Non-blind area PAL system design based ondichroic filterYujie Luo,2 Jian Bai,2Xiangdong Zhou,2 Xiao Huang,'2 Qun Liu,'2 Yuan YaoCollege of Optical Science and Engineering.Zhejiang University,Hangzhou,Zhejiang.ChinaState Key Laboratory of Modern Optical Instrumentation,Zhejiang University.Hangzhou.Zhejiang.Chinabai@zju.edu.cnAbstract:An inevitable problem of Panoramic Annular Lens (PAL)System is the blind area in the central part of image plane,which vastlyreduces the pixel efficiency of detector.Here we present a design of PALsystem based on dichroic filter,which can combine two channels of rays indifferent bands and image them on the same detector.The dichroic filter inthis paper is designed to make ultraviolet rays reflect and visible raystransmit,and the ultraviolet rays are imaged on the PAL imaging area whilethe visible rays are imaged on the blind area of conventional PAL imageplane.In order to image both bands of rays on the same plane,we add acorrecting lens group in front of PAL system.As a result,the FOV ofultraviolet channel is ranging from 30~90 and the one of visible channelis ranging from0°-l0°.OCIS codes:(220.3620)Lens system design;(080.2740)Geometric optical design;(120.4570)Optical design of instruments.References and links1.Z.Huang,J.Bai,T.X.Lu,and X.Y.Hou,"Stray light analysis and suppression of panoramic annular lens,"0 pt.Express219),10810-108202013).2.I.Powell,"Design study of an infrared panoramic optical system,"Appl Opt.35(31),6190-6194(1996).3.S.Niu,J.Bai,X.Y.Hou,and G.G.Yang,"Design of a panoramic annular lens with a long focal length,"Appl0pt.4632,7850-7857(2007).4.S.Thibault,"Panoramic lens applications revisited,"Proc.SPIE Vol.7000,70000L1-8(2008).5.E.J.Tremblay,D.L.Marks,D.J.Brady,and J.E.Ford,"Design and scaling of monocentric multiscaleimagers,"Appl0pt.51(20,4691-4702(2012).6.annular center of field of view,"Opt.Express 19(5),3843-3853 (2011).Y.J.Luo,J.Bai,and Y.Yao,"Design of vari-focal panoramic annular lenses based on Alvarez 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systems,"AppL.0pt.51(17),3776-37862012).16.B.Lin,T.Y.Yu,D.Q.Liu,and F.S.Zhang,"Design and deposition of infrared/visible wide-band colorseparation filters,"Joumal of Infrared Millim.Waves 23(5),393-395(2004)(in Chinese).17.Datasheet of Product CMV4000,CMOSIS.#256869Received 7 Jan 2016;revised 16 Feb 2016;accepted 17 Feb 2016;published 26 Feb 20167Mar20161Vol.24,No.51D0:10.1364/OE.24.0049131OPT1 CS EXPRESS491318.Chinese CDGM optical glass products database1.IntroductionAll of the panoramic annular lens,fisheye lens and catadioptric system can realize the featureof imaging a wide field of view (FOV),even over 90,which are mostly used in the satellitenavigation system,robotic vision system and surveillance system,each with their merits andshortcomings [1].The main advantages of PAL system are its compact size,small f-thetadistortion and simple fabrication structure.In 1994,Canadian researcher Powell designed aninfrared panoramic lens system which utilized two lens groups to project a full 360cylindrical field of view onto a two-dimensional annular plane [2].During the past twodecades,other researchers endeavored lots of improved designs towards PAL system,forinstance,extending the usage of PAL to visible and ultraviolet bands,introducing asphericalsurface into the PAL system to shrink the blind area,adding several detectors on the imageplane to get a high resolution picture,utilizing free-form surface to realize the zoomingfunction,and so on [3-8].However,there is still a fatal problem for us to conquer,the blindarea.(a)(c)Fig.1.Principle of FCP and ray-tracing in PAL.(a)FOV distribution,where Z represents theoptical axis,a and B denote the minimal and maximal FOV respectively,while 0 indicatesthe orthogonal FOV to the optical axis.(b)Image plane.The imaging area of PAL system withFCP principle is a ring,the inner and outer solid circles are corresponding to the FOV a andB respectively,and the dashed circle represents the orthogonal FOV.The FOV a and B areexpanded by this orthogonal FOV.(c)Typical structure of PAL system,where Ti,T2 representthe transmitted surface 1 and transmitted surface 2,and R,R2 represent the reflected surface Iand reflected surface 2.According to the principle of flat cylinder perspective (FCP),while utilizing the PALsystem,the existence of surface R2 in Fig.1(c)will obstruct the rays in front of this system,thus the pixels in the central part of detector cannot capture any ray,forming a round blindarea [9].And this problem directly weakens the efficiency of detector and limits the usage ofPAL system.In order to make up the blind area on image plane,there must be the real objectpoints conjugated to the image points in the central part of detector.With the purpose oflooking for the conjugative object points,Chinese researcher Z.Huang proposed to coat a#256869Received 7 Jan 2016;revised 16 Feb 2016;accepted 17 Feb 2016;published 26 Feb 20167Mar20161Vol.24,No.51D0L:10.1364/0E.24.0049131OPT1 CS EXPRESS4914ring-shaped reflective film on surface R2 of PAL block in 2012,which can make the rays withthe conventional PAL FOV reflect and let the incident rays which usually have a small FOVin front of the system transmit into the PAL block through the hole in the central part of thisfilm [10].By this design,the blind area of typical PAL system can be made up.Besides,in2015,researcher C.Gong proposed a new structure of panoramic system without centralblindness based on a special aspherical lens [11].And the essence of both methods mentionedabove is the same that there is a ring-shaped film coated on a designated surface.However,there still mainly have two drawbacks about this structure:1)it is difficult to coat a ring-shaped reflective film with smooth inner round boundary on a non-planar surface,and 2)thesmall size of this hole makes it harder to enlarge the front FOV and weakens the energy of theincident rays.PALRing-shapedFront imagingareaPAL imagingarenRelay lensImage planeNon-imagingPAL imaging track一Front imaging trackFig.2.The schematic diagram of non-blind area PAL system with a ring-shaped reflectivefilm.The gray line on surface R2 represents the ring-shaped reflective film,which can only letthe front rays within the area of central hole pass through,and make the rays in any wavelengthreflect at other parts of this film.In recent decades,the technique of film coating has obtained an extraordinarydevelopment,especially on the coating of wavelength-based film.Dichroic filter is a kind ofselective film,which can make the rays of some specific wavelength bands reflect and makerays of other bands transmit [12].In this paper,we propose to apply the technique of dichroicfilter into PAL system,which can solve the problems mentioned above and even get somenew features,for example,the dual-band imaging.Here we take the optical system of star-tracker as the prototype,and get an achievable design of dual-channel PAL system withdifferent bands,which can confirm the feasibility of this structure and provide a new schemeto miniaturize the size of multi-channel star-tracker system.This paper is organized as follows.Section 2 presents the application background and thedetailed principle of non-blind area PAL system based on dichroic filter.Section 3 presentsthe design procedure step by step,including the design analysis and results.Section 4concludes the optical system proposed in this paper and discusses the other application area ofthis kind ofnon-blind area PAL system.2.The principle of non-blind area PAL system based on dichroic filter2.1 Application backgroundStar-tracker is an avionics instrument used to provide the absolute 3-axis attitude of aspacecraft utilizing star observations,which consists of an optical system and associatedprocessing electronics.The processor has the capability to perform star identification utilizingautonomously [13].The prototype of optical design in this paper is a kind of new dual-#256869Received 7 Jan 2016;revised 16 Feb 2016;accepted 17 Feb 2016;published 26 Feb 20167Mar20161Vol.24,No.51D0:10.1364/0E.24.00491310PT1 CS EXPRESS4915