SPIE Reviews

Principles and techniques of digital holographic microscopy

[+] Author Affiliations
Myung K. Kim

University of South Florida, Department of Physics, 4202 E. Fowler Avenue, Tampa, Florida33620 mkkim@cas.usf.edu

SPIE Reviews. 1, 018005 (May 14, 2010). doi:10.1117/6.0000006
History: Received December 05, 2009; Accepted March 17, 2010; Published May 14, 2010
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Digital holography is an emerging field of new paradigm in general imaging applications. We present a review of a subset of the research and development activities in digital holography, with emphasis on microscopy techniques and applications. First, the basic results from the general theory of holography, based on the scalar diffraction theory, are summarized, and a general description of the digital holographic microscopy process is given, including quantitative phase microscopy. Several numerical diffraction methods are described and compared, and a number of representative configurations used in digital holography are described, including off-axis Fresnel, Fourier, image plane, in-line, Gabor, and phase-shifting digital holographies. Then we survey numerical techniques that give rise to unique capabilities of digital holography, including suppression of dc and twin image terms, pixel resolution control, optical phase unwrapping, aberration compensation, and others. A survey is also given of representative application areas, including biomedical microscopy, particle field holography, micrometrology, and holographic tomography, as well as some of the special techniques, such as holography of total internal reflection, optical scanning holography, digital interference holography, and heterodyne holography. The review is intended for students and new researchers interested in developing new techniques and exploring new applications of digital holography.

Digital holography (DH) is an emerging technology of new paradigm in general imaging applications. By replacing the photochemical procedures of conventional holography with electronic imaging, a door opens to a wide range of new capabilities. Although many of the remarkable properties of holography have been known for decades, their practical applications have been constrained because of the cumbersome procedures and stringent requirements on equipment. A real-time process is not feasible, except for photorefractives and other special materials and effects. In digital holography, the holographic interference pattern is optically generated by superposition of object and reference beams, which is digitally sampled by a charge-coupled device (CCD) camera and transferred to a computer as an array of numbers. The propagation of optical fields is completely and accurately described by diffraction theory, which allows numerical reconstruction of the image as an array of complex numbers representing the amplitude and phase of the optical field. Digital holography offers a number of significant advantages, such as the ability to acquire holograms rapidly, availability of complete amplitude and phase information of the optical field, and versatility of the interferometric and image processing techniques. Indeed, digital holography by numerical diffraction of optical fields allows imaging and image processing techniques that are difficult or not feasible in real-space holography.

Holography was invented in 1948 by Dennis Gabor in an effort to improve the resolution of the electron microscope, where the correction of electron lens aberrations posed increasing technical difficulty. Instead of attempting to perfect the electron imaging lens, Gabor dispensed it altogether and realized that the diffraction pattern of the electron beam contained complete information regarding the amplitude and phase of the electron wave. The record of the electron wave diffraction is then used to optically synthesize the object field. This allowed the use of the optics of visible light for image formation, and was much easier and more developed compared to electron optics. He named the new imaging principle holography, for its ability to record the whole optical field 13.

The holography principle was immediately applied to recording and imaging by visible light 4. But it had to wait for two critical inventions before its full potential was to be realized. One was the powerful coherent source of light in lasers to provide high quality interference contrast. The other, due to Leith and Upatnieks 57, was off-axis illumination with a separate reference wave, thus eliminating the problem of the zero-order and twin images of the Gabor's on-axis configuration. Soon many new techniques and applications of holography began to develop. Holography is now a mature field, and an excellent survey is given, for example, in Ref. 8. It was also realized early on that the use of nonplanar structures of the reference beam can lead to various manipulations of the holographic output, ranging from image magnification to more complex image processing, such as encryption, pattern recognition, associative memory, and neural networks 9. Instead of photographic plates or films, real-time holography is possible with photorefractives and other nonlinear optics materials. In fact, we now understand much of nonlinear optics as generalizations of the holography principle, including phase conjugation, four-wave mixing, spectral hole burning, and photon echo 10. The holography process is being developed for other regions of the electromagnetic spectrum, especially x-ray holography, with its prospect of atomic resolution 11, as well as for microholography of living organisms 12.

For many application areas, real-time operation is of critical importance but is difficult with conventional holography. Even photorefractives and other nonlinear optics systems require substantial equipment and technical care to implement them, and have seen only limited practical applications. Digital holography replaces physical and chemical recording processes with electronic ones, and the optical reconstruction process with numerical computation. The propagation of optical fields is completely and accurately described by diffraction theory, and in 1967, Goodman and Lawrence demonstrated the feasibility of numerical reconstruction of an image from a Fourier hologram detected by a vidicon camera 13. Schnars and Jueptner, in 1994, were the first to use a CCD camera directly connected to a computer as the input, and compute the image in a Fresnel holography setup 14. In what is now called digital holography (DH), holographic interference is produced by optical processes in real space, while reconstruction is by numerical computation. Conversely, in computer-generated holography (CGH), the hologram can be produced by numerical computation inside a computer, followed by printing or other outputs to real space 15. Reconstruction is then carried out by optical means. CGH has many interesting properties and applications, such as the ability to arbitrarily prescribe desired amplitude and phase properties of the output optical field starting from fictitious objects. CGH is not a subject of this review 16.

By direct electronic recording of holographic interference, and because of the increasing speed of holographic computation, real-time holographic imaging is now possible, and more importantly, the complete and accurate representation of the optical field as an array of complex numbers allows many imaging and processing capabilities that are difficult or infeasible in real-space holography 17. Various useful and special techniques have been developed to enhance the capabilities and extend the range of applications. In digital holographic microscopy, a single hologram is used to numerically focus on the holographic image at any distance 1819. Direct access to the phase information leads to quantitative phase microscopy with nanometer sensitivity of transparent or reflective phase objects, 2021 and allows further manipulations such as aberration correction 22. Multiwavelength optical phase unwrapping is a fast and robust method for removing 2π-discontinuities compared to software algorithm-based methods 23. A significant constraint of digital holography is the pixel count and resolution of the imaging devices. Suppression of the zero-order and twin images by phase-shifting digital holography allows efficient use of the pixel array 24. Digital Gabor holography, without separate reference beams, is useful for particle imaging applications by providing 4-D space-time records of particle fields 25. Digital holography naturally evolved from the effort to utilize electronic imaging in interferometry, such as in electronic speckle pattern interferometry (ESPI) 26. Metrology of deformations and vibrations is a major application area of digital holography 27. Optical processing, such as pattern recognition and encryption, by digital holography also offers new capabilities 28.

Basic principles of diffraction and general holography are outlined in Sec. 2, and a general description of digital holographic microscopy (DHM) is given in Sec. 3, with an emphasis on the quantitative phase microscopy by DHM. Methods of numerical calculation of diffraction are described and compared in Sec. 4, and a number of main types of interferometer configurations used in digital holography experiments are given in Sec. 5. There are many numerical techniques that lead to the unique and powerful capabilities of digital holography, described in Sec. 6. Then in Sec. 7, a survey is given of the application areas of DHM, as well as special techniques that expand the capabilities and applications of digital holography. This review has an emphasis on microscopy applications of digital holography, and therefore omits some major areas of digital holography development, such as metrology of macroscopic systems and image processing of holographic data 17.

Scalar Diffraction Theory

First, some of the main results of scalar diffraction theory are recalled and applied to the description of basic holographic image formation. We start by writing down the Fresnel-Kirchoff diffraction formula for the general problem of diffraction from a 2-D aperture depicted in Fig. 129.

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Geometry of diffraction. E1: input plane and E2: output plane.

Huygens convolution

Given the optical field E0(x0,y0) over the input plane Σ0 at z=0, the field E(x,y) over the output plane Σ at z is, with the wavelength λ=2π/k, Display Formula

1E(x,y;z)=-ik2πzΣ0dx0dy0E0(x0,y0)exp[ik(x-x0)2+(y-y0)2+z2].
This is a convolution integral: Display Formula
2E(x,y;z)=E0SH,
where the point spread function (PSF) is Display Formula
3SH(x,y;z)=-ik2πzexp[ikx2+y2+z2],
representing the Huygens spherical wavelet.

Fresnel transform

For paraxial approximation, valid for z3k8[(x-x0)2+(y-y0)2]max2, the Fresnel PSF is Display Formula

4SF(x,y;z)=-ik2πzexp[ikz+ik2z(x2+y2)].
For example, for λ=0.6μm and [(x-x0)2+(y-y0)2]max=5mm, one needs z>zmin=93mm. Then the diffraction is described with a single Fourier transform, as Display Formula
5E(x,y;z)=(2π)exp[ik2z(x2+y2)]F{E0(x0,y0)SF(x0,y0;z)}[kx,ky].
We denote the Fourier transform of a function f(x,y) with respect to spatial frequencies (kx,ky) as Display Formula
6F{f(x,y)}[kx,ky]=12πdxdyf(x,y)exp[-i(kxx+kyy)]=f̃(kx,ky).
The spatial frequencies in Eq. (5) are Display Formula
7kx=kxz;ky=kyz.

Angular spectrum

An alternative approach to describe diffraction is by analysis of the angular spectrum. Given the field E0(x0,y0) at the input plane Σ0(z=0), its angular spectrum is defined as the Fourier transform Display Formula

8A0(kx,ky)=F{E0}=12πΣ0dx0dy0E0(x0,y0)exp[-i(kxx0+kyy0)].
Then of course, the input field E0(x0,y0) is the inverse Fourier transform Display Formula
9E0(x0,y0)=F-1{A0}=12πΣ0dkxdkyA0(kx,ky)exp[i(kxx0+kyy0)].
The exponential phase factor is the (x0,y0) projection of a plane wave with a wave vector k=(kx,ky,kz), where kz=k2-kx2-ky2. After propagation over a distance z, the plane wave acquires an additional phase factor exp{ikzz}, so that the field E(x,y) at Σ(z) is Display Formula
10E(x,y;z)=12πΣ0dkxdkyA0(kx,ky)exp[i(kxx+kyy+k2-kx2-ky2z)]circ(kx2+ky2k)=F-1{A0(kx,ky)exp[ik2-kx2-ky2z]circ(kx2+ky2k)}[x,y].
The circle function circ, whose value is one where the argument is less than one and is zero otherwise, is necessary to restrict kz to be real. Ordinarily, k2kx2+ky2, and the circle function can be dropped. We can also express Eq. (10) as a convolution. To save space, all (x,y) terms are abbreviated with (x). Implied (y) terms should be clear from the context. Display Formula
11E(x,y;z)=1(2π)2Σ0dx0E0(x0)Σ0dkxexpi[kx(x-x0)]exp(ik2-kx2z)=12πΣ0dx0E0(x0)F-1{exp(ik2-kx2z)}[x-x0]=E0SA,
Display Formula
12SA(x,y;z)=12πF-1{exp(ik2-kx2-ky2z)}[x,y].
Note that the Fresnel PSF can be expressed as Display Formula
13SF(x,y;z)=12πF-1{exp[ikz-iz2k(kx2+ky2)]}.
Under paraxial approximation, the Fresnel transform and angular spectrum methods are equivalent.

Holography of Point Sources

It is useful to consider holographic imaging by point sources 3031. Referring to Fig. 2, suppose two point sources E1δ(x-x1,y-y1,z-z1) and E2δ(x-x2,y-y2,z-z2) emit spherical waves toward the hologram plane Σ0(x0,y0) at z=0. Again, (x,y) pairs of expressions are mostly abbreviated with (x) only. Using Fresnel approximation, the fields at z=0 are Display Formula

14E1(x0,y0)=E1exp[-ikz1-ik2z1(x0-x1)2]E2(x0,y0)=E2exp[-ikz2-ik2z2(x0-x2)2].
The intensity on the hologram plane is Display Formula
15I12=|E1+E2|2=|E1|2+|E2|2+2E1E2cos[k(z1-z2)+k2z12(x0-x12)2+kζ12],
where Display Formula
161z12=1z1-1z2;x12z12=x1z1-x2z2;ζ12=12(x1-x2)2z1-z2.
This is a Fresnel zone pattern of a point source located at (x12,y12,z12). Now illuminate the hologram with a third spherical wave of a possibly different wavelength λ=2π/k(μk/k) from the point source E3δ(x-x3,y-y3,z-z3): Display Formula
17E3(x0,y0)=E3exp[-ikz3-ik2z3(x0-x3)2].
The optical field at another plane Σ(x,y) at an arbitrary z is calculated using the Fresnel diffraction formula. We calculate only the twin image terms arising from Display Formula
18I12±=E1E2exp[±ik(z1-z2)±ik2z12(x0-x12)2±ikζ12].
Then, Display Formula
19E±(x,y;z)=-ik2πzexp(ikz)Σ0dx0I12±E3exp[ik2z(x-x0)2]=-ik2πzE1E2E3exp[±ik(z1-z2)-ik(z3-z)±ikζ12]×Σ0dx0exp[-ik2(1Z±-1z)x02+ik(X±Z±-xz)x0+ik2(±x122z12-μx32z3+μx2z)].
After some algebraic effort, we obtain Display Formula
20E±(x,y;z)=α±E1E2E3exp[±ik(z1-z2)-ik(z3-z)]exp[ik2(x-X±)2z-Z±+iΦ±],
where Display Formula
211Z±=1μz12+1z3=1μz1±1μz2+1z3X±Z±=x12μz12+x3z3=x1μz1±x2μz2+x3z3,Φ±=k2[(x1-x2)2μz1z2±(x1-x3)2z1z3(x2-x3)2z2z3]Z±
and α±=(1-zZ±)-1.

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Holography of point sources. Σ0 and E3: point sources for recording a hologram on Σ plane. 100×100μm2: point source for reading the hologram. 256×256: image plane.

The results show that the fields E±(x,y;z) are spherical waves centered at (X±,Y±,Z±). Various magnifications can be calculated. The lateral, longitudinal, and angular magnifications are, respectively, Display Formula

22Mx±=X±x1=Z±μz1=11±z1z2+μz1z3;Mz±=Z±z1=Z±2μz12=μMx2;Mx/z±=(X±/Z±)(x1/z1)=1μ.
For the case of equal wavelengths, μ=1, and the reference sources on the optical axis, (x2,y2)=(x3,y3)=(0,0), the prior expressions simplify to Display Formula
231Z±=1z1±1z2+1z3;X±=x1Z±z1Φ±=kx122z1(1z2±1z3)Z±;α±=11-zZ±.

These results are based on the quadratic (Fresnel) approximation in Eq. (4). If higher order terms are included, then one obtains the third-order aberration terms: spherical aberration, coma, astigmatism, field curvature, and distortion 3031. With the higher order terms, magnification or wavelength mismatch can introduce aberrations. For the most part, the following theoretical descriptions will be within the Fresnel approximation. If the finite size of the hologram is taken into account, the image point has a finite spread determined by the numerical aperture of the hologram 32.

General Description of Digital Holographic Microscopy

A basic digital holographic microscopy (DHM) setup consists of an illumination source, an interferometer, a digitizing camera, and a computer with necessary programs. Most often a laser is used for illumination with the necessary coherence to produce interference. All different types of lasers have been used, from ubiquitous HeNe lasers and diode lasers, to diode-pumped and doubled YAG lasers (often referred to simply as a solid-state laser), argon lasers, as well as tunable dye lasers and Ti:sapphire femtosecond lasers. For multiwavelength techniques, two or more different lasers can be coupled into the interferometer, or a tunable laser can be employed. There are also low-coherence techniques for the purpose of reducing speckle and spurious interference noise, or generating contour or tomographic images. A short-pulse (picosecond or femtosecond) laser can be used, or a tunable laser can be turned into a broadband source by removing the tuning element. Even an LED typically has 10 μm or so coherence length, which can be sufficient for holographic microscopy. DHM using a 10.6-μmCO2 infrared laser 33, deep UV (193 nm), 34 and 32-nm soft x-ray 35 has been demonstrated.

Two main types of interferometers, the Michelson interferometer for reflective objects and the Mach-Zehnder interferometer for transmissive objects, are depicted in Fig. 3. In each diagram, the light-green beams are the input from the laser, the light blue is the reference beam path, and the light red depicts image formation of an object point. In both designs, the object is illuminated with a plane wave, and the reference arrives at the CCD plane with the same wavefront curvature as the object wave, except for an offset in the angle of incidence for off-axis holography. The Mach-Zehnder types require more components but offer more flexibility in alignment, especially when microscopic imaging optics are used. Interferometers can also include various apertures, attenuators, and polarization optics to control the reference and object intensity ratio. Polarization optics can also be used for the explicit purpose of birefringence imaging. There can also be various types of modulators such as piezo-mounted optics, liquid crystal phase modulators, acousto-optic, or electro-optic modulators to establish modulated signals. Techniques such as the lensless Fourier holography 36 configuration can be used for magnification, but in practice achievable magnification is limited and explicit magnification by microscope objective lenses is preferred and necessary. Another lens can be used in the reference arm to match the curvatures of the object and reference wavefronts. There are many versatile techniques in digital holography that compensate for various types of aberrations and imperfections of the optical system (see Sec. 6d), and therefore, in comparison with conventional holography, the optical and mechanical requirements can be significantly less stringent.

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(a) Michelson interferometer for digital holographic microscopy of reflective specimen. (b) Mach-Zehnder interferometer for digital holographic microscopy of transmissive specimen. BS: beamsplitters; L: lenses.

Typically a CCD, or more recently CMOS, cameras are used to capture and digitize a holographic interference pattern. The pixel size of these devices is several microns with pixel counts of 10002 or so. These parameters are the main limiting factors in DHM resolution and prescribes the range of applications, but one would expect them to continue to improve in the coming years. The captured hologram pattern is digitized by the camera, or a frame grabber, and input to the computer as a 2-D array of integers with 8-bit or higher grayscale resolution. The main task of the computer is to carry out the numerical diffraction to compute the holographic image as an array of 2-D complex numbers. In addition, the computer program handles a number of other tasks, such as pre- and postprocessing of the images, rendering and storage of images, as well as timing and other necessary controls of the apparatus.

An example of the DHM process is shown in Fig. 4 using a resolution target with field of view (FOV) = 200×150μm2. Figure 4 is the hologram, with detail shown in the inset, where the interference fringes are visible. Figure 4 is the angular spectrum (Fourier transform), showing the zero-order and twin image peaks. One of the twin terms is selected with a numerical bandbass filter (yellow circle). The filtered hologram is then used for numerical diffraction over an appropriate distance, which results in the reconstructed holographic image as a 2-D array of complex numbers. The amplitude and phase images in 4, respectively, are obtained by taking the absolute magnitude and phase of the complex array. The phase image represents a phase profile of the optical field reflected from the object surface or transmitted through a thickness of a transparent object. The phase profile has the precision of a fraction of optical wavelength, and therefore reveals nanometric variations of the surface or the optical thickness of the specimen. In Fig. 4, minute smudges of some kind are visible, apparently some fraction of wavelength thickness, which the amplitude image completely misses. The phase image color scale ranges 2π from blue to red. The object surface is slightly tilted with respect to the reference wavefront, and such tilt and other aberrations can readily be compensated by numerical techniques described later.

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Digital holographic microscopy process (resolution target) (FOV = 200 × 150 μm, 1024 × 768 pixels): (a) hologram, with detail shown in inset; (b) angular spectrum, with the yellow circled area pass-filtered for reconstruction; (c) amplitude image; and (d) phase image.

A well-known distinctive feature of holography is the 3-D content of the image information. In DHM, a single hologram is used to reconstruct the optical field at any distance from the hologram, within the limitation of the approximation method used. For example, Fig. 5 shows a hologram of a paramecium. From the single hologram, the image is calculated at various distances, which are then assembled into a video clip in 5. It shows the paramecium image going through a best focus, preceded and followed by more defocused images, emulating the turning of a focusing knob on a conventional microscope. In Fig. 6, a series of holograms of a live paramecium and several euglenas are captured. In the scene, the paramecium and the euglenas swim not only in lateral directions but also in varying depths. In conventional video microscopy, the focal plane would be fixed and whatever happened to be in that plane would be recorded, but information on objects not in the focal plane would be permanently lost. With DHM, the holographic movie is processed by calculating the images while adjusting the reconstruction distances to track a particular specimen as it swims up and down in the 3-D object volume. Movies of thus calculated amplitude and phase images are shown in Fig. 6 and 6and respectively. In effect, the holographic movie is a complete 4-D space-time record of the object volume.

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DHM numerical focusing on paramecium: (a) a single hologram captured by the camera (FOV = 250 × 250 μm, 464 × 464 pixels); and (b) 5 of a series of holographic images calculated at varying distances.10.1117/6.0000006.1

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Holographic movies of paramecium and euglena: (a) holograms (6a), (b) amplitude images (6b), and (c) phase images (6c). (FOV = 250 × 250 μm, 464 × 464 pixels). 10.1117/6.0000006.210.1117/6.0000006.310.1117/6.0000006.4

Quantitative Phase Microscopy

Many microscopic biological specimens, such as living cells and their intracellular constituents, are mostly transparent and therefore problematic for conventional bright-field microscopy. A number of techniques have been developed for rendering transparent phase objects visible 37 that have played very important roles in the development of modern biology and medicine. For example, in dark-field microscopy, only the scattering centers and boundaries contribute to the image signal against a zero background. In the Zernike phase contrast microscope, the phase variation is converted into amplitude variation by use of a phase plate and spatial filtering. In differential interference contrast (DIC) microscopy, the interference of two sheared polarization components results in images that have a shadow effect and thus gives a 3-D perception of the object. Interference microscopy, using a Michelson objective for example, produces fringes of equal thickness of a transparent object. Although these techniques are effective in making transparent objects visible, the phase-to-amplitude conversion is nonlinear, and there are significant artifacts in the images such as the halo in Zernike phase contrast and the disappearance of contrast along the direction perpendicular to shear in DIC. These techniques do not produce quantitative phase images.

The optical phase of the light transmitted through transparent objects can convey quantitative information about the object, such as its physical thickness and index of refraction 38, which in turn are functions of physical density or chemical concentration properties. High precision measurements of optical phase can thus reveal subtle changes in these parameters that accompany cellular processes. To obtain quantitative phase images, one can perform an interferometric measurement of a focused beam of light on an object, and scan the beam over the object in a raster fashion. Optical profilers based on scanning interferometers are especially well suited for imaging applications in materials science, as in MEMS and nanofabrication, because of the high precision obtainable and the static nature of the objects being imaged 3940. On the other hand, the speed constraint and mechanical complexity of scanning interferometers can significantly restrict the range of applications in biomedical imaging 41, where one needs to make observations of dynamic processes under widely varying environments. There have been some recent developments in 2-D quantitative phase microscopy. In phase-shifting interference microscopy 4243, the quantitative phase image is obtained from a combination of three or more interferograms. There is also a noninterferometric method to extract quantitative phase images from differential focusing properties of bright-field intensity images alone 4445.

Digital holography is a very effective process for achieving high-precision quantitative phase microscopy. The phase image is immediately and directly available as soon as the 2-D complex array of the holographic image is calculated. A single hologram exposure is required. It does not involve raster scanning. Most importantly, the phase image is a quantitative representation of the object profile with nanometer, and even subnanometer, precision 2021,4648. An example of DHM imaging of a layer of onion cells is shown in Fig. 7, where Fig. 7 is the hologram and Fig. 7 is its angular spectrum. Because of the structure of the specimen, the spectral peaks are more diffuse compared to Fig. 4. Figure 7 is the amplitude image, analogous to what one would see through a conventional microscope, and Fig. 7 is the phase image. The onion cells apparently have thicknesses of several microns, and therefore the phase profile varies by several cycles of 2π radians. A public-domain phase unwrapping algorithm is used to remove the 2π discontinuities in Fig. 7, and it is rendered in pseudocolor pseudo-3-D perspective in Fig. 7. Figure 7 is pseudo-3-D in the sense that the apparent height profile is the profile of optical thickness that includes both physical thickness and index variation, and one needs to use caution in interpreting such an image. Figure 8 displays a few more examples of quantitative phase microscopy (QPM) images by DHM. Figure 8 is one group of three bars on a resolution target. The noise level in the flat area of the image is measured to be 3 nm and the thickness of the chromium film is measured to be about 50 nm, consistent with the manufacturer's estimate. Figures 8 are fixed SKOV-3 ovarian cancer cells, where one can discern several intracellular components such as the nuclear membrane and chromosomes. Figure 8 shows several red blood cells, while in Fig. 8 one can notice a fold of the cheek epithelial cell, as well as its nucleus and mitochondria. Figure 8 is an image of a small quartz crystal in common sand.

Grahic Jump LocationF7 :

Digital holographic microscopy process (onion cells) (FOV = 100 × 100 μm, 416 × 416 pixels): (a) hologram, (b) angular spectrum, (c) amplitude image, (d) phase image, (e) unwrapped phase image, and (f) phase image in pseudo-3-D view.

Grahic Jump LocationF8 :

Examples of quantitative phase microscopy by digital holography: (a) resolution target (25 × 25 μm, 452 × 452 pixels); (b) SKOV-3 ovarian cancer cells (60 x 60 μm, 404 × 404 pixels); (c) SKOV-3 ovarian cancer cell (60 × 60 μm, 404 × 404 pixels); (d) red blood cells (50 × 50 μm, 404 × 404 pixels); (e) cheek epithelial cell (60 × 60 μm, 404 × 404 pixels); and (f) quartz crystal of sand (60 × 60 μm, 404 × 404 pixels).

Comparisons of Analog and Digital Holographic Microscopy

There are a number of significant distinctions between analog (AH) and digital (DH) holographies. Most obviously, DH does not involve photochemical processing. Therefore, DH is orders of magnitude faster and can be performed at video rates. Additional hardware required in DH is the CCD camera and a computer, while the need for dark room facilities and a supply of chemicals is unnecessary. Furthermore, because of the high sensitivity of CCD compared to photographic emulsion, the exposure time is reduced by orders of magnitude. For example, a CCD pixel area of 100 μm2 can detect as few as several photons, whereas a similar area of a high-sensitivity photographic plate requires many millions of photons. Short exposure time in turn implies much reduced requirement on the mechanical stability of the apparatus. Heavy optical tables with vibration isolation are often not critical. On the other hand, the main issue of DH is low resolution. A typical CCD pixel is several microns across, while the grains on a photographic emulsion may be 2 orders of magnitude finer. This limits the spatial frequency of the fringes and therefore the angular size of the object to a few degrees for DH, while a full 180 deg is possible for AH. The familiar parallax effect of display holograms of AH is currently not feasible in DH 49. The real strength of DH, however, is the whole range of powerful numerical techniques that can be applied once the hologram is input into a computer. One simple but significant example relates to Fig. 3, where a lens is used to magnify the hologram FOV to match the CCD size. Once the computer reads the hologram into an array, one only needs to specify the dimension of the FOV and the wavelength, and proceed to compute the numerical diffraction. In AH, however, to properly read out the magnified or demagnified hologram, the wavelength also needs to be scaled proportionately, a task that is highly cumbersome at the least and infeasible in most cases. Another example is holographic interferometry using multiple wavelengths. In AH interferometry, multiple holograms are produced and repositioned exactly, and ideally each hologram needs to be illuminated with a different wavelength, which can be physically impossible. Most often the superposed holograms are illuminated with a single wavelength, and the resulting aberrations are unavoidable. In DH, however, the superposition simply consists of an addition of several numerical arrays. There is no limitation on the number of arrays, and furthermore, there are ways to preprocess the arrays to compensate for chromatic and other aberrations if present. More examples of the power of numerical processing in DH will become evident in the following discussions.

Because of its sensitivity and technical versatility, quantitative phase microscopy is a very important and active area of research and applications in digital holography 2021,46. Aberrations or other deformations of wavefronts can easily be compensated by using a matching reference wave 22,5051. Multiwavelength optical phase unwrapping (see Sec. 6c) allows nanometric-precision phase imaging over a range of many micrometers without many problems associated with common software-based unwrapping methods 23,52. Biomedical microscopy application is an area that can benefit significantly from the new capabilities of digital holography by providing label-free, minimally invasive, and highly sensitive methods of imaging subtle changes in the physical and physiological states of cells and tissues 19,47,5355. Materials and MEMS technologies can also utilize digital holography in characterization and testing of various structures 5661.

Numerical Diffraction