An out-of-plane sensitive electronic speckle pattern interferometer (ESPI) using holographic optical elements (HOEs) for studying rotations and vibrations is presented. Phase stepping is implemented by modulating the wavelength of the laser diode in a path length imbalanced interferometer. The time average ESPI method is used for vibration measurements. Some factors influencing the measurements accuracy are reported. Some advantages and limitations of the system are discussed.
A new low-toxicity diacetone acrylamide-based photopolymer is developed and characterized. The environmentallycompatible
photopolymer has been modified with the inclusion of glycerol. The incorporation of glycerol results in a
uniform maximum refractive index modulation for recording intensities in the range of 1-20 mW/cm2. This may be
attributed to glycerol's nature as a plasticizer, which allows for faster diffusion of un-reacted monomer within the grating
during holographic recording. An optimum recording intensity of 0.5 mW/cm2 is observed for exposure energies of 20-
60 mW/cm2. The modified photopolymer achieves a refractive index modulation of 2.2×10-3, with diffraction efficiencies
up to 90 % in 100 μm layers. The photopolymer layers containing glycerol have improved stability and optical quality.
The application of an out-of-plane sensitive electronic speckle pattern interferometer (ESPI) using holographic optical
element (HOE) to vibration amplitude and phase mapping is reported. The novelty of the proposed system is the use of a
speckle reference wave stored in a reflection holographic optical element (HOE). The incorporation of a HOE minimizes
the alignment difficulties. The HOE based ESPI system is compact containing only a diode laser, HOE and a digital
CMOS camera. The measurement technique is a combination of time averaged ESPI and reference beam phase
modulation in an unbalanced interferometer. The reference beam phase modulation is implemented by modulating the
drive current of the diode laser. The presented HOE based ESPI system is easy to align and compact and thus suitable for
industrial non-destructive testing and vibration analysis.
A simple electronic speckle pattern interferometer (ESPI) using a transmission holographic optical element (THOE) is
presented. The THOE is designed to create a speckled reference beam in the interferometer. It is a transmission
hologram of a diffusely transmitting glass plate. A specific requirement for the fabrication of the THOEs is for them to
be recorded at one wavelength, at which the recording material is photosensitive and reconstructed using a near-infrared
laser diode which can be current modulated for phase shifting purposes. A partially reflective glass plate provides
illumination of the object along the normal to its surface, ensuring that the system is sensitive only to out-of-plane
displacement of the object. The intensity of the object beam can be controlled by using reflective glass plates with
different reflection coefficients. It is demonstrated that the HOE based system can be used for vibration measurements
and modal analysis. A big advantage of the system is its simplicity.
Ocular Microtremor (OMT) is a continual, high frequency physiological tremor of the eye present in all subjects even
when the eye is apparently at rest. OMT causes a peak to peak displacement of around 150nm-2500nm with a broadband
frequency spectrum between 30Hz to 120Hz; with a peak at about 83Hz. OMT carries useful clinical information on
depth of consciousness and on some neurological disorders. Nearly all quantitative clinical investigations have been
based on OMT measurements using an eye contacting piezoelectric probe which has low clinical acceptability. Laser
speckle metrology is a candidate for a high resolution,
non-contacting, compact, portable OMT measurement technique.
However, tear flow and biospeckle might be expected to interfere with the displacement information carried by the
speckle. The paper investigates the properties of the scattered speckle of laser light (λ = 632.8nm) from the eye sclera to
assess the feasibility of using speckle techniques to measure OMT such as the speckle correlation. The investigation is
carried using a high speed CMOS video camera adequate to capture the high frequency of the tremor. The investigation
is supported by studies using an eye movement simulator (a bovine sclera driven by piezoelectric bimorphs). The speckle
contrast and the frame to frame spatiotemporal variations are analyzed to determine if the OMT characteristics are
detectable within speckle changes induced by the biospeckle or other movements.
Electronic speckle pattern interferometry (ESPI) is a full-field measurement technique, capable of displaying vibrational
mode shapes. Two electronic speckle pattern interferometers using reflection holographic optical elements (RHOEs) are
presented. In the first ESPI system the RHOE is designed to create the speckled reference beam. A partially reflective
glass plate provides illumination of the object along the normal to its surface, ensuring that the system is sensitive only to
out-of-plane displacement of the object. It is demonstrated that the HOE-based system can be used for vibration
measurements. Phase shifting can be implemented for fringe analysis.
In the second ESPI system a reflection holographic optical element of a flat diffusely reflecting surface serves a dual
purpose. On reconstruction, a diffuse beam of laser light is produced to act as a reference beam in the ESPI system.
Undiffracted light passing through the RHOE serves to illuminate the object. This system is not completely insensitive to
in-plane displacement but the illumination and observation directions can be made nearly collinear.
The systems are compared in terms of flexibility in their adjustment, sensitivity, suitability and limitations for different
applications.
The introduction of holographic optical elements in ESPSI systems gives the advantage of using high aperture optical
elements at relatively low cost. Both systems are suitable for out-of-plane vibration studies. The results obtained are
promising for future applications of RHOEs in alternative laser Doppler vibrometry systems.
Electronic speckle pattern interferometry (ESPI) is a full-field measurement technique, capable of displaying vibrational
mode shapes. A simple optical set-up for an ESPI system using a holographic optical element (HOE) is presented. The
HOE is designed to create a speckled reference beam in the interferometer. A partially reflective glass plate provides
illumination of the object along the normal to its surface, ensuring that the system is sensitive only to out-of-plane
displacement of the object. It is demonstrated that the HOE-based system can be used for vibration measurements. Phase
shifting can be implemented for fringe analysis. A big advantage of the system is its simplicity. It requires a small
number of components: a coherent light source, a holographic optical element, a glass plate and a CCD camera.
Introducing holographic optical elements in ESPI gives the advantage of large aperture optical elements at relatively low
cost.
An important area of application of holographic optical elements (HOEs) is in optical and electronic speckle pattern
interferometry. The design, fabrication and characterization of holographic optical elements (HOEs) for electronic
speckle pattern interferometry are presented. Reflection HOEs (RHOEs) were fabricated for use in electronic speckle
pattern interferometers (ESPI) and laser Doppler vibrometers (LDV). The HOE-based interferometer is sensitive to out-of-plane displacements only. The results obtained are promising for future applications of the system for modal analysis.
Three electronic speckle pattern shearing interferometers (ESPSI) using photopolymer holographic gratings to produce
the sheared image are presented. In the first ESPSI system two holographic gratings are used. The gratings are placed
between the object and an imaging lens in front of the CCD camera. In the second ESPSI system one grating is used in
combination with a sheet of ground glass. The sheared images on the ground glass are further imaged onto a CCD
camera. In the third ESPSI system only one grating is used - it is placed in front of the object. The image and the sheared
image are imaged onto the CCD camera, whose optical axis coincides with the normal to the object surface. The
introduction of photopolymer holographic gratings in ESPSI systems gives the advantage of using high aperture optical
elements at relatively low price. The systems are compared in terms of flexibility in their adjustment, sensitivity,
suitability and limitations for different applications.
An optical set-up for electronic speckle pattern shearing interferometry (ESPSI) using a photopolymer diffractive optical element as a shearing element, is presented. A laser beam illuminates the object at an angle to the normal to the object surface. The holographic diffraction grating is placed in front of the object. The zero and the first order of diffraction form the image and the sheared image of the object. The images are imaged onto the CCD camera, whose optical axis coincides with the normal to the object surface. The field of view is limited only by the dimensions of the photopolymer plate. The photopolymer diffractive element is characterised by low level of light scatter and diffraction efficiency of 60%. The simplicity of the proposed new shearing interferometer is manifested by the extremely small number of components required - a coherent light source, a holographic optical element and a CCD camera.
A study of the light induced surface relief modulation in thin photopolymer layers is reported. Due to the nature of the photopolymer used no additional post-processing is required after holographic recording. An investigation of the dependence of the amplitude of the photoinduced relief modulation on different parameters of recording such as spatial frequency, intensity of the beams and times of exposure has been carried out. The surface relief modulation is characterized by white light interferometry. Photopolymer layer thickness ranges from 1-5 μm. A model of the mechanism of surface relief formation is proposed on the basis of the measured dependencies. A variety of patterns are inscribed in order to demonstrate the potential of this photopolymer in the design of different diffractive optical elements.
Thermal sprayed coatings have wide engineering applications. There now exists a wide range of destructive and nondestructive testing (NDT) methods for surface coating inspections. This paper describes an application of Electronic Speckle Pattern Shearing Interferometry (ESPSI) for NDT of thermal sprayed surface coatings. In contrast to other conventional methods such as eddy current, ultrasonic or X-ray, ESPSI allows fast and large survey area inspection. Experimental results of shearographic measurements are presented. Thermal sprayed coatings were tested using ESPSI. Delaminations of the coatings were detected and the fringe patterns were captured using this method. It is shown that the shearography technique can be applied successfully to surface coating quality inspection and it is very effective for delamination detection.
Electronic speckle pattern shearing interferometry (ESPSI) is superior to Electronic speckle pattern interferometry (ESPI) when strain distribution, arising from object deformation or vibration, need to be measured. This is because shearography provides data directly related to the spatial derivatives of the displacement. Further development of ESPSI systems could be beneficial for wider application to the measurement of mechanical characteristics of vibrating objects. Two electronic speckle pattern shearing interferometers (ESPSI) suitable for vibration measurements are presented. In both ESPSI systems photopolymer holographic gratings are used to shear the images and to control the size of the shear. The holographic gratings are recorded using an acrylamide-based photopolymer material. Since the polymerization process occurs during recording, the holograms are produced without any development or processing. The ESPSI systems with photopolymer holographic gratings are simple and compact. Introducing photopolymer holographic gratings in ESPSI gives the advantage of using high aperture optical elements at relatively low cost. It is demonstrated that both ESPSI system can be used for vibration measurements. The results obtained are promising for future applications of the systems for modal analysis.
Recently there has been an increasing interest in the application of shearography for modal analysis of vibrating objects. New interferometric systems, which are simple and flexible are of interest for engineering and industrial applications. An electronic speckle pattern shearing interferometer (ESPSI) with a very simple shearing device is used for study of vibrations. The shearing device consists of two partially reflective glass plates. The reflection coefficients of the coatings are 0.3 and 0.7 respectively. The distance between the two glass plates controls the size of the shear. The versatility of this simple shearing interferometer is shown. It is demonstrated that the ESPSI system can be used for vibration measurements and phase-shifting implemented for fringe analysis. The results obtained are promising for future applications of the system for modal analysis.
Two optical set-ups for electronic speckle pattern shearing interferometry (ESPSI) using photopolymer diffractive optical elements are presented. Holographic gratings are recorded using an acrylamide based photopolymer material. Since the polymerisation process occurs during recording, the holograms are produced without any development or processing. In both ESPSI systems the photopolymer gratings are used to shear the image. In the first ESPSI system only one grating is used in combination with a sheet of ground glass. The distance between the grating and the ground glass can be used to control the amount of the shear. The sheared images on the ground glass are further imaged onto a CCD camera. In the second ESPSI system two gratings are used to shear the image. The gratings are placed between the object and an imaging lens in front of the CCD camera. The distance between the two gratings controls the size of the shear. The ESPSI system with two photopolymer holographic gratings is compact and suitable for industrial applications. Introducing photopolymer holographic gratings in ESPSI gives the advantage of using high aperture optical elements at relatively low price. Both of these interferometric systems are simple and flexible.
The use of electronic speckle pattern interferometry (ESPI) for non-destructive characterization of unplasticized polyvinylchloride (uPVC) pipes is presented. When pipes are inflated slightly by internal pressurization ESPI may then be used to measure the resulting displacement over the area imaged by the video camera. The interferometric method used is non-contact and gives high-confidence results for strain and Young's modulus of thick PVC pipes. As a step towards using ESPI in dynamic rather than static applications, the ESPI data were recorded during the application of pressure increase. At the same time the change in pressure in the pipe was monitored using a pressure transducer and a specially written Labview Program so that the deformation caused could be modelled using Finite Element Analysis. A finite element model was developed for prediction of the mechanical behavior of the pipes undergoing transient pressure change. The model data have been used here for comparison with ESPI results for slow dynamic deformation of the pipe under test. Demonstration of the possibility of carrying out similar analyses in a dynamic system will greatly expand the applications for this technique.
An electronic speckle pattern interferometry (ESPI) system for detection of cracks and defects is presented. In the first stage a holographic optical element (HOE) is recorded using a photopolymer material. Since the polymerization process occurs during recording, the holograms are produced without any development/processing. In the second stage the HOE is used in an ESPI configuration for detection of cracks and defects. Due to the introduction of the HOE in the ESPI set-up, precise alignment of the optical elements is not necessary. For this reason the system is well suited for industrial applications.
The use of electronic speckle pattern interferometry (ESPI) for non-destructive material characterization of thick and thin unplasticized polyvinylchloride (uPVC) pipes is presented. Pipes are tested by internal pressurization and ESPI gives a complete mapping of the resulting displacement field over the area imaged by the video camera. The results for the strain derived from ESPI data and from the standard mechanical method using strain gauges agree very well with each other. The interferometric method used is non-contact and gives high-confidence results for Young's modulus of uPVC pipes. The fringe counting method gives the total diplsacment over the field of view imaged by the CCD camera and is subject to a fringe error of 0.5. This simple approach is valid when the displacement behavior of the sample is known. When this is not the case then it is necessary to calculate the phase map of the displacement of digitally shifting the phase difference between the two beams in the interferometer. We have implemented this technique by modulating the laser diode drive current to alter the wavelength of the laser very slightly between frames. A linear phase map of the displacement is always obtained in the present case.
Spectral characteristics based on in-vivo fluorescence measurements were taken of five laboratory uni-algal cultures: Isochrysis galbana, Nannochloris atomus, Tetraselmis chui, Tetraselmis pseuicia and Tetraselmis seucica. Two diffrent techniques for fluorescence measurements have been used to detection of fluorescence of marine algae. The techniqeus used were Confocal Microscope LSM 510 and LabRam 1B Spectroscopic Microscope. Both instruments were able to detect the in vivo fluroescence signal of all uni-algal cultures. Different excitation wavelengths have been used. The excitation wavelength of 488 nm produced the fluorescence signal with highest intensity. Confocal fluorescence microscopy shows that the samples are spectrally uniform from cell to cell. This result suggests that fluorescence measurements can be used for identification of algal species. Raman spectroscopy is employed to identify differences in molecular structure between the samples. Chlorophyll fluorescence of different species shows a change. As fluorescence is directly linked to the physiological state of the photosynthetic system and very sensitive to stress being experienced further research in the direction of fluorescence detection of toxic algae species will be of interest.
Non-destructive testing of rubber by optical means presents a challenge because rubber has different mechanical properties from conventional solids when subjected to stress. The size of flaws introduced in rubber during the manufacturing process is an important characteristic as they increase the stress in the specimen when it is in use. The use of electronic speckle pattern interferometry, electronic speckle pattern shearing interferometry and white light interferometry for non-destructive material characterization of rubber is presented. It is shown that electronic speckle pattern shearing interferometry can be of some use for visualization of flaws on the rubber surface under thermal stress. White light interferometry gives a complete profiling of the elastomer surface and it is only one of the three interferometric techniques for measuring flaws on the rubber surface.
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