Time-lapse interferometry and contouring using television systems

History and metrology applications of a game-changing technology: digital holography [Invited]

In digital holography (DH), information in the hologram is recorded and stored in digital format in discrete bits. Like its parent, holography, DH evolved over many years with periods of dormancy and revival. Almost abandoned, multiple times, unanticipated events or developments in separate industries revived it with explosive, quantum jumps, making it useful and popular to a wide audience. Although its history has been treated in many papers and books, the field is dynamic and constantly providing new opportunities. Having been born long before low-cost, fast, powerful digital computers and digital detectors were available, DH was confined to the academic world, where practical applications and commercial opportunities were few if any. Consumer demand that led to low-cost personal computers, high-resolution digital cameras, supporting software, and related products changed the situation drastically by providing every potential researcher affordable, powerful hardware and software needed to apply image processing algorithms and move DH to new practical application levels. In this paper, as part of the sixtieth anniversary of off-axis holography, we include a brief introduction to the fundamentals of DH and examine the history and evolution of DH during its periods of rise and fall. We summarize many new emerging techniques, applications, and potential future applications along with additional details for metrological examples from the authors' research.

Microshape Measurement Method Using Speckle Interferometry Based on Phase Analysis

A method for the measurement of the shape of a fine structure beyond the diffraction limit based on speckle interferometry has been reported. In this paper, the mechanism for measuring the shape of the fine structure in speckle interferometry using scattered light as the illumination light is discussed. Furthermore, by analyzing the phase distribution of the scattered light from the surface of the measured object, this method can be used to measure the shapes of periodic structures and single silica microspheres beyond the diffraction limit.

Nonimaging speckle interferometry for high-speed nanometer-scale position detection

We experimentally demonstrate a nonimaging approach to displacement measurement for complex scattering materials. By spatially controlling the wavefront of the light that incidents on the material, we concentrate the scattered light in a focus on a designated position. This wavefront acts as a unique optical fingerprint that enables precise position detection of the illuminated material by simply measuring the intensity in the focus. By combining two fingerprints we demonstrate position detection along one in-plane dimension with a displacement resolution of 2.1 nm. As our approach does not require an image of the scattered field, it is possible to employ fast nonimaging detectors to enable high-speed position detection of scattering materials.

Adaptive correction method of inhomogeneous light intensity for digital speckle interferometry by subtraction

In digital speckle interferometry, subtracted fringe patterns are always influenced by inhomogeneous light that is reflected from the tested object and received by the CCD. In this paper, by analyzing speckle's statistic property, we propose a numerical processing method to correct this nonuniform light intensity distribution within adaptive windows. This method includes estimating light intensity distribution of the tested object, constructing an adaptive window for every pixel, and correcting the intensity in the adaptive windows. By applying this method to our experiment, we find it is valid for intensity correction without changing necessary phase information.

Mathematical refocusing of images in electronic holography

This paper presents an illustration of mathematical refocusing of images obtained by the HoloFringe300K electronic holography program. The purpose is to demonstrate that this form of electronic holography is equivalent to image-plane, phase-stepped digital holography. The mathematical refocusing method used here differs from those in common use and may have some advantages.

Theoretical analysis of three-dimensional imaging and recognition of micro-organisms with a single-exposure on-line holographic microscope

Single-exposure on-line (SEOL) digital holography is a recently proposed technique for monitoring, visualization, and recognition of three-dimensional (3D) objects. In contrast to traditional multi-exposure on-line digital holography, it uses only one exposure, which makes it particularly suitable for imaging and recognizing moving micro-organisms. However, the cost of using only one exposure is the superposition of a conjugate image on the desired reconstructed image. The influence of the conjugate image on the visualization and recognition performance is investigated. The conditions for which the cross-talk noise induced by the conjugate image is negligible are derived. It is demonstrated that with conditions common in imaging of microscopic 3D biological objects, SEOL digital holography is highly tolerant of cross-talk noise induced by the conjugate image.

Automatic identification of biological microorganisms using three-dimensional complex morphology

We propose automated identification of microorganisms using three-dimensional (3-D) complex morphology. This 3-D complex morphology pattern includes the complex amplitude (magnitude and phase) of computationally reconstructed holographic images at arbitrary depths. Microscope-based single-exposure on-line (SEOL) digital holography records and reconstructs holographic images of the biological microorganisms. The 3-D automatic recognition is processed by segmentation, feature extraction by Gabor-based wavelets, automatic feature vector selection by graph matching, training rules, and a decision process. Graph matching combined with Gabor feature vectors measures the similarity of complex geometrical shapes between a reference microorganism and unknown biological samples. Automatic selection of the training data is proposed to achieve a fully automatic recognition system. Preliminary experimental results are presented for 3-D image recognition of Sphacelaria alga and Tribonema aequale alga.

Strain and mechanical behavior measurements of soft tissues with digital speckle method

Soft tissues of the body are composite, typically being made up of collagen and elastin fibers with high water contents. The strain measurement in soft tissues has proven to be a difficult task. The digital speckle method, combined with the image processing technique, has many advantages such as full field, noncontact, and real time. We focus on the use of an improved digital speckle correlation method (DSCM) and time-sequence electric speckle pattern interferometry (TSESPI) to noninvasively obtain continual strain measurements on cartilage and vessel tissues. Monoaxial tensile experiments are well designed and performed under constant temperature and the necessary humidity with smart sensors. Mechanical behaviors such as the tensile modulus and Poisson ratio of specimens are extracted based on the deformation information. A comparison of the advantages and the disadvantages of these techniques as well as some problems concerning strain measurements in soft tissues are also discussed.

Fiber electronic speckle pattern interferometry and its applications in residual stress measurements

A two-dimensional in-plane displacement-sensitive electronic speckle pattern interferometer has been developed. With a fiber coupler with one input and four outputs, two sets of dual-beam interferometric configurations in orthogonal directions are constructed to determine in-plane displacements completely. When a CCD camera with a zoom lens is located at an adequate distance from the specimen, a testing area ranging from 1.4 mm x 1.0 mm to 30.0 mm x 24.0 mm can be examined in quasi-real-time. Incorporated with the hole-drilling technique, it has currently been demonstrated in residual stress measurements. One application is for determining the residual stress of a thick cylinder consisting of two concentric circular tubes with interference fit. The other is for analyzing the residual stress distribution of a recordable optical compact disc. A simple approach to interpreting the values of residual stresses from the displacement contours is presented.

Measuring mixing dynamics of transparent fluids with electronic speckle pattern interferometry

Electronic speckle pattern interferometry (ESPI) combined with phase-shifting techniques is used for studying the mixing dynamics of transparent fluids. The potentials of the technique for studying fluid mixing are illustrated for simple examples of water flow and moving water droplets in water. With ESPI we could actually follow water droplets moving in water and water flowing in water on a television monitor at the video rate. A He-Ne laser (lambda(0) = 632.8 nm) was used as the light source, and phase stepping was applied, giving an interferometric sensitivity better than lambda(0)/10. The observed phase changes are due to changes in the refractive index caused by small temperature differences between the droplets and the surrounding water. Temperature differences of less than 0.1 K are detectable for droplets of a diameter of 4 mm.

Vibration analysis of logs with electronic speckle pattern interferometry

Electronic speckle pattern interferometry combined with phase-shifting techniques for vibration analysis of logs is presented. Changes in the vibration pattern were followed on a television monitor at the video rate. We determined resonant vibrations by scanning through the frequency range of interest and by changing the point of excitation. We observed bending and torsional modes of vibration by changing the support and point of excitation. The vibrational modes reflect the structural properties of the material. The longitudinal modulus of elasticity and the shear modulus were calculated, giving valuable information for the strength grading of logs.

Measurement of plant movement in young and mature plants using electronic speckle pattern interferometry

The use of electronic speckle pattern interferometry (ESPI) to monitor and measure the surface movement of plants is presented. We chose to study the gravitropical response of a coleoptile (the shoot from a growing seed) to illustrate the potentials of the technique for botanical measurements. A coleoptile represents a very fast growing translucent biological object, a difficult object to record interferometrically Traditional holographic interferometry is not suited to the study of objects with such rapid fringe decorrelation. However, ESPI with its short exposure time, fast sampling rate, and high sensitivity makes it possible to obtain fringes even on the very tip of the coleoptile where the microstructure changes most rapidly.

Double-pulse characteristics of a single-oscillator Nd:YAG laser affecting its performance in TV holography

The relevance of several double-pulse laser parameters for the fringe quality in TV holography is emphasized theoretically. It is demonstrated that single-oscillator Nd:YAG lasers can show significantly reduced characteristics in double-pulse operation, limiting the performance of these lasers in double-pulse TV holography.

Real-time measurements of spatial velocity distribution with a laser Doppler imaging system

A new laser Doppler imaging system with a TV camera has been constructed, which brightly displays 1-D velocity distribution on a monitor. Some characteristics of this system have been experimentally investigated from measurements of simple velocity distribution on a constantly rotating ground glass disk. From an adaptation to fluid flow, it has been shown that the measurements of spatial velocity distribution can be achieved in real time.

Digital speckle-pattern shearing interferometry

An application of digital image processing techniques to speckle-shearing interferometry is described. A present system consists of an image-shearing camera using a Fresnel biprism of small angles and a digital TV-image processing facility. This interferometer makes it easy to measure in quasi-real-time spatial derivatives of surface displacement and modal vibration amplitude of objects. A statistical theory is applied to analyze the formations of these fringes due to 3-D displacement. Interpretation of the result leads to conditions of maximum fringe contrast and the limitation of this technique. Experimental results in the three cases of slope of normal displacement, surface strain, and slope of vibration amplitude measurements are presented.

Electronic speckle pattern interferometry using digital image processing techniques

The use of digital image processing techniques for electronic speckle pattern interferometry is discussed. A digital TV-image processing system with a large frame memory allows us to perform precise and flexible operations such as subtraction, summation, and level slicing. Digital image processing techniques make it easy compared with analog techniques to generate high contrast fringes. Some experimental verifications are presented in the cases of surface displacement and vibration amplitude measurements.

Electronic speckle pattern interferometric system based on a speckle reference beam

A new electronic speckle pattern interferometric (ESPI) technique is introduced. The technique is based on a reference beam combined with a large aperture optical system. The basic principles are described and compared with conventional ESPI setups. The new interferometer is easy to adjust, it is invulnerable to dust and scratches on the optical components, and is very compact. It is well-suited for practical engineering applications. Light sensitivity and fringe quality are comparable with the conventional ESPI features. Superior fringe pattern can be obtained by use of a new speckle reduction technique to be described.

Use of chopped laser light in electronic speckle pattern interferometry

Shortened exposure of each TV frame is used in conjunction with time-average ESPI to study the sinusoidal vibrations of extremely unstable objects. The effect of short exposures upon the fringe functions is calculated. It is shown how sinusoidal phase modulation techniques can be used at all realistic exposure values. We also discuss how double-exposure ESPI with a cw laser can be used to study more random movements, e.g. of biological objects. The lag characteristic of the TV target is used to extend the time separation. Fringe patterns illustrating applications of these techniques are presented.

Superresolution microscope using electrical superposition of holograms

By combining a microscope and television systems, a superresolving microscope is constructed. The basic idea of this system is to convert the hologram signal of the image to an electrical video signal using television systems and to perform operations necessary to carry out the superresolution electrically. As a result images which are superresolved twice in one dimension are displayed on a TV monitor in 5 sec.