity discharge (HID) lamps that include metal-halide
`lamps, UHP lamps, and xenon lamps [1]. Tungsten-halogen
`lamps are incandescent lamps with halogen atoms incorporated
`in the gaseous fill surrounding the filament. Light is emitted
`from the tungsten filament at around 2800 K to 3200 K. The
`advantages of these lamps are radiant cooling and low cost
`(only 0.0005 $/lumen). The disadvantages are low efficiency
`( 15 lm/W) and short lifetime. Also, the fundamental drawback
`is that it cannot achieve daylight color (color temperature
`6500 K) due to the lack of blue light in the spectrum. Thus,
`these lamps are not used in high-performance high-brightness
`projection systems. The HID lamp, also known as the arc lamp,
`consists of a sealed envelope containing a high-pressure gas
`and two electrodes. The fill gas can be xenon. A liquid such as
`mercury or a solid such as metal-halide salt is usually added to
`increase the efficiency. These lamps have very high efficiency
`(100 lm/W), and the correlated color temperature (CCT) can be
`as high as 8500 K. The major drawbacks are that they are quite
`
`Manuscript received July 30, 3006; revised September 12, 2006. This work
`was supported by the Hong Kong Government Innovation and Technology
`Fund.
`The authors are with the Center for Display Research, Department of Elec(cid:173)
`trical and Electronic Engineering, Hong Kong University of Science and Tech(cid:173)
`nology Kowloon, Hong Kong (e-mail: eekwok@ust.hk).
`Digital Object Identifier 10. !109/JDT.2007.901560
`
`expensive and they produce strong UV!IR emissions. As well,
`mercury-containing lamps are environmentally unsuitable. Arc
`lamps are also dangerous as the glass envelope and electrodes
`can break easily. Despite these drawbacks, arc lamps are used
`widely in high-brightness projection systems.
`As the projection market is increasing, alternative light
`sources are being explored. There is also a demand for portable
`low-cost projection systems which do not require very high
`brightness. Optical performance, portability, and low cost are
`crucial. HID lamps can provide good optical performance, but
`they are very expensive and not portable. Tungsten-halogen
`lamps satisfy the requirement on the cost, but the luminous
`efficiency and the color-rending ability are unacceptable for
`most applications. Thus, a novel illumination system needs to
`be developed for projection systems to satisfy all requirements:
`low cost, good color rendering, high luminous efficiency, and
`long lifetime.
`Light-emitting diode (LED) technology is ideally suited for
`these applications [2]-[4]. It has the advantages of excellent
`color gamut (> 95%NTSC), long lifetime (> 50 K·h), high
`brightness(> 10 K·nit), and ease of control and is environmen(cid:173)
`tally friendly. The controllability leads to novel ideas such as
`blinking light source and field sequential color. The continu(cid:173)
`ously improved efficiency makes it one of the best candidates
`for illiunination systcnts [4]. For the case of projection light
`sources, generally an LED array has to be used. There have
`been proposals of single-chip LED projectors as well as LED
`arrays for high luminous projectors with 400-lm output [2], [3].
`For such LED light sources, details, such as illumination optics
`and LED packaging for heat dissipation, have to be investigated
`thoroughly.
`In this paper, we shall first develop a novel method on LED
`array optical design for projection systems. Based on this
`method, the LED lens-array and compound parabolic concen(cid:173)
`trator (CPC)-array illumination system will be designed and
`developed, respectively. The package for the LED atTay will be
`also studied. Then, several possible architectures on LED-based
`liquid crystal on silicon (LCOS) projection systems will be
`suggested. As an illustration, a one-panel LCOS system will be
`tested experimentally, and the performance will be reported.
`
`II. OPTICS OF LED ARRAYS
`
`The fundamental questions (or requirements) in any illumi(cid:173)
`nation system are: etendue or F/# of the light source, light flux
`needed, and the acceptable light efficiency. The solution to the
`first requirement is given by the etendue limit [ 1]. If the area of
`the LED light source is ALED with the averaging emitting solid
`angle of ~kEo, the etendue ( ELED) of this light source can be
`written as
`
`( 1)
`
`1551-319X/$25.00 © 2007IEEE
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`JOURNAL OF DISPLAY TECHNOLOGY, VOL. 3, NO.3, SEPTEMBER 2007
`
`f
`
`/\
`i
`l \
`I i
`i I
`I
`
`, ....... .
`
`Fig. I.
`
`Illumination system with I 00% and 'ILED light utilization efficiency.
`
`Ll: D l 1ght Svuru:
`\S,,,,{:
`1, 1
`L'
`·lut.J) !·':·:
`
`The image of this light source through the light collection
`system at the position of the light valve has the total area of Ar
`and an averaging solid angle of Dr. For any optical systems,
`the etendue never decreases. Thus, the size of the light source
`is given by
`
`(2)
`
`For the desired light collection system, as shown in Fig. I (a), the
`total light flux of the LED source is completely projected onto
`the light valve with the requested F/#, that is, the area of A 1 is
`equal to the area of the light valve AL. As well, the solid angle
`~lr should be equal to or Jess than the requested solid angle f2L
`for the light valve. Since the LED source should be compact,
`and the total area of LED light source is given, then
`
`ALED = N • Apack
`
`(3)
`
`where N is the number of LEDs in the an·ay and A pack is the
`area of each LED package. Thus, the LED maximum number
`N is given by
`N<~~<~~-
`-
`nLED Apack -
`nLED Apack
`
`(4)
`
`For the case described by (4), the illumination system has the
`highest light utilization efficiency ( 100% ), which means all of
`the light from the light source can be utilized.
`If the LED number is increased, the total flux of the LED
`source will increase as well. However, at the same time, the
`light utilization efficiency will decrease since the etendue will
`be larger. The question is whether the flux of the light that can
`enter the light engine with the desired F/# also increases.
`Assume that light flux <I>LED can enter the light engine and
`be utilized completely, as shown in Fig. I (b). This flux 1>LED
`is contained in a solid angle of n%LED. The light utilization
`efficiency is
`
`'/]LED=-=----(cid:173)
`(!)LED-total
`
`(5)
`
`where <PLED-total is the total flux of the single LED. Then, the
`maximum LED number N for the illumination system with the
`utilization efficiency of TILED is given by
`
`N < DL
`AL
`-
`no/oLEO Apack
`
`(6)
`
`which is similar to (4) except for the new solid angle D%LED·
`The maximum usable light flux F is
`DL
`AL
`y-riLED(j)LED-total
`pack
`
`%LED
`
`F :::; D
`( D TILE~
`
`) DLAL<PLED-total·
`%LED pack
`
`(7)
`
`(8)
`
`It should be noted that the calculations above are based on
`the estimated etendue given by (I). The more accurate etendue
`calculation [ 1] is given by
`
`E = ll cos OdrldA.
`
`This calculation strongly depends on each specified optical
`system. Generally, no analytical equation can be derived. The
`analysis based on the accurate etendue calculation is quite
`complicated and sometimes impossible. Thus, it only can be
`done qualitatively. The results of our estimated calculations
`can be very good initial designs for the LED light sources. This
`will be verified later.
`Note that the etendue does not refer to the light intensity
`within the distribution of interest: it only refers to the distri(cid:173)
`bution's geometric boundaries. Thus, the second and third re(cid:173)
`quirements in the illumination design are related to flux and ef(cid:173)
`ficiency. If the efficiency of the system light engine is TILE, then
`the maximum final output <Pun-screen on the screen will be
`
`<Pun-screen ='I]LEF
`='I]LE ~ ( ___ !]LE_D _
`\
`\ D%LRf)Apack}
`
`X DLAL<PLED-total·
`
`(9)
`
`If the input power of each LED is 1 W, then the system luminous
`efficiency AI is
`
`"'( =
`
`'l?on-screen
`N . l
`
`= (j)LED-totai'IILED'TILE·
`
`(10)
`
`Rewriting (9) and (1 0) gives
`
`<]?on-screen
`
`(!)LED-total
`
`(11)
`
`(12)
`
`Equations (11) and (12) are the key rules for the LED-array
`based illumination system design. The factors nLAL, TILE· and
`<PLED-total depend on the system requirements (panel size,
`F/#, and light engine efficiency) as well as the LED die itself
`( <I>LED-total), while TILED I Apackn%LED is dependent on the
`LED optical performance and design.
`Based on these rules, any LED mny designs can be
`cauied out in a parameter space. The factor ((riLED) I
`cn%LEDApack))DLAL'/]LE for different LED optical designs
`is plotted as a function of the LED light collection efficiency
`'/]LED. (<Pan-screen) I (<PLED-total) and (r) I ( (j)LED-totalTILE)
`are calculated according to the system requirements, and they
`give the minimum requirements or the boundaries in the param(cid:173)
`eter space, as shown in Fig. 2. Thus, the solutions will be in the
`shaded region. Table I lists these two boundaries with the light
`
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`297
`
`<1> un-scn!en
`<l> I.FJ)
`
`:~
`t I~
`---1-------
`
`Thus, Dmax = 2R when t 2: R. Thus, the only variable is the
`lens-array thickness (t). For each l, (r/LED)I(D%LEDApack)
`can be easily calculated with commercial optical design soft(cid:173)
`ware, such as TracePro, ASAP, or LightTools. Fig. 4 shows
`(('IJLEo)I(D%LEDApack))DLAL77LE versus 'IJLED parameter
`space as a function of the lens-mny thickness t with different
`R (or D). Fig. 5 shows the maximum LED number N based on
`(6) versus 'IJLED parameter space as a function of the lens-array
`thickness t with different R (or (D). The light valve used here
`is 0.87-in LCOS panel with F/2 optics, and the light engine
`efficiency is 8%.
`requirements of 8%
`the m1mmum
`According
`to
`I,
`efficient LCOS systems
`listed
`in Table
`the bound(cid:173)
`aries in
`the parameter space are: 1)
`the flux boundary
`( c!>on-screen) I ( cpLED-total) > 0.66 and 2) the efficiency
`(I') I ( c!>LED-totul'IJLE) > 0.42 if the
`boundary 'IJLED =
`required final output is 20 lm with the system luminous
`efficiency of 1 lm/W and assuming 30 lm/W LED dies are
`used. Thus, the design solution in the parameter space should
`be above these two boundary planes. From Fig. 4, then the
`proper t and R(D) can be obtained (physical parameters of
`each LED optical structure). Once these parameters are known
`(D, R, t and 'IJLEo), from Fig. 5, the number of LED N can be
`found. For the lens-anay solution, in order to achieve the final
`performance of 20-40 lm with the system luminous efficiency
`of 0.6-1.4 lm/W, 15-40 LEDs with 50 1m/LED will have to
`be used. The questions on LED illumination design, including
`the structure of the lens arrays (D, R, t), number of LEDs,
`and the light collection efficiency '/]LED, can be obtained in the
`parameter space.
`The probiem for LED iens-mny method is that it needs many
`LEDs to achieve a large final output. This will result in a low
`system luminous efficiency, if the light engine efficiency is not
`high enough. The reason for this is that the lens mny is not a
`perfect optical component for light collection.
`In order to solve this problem, a solid CPC-array has been
`designed. The circular CPC was developed recently in an LED
`illumination system [6]. However, because the LED and the
`light valve (TFT, LCOS, and DLP) are all rectangular, the CPC
`should also be rectangular in order to obtain a high light collec(cid:173)
`tion efficiency T/LED. Our rectangular CPC and LED CPC-array
`structures (2 x 3) are shown in Fig. 6. There are f1ve shape pa(cid:173)
`rameters for this CPC: focal length f of the parabolic shape, tilt
`angle u of the parabolic axis, the front length FL, the back length
`BL, and focal shift DF. From a design point of view, the output
`diameter (Dout) and the input diameter (Din) of a CPC is more
`important, and both are functions of the five shape parameters.
`The input diameter (Din) is limited by the size of LED dies and
`packaging of the LED dies. Thus, the main design parameters
`will be Dout and the total length (LF-LB).
`Similar to the lens-mny design, by varying these five
`parameters of the CPC, the design parameters (Dout and
`LF-LB) can be calculated. For different design parameters,
`((T/LED)I(S'2%LEDAPack))nLALJ/LE can be plotted as a func(cid:173)
`tion of the LED light collection efficiency T/LED. This result is
`shown in Fig. 7 as a function of Dout of the CPC. Fig. 8 shows
`the maximum LED number N based on (6) versus '1/LED pa(cid:173)
`rameter space as a function of the output diameter of the CPC.
`
`Fig. 2. Parameter space with minimum requirements or boundaries.
`
`TABLE I
`BOUNDARIES FOR DIFFERENT <l>ou-oc..-ccu; <l>~oED-tolnl WITH 8% 1/GE
`
`f/U.
`
`c/JOII-SCreen
`
`I
`I C/Ju:JJ-toral
`
`¢011 ·· .q:ret•Jf
`<f> LED-lout!
`y
`<I> u:IJ-tow/7 u:
`
`8%
`
`20 Jm
`50 lm
`30 lm
`0.4
`0.66
`
`40 lm
`50 lm
`80 lm
`0.8
`0.5
`
`0.42y
`
`0.25y
`
`0.25y
`
`0.16y
`
`'
`:
`:
`
`'
`: /']Lens <liTH)
`
`~ '
`
`: 0 : YR
`. . ...
`:r·---\: 1- ------ --.:.-.:.~:.~:.:.-.:.-.:---r
`'"~ j
`(~\::::::mm t
`
`I
`
`11001 Si
`
`_________ _
`
`fig. 3. Structure of LED lens array.
`
`engine efficiency 'IJLE of 8% for different required final output
`c!>on-scrcen. the total flux cj)LED-total of the single LED.
`The objective of the LED illumination design is to find the
`LED optical package that satisfies these two minimum require(cid:173)
`ments. In this paper. the LED lens-array and CPC-array will be
`designed according to these laws.
`The structure of the LED lens-array is shown in Fig. 3. The
`1-W l-mm 2 LED dies are placed inside the wet-etched [100]
`Si V-grooves. On top of the LED-groove substrate, the lens
`array is then fabricated after wire bonding of the LED dies. The
`fabrication method for the lens array is a so-called "molding"
`method, which is originally developed for the micro-lens array
`used in the organic LEDs [5]. There are four physical param(cid:173)
`eters in the LED lens-array structure: the aperture (D), the
`radius (R), and the thickness (t) and refractive index (n) of the
`materials, as shown in Fig. 3. By varying these four parameters
`of the lens-array, ((77LED)I(D%LEDApack))fhAL'l7LE can be
`plotted as a function of the LED light collection efficiency
`1/LED. Take a 1-mm radius (R) silicone-based lens array
`(n = 1.49) as an example. In this case, only when t 2: R, light
`from the LED can be collected by this lens array efficiently.
`
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`JOURNAL OF DISPLAY TECHNOLOGY, VOL. 3, NO.3, SEPTEMBER 2007
`
`R= 1.5
`
`R= 1
`
`0,8
`
`lil
`><' 0.6
`
`::l u: 1 0.4
`J 0.2
`" u:
`
`0
`0
`
`0.2
`
`0.4
`
`Efft.0
`
`',>
`0.6
`
`-----
`0.8
`
`1.5
`
`1
`
`0.5
`
`0.8
`
`0.7
`
`lil 0.6
`><' " 0.5 ·'
`u:
`) 0.4
`~ 0.3
`,}
`" u: 0.2
`0.1
`
`0
`
`3
`
`2.5
`
`Fig. 4.
`
`((17r,Eo)/(S1%LEDAPack))Ou'l.r,I7LE versus 1/LED parameter space for the lens array.
`
`\
`
`R=3
`R=2 R=2.5
`
`R=i 3.5
`
`0.6
`
`0.8
`
`Eft\. eo
`
`1
`
`0
`
`80
`
`60
`
`40
`
`20
`
`~
`
`z
`
`'
`
`'••A
`
`0
`0
`
`R= 2~" a R,.3.5
`
`R= 2
`
`i
`z
`
`80
`
`60
`
`40
`
`20
`
`0
`0
`
`3
`
`2.5
`
`2
`
`8
`
`6
`
`6
`
`0.6
`
`Efflro
`
`0.8
`
`1
`
`0.5
`
`0.6
`
`0.8
`
`1 / 1
`
`2
`
`Fig. 5. Maximum LED number ;Y versus 1/LED parameter space for the lens array.
`
`Compared with the lens-array parameter space, for the same
`boundary (e.g., (<DoH-screen) I (<PLED-total) 2: 0.66; ?]LED =
`(r) I ( <PLED-totaJ17LE) 2: 0.42), the CPC system has solutions
`with high system luminous efficiency. For the same engine
`system and a final output of 20 lm, the CPC needs only six
`LEDs with an overall system luminous efficiency of> 3 lm/W.
`
`Ill. LED ILLUMINATION SYSTEM
`
`A. Packaging
`
`After the optical design of the LED mny is done, the LED
`illumination system can be packaged. Although the lens-array
`system has lower efficiency than the CPC one, we still use it in
`our investigation due to the ease of fabrication. According to the
`lens-array design, the LED has to be packaged very close to each
`other. The most important issue for this ultracompact LED light
`source is heat dissipation. A l-mm2 LED dissipating 1 W corre(cid:173)
`sponds to 100 W/cm2 of heat flux, which is twice the amount of
`the heat flux generated in a conventional microprocessor chip.
`Therefore, the effective removal of heat (to maintain a safe junc(cid:173)
`tion temperature) is the key to LED light source package. Nowa-
`
`days, t1ip-chip technology (FCT) is one of the best solutions for
`heat dissipation. It can be used in our package.
`We also developed a normal package for the LED lens array.
`The idea is that, using a silicon wafer as the substrate, heat
`can be dissipated efficiently due to its high them1al conduc(cid:173)
`tivity. The LED dies are placed on the bottom of the wet-etched
`[100] silicon V-shape groove as shown in Fig. 3. The surfaces
`of these V -shape grooves are coated with a metal such as alu(cid:173)
`minum, which serves as a mirror. The shape of the V -shape
`groove is important optically. Then, the lens array with the de(cid:173)
`sired dimension based on our design is formed on top of the op(cid:173)
`timal LED-groove substrate. The LED dies are mounted on the
`V -groove substrate by using thermally conductive epoxy. The
`same epoxy is also used to glue the silicon substrate onto the
`aluminum heat sink.
`The heat flow through the conducting layers attached in par(cid:173)
`allel or series can be analyzed in the equivalent thermal circuit
`[7]. Thermal conductivities of the various materials used in our
`package must be maximized. Of particular interest are the var(cid:173)
`ious interface materials, which constitute a large portion of the
`thermal resistance. In most cases, contact resistances rather than
`the bulk resistance of the interface material dominate the overall
`
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`299
`
`Fig. 6. Rectangular CPC and LED CPC-array structures.
`
`0.8
`
`fJ
`~ 0.6
`
`!!; • ~ 0.4
`,.f
`::J u:: 0.2
`
`0
`
`0
`
`0.2
`
`0.4
`
`Eff\.eo
`
`0.6
`
`'--,_ '
`0.8
`
`1 4
`
`( ( IJu;u) / (\ltYoLEDA.,.,ck) )DL AL 1/LE versus 1/LED parameter space
`Fig. 7.
`for the CPC array.
`
`.. I
`
`' '
`
`,!
`
`,._
`
`\,
`l
`
`L
`
`l
`i'
`
`'
`
`I
`·I
`
`100
`
`80
`
`60
`
`40
`
`20
`
`~ z
`
`0 -v.,
`0
`
`0.2
`
`Effh0
`
`0.6
`
`10
`
`Fig. 8. Maximum LED number N versus 1/LED parameter space for the CPC
`array.
`
`thermal resistance. By selecting the epoxy at the interfaces and
`other package materials, the overall thermal resistance of our
`
`Fig. 9. LED lens-array light source package with 40 1-W LEOs.
`
`10
`
`LED lens-array package is only limited by the LED sapphire
`substrate, which is 30 "CNv. The signiflcanl decrease iu thermal
`resistance (compared with the traditional 5-mm package T-1
`and T-1 3/4,Rthjp > 200°C/W) allows high-current-density
`operation.
`It is estimated that the maximum junction temperature for a
`1-mm2 LED die is around 120 °C. The measured package tem(cid:173)
`perature is < 40 ° C for 40 1-W LEDs packaged at the area
`of 160 mm2 • Thus, the permissible dissipation of our package
`is approximately 2 W. Fig. 9 shows the LED lens-array light
`source with 40 (8 x 5) 1-W LEDs (blue LEDs plus yellow phos(cid:173)
`phor) packaged at the area of 160 mm2 . The bare LED chips are
`from Epistar Corporation, with 27-30 1m/LED. The total output
`of this light source is 1000-11 00-lm white light at a driving cur(cid:173)
`rent of 350 mA for each LED. The R, G, and B LEDs are also
`tested on our package, and similar performance is obtained.
`For the CPC array solution, the LED package is much easier
`since less LEDs are used and LEDs are placed several millime(cid:173)
`ters ("" 7 mm in our design) away from each other. Thus, both
`FCT and our normal package can be used, and the heat dissi(cid:173)
`pation will not be a problem. We also package the CPC system
`with a 2x3 LED array.
`
`B. Polarization Conversion
`
`If the LED illumination system is employed for the projection
`system based on a liquid crystal light valve, polarization conver(cid:173)
`sion is needed [8]-[12]. The simple reflective polarizer and the
`broadband quarterwave plate (QWP) can be used as the polar(cid:173)
`ization converter for our LED source, as illustrated in Fig. 10,
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`
`: ..
`-converter
`
`--Polarizer
`
`:v
`
`!V""
`
`100%
`
`90%
`
`80%
`
`70%
`
`60%
`
`50%
`
`40%
`
`30%
`
`20%
`
`10%
`
`QWP
`
`polarizer
`
`O% 400
`
`430
`
`460
`
`490
`
`520
`
`550
`
`580
`
`610
`
`640
`
`670 700
`
`Fig. I 0. Structure of the polarization converter and its polarization conversion efficiency.
`
`TABLE II
`LJGIIT COLLECTION EFFICIENCY 'ILED FOR
`SEVERAL 2 X 3 CPC-ARRAY SYSTEMS
`
`CPC
`
`4.9mm (Dout)
`
`6.4mm (Duut)
`
`9.5mm (Dout)
`
`Lens number 0
`
`I
`
`2
`
`Ideal 0
`
`I
`
`2
`
`Ideal
`
`0
`
`I
`
`2
`
`Ideal
`
`0.6 inch
`
`22~1. 30% 33% 38% 27% 30% 33% 36% 18% 33% 35% 37%
`
`o.s7 inch b9"/ol62%179%18s%154%163% lno~olss% 147% 16s%lso% ss% 1
`i
`i
`i
`i
`i
`i
`i
`i
`i
`
`I
`
`which also shows its experimental polarization conversion effi(cid:173)
`ciency. The single reflective polarizer has 50% conversion effi(cid:173)
`ciency, obviously. The efficiency for our polarization converter
`is around 64%.
`
`C. Light Collectionfmm LED Light Sources
`The LED light collection efficiency 77LED obtained from the
`parameter space is the maximum value that can be achieved,
`which is calculated according to the etendue law. In the prac(cid:173)
`tical system, the simplest way to achieve this value is to use
`condenser lenses. We have done the optimizations on the
`condenser lenses for both lens-array and CPC-atTay systems.
`Table II summarizes the real light collection efficiency TiLED
`by using zero, one, and two condenser lenses for several
`CPC-array systems, compared with the ideal value from our
`parameter space method, for different panel sizes (0.6 and
`0.86 in). For example, for Dout of 6.4-mm CPC, from our
`parameter space estimation, a 2 x 3 CPC mny will be cor(cid:173)
`responding to 36% light collection efficiency for a 0.6 LCOS
`panel. By using a different number of condenser lenses, the
`maximum practical efficiency can be 33%. Thus, it can be
`seen that the real efficiency is close to the ideal one by using
`two condenser lenses. Sometimes, even one condenser lens is
`acceptable for the simple system configuration. In our practical
`systems, either one or two general BK7 condenser lenses are
`employed depending on the panel size used.
`
`IV. LED-BASED LCOS PROJECTION SYSTEMS
`Based on our optical designs and packages on LED illumi(cid:173)
`nation systems, several LED-based LCOS projection systems
`are suggested. In these systems, a 0.87-in 1280 x 768 color
`filter (CF) LCOS panel is used as the LCOS light valve [13]. 1
`Fig. 11 shows the LED-based one-panel system using either
`blue LED and yellow phosphor or RGB-LED combination. For
`the RGB-LED combination, they can be separated onto three
`blocks as shown. The red-, green-, and blue-LED light sources
`can be combined with an X-cube. Since the red LEDs have the
`highest luminous efficiency and the blue light is not very impor(cid:173)
`tant in the projected images, the red and blue LEDs can also be
`placed on the same block in a two-block system, as shown in
`Fig. 11 (b). This will simplify the optical design somewhat.
`Table III summmizes the expected final outputs of one-panel
`systems based on current LED technologies (late 2005), ac(cid:173)
`cording to our optimization on LED illumination system from
`the parameter space method. In this calculation, 77LE is assumed
`to be 8%. With more efficient LEDs, this table needs to be mod(cid:173)
`ified accordingly. This modification should be rather straight(cid:173)
`forward.
`It can be seen that the LED-based one-panel system can pro(cid:173)
`vide the final output of 15-40 lm. The blue LED plus yellow
`phosphor combination is simple and low-cost. Based on this so(cid:173)
`lution, the final output of 20 lm is still good for the application
`where the projected screen diagonal is smaller than I 0 in. For
`2x more efficient LED, which is available currently, the final
`projector output will be more than 2 x, since a smaller number
`of LED can be used, which improves the etendue requirement. It
`is estimated that the final output can be more than 3 x or 120 lm.
`One of our targets is a 10-in LED-based mini-projector.
`Fig. 12 shows our practical LED lens-array system setup for
`this target. The 40-LED light source demonstrated above is
`applied to this system integration. The total flux of our 40-LED
`
`1Color-filter LCOS (CF-LCOS) panels are obtained from Integrated Mi(cid:173)
`crodisplays Limited (iMD) of Hong Kong. Descriptions of CF-LOCS panels
`can be found in iMD website www.hkimd.com
`
`Energetiq Ex. 2086, page 6 - IPR2015-01279
`
`

`
`YU eta/.: LED-BASED PROJECTION SYSTEMS
`
`301
`
`Projection
`Len~
`
`1\na!yzcr
`
`PBS
`
`CFI.COS
`(a)
`
`GIXD
`
`CF LCOS
`
`(b)
`
`Fig. II. LED-based one-panel system.
`
`TABLE III
`EXPECTED FINAL OUTPUTS Of ONE-PANEL SYSTEMS BASED ON CURRENT LED TECHNOLOGIES. 1/LE IS ASSUMED TO BE 8% IN THIS CALCULATION
`
`Array
`
`LED Technology
`
`(j'JLED-total
`
`LED
`
`1/LED
`
`Projector Output
`
`Number/Chip
`
`(lm)
`
`lens
`
`array
`
`Blue LED+ Phosphor
`
`Blue LED+ Phosphor
`
`RGB-LED +Dichroic
`
`Mirror
`
`RGB-LED +Dichroic
`
`Mirror
`
`50
`
`50
`
`R:G:B
`
`50:50:16
`
`R:G:B
`
`50:50:16
`
`Blue LED+ Phosphor
`
`Blue LFD+ Phosphor
`
`~-----
`
`50
`
`50
`
`array
`
`Mirror
`
`ROB-LED+ Dichroic
`
`Mirror
`
`I
`
`R:G:B
`
`50:50:16
`
`R:G:B
`
`50:50:16
`
`40
`
`15
`
`40
`
`15
`
`6
`
`12
`
`6
`
`12
`
`15%
`
`37%
`
`15%
`
`37%
`
`85%
`
`50%
`I 85% I
`
`I
`
`50%
`
`24
`
`22
`
`40
`
`37
`
`20
`
`24
`
`34
`
`40
`
`Fig. 12. Practical blue-LED + yellow-phosphor-based one-panel system and its projected image.
`
`light source provides > 1000 lm. The measured TJLED is 15%
`for our present system. The practical f/LE is 8%. So, the final
`output should be 12 lm according to our estimation based on
`(7 .I) and the measured T/LE and T/LED. The final experimental
`output is 11 lm, which is consistent with our design value.
`The practical projected image of our LED-based one-panel
`projector is also shown in Fig. 12.
`If more flux output and larger color gamut are requested, an
`RGB-LED light engine will have to be used. Fig. 13 shows the
`practical LED lens-array system for this target and its associ(cid:173)
`ated projected image. The medium-flux RGB LEDs are used in
`
`this system integration. This system has the potential to provide
`40-lm output, as shown in Table III.
`The LED lens-an·ay system achieves the required target by
`using more LEDs corresponding to low system luminous effi(cid:173)
`ciency, while an LED CPC-array system shows the same per(cid:173)
`formance by using less than ten LEDs. In our demonstration, an
`2 x 3 LED CPC-array is used. This system can provide > 20 lm
`output by just using six LEDs.
`The final flux outputs of these two systems are still not high
`enough for the large-screen (> 40 in) rear projection television
`(RPTV) applications. If the LED light source is still used for
`
`Energetiq Ex. 2086, page 7 - IPR2015-01279
`
`

`
`302
`
`JOURNAL OF DISPLAY TECHNOLOGY, VOL. 3, NO.3, SEPTEMBER 2007
`
`Fig. 13. Practical RGB-LED-based one-panel system and its projected image.
`
`PBS
`
`i'rojc\:lion I .Cll'i
`
`R-I.COS -I
`
`·LI'[)
`
`K' B-LCOS
`
`Fig. 14. Two-/three-panel system configurations.
`
`R U LLDs
`
`TABLE IV
`EXPECTED FINAL OUTPUTS OF TWO-/THREE-PANEL SYSTEMS
`
`Array
`
`LED Technology
`
`l/Ju::D-rotal
`
`lens
`I aJTay
`
`I
`
`RGB-LED
`
`G: 50 1m
`
`LED
`Number/Chip
`15
`
`7JLED
`
`37%
`
`Projector Output
`(lm)
`\60
`
`--·-
`
`the large-screen RPTV application due to its good color ren(cid:173)
`dering and low cost, then the LED-based two-/three-panel sys(cid:173)
`tems are suggested. The system configurations are shown in
`Fig. 14. These two systems have a similar efficiency, because
`the efficiency of the green light dominates the overall system
`efficiency and both systems have the same efficiency for the
`green light. Table IV summarizes the expected final outputs of
`the systems based on current LED technologies. In this calcu(cid:173)
`lation, T/system is assumed to be 34%. It can be seen that these
`two systems can provide > 150-lm output that will be good for
`HDTV applications. Due to the cost and complexity of different
`systems, only a one-panel system is tested expetimentally in this
`paper. Two-/three-panel systems based on a CPC anay will be
`concentrated in future studies of HDTV applications.
`
`(!)I ( <.PLED-total''lLE)) in the above parameter space, various
`solutions can be obtained.
`According to this law, the LED lens-array and CPC-array
`systems have been designed and developed. The optical perfor(cid:173)
`mances and packages of these systems are examined. The final
`output > 1000-lm LED light source is built. Based on the de(cid:173)
`signs of the lens-atTay and CPC-array illumination systems, sev(cid:173)
`eral LED-based LCOS projection systems are suggested.
`Due to the cost and complexity of different systems, in this
`paper only a one-panel LCOS projection system is tested to
`prove our optical designs. The results show that our method is
`useful in the design of LED-array illumination systems. The
`output obtained agrees with that from both simulations and
`experiments.
`
`V. CONCLUSION
`
`ACKNOWLEDGMENT
`
`A novel design method of an LED-array illumination
`system for projection displays has been developed. By com(cid:173)
`bining etendue limitation, intensity, and efficiency require(cid:173)
`ments, a parameter space method is proposed. The factor
`((77LED)I(D%LEDApack))DLA£'17LE is plotted as a function of
`the LED light collection efficiency T/LED with varying physical
`parameters of the different LED optical designs. According to
`the minimum requirements (( <.Pon-screen) I (<.PLED-total) and
`
`The authors would like to thank Epistar Corporation for sup(cid:173)
`plying the LED chips in this study.
`
`REFERENCES
`
`[I) E. H. Stupp and M.S. Brennesholtz, Projection Displays. New York:
`Wiley, 1998.
`[2] M. H. Keuper, G. Harbers, and S. Paolini, "ROB LED illuminator for
`pocket sized projectors," Proc. Soc. lnf Disp. (SID '04 }, p. 943, 2004.
`[3] W. Folkerts, Proc. Soc.lnf Disp. (SJD'04), p. 1226,2004.
`
`Energetiq Ex. 2086, page 8 - IPR2015-01279
`
`

`
`YU eta/.: LED-BASED PROJECTION SYSTEMS
`
`303
`
`(May
`[4[ Sandia, Solid-State Lighting Archived Headline News
`11, 2004)
`[Online[. Available: http://lighting.sandia.gov/Xlight(cid:173)
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`[61 H. Zou, A. Schleicher, andJ. Dean, "Single panel LCOS color projector
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