Optical Design of DL P Optical E ngine B ased on L ED Light Source3Q I U S ong13,W A N G W ei2s heng1,W A N G S hu2x i an1,Z H U B i ng1,TO N G Yan1,L I U J i n2gao1,M A O M i ng1,Z H A N G M i ng2j ie2,X U J i a21.School of I nf ormation,Science&Technolog y,East China N ormal Universit y,S hanghai200062,China;2.School of S cience&Engineer,East China N ormal Universit y,S hanghai200062,ChinaAbstract:A new DL P Optical Engine based on L ED Light Source in t he illuminating system for rear p rojec2 tion television(RP TV)is p roposed.The prototype employs HD2+Digital Micromirror Device(DMD)as t he image chip,and t he Fly2Lens as t he homogeneous element s.The p rinciple of t he optical design met hod based on etendue is analyzed to design t he system st ruct ure.The Fly2Lens illumination system has been employed to realize high light collection efficiency and t he optical uniformities and t he optical component design is discussed.The simulation result is also p ropo sed.Finally,t he prototype is manufact ured and t he test result is compared to t he simulation result.The white field outp ut flux is112.3lm,and t he homo2 chromatic outp ut flux R226.1lm,G251.4lm and B219.0lm is achieved wit h timing sequence distribution for t he t hree group s of L EDs R:G:B(10:15:11).The uniformities are below±20%.K ey w ords:L ED;DL P;Fly2Lens;DisplayEEACC:7260;41基于L ED光源的DL P光学引擎的光学设计3邱 崧13,王蔚生1,王淑仙1,朱 兵1,童 雁1,刘锦高1,毛 敏1,张鸣杰2,徐 佳21.华东师范大学信息科学与技术学院,上海200062;2.华东师范大学理工学院,上海200062摘 要:设计了一种基于L ED光源照明系统的DL P背投电视光学引擎,样机以HD2+数字微镜(DMD),蝇眼透镜,X棱镜作为主要光学系统构件.介绍了基于光学扩展量etendue的光学系统设计方法,采用了蝇眼透镜阵列作为匀光方案来提高光能利用率和光源均匀度,分析了照明系统的工作原理和系统结构,并给出了仿真结果.样机在三组L ED占空比为10:15:11的时序工作模式下白场输出光通量达到了112.3lm,单色输出光通量分别为R226.1lm、G251.4lm、B219.0lm,光场均匀度小于±20%.关键词:L ED;DL P;蝇眼透镜;显示中图分类号:TN94 文献标识码:A 文章编号:100529490(2008)0621712204 Nowadays,display technology develops quickly. The flat display such as the liquid crystal display (LCD)and the plasma display panel(PDP)have the trend to take the place of the traditional CRT.Among the large screen display whose size is above50inch, projectors based on the micro2display technology,in2 cluding the front and rear projectors,will still be the mainstream owing to the cost.For all we know t hat t he light source is t he main p roblem t hat limit s t he develop ment of micro2 display RP TV(MD2RP TV).The short lifetime of ult ra2high performance(U HP)lamp result s in t he high cost of t he light source[1].This is t he reason why t he sale of MD2RP TV was good at first,but fell sharply after a few years.L ED is a new promising light source of t hep rojection system ,because t he power per L ED andt he luminous flux per Watt increase wit h t he im 2provement of L ED ’s st ruct ure and t hermal disper 2sion [226].Meanwhile ,L ED has t he advantage of long lifetime ,wide color range ,harmless light ,environmental 2friend and high switch speed.St udy of t he L ED light source projectors focu 2ses on two point s :pocket projectors and MD 2RP TV.Wit h t he improvement of L ED ’s luminous flux ,small and portable p rojectors can also use L ED as it s light source [729].The pocket projectors require small size ,low power consumption and t he outp ut flux of several and dozens of lumen while t he st udy of t he MD 2RP TV aims at t he large out 2p ut flux.Generally speaking ,t he lowest outp ut flux for RP TV should be 200lm.The size of t he optical engine for RP TV is not so important since it can be p ut in t he racks.SUMSAN G ,A KA I ,SAN YO and J VC showed t he L ED light source RP TV on t he CES exhibition held in Las Vegas ,t he U nited States in J anuary of 2006.In the paper ,we proposed an optical engine pro 2totype based on L ED light source for RPTV ,the structure of the optical system ,the operational princi 2ple and the technological characters were introduced.1 K ey Components and Principle ofDesign1.1 Choice of the K ey ComponentsNowadays ,t here are DMD ,L CD and Liquid Crystal on Silicon (L COS )in t he digital micro 2dis 2play chip family.The DMD HD2+chip ,who se display definition is 1280×720,is employed in t his p rototype because it has t he following advantages :(1)DMD utilizes t he “none 2polarized ”light ,whereas L CD and L COS belong to t he liquid crys 2tal device and t he polarized transformation of t he light will complicate t he optical system st ruct ure.(2)In t he polarized transformation of t he light ,t he etendue (etendue is a geomet rical p rop 2erty of t he optics related to t he beam divergence and t he area of t he beam )of illumination light beam will double ,unless only using a polarized component.This p roblem won ’t affect t he U HP lamp because it has high power density and small etendue.By comparison ,due to L ED ‘s lower power density ,it will directly result in t he de 2crease of t he light energy utilization.Thus ,for t he micro 2display chip of t he same size ,t he outp ut flux of Single 2Panel DL P system can reach almost t he same as or even more t han t hat of Three 2Panel L CD or L COS system.(3)In Single 2Panel DL P projection system ,t hree group s of L ED are working sequentially ,t hat is ,only one of t he R ,G and B group s is lit at t he same time.The power consumption of t he system can be lower t han t hat of Three 2Panel system.(4)L ED ’s long lifetime makes it a good match wit h DMD.To make t he outp ut flux wit hin t he range of t he etendue ,t he L umileds Corporation ’s Green ,Blue and Red L ED were chosen as t he light source of t he prototype.1.2 Principle of the System Structure DesignTo design t he system st ruct ure ,t he DMD ’s etendue must be calculated to match t he L ED ’s one.Etendue can be calculated by t he following e 2quatio n :E =n 2·π·A ·sin 2θ(1)where ,E is etendue ,n is t he ref ractive index ofmedium ;A is t he lighting area of L ED or DMD ;θis half of t he largest divergence (convergence )an 2gle of t he usable beam in t he medium.In ideal situation ,L ED ’s etendue should be equal to DMD ’s i.e.when the light transportation efficiency is the highest ,all the optical energy can be projected on the DMD within this etendue.Actually ,due to the aberration or other reasons ,the energy utilization de 2creases.According to the equation of the etendue ,the number of the L ED (m )and corresponding useable an 2gle (θ)can be figured out.E L ED =m ·n 2·π·A L ED sin 2θL ED =E DMD (2)Analyzing t he light energy wit hin t he etendueof different number of L ED ,we will select t he number according to t he rule t hat more optical en 2ergy can be gotten from t he less L ED.Apart f rom t he etendue and light energy ,t he optical system struct ure ,mechanical struct ure and ot her factors should also be considered.On second t hought s ,eighteen L EDs per color were chosen as t he light source of t he prototype.Figure 1illust rates t he optical system st ruc 2t ure of t he optical engine.L ED Array ,a Collima 2ting Lens corresponding to L ED one by one ,a3171第6期邱 崧,王蔚生等:基于L ED 光源的DL P 光学引擎的光学设计2012-05-16########################2012-05-16########################2012-05-16Front Fly 2Lens ,a Back Fly 2Lens and Converging Lens const ruct a basic lighting system.There are t hree group s of similar lighting system for R G B light source.In t his figure only one group is showed out and t he ot her two group s are located at t he up and down ends of t he X Prism t hat combines R 、G 、B t ricolor to white color.The combinative light reflect s from t he bottom one of t he TIR Prism group to t he DMD for image modulatio n.Afterwards ,t he modulated light beam reflect s f rom DMD ,goes t hrough t he TIR Prism and Pro 2jection Lens and t hen project s image to t hescreen.12L ED Array ,22Collimating Lens ,32Front Fly 2lens.42Back Fly 2lens ,52C onverging Lens ,62X Prism ,72T op and Bottom Reflector ,82TIR Prism ,92DMD ,102Projection LensFig.1 Optical structure of the optical engine2 Design and Simulation of the Illu 2mination System The design of illumination systems needs to real 2ize high light collection efficiency and enough uniformi 2ty.There are two methods ,Fly 2Lens and optical beam ,to change the uniformity.The former features less optical component ,shorter optical path and high light collection efficiency while it has shortcomings such as die sinking and more initial investment.How 2ever ,the latter is on the contrary.The former is em 2ployed in our system because high light collection effi 2ciency is in the first priority.2.1 Design of the Fly 2LensThe system adopt s Fly 2Lens lighting unit composed of a Front Fly 2Lens and a Back Fly 2Lens.The p rinciple of light uniform illumination is t hat t he Fly 2Lens divides light beam into several ti 2ny light beams ,which image independently and in 2tegrate toget her to make t he whole light field uni 2form.One L ED is correspondent to one group of Fly 2Lenses.The number of t he unit s sho uld be minimized to meet t he requirement of t he uniformi 2ty.The p rinciple narrated above is shown in Fig 2ure 2.There are two object 2image relations.One ist hat L ED ’s beam goes t hrough t he packaging of it 2self ,a collimation lens and t he Front Fly 2Lens to image on t he Back Fly 2Lens ;t he ot her is between t he unit of Front Fly 2Lens and DMD.The Front Fly 2Lens is located on t he focal plane of t he Back one while micro 2display chip is located on t he focal plane of a converging lens.The details of t his struct ure refer to t he reference [10].Fig.2 The principle of uniform illuminationTo obtain t he object 2image relations described above ,certain eccent ricity should be made for t he Front and Back Fly 2Lens unit s.The eccentricity of t he Fro nt Fly 2Lens makes t he center of t he beam located in t he center of t he corresponding Back Fly 2Lens unit.The details refer to t he reference [10].The eccent ricity of t he Back Fly 2Lens is to pre 2cor 2rect sp herical aberration of t he Converging Lens.When t he light beam goes t hrough t he Back Fly 2Lens and t he Converging Lens ,t he sp herical aber 2ratio n will cause t he image of t he Front Fly 2Lens deviating from t he center of t he DMD.Anot her so 2lution is to design t he Converging Lens as an as 2p heric lens.However ,t he system cost will rise.Figure 3shows t he Fly 2Lens unit s ,t he front unit s on t he left and t he back ones on t he right.There are eight lens unit s in one group correspond 2ing to one L ED ,and t he number of t he f ront lens unit s is t he same as t he back ones.The distance f rom t he single lens unit to t he center of t he entire Fly 2Lens decides t he eccent ricity.More distance makes more eccentricity.Fig.3 Photo of Front and Back Fly 2Lens4171电 子 器 件第31卷2012-05-16########################2012-05-16########################2012-05-16In order to decrease t he size of RP TV ,t he fo 2cal lengt h of Projectio n Lens is required to be short enough t hat p rojection lengt h is short.In usual de 2signs ,t he asp heric plastics lens close to t he screen accumulates dust easily because it is upwardly placed and can’t tolerate cleaning.In addition ,t he attached p rotecting glass may bring ext ra cost and light energy lo ss.To overcome t he above short 2comings ,we substit ute t he plastic lens wit h t he glass ones who se basic unit still owns eight lenses.And t he imp rovement was made t hat t he two smal 2lest lenses employ asp heric lenses.Figure 4shows t he t ransfer f unction curve of t he glasslens.Fig.4 Transfer f unction curve of the glass lens2.2 Design of other ComponentsOt her main component s are X Prism ,Reflec 2tor and TIR Prism.Different f rom t he X Prism in Liquid Crystal Projector illuminated wit h polarized light ,t he one in t he DL P system is coated wit h not t he polariza 2tion film but t he depolarization film because DL P employs non 2polarized light.The Reflector not only avoids t he interference between t he PCB ,on which t he DMD is mounted ,and light pat h but also adjust s t he light pat h.TIR Prism is a special component to separate reflection ray f rom incident light.There is a 24o angle between reflection ray and incident light t hat is usually enlarged a bit to imp rove t he cont rast.In addition ,t he two light beams have certain widt h and angle.TIR Prism has two p risms cemented.There is a several 2micron air gap between t he ce 2mented surfaces.Also ,t he anti 2reflective film on t he cemented surface should perform well because some light s have more t han 80o incident angles to t he cemented surface.2.3 Simulation of Illumination E ffectIllumination effect on DMD was simulated asshown in Figure 5.No obvio us bright or dark are 2as are found ,which indicates good uniformity in light field.If t here is no timing sequence distribu 2tion for t he t hree group s of L EDs ,t he total outp ut flux from DMD is 615.2lm.The flux of R ,G and B is 160lm ,338.9lm and 116.3lm.It is esti 2mated t hat screen flux is 363lm on account of t he loss of DMD ,TIR Prism and ProjectionLenses.Fig.5 Simulation of illumination effect on DMD3 T est R esultsThe color coordinate values of t he tricolor L ED are R (0.6964,0.3034),G (0.2991,0.6734),B (0.1381,0.0568),depending on which t he screen flux can be estimated when t he p roto 2type is working wit h different color temperat ures of white filed.When t he white filed is 6500K ,t he screen flux is 147.1lm.In t he case of 9300K ,t he screen flux is 145.2lm.The circuit of t he prototype is redesigned based on t he reference design for U H P lamp and color wheel provided by TI.Testing condition is described as following :(Here all t he time se 2quences is rep resented by angle ).Timing sequences distribution for R ,G,B L EDsare R (50°),G (75°),B (55°),R (50°),G (75°),B (55°),which means the total angle of 360°is distribu 2ted to six segments for three groups of L EDs.Time for Switching Color (TSC )is 7°.TSC denotes the time for which DMD needs to be shut down while the L ED source changes its color sequentially.In t he test ,t he homochromatic outp ut flux is R (26.1lm ),G (51.4lm )and B (19.0lm ).The white field o utp ut flux is W (112.3lm )and t he u 2niformities are below ±20%.(下转第1719页)5171第6期邱 崧,王蔚生等:基于L ED 光源的DL P 光学引擎的光学设计2012-05-16########################2012-05-16########################2012-05-16阵列放在恒温箱中连续工作12h ,采用16路接触式测温系统测量选定点A 、B 、C 的温度,测温结果分别为58.1℃、56.8℃和57.5℃,比仿真结果稍低,但是在允许误差范围内,热试验结果验证了仿真结果的正确性。
