Additive-Subtractive Two-Wavelength ESPI Contouring by Using a Synthetic Wavelength Phase Shift

The addition correlation of two speckle fields by simultaneousillumination at different wavelengths is used for object contouring ina Twyman-Green-type interferometer. Fringe visibility is enhancedwhen the stochastic speckle background intensity obtained from areference plane modulation is subtracted. We calculate the contourphase map by using a phase-shift algorithm in the syntheticwavelength. A comparison with a sequential illumination, phasedifference method based on a laser wavelength phase shift isgiven. The test setup does not need to be stable on aninterferometric scale, and therefore a method is provided that lendsitself to applications in noisy environments.     

Speckle suppression in projection displays by using a motionless changing diffuser

Speckle suppression in projection displays with a laser light source can be achieved by imaging a changing diffuser with random phase cells onto the screen. Theoretical expressions for the speckle contrast in this method have been earlier obtained in the case when different realizations of the phase diffuser produced statistically independent patterns of the light field on the screen. In the present paper, these expressions are generalized in the case when different realizations of the phase diffuser produce partly correlated speckle patterns. The possible structure of a motionless changing diffuser is presented. It includes a dynamic diffractive optical element (DDOE) and a light homogenizer. The DDOE can be based on the electrically controlled spatial light modulator (SLM) with a deformable polymer layer. This type of SLM can handle high light power and, therefore, can be used in projection displays with powerful laser beams.     

De-correlation Effects in Speckle-pattern Interferometry. 1. Wavelength change dependent de-correlation with application to contouring and surface roughness measurement

A theoretical investigation of the de-correlation of speckle pattern with change of wavelength has been performed with particular reference to the electronic speckle-pattern interferometer. A method of measuring surface roughness using speckle de-correlation is proposed and some preliminary results presented. In addition, the loss of fringe visibility in two-wavelength speckle-pattern interferometric contouring is investigated and experimental confirmation of the theory given.     

Demonstration of a home projector based on RGB semiconductor lasers

In this paper, we demonstrate a high-definition 3-liquid-crystal-on-silicon (3-LCOS) home cinema projection system based on RGB laser source modules. Both red and blue laser modules are composed of an array of laser diodes, and the green laser is based on an optically pumped semiconductor laser. The illumination engine is designed to realize high energy efficiency, uniform illumination, and suppression of speckle noise. The presented laser projection system producing 1362 lm D65 light has a volume of about 450×360×160 mm3.     

Description and simulation of an active imaging technique utilizing two speckle fields: iterative reconstructors

Quasi-monochromatic light will form laser speckle upon reflection from a rough object. This laser speckle provides information about the shape of the illuminated object. In a prior paper [J. Opt. Soc. Am. A 19, 444 (2002)], it was shown that two intensities of two speckle patterns and their interference are sufficient to produce an unambiguous (except for object translation) band-limited image of the object, based on a root-matching technique described therein, in the absence of measurement error and in the case of distinct roots of the field polynomials and their complex conjugates. On the other hand, algorithms based on the root-matching technique are found to be slow and sensitive to noise. So motivated, several other techniques are applied to the problem, including phase retrieval, expectation maximization, and statistical maximization. The phase-retrieval and expectation-maximization techniques proved to be most effective for reconstructions of complex objects larger than 10 pixels across, and high-quality images were formed by using three independent sets of two-field data (three frames of two-wavelength data), each comprising two speckle intensity patterns and their interference. Two additional results of note are reported. First, the expectation-maximization algorithm produced relatively good images when three or more frames each of only one speckle intensity pattern (data at just one wavelength) were used and second, the phase-retrieval algorithm when only the object autocorrelation was used also produced relatively good images for the chosen test object.     

X-Ray Efficiency and Modulation Transfer Function of Fluorescent Rare Earth Screens,Determined by the Monte Carlo Method

The overall, X-Rays to light conversion efliciency and the modulation transfer function of radiographic single front screens have been computed using the Monte Carlo method to simulate the generation and the multiple-scattering diffusion and absorption of light photons in the screen. The computation is based on a set of probability functions related to the absorption of X-Ray photons, free-path lengths and absorption of light photons and reflection conditions at the boundaries. A constant scattering angle has been used for light photons colliding with phosphor grains. For a given phosphor volume fraction, results are shown as a function of phosphor grain size, screen thickness and light wavelength. Some computed results have been checked experimentally.

A comparison is made between screens based on mainly green emitting terbium activated gadolinium oxysulphide, and conventional screens based on blue emitting lead activated calcium tungstate. Screens having equal packing volume, thickness and phosphor grain size show a much higher conversion efficiency and improvement in modulation transfer functions when made of Gd₂O₂S:Tb.     

Statistical Properties of Phase Variation in Shearing Speckle Interferometry and Its Effect on Fringe Visibility

Abstract

The statistical properties of two interfering speckle fields indicate a reduction in phase variation as the coherent correlation between them increases. In this paper, the impact of this phenomenon on fringe visibility in shearing speckle interferometry, where two laterally shifted speckle fields are super-imposed, is studied. Work is also done to correlate fringe visibility with the amount of lateral shear based on the limitations imposed by phase variance statistics.     

Aspect ratio of elongated polychromatic far-field speckles of continuous and discrete spectral distribution with respect to surface roughness characterization

Here polychromatic speckle patterns generated either by a polychromatic light source that emits at discrete frequencies or by a light source showing a continuous narrow-band spectral distribution are studied. The purpose here is the application of polychromatic speckle-pattern analysis to an in-process surface roughness characterization. To compare the coherence properties of the different polychromatic light sources, first a modified definition of the coherence length is introduced. Furthermore, the relevant optical phenomena, namely, the speckle elongation caused by the angular dispersion and the roughness-dependent speckle decorrelation, are summarized. It is shown that light sources with a continuous spectral distribution have essential advantages in comparison with discrete wavelength sources. The theoretical results are confirmed by experimental investigations based on a digital algorithm for the evaluation of CCD images of polychromatic speckle patterns, which are recorded in the Fourier plane of a Fourier-transforming lens.     

Simple model for image-plane polychromatic speckle contrast.

A simple geometrical model was developed for calculation of the contrast of a polychromatic image-plane speckle pattern from a source of light with high spatial coherence. It is based on counting the number of independent speckle patterns that contribute to a given point in the image plane. This results in a simple equation for the contrast as a function of imaging geometry; relative orientation of the projection direction, observation direction, and specimen normal; bandwidth of the light source; and surface roughness. Its validity was established by comparison with an exact solution: rms errors in the calculated contrast were only 0.033 over a wide range of parameter values likely to be encountered in practice.     

Analysis and correction of ultrasonic wavefront distortion based on a multilayer phase-screen model

A model is introduced that incorporates the cumulative wavefront distortion effects caused by spatial heterogeneities along the path of propagation, and a corresponding model-based wavefront distortion-correction method is presented. In the proposed model, a distributed heterogeneous medium is lumped into a series of parallel phase screens. The distortion effects can be compensated-without a priori knowledge of the distorting structure-by backpropagation of received wavefronts through hypothetical multiple phase screens located between the imaging system and targets, while each pointwise time shift is adjusted iteratively to maximize a specified image quality factor at the final layer. Theoretical analyses indicate that the mean speckle brightness decreases monotonically with the root-mean-square value of distributed phase distortions; therefore, the speckle brightness can be used as an image quality factor. Experimental one-dimensional (1-D) array data with simulated distortion effects based on a real 2-D abdominal-tissue map were used to evaluate the performance of the proposed method and existing aberration-correction techniques. The simulated characteristics of wavefront distortion and relative performance of existing correction techniques were similar to reports based on abdominal-wall data and breast data. This investigation shows that the proposed method provides better compensation for wavefront distortion.     

Speckle in the 4F optical system

We provide a comprehensive analysis of the space and wavelength dependence of speckle generated by a thin diffuser and imaged by a 4F optical system. The use of this space-invariant system is shown to lead to the well-known features of speckle patterns in an analytically simple and elegant manner, thereby providing a clear insight into speckle in an optical configuration that includes the Fresnel zone, the optical Fourier transform plane, and the image plane. In our analysis we assume a white-noise diffuser. The spatial variation mainly depends on the imaging system, whereas the wavelength dependence is related to diffuser heights. Motion dynamics of speckle are also included.     

Recording of In-plane Surface Displacement by Double-exposure Speckle Photography

Local displacements in the plane of an object surface illuminated by coherent light may be measured by recording double-exposure photographs of it. When the surface is illuminated symmetrically by two oblique beams, ‘speckle correlation fringes’ appear in the doubly exposed negative, due to the non-linear nature of the photographic recording. Factors affecting the visibility of these fringes and the range of displacement which can be measured are discussed. The fringe visibility falls to zero for displacements larger than a speckle width, but measurements can then be performed upon the optical transform of the negative image. With uni-directional object illumination, the in-plane displacement can be measured on a point-by-point basis in magnitude and in direction from Young's fringes observed. With two symmetrical oblique illuminating beams the effect of a small surface strain is displayed, even if large lateral displacement has occurred, by using spatial filtering when viewing the photographic image.     

Additive-subtractive phase-modulated electronic speckle interferometry: analysis of fringe visibility

Fringe-visibility issues of additive-subtractive phase-modulated (ASPM) electronic speckle pattern interferometry (ESPI) are explored. ASPM ESPI is a three-step method in which additive-speckle images are acquired rapidly in an analog fashion in every frame of a video sequence, a speckle phase modulation is intentionally introduced between frames, and a digital subtraction of consecutive pairs of additive-speckle images is performed. We show that this scheme has the good high-frequency noise immunity associated with additive-ESPI techniques as well as the good fringe visibility associated with subtractive-ESPI techniques. The method has better fringe visibility than can be obtained with purely additive ESPI and also does not suffer from the fringe distortions that can occur with subtractive ESPI in the presence of high-frequency noise. We show that even if full speckle decorrelation were to occur between the two additive speckle images that are to be subtracted, the visibility of ASPM ESPI fringes can be made to approach unity by suitable adjustment of the reference-to-object beam-intensity ratio.     

Optical authentication method using a three-dimensional phase object with various wavelength readouts

An optical system for authentication using a 3D (3D) random phase object with various wavelength readouts is proposed. The 3D phase object without surface modulation is secure when the scattering is strong enough because it prevents from the interferometric measurement. The identification is implemented by the correlation between a measured speckle pattern of the 3D phase object and stored speckle patterns. For accurate identification, two speckle patterns of the 3D object obtained by illuminating two wavelengths are used. Experimental demonstrations and numerical evaluations of wavelength selectivity are presented.     

Speckle contrast of the sum of N partially correlated speckle patterns

In this paper a general method is presented for calculating the theoretical speckle contrast of a sum of correlated speckle patterns, motivated by the need to suppress the presence of speckle in laser projection displays. The method is applied to a specific example, where correlated speckle patterns are created by sequentially passing light through partially overlapping areas on a diffuser, before being projected onto a screen. This design makes it possible to find a simple expression for the correlation between speckle patterns. When the set of correlations involves symmetry, it is shown that the expression for the speckle contrast becomes simpler. The difference in performance between discretely and continuously varying speckle patterns is also investigated. In an example with speckle reduction by a rotating sinusoidal grating, it is found that continuous variation gives a speckle contrast that is 0.61 times the contrast obtained by discretely summing the maximum number of independent patterns.     

Localizable imaging study for the noninvasive diagnosis of lesions with the backscattering polarized light

The asymmetry appearing in the degree of polarization (DOP) distribution of the backscattering polarized light from tissues is investigated by using polarized Monte Carlo simulation. When the incident point is close to the boundary of the lesion inside the tissue, high asymmetry emerges regardless of the polarized direction of the incident light. A noninvasive method based upon the DOP asymmetry of the backscattering light is proposed to locate lesions hidden in live tissues by scanning a point light source. Imaging of the front projection on complicated lesion structures is demonstrated with this method. Its transverse resolution, which is affected by the wavelength of incident light and the size of the scattering particle, can reach the diameter of the lesion scattering particle theoretically while the best longitudinal detection depth can be achieved by choosing a suitable incident wavelength according to the scattering characters of the tissue.     

Adaptive correction to the speckle correlation fringes by using a twisted-nematic liquid-crystal display

An innovative technique for reducing speckle noise and improving the intensity profile of the speckle correlation fringes is presented. The method is based on reducing the range of the modulation intensity values of the speckle interference pattern. After the fringe pattern is corrected adaptively at each pixel, a simple morphological filtering of the fringes is sufficient to obtain smoothed fringes. The concept is presented both analytically and by simulation by using computer-generated speckle patterns. The experimental verification is performed by using an amplitude-only spatial light modulator (SLM) in a conventional electronic speckle pattern interferometry setup. The optical arrangement for tuning a commercially available LCD array for amplitude-only behavior is described. The method of feedback to the LCD SLM to modulate the intensity of the reference beam in order to reduce the modulation intensity values is explained, and the resulting fringe pattern and increase in the signal-to-noise ratio are discussed.     

Ultrasound-modulated optical tomography of biological tissue by use of contrast of laser speckles

Ultrasound-modulated optical tomography based on the measurement of laser-speckle contrast was investigated. An ultrasonic beam was focused into a biological-tissue sample to modulate the laser light passing through the ultrasonic column inside the tissue. The contrast of the speckle pattern formed by the transmitted light was found to depend on the ultrasonic modulation and could be used for imaging. Variation in the speckle contrast reflected optical inhomogeneity in the tissue. With this technique, two-dimensional images of biological-tissue samples of as much as 25 mm thick were successfully obtained with a low-power laser. The technique was experimentally compared with speckle-contrast-based, purely optical imaging and with parallel-detection imaging techniques, and the advantages over each were demonstrated.     

Contrast enhancement for electronic speckle pattern interferometry fringes by the differential equation enhancement method

Electronic speckle pattern interferometry fringe patterns usually have poor contrast so it is important to enhance fringe contrast for the extraction of phase from a single fringe pattern. We present new enhancement methods based on differential equations (called DE enhancement methods) to electronic speckle pattern interferometry fringes. The DE enhancement methods transform the image processing to solve differential equations. With the proposed methods, the visibility of the correlation speckle fringe patterns can be improved significantly. We tested the proposed methods on computer-simulated speckle correlation fringes and experimentally obtained fringes, and we compared the new method with other contrast enhancement techniques. The experimental results illustrate the performance of this approach.