Speckles are a statistical interference phenomenon which arises when coherent light is scattered by an optically rough surface. The resulting speckle field can be examined using statistical techniques and some results that arise are related to the form of the resulting speckles. Characteristic features are the so-called lateral and longitudinal speckle size as well as the orientation of speckles in the z-direction. It has been shown that the 3D orientation of speckles in free-space and in Fourier transforming systems are different from each other. Here we review some of Sheridan's work in this area. We first examine the speckle field formed by a diffuser in free space over a 3D volume. Next an Optical Fourier Transform (OFT) system is assumed, where the diffuser is placed in the front focal length of a Fourier transforming lens and we examine the resulting speckle field in the back focal plane of this OFT system. We compare and contrast the properties of the speckles from both setups free-space and Fourier.
In this paper, we assess the quality of three speckle image processing techniques - namely Laser Speckle Contrast Imaging (LSCI), Fuji method and Generalised Differences (GD) - in the detection of veins in various plant leaves. We develop a simple NIR laser speckle imaging set-up and find that the Fuji algorithm is best suited for producing highly contrasted and spatially resolved veins in leaf tissue, with the LSCI algorithm producing lower resolution results and GD failing to locate fine vascular structure. Determining the practicality of various speckle image processing techniques for vein detection aides future research for possible subcutaneous blood ow detection in non-invasive biomedical applications.
Recently, a method for synthesizing a hologram of three dimensional (3D) objects from captured light field array is demonstrated. The 3D objects can be captured under incoherent light illumination using a micro lens array and their orthographic projection view images are generated from the captured elemental images. The synthesized orthographic projection view images are then multiplied by the corresponding phase functions and combined to form a digital hologram. For the first time, we analysis the performance of synthesized hologram under photon counting (low light imaging) conditions. The feasibility of this technique is experimentally verified by recording the orthographic projection images using a micro lens array and the reconstructed photon counted hologram is presented with varying photoncounting measurements.
The linear canonical transform (LCT) is used in modeling a coherent light-field propagation through first-order optical systems. Recently, a generic optical system, known as the quadratic phase encoding system (QPES), for encrypting a two-dimensional image has been reported. In such systems, two random phase keys and the individual LCT parameters (α,β,γ) serve as secret keys of the cryptosystem. It is important that such encryption systems also satisfy some dynamic security properties. We, therefore, examine such systems using two cryptographic evaluation methods, the avalanche effect and bit independence criterion, which indicate the degree of security of the cryptographic algorithms using QPES. We compared our simulation results with the conventional Fourier and the Fresnel transform-based double random phase encryption (DRPE) systems. The results show that the LCT-based DRPE has an excellent avalanche and bit independence characteristics compared to the conventional Fourier and Fresnel-based encryption systems.
KEYWORDS: Digital holography, Systems modeling, Signal analyzers, Digital Light Processing, Digital signal processing, Holograms, Fourier transforms, Data modeling, Radium, Data conversion, Analytics, Electronics engineering, Solar energy
The 2D non-separable linear canonical transform (2D-NS-LCT) can model a range of various paraxial optical systems. Digital algorithms to evaluate the 2D-NS-LCTs are important in modeling the light field propagations and also of interest in many digital signal processing applications. In [Zhao 14] we have reported that a given 2D input image with rectangular shape/boundary, in general, results in a parallelogram output sampling grid (generally in an affine coordinates rather than in a Cartesian coordinates) thus limiting the further calculations, e.g. inverse transform. One possible solution is to use the interpolation techniques; however, it reduces the speed and accuracy of the numerical approximations. To alleviate this problem, in this paper, some constraints are derived under which the output samples are located in the Cartesian coordinates. Therefore, no interpolation operation is required and thus the calculation error can be significantly eliminated.
Propagation and diffraction of a light beam through nonlinear materials are effectively compensated by the effect of selftrapping. The laser beam propagating through photo-sensitive polymer PVA/AA can generate a waveguide of higher refractive index in direction of the light propagation. In order to investigate this phenomenon occurring in light-sensitive photopolymer media, the behaviour of a single light beam focused on the front surface of photopolymer bulk is investigated. As part of this work the self-bending of parallel beams separated in spaces during self-writing waveguides are studied. It is shown that there is strong correlation between the intensity of the input beams and their separation distance and the resulting deformation of waveguide trajectory during channels formation. This self-channeling can be modelled numerically using a three-dimension model to describe what takes place inside the volume of a photopolymer media. Corresponding numerical simulations show good agreement with experimental observations, which confirm the validity of the numerical model that was used to simulate these experiments.
The 2D non-separable linear canonical transform (2D-NS-LCT) can describe a variety of paraxial optical systems. Digital algorithms to numerically evaluate the 2D-NS-LCTs are not only important in modeling the light field propagations but also of interest in various signal processing based applications, for instance optical encryption. Therefore, in this paper, for the first time, a 2D-NS-LCT based optical Double-random- Phase-Encryption (DRPE) system is proposed which offers encrypting information in multiple degrees of freedom. Compared with the traditional systems, i.e. (i) Fourier transform (FT); (ii) Fresnel transform (FST); (iii) Fractional Fourier transform (FRT); and (iv) Linear Canonical transform (LCT), based DRPE systems, the proposed system is more secure and robust as it encrypts the data with more degrees of freedom with an augmented key-space.
Terahertz radiation lies between the microwave and infrared regions in the electromagnetic spectrum. Emitted frequencies range from 0.1 to 10 THz with corresponding wavelengths ranging from 30 μm to 3 mm. In this paper, a continuous-wave Terahertz off-axis digital holographic system is described. A Gaussian fitting method and image normalisation techniques were employed on the recorded hologram to improve the image resolution. A synthesised contrast enhanced hologram is then digitally constructed. Numerical reconstruction is achieved using the angular spectrum method of the filtered off-axis hologram. A sparsity based compression technique is introduced before numerical data reconstruction in order to reduce the dataset required for hologram reconstruction. Results prove that a tiny amount of sparse dataset is sufficient in order to reconstruct the hologram with good image quality.
The accurate measurement of optical phase has many applications in metrology. For biological samples, which appear transparent, the phase data provides information about the refractive index of the sample. In speckle metrology, the phase can be used to estimate stress and strains of a rough surface with high sensitivity. In this theoretical manuscript we compare and contrast the properties of two techniques for estimating the phase distribution of a wave field under the paraxial approximation: (I) A digital holographic system, and (II) An idealized phase retrieval system. Both systems use a CCD or CMOS array to measure the intensities of the wave fields that are reflected from or transmitted through the sample of interest. This introduces a numerical aspect to the problem. For the two systems above we examine how numerical calculations can limit the performance of these systems leading to a near-infinite number of possible solutions.
We demonstrate theoretically and experimentally that the light can be self-focused and self-trapped in a self-written optical waveguide in a bulk acrylamide/polyvinyl alcohol (AA/PVA) solid photopolymer material volume. The manufacture method, i.e., how to prepare the AA/PVA photopolymer material is detailed. In our experimental observation the refractive index changes induced are permanent. The resulting optical waveguide channel has good physical stability and can be integrated with optoelectronic devices as part of integrated optical systems. The theoretical model developed predicts the formation/evolution of the observed self-written waveguides inside the bulk material. The model involves appropriately discretizing and then numerically solving the paraxial wave equation in Fourier space and the material equation in time space.
We report on the combined far-field measurement of the three involuntary eye movements, drift, microsaccades, and ocular microtremor (OMT), using a noncontact far-field optical method. We review the significance of the smallest and least measured, and thus least understood, of the three, OMT. Using modern digital imaging techniques, we perform detailed analysis, present experimental results, and examine the extracted parameters using a noncontact far-field sensor. For the first time, in vivo noncontact measurements of all three fixational in-plane movements of the human eye are reported, which simultaneously provide both the horizontal (left-right) and vertical (up-down) displacement results.
Digital holographic microscopy is suitable for the detection of microbial particles in a rapidly flowing fluid since in this technique the focusing can be carried out as post-processing of a single captured image. This image, known as a digital hologram, contains the full complex wave front information emanating from the object which forms an interference pattern with a known reference beam. Post-processing is computationally intense and it constitutes a bottleneck for real time inspection of fast moving scenes. In the current work, GPU computation is used to accelerate the post-processing of the holographic images captured by digital holographic microscopy. Efficiency and reliability of a pre-processing step in order to eliminate low information content holographic images is also investigated.
An compact multispectral digital in-line holographic microscope (DIHM) is developed that emulates Gabor's original holographic principle. Using sources of varying spatial coherence (laser, LED), holographic images of objects, including optical fiber, latex microspheres, and cancer cells, are successfully captured and numerically processed. Quantitative measurement of cell locations and percentage confluence are estimated, and pseudocolor images are also presented. Phase profiles of weakly scattering cells are obtained from the DIHM and are compared to those produced by a commercially available off-axis digital holographic microscope.
Access to both the phase and intensity of an image can be provided using digital holographic (DH) imaging techniques. Recently, the difference of two holograms captured with two appropriately related wavelengths was demonstrated to produce the Laplacian of an object field. Applying telecommunication lasers the feasibility of infrared (IR) DH and DH Laplacian reconstruction and the associated theoretical analyses are presented. This is achieved by combining a tunable mid-IR laser source and mid-IR sensitive InGaAs-based digital camera.
Holography is the process where two coherent wavefields interfere resulting in an interference pattern from which
whole field information can be retrieved. Digital holography is the process where the intensity of the superposition of
the two waves is recorded using a light sensitive opto-electronic detector array such as a CCD or CMOS camera. From
this recorded hologram it is possible to numerically reconstruct the object wavefield.
When an optical beam is focused on a pinhole whose diameter is of the order of a few times the wavelength of the
illumination beam, a spherically divergent wavefield is emitted. We use the emitted optical beam to illuminate weakly
scattering objects resulting in a geometrically magnified diffraction pattern at the camera face. Scattered light from the
object is the called the object wavefield, while unscattered light acts as the reference wavefield. The hologram is
captured digitally before numerical reconstruction is applied to yield whole field information about the object.
It is possible to reconstruct the objects wavefield using this method from coherent laser or incoherent LED illumination.
The emitted light from the pinhole acts a pointsource of spatially coherent light enabling holography. This, in
combination with the use of multiple wavelength LED's multispectral amplitude images can be reconstructed.
The multispectral lensless DIHM described here can be used to holographically image biological specimens such as
cells grown for use in the biopharmaceutical industry or for research purposes. In analysing cell viability based on the
trypan blue assay, the outer membrane of non-viable cells is penetrated by violet blue dye. Using such a Digital In-line
Holographic Microscope as described here, automatic classification of viable and non-viable cells could be performed.
A new sampling criterion for the linear canonical transform (LCT) was recently proposed by the authors. This criterion
was based on an analysis of the consequences of sampling the LCT of a discrete signal. Previous LCT sampling work in
the literature considered only sampling the LCT of continuous, band limited signals. This analysis has great significance
for numerical simulations of first order optical systems using the LCT, as these simulations must necessarily consider a
sampled input function, and evaluate only a sampled output function. It is also significant for the analysis of periodic
structures such as gratings. We present a review of this new sampling criterion and the associated analysis. We clarifying
its meaning and consider its applications. In particular, we consider the consequences of the analysis for fast LCT
algorithms, and for the use of the discrete Fresnel transform in digital holography. We conclude that the spectral method
of calculating Fresnel transforms may benefit greatly from recent advances to LCT sampling theory.
KEYWORDS: Image segmentation, Digital holography, 3D image processing, Holography, Visualization, Microscopy, Holograms, 3D image reconstruction, Glasses, 3D modeling
This paper demonstrates a technique that could prove useful for extracting three-dimensional (3D) models
from a single two-dimensional (2D) digital in-line holographic microscopy (DIHM) recording. Multiple intensity
images are reconstructed at a range of depths through a transmissive or partially transmissive scene recorded
by DIHM. A two step segmentation of each of these reconstructed intensity images facilitates the construction
of a data set of surfaces in 3D. First an adaptive thresholding step and then a border following step are
implemented. The surfaces of segmented features are rendered in 3D by applying the marching cubes algorithm
to polygonize the data set. Experimental results for a real world DIHM capture of a transmissive glass sample
are presented to demonstrate this segmentation and visualization process.
Digital holography is the process where an object's phase and intensity information is retrieved from intensity images obtained using a digital camera (CCD or CMOS sensor). Unlike off-axis holography, object information is not modulated onto carrier fringes, thus in-line digital holography makes optimum use of the recording device's sampling bandwidth resulting in higher resolution digital holograms. However, reconstructed images are obscured by the linear superposition of the unwanted out of focus twin images. In addition to this, speckle noise degrades overall quality of the reconstructed images. The speckle effect is an optical phenomenon arising when laser sources are used in digital holographic systems. Minimizing the effects due to speckle noise, removal of the twin image and using the full sampling bandwidth of the capture device, aids overall reconstructed image quality. Using interferometric techniques, it is possible to record whole field information about an object. Digital processing of the reconstructed holograms can remove or suppress the twin image while effects from speckle noise can also be reduced numerically. Machine vision techniques can then be applied to segment and distinguish objects of interest in the hologram. Coding the resulting phase information onto a spatial light modulator (SLM), real world, three dimensional images of objects can be reconstructed using the computer generated hologram.
Digital holography is the process where an object's phase and amplitude information is retrieved from intensity images
obtained using a digital camera (e.g. CCD or CMOS sensor). In-line digital holographic techniques offer full use of the
recording device's sampling bandwidth, unlike off-axis holography where object information is not modulated onto
carrier fringes. Reconstructed images are obscured by the linear superposition of the unwanted, out of focus, twin
images. In addition to this, speckle noise degrades overall quality of the reconstructed images. The speckle effect is a
phenomenon of laser sources used in digital holographic systems. Minimizing the effects due to speckle noise, removal
of the twin image and using the full sampling bandwidth of the capture device aids overall reconstructed image quality.
Such improvements applied to digital holography can benefit applications such as holographic microscopy where the
reconstructed images are obscured with twin image information. Overcoming such problems allows greater flexibility in
current image processing techniques, which can be applied to segmenting biological cells (e.g. MCF-7 and MDA-MB-
231) to determine their overall cell density and viability. This could potentially be used to distinguish between apoptotic
and necrotic cells in large scale mammalian cell processes, currently the system of choice, within the biopharmaceutical industry.
Ocular microtremor (OMT) is a physiological high frequency (up to 150Hz) low amplitude (150-2500nm) involuntary
tremor of the human eye. It is one of the three fixational ocular motions described by Adler and Fliegelman in 1934 as
well as microsaccades and drift. Clinical OMT investigations to date have used eye-contacting piezoelectric probes or
piezoelectric strain gauges. Before contact can be made, the eye must first be anaesthetised. In some cases, this induces
eyelid spasms (blepharospasm) making it impossible to measure OMT. Using the contact probe method, the eye motion
is mechanically damped. In addition to this, it is not possible to obtain exact information about the displacement. Results
from clinical studies to date have given electrical signal amplitudes from the probe. Recent studies suggest a number of
clinical applications for OMT, these include monitoring the depth of anaesthesia of a patient in surgery, prediction of
outcome in coma, diagnosis of brainstem death. In addition to this, abnormal OMT frequency content is present in
patients with neurological disorders such as Multiple sclerosis and Parkinson's disease. However for ongoing clinical
investigations the contact probe method falls short of a non-contact accurate measurement solution. In this paper, we
design a compact non contact phase modulating optical fiber speckle interferometer to measure eye motions. We present
our calibration results using a calibrated piezoelectric vibration simulator. Digital signal processing is then performed to
extract the low amplitude high frequency displacement information.
Ocular microtremor (OMT) is a physiological high-frequency (up to 150 Hz) low-amplitude (25-2500 nm peak-to-peak) involuntary motion of the human eye. Recent studies suggest a number of clinical applications for OMT that include monitoring the depth of anesthesia of a patient in surgery, prediction of outcome in coma, and diagnosis of brain stem death. Clinical OMT investigations to date have used mechanical piezoelectric probes or piezoelectric strain gauges that have many drawbacks which arise from the fact that the probe is in contact with the eye. We describe the design of a compact noncontact sensing device to measure OMT that addresses some of the above drawbacks. We evaluate the system performance using a calibrated piezoelectric vibrator that simulates OMT signals under conditions that can occur in practice, i.e., wet eye conditions. We also test the device at low light levels well within the eye safety range.
Ocular microtremor (OMT) is a biological high frequency (up to 150Hz) low amplitude (25-2500nm peak to peak)
involuntary motion of the human eye. Clinical OMT investigations to date have used eye-contacting mechanical
piezoelectric probes or piezoelectric strain gauges. Before contact can be made, the eye must first be anaesthetized. In
some cases, this eyelid spasms occur making it impossible to measure OMT. Using the contact probe method, the eye
motion is mechanically loaded. Results from clinical studies with this method to date have given electrical signal
amplitudes from the probe proportional to the displacement, but not the exact displacement information. Recent studies
suggest a number of clinical applications for OMT, these include monitoring the depth of anesthesia of a patient in
surgery, prediction of outcome in coma, diagnosis of brain stem death. In addition to this, in patients with neurological
disorders such as Multiple Sclerosis and Parkinson's disease, abnormal OMT frequency content is present. In this paper,
we design a compact non-contact phase modulating optical fiber speckle interferometer to measure eye motions. We
simulate OMT motion using a calibrated piezoelectric vibration simulator and compare results produced using a contact
method with those using our optical non-contact method.
KEYWORDS: Speckle, Interferometers, Digital signal processing, Signal to noise ratio, Sensors, Modulation, Signal processing, Motion measurement, Calibration, Demodulation
Contact techniques exist to measure low amplitude low frequency mechanical vibration, however, by mechanically
loading the system of interest, they affect the measured results. In this paper, we design a compact non-contact optical
fiber speckle interferometer to measure inplane displacements. We implement this under laboratory conditions, and
present our calibration results, measuring low-amplitude microvibrations from 0.34 nm to 1.5 μm over a frequency range
from 10 Hz to 150 Hz.
The main advantage of the double random phase encryption technique is its physical implementation however to
allow us to analyse its behaviour we perform the encryption/decryption numerically. A typically strong encryption
scheme will have an extremely large key-space, which will make the probable success of any brute force attack on
that algorithm miniscule. Traditionally, designers of optical image encryption systems only demonstrate how a small
number of arbitrary keys cannot decrypt a chosen encrypted image in their system. We analyse this algorithm from a
key-space perspective. The key-space of an encryption algorithm can be defined as the set of possible keys that can
be used to encode data using that algorithm. For a range of problem instances we plot the distribution of decryption
errors in the key-space indicating the lack of feasibility of a simple brute force attack.
Micro-optical elements are becoming more and more important in both consumer and industrial products. Developments such as the tunable liquid lens and the silicon laser will lead to further development of these technologies. In this paper we shall discuss a method that enables perturbation of the shape of a UV curable liquid droplet using an applied electrostatic field. This method provides a novel method for the control of the pre-cured lens profile and hence the final solid lens optical properties. This method also allows the fabrication of aspheric lenses using the UV-curable technique with the degree of aspericity controlled in real time by varying the applied electric field. The analysis of this fabrication method requires the development of an accurate in-situ lens profile measurement system. A range of techniques can be used to examine the resulting solid lens including mechanical techniques such as Dektak and Talysurf profilometry and optical techniques such as laser profilometry and interferometric techniques. We note that in order to fully characterise this fabrication technique it is necessary to measure the surface profile of the lens both post-curing and also when it is in the liquid state. The method chosen to examine the liquid lens is interferometry. In this study the surface profile of the microlens is examined using a Mach-Zehnder interferometer. The development of this interferometric measurement system and the analysis software are discussed. The application of this system in examining the E-field induced perturbation of the UV curable resin lenses will also be discussed and initial attempts to control the optical properties of the lens by preshaping the lens using the E-Field technique are discussed.
Digital Holography is the technique of numerically reconstructing a three-dimensional (3D) image containing both amplitude and phase information from a two dimensional (2D) interference pattern recorded by the CCD. In this paper, we study the effects of varying the coherence length by using light from two types of source's (1) coherent laser light and (2) spatially filtered incoherent light from a Light Emitting Diode (LED). We present results using both calibrated test objects and biological samples with view to developing a 3D object recognition and classification system.
Micro-optical elements and related technologies are becoming increasingly important in a number of areas. There
are many different methods of fabricating these micro-optical elements. One such technique to produce microlenses
is the UV-curable resin method. This method involves the application of droplets of UV-curable resin to a substrate,
which can then be cured using UV light. The quality of the lenses produced using this method can vary due to a
number of experimental difficulties. These include the fact that the UV resin shrinks as it is cured. This shrinkage
effect is a difficult problem, as it is a property of the UV material and the substrate. The ability to pre-shape the
liquid droplet may enable the manufacturer to compensate for this shrinkage.
In this paper we propose a method to generate a controlled perturbation of the liquid profile and hence the optical
properties of the final lens. We describe our method of inducing variations in the droplet profile using an applied
electric field. This method enables the fabrication of aspheric lenses. The analysis of this fabrication method
requires the development of an accurate lens profile measurement system. A number of techniques can be used to
examine the resulting lens elements including mechanical techniques such as Dektak profilometry and optical
techniques such as interferometric techniques including laser profiler and white light profiler. In this paper we give
the details of the modified Mach-Zehnder interferometric measurement system used, discuss the optical properties of
the lens and show a demonstration of the perturbation of the lens profile.
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