holographic methods

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A overview of holography and its methods

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Review of digital holographicmicroscopy for three-dimensionalprofilingand trackingXiao YuJisoo HongChanggeng LiuMyung K. KimReview of digital holographic microscopy forthree-dimensional profiling and trackingXiao Yu,* Jisoo Hong, Changgeng Liu, and Myung K. KimUniversity of South Florida, Digital Holography and Microscopy Laboratory, Department of Physics, Tampa, Florida 33620Abstract. Digital holographicmicroscopy(DHM)isapotent tool toperformthree-dimensional imagingandtracking. We present a review of the state-of-the-art of DHM forthree-dimensional profiling and tracking withemphasisonDHMtechniques, reconstructioncriteriafor three-dimensional profilingandtracking, andtheirapplicationsinvariousbranchesof science, includingbiomedical microscopy, particleimagingvelocimetry,micrometrology, and holographic tomography, to name but a few. First, several representative DHM configu-rations are summarized and brief descriptions of DHM processes are given. Then we describe and compare thereconstruction criteria to obtain three-dimensional profiles and four-dimensional trajectories of objects. Details ofthesimulatedandexperimental evidencesofDHMtechniquesandrelatedreconstructionalgorithmson par-ticles,biological cells,fibers,etc.,withdifferentshapes,sizes,andconditionsarealsoprovided.Thereviewconcludes with a summary of techniques and applications of three-dimensional imaging and four-dimensionaltracking by DHM. The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction ofthis work in wholeor inpart requires full attribution of the original publication,including its DOI. [DOI:10.1117/1.OE.53.11.112306]Keywords: digital holography; microscopy; three-dimensional profiling; four-dimensional tracking; microparticles; nanoparticles;microfibers; cells.Paper 140175SSreceived Jan. 29, 2014; revised manuscript received Feb. 28, 2014; accepted for publication Mar. 4, 2014; publishedonline Apr. 3, 2014.1IntroductionNowadays, three-dimensional (3-D) profilingandtrackingmicroscale objects have been receiving much attentiondue to their wide applications. For example, microsphere ormicrobubble counting and locating in a 3-D space occur inthe fields of microfluidics, suspension rheology, crystalliza-tion, etc.111Many microorganisms swim in 3-D and helicalpaths. Thus, measuring their 3-D trajectories in time is essen-tial to obtain detailed information on biophysical processes,such as motile behavior and dynamic performance.1214Quantitativeanalysesofcancercell locomotionandshapechange in a 3-D environment reveal the biological character-isticsfor clinical need.15203-Dimagingofrandomlyori-entedmicrofibers andtheir interactions withsurroundingfree-swimmingcells opens upnewperspectives incaseswhere quantitative measurement of characteristics (size, length,orientation, speed, displacement, etc.) is of great interest.21Various approaches have been demonstrated for 3-Dprofilingandtrackingof micro-sizedobjects, andopticaltechniques and numerical localization algorithms have beenwidely chosen as remarkable tools since they have the advan-tages of being full-field, label-free, noncontact, noninvasive,etc.2243Digital holographic microscopy (DHM) is an emerg-ingtechnologyofanewparadigmingeneralimagingandbiomedical applications.44,45Various techniques of DHM,includingoff-axisDHM, in-lineDHM, quantitativephasemicroscopybydigital holography(DH-QPM), etc., havebeenproventobepotent toolstoprofileandtrackmicro-sizedobjectsin3-Dvolumes. Inconventional microscopytechniques, only two-dimensional focused images on a fixedplane can be recorded, while information not in the focal planewouldbepermanentlymissed. However, DHMprovidesatool torefocusanobject andisabletorecordahologramcontainingall thereal-time, 3-Dstructuresofanobject inthe absence of mechanical focus adjustment. And the infor-mation is available in digital form for postprocessing. DHMhas been shown to be the key to 3-D particleimage veloc-imetry (PIV)4650since the conventional PIV techniques havetheinherent limitations of thindepthof field, instrumentcomplexity, and impossibility of real-time imaging.5153However, DHM is able to overcome these and has the sim-plicity of the microscope and sample preparation, maximuminformation and resolution, reduction in time and dataamount, etc. Aberrations and background distortions ofthe optical field can be minimized by available DHM tech-niques. The DHM for 3-D profiling and tracking generally isa two-step process: first a hologram is recorded digitally, andthenthe hologram is numerically reconstructed in differentdepths to yield an image of the object by various numericaldiffractionmethods, suchasangularspectrummethod,44,45Kirchhoff-Helmholtz transform,54Fresnel transform,55,56Huygens convolution,44,45etc. This reconstruction resultsin complex field of object, and one can extract its amplitudeor phase profile to represent the object. Investigations of thethird dimension based on the reconstructed data are then per-formed to determine focal planes and complete the 3-D pat-tern recognition. For a moving object, the reconstruction ofsuccessiveholographicdatayieldsacompletefour-dimen-sional space-timerecordofthedynamicprocess. DHMisdemonstrated to have the capacity of monitoring the 3-D dis-tributionandmotionpatternof particles, livingcells, andfiberswithdifferent shapes(spherical, needleshaped, andrandomly oriented),size(fewto hundredmicrometer),andconditions (static, suspended, and flow-through) in real time.Description of several representative DHM techniques isintroducedinSec.2,andtheholographicinformationpro-cessingmethodsfor 3-Dprofilingandtrackingaregiven *Address all correspondence to: Xiao Yu, E-mail:xyu4@mail.usf.eduOptical Engineering 112306-1 November 2014 Vol. 53(11)Optical Engineering 53(11), 112306 (November 2014)REVIEWin Sec. 3. Wide and active field of applications on 3-D profil-ingandtrackingofdifferent samples, includingmicropar-ticles, bubbles,biologicalcells,microfibers,etc., byDHMtechniquesareintroducedinSec. 4, andtheconclusionisprovided in Sec. 5. This review has an emphasis on applica-tions of DHM in the field of 3-D profiling and tracking and,therefore,omits somemajorareas,suchas digital hologra-phy principle and development, theoretical studies of digitalholography,specialdigitalholography techniques,etc.2DHM ConfigurationsAbasicDHMsetupconsistsofanilluminationsource, aninterferometer, a digitizing camera, and a computer with nec-essary programs. Most often a laser is used for illuminationwiththenecessarycoherencetoproduceinterference. Formultiwavelengthtechniques, twoor more different laserscan be coupled into the interferometer, or a tunable laser canbeemployed.Therearealsolow-coherencetechniquesforthepurposeofreducingspeckleandspuriousinterferencenoise or generating contour or tomographic images. Even anLED typically has 30 m or so coherence length, which canbesufficient forholographicmicroscopy.2.1In-Line DHMIn this review, we refer to Gabor holography as in-line holog-raphy.57In-line DHM is a type of microscopy without objec-tive lenses,and as illustrated in Fig.1, a singlelight beamdirected onto a pinhole of a diameter of the order of a wave-length illuminates the object, typically several thousandwavelengthsawayfromthepinhole, andtheobject beamis the part of the incident light that is scattered by the objectand the unscattered remainder is taken as the reference beam.If the light source is coherent, the pinhole is not necessary.The object field is automatically in alignment with the refer-ence beam, and the interference of the object and referencebeams results in the holographic diffraction pattern, which isrecorded by CCD camera and then transferred to a computerfornumericalreconstruction.Byremovingbackgroundeffects first andthenrecon-structingtheobject fieldat different planes, a3-Dimagecan be built up from a single two-dimensional image, and theentire hologram pixel count is utilized, which also leads toshorter minimum distances for reconstruction and higher res-olutionof theresultant image. Withthesimplicityof theapparatus and large depth of field, the in-lineDHM is par-ticularly useful for particle image analysis. In conjunction withreconstruction algorithm, the 3-D position of a particle can bedetermined andthen appliedtoparticle velocimetry,such astracking small particles or swimming cells in a liquid flow.2.2Off-Axis DHMIn an off-axis DHM setup, a Mach-Zehnder interferometer ischosensince it offers flexibilityinalignment, especiallywhen microscopic imaging optics is used. One microscopeobjective lens (MO) is needed for object magnification andanother one is used in the reference arm to match the curva-tures of the object and reference wave fronts. We illustratedtheoff-axis holographicmicroscopysetupinFig. 2. Theobject arm contains a sample stage and an MO that projectsamagnifiedimageoftheobject ontoaCCDcamera. Thereference arm similarly contains another MO, and the refer-enceandobject wavesareoffset byanangletoavoidtheoverlapof thereferenceandthetwinimages, sothat theholographic interference pattern contains fringes due to inter-ferencebetweenthediffractedobjectfieldandtheoff-axisreference field. The captured holographic image is numeri-callyconvertedintoFourier domaintoobtaintheangularspectrum,44,45andaspatial filter isthenappliedtoretainthereal imagepeakalone. Thefilteredangular spectrumis propagatedtoappropriate distance and, byaninverseFourier transform, reconstructed as an array of complexnumbers containing the amplitude and phase images ofthe sample. Both amplitude and phase profiles can be furtheranalyzed to quantitatively determine the positions of objectsina 3-Dspace.2.3QuantitativePhase Microscopy by DigitalHolographyOff-axis DHMis a very effective process for achievinghigh-precision QPM since it allows measurement of opticalFig. 1In-line digital holographic microscopy (DHM) setup. MO, micro-scope objectives; P, pinhole; S, sample object.Fig. 2O