John R. Tyrer

An electronic speckle pattern interferometer for complete in-plane displacement measurement

Measurement of plane-strain surface displacement with ESPI requires at least two in-plane illumination geometries. For static loading conditions it is acceptable to record these two interferograms sequentially. However, for time-dependent strain fields, it is necessary to use both illumination geometries simultaneously so that a recording is made with identical strain conditions existing for both. The authors describe a new interferometer that has been devised to measure two in-plane interferograms at the same time. The determination of the method of operation and experimental verification of the technique are given.

Age-related differences in the elasticity of the human cornea.

PURPOSE The goal of this study was to determine age-related variation in the elasticity of the human cornea using nondestructive means. METHODS Organ cultured human corneoscleral buttons were studied. Changes in strain were measured with a radial shearing speckle pattern interferometer after an increase in intraocular pressure from 15.0 to 15.5 mm Hg. Changes in central corneal displacement were calculated by integration, and a bulk corneal Youngs modulus was derived by mathematical analysis. RESULTS Fifty corneas, including 17 pairs, were studied. Donors were aged between 24 and 102 years (mean, 73.1); 29 (58%) specimens were from male donors and 21 from female donors. Youngs modulus of the cornea increased with age, with the line of best fit indicating an approximate doubling from 0.27 MPa at age 20 years (95% confidence interval, 0.22-0.31) to 0.52 (0.50-0.54) MPa at age 100 years (R² = 0.70). CONCLUSIONS The stiffness of the human cornea increases by a factor of approximately two between the ages of 20 and 100 years. This variation is relevant to the algorithms used to predict the response to incisional and ablative refractive surgery and will also affect the formulas used to calculate intraocular pressure by applanation.

Two-dimensional strain measurement with ESPI

Abstract Optical techniques have been applied to the measurement of solid deformations in many instances. Practical difficulties with a particular technique can be overcome, in some circumstances, with attention to experimental detail. However, a common problem is the quantitative interpretation of the optical data thus obtained: this can be a time-consuming process and depends on the operators skill. This paper describes an electronic speckle pattern interferometer that measures two in-plane displacement components simultaneously. The (phase-stepping) procedures implemented for automated displacement and strain analysis are described. Simultaneous measurement of the two displacement components is particularly important for shear strain measurement, because data from the two in-plane views must be combined. The accuracies of displacement and strain measurements are shown to be ±0·03 μm and ±6 μstrain, respectively. Results are presented for a compact tension specimen.

Effects of variation in depth and side cut angulations in LASIK and thin-flap LASIK using a femtosecond laser: a biomechanical study.

PURPOSE To study the corneal biomechanical effects of varying LASIK flap depth and side cut angulations and evaluate the relative contribution of the lamellar and side cuts using a femtosecond laser and radial shearing speckle pattern interferometry (RSSPI). METHODS Forty-two organ-cultured human corneas were divided into a control group and three investigative groups, each undergoing different incision types at both 90- and 160-μm depth using a femtosecond laser. In the first group, typical LASIK flaps were created; in the second group, only the bed was cut (delamination); and in the third group, side cuts alone were affected. Corneal strain was measured using RSSPI before and after treatment following an increase in hydrostatic pressure from 15.0 to 15.5 mmHg and again after 1 week of incubation in culture medium. RESULTS The flap group demonstrated a weakening of strength related to the depth of cut, with strain increasing by 9% and 32% at 90 and 160 μm, respectively. Similar changes, 9% and 33%, were observed following execution of side cuts to the same depths. By contrast, strain increase following delamination showed no relationship with depth, increasing by 5% in both instances. When the side cut angle was made more acute, strain decreased with a 2% strain increase being measured after a 90-μm, 150° side cut was created. No significant changes occurred during the period of organ culture. CONCLUSIONS Vertical side cuts through corneal lamellae rather than horizontal delamination incisions contribute to the loss of structural integrity during LASIK flap creation. Angulating side cuts such that the stromal diameter of the flap exceeds its epithelial diameter can decrease this effect.

Manipulation of the fourier components of speckle fringe patterns as part of an interferometric analysis process

Abstract A Fourier filtration algorithm has been applied to speckled fringe patterns, as obtained with the electronic speckle pattern interferometer. The Fourier power spectrum components found are then conveniently manipulated to extract noise that is inherent to the speckled fringes, thus rendering holographic-quality displays. The use of this technique facilitates the computer interferometric analysis of such fringe patterns. Some examples of the phase fringes and the corresponding object displacements obtained using this method are shown.

Extraction of phase data from electronic speckle pattern interferometric fringes using a single-phase-step method: a novel approach

A new technique has been developed for extracting both deformation and shape information from electronic speckle pattern interferometry fringes that involves a significant improvement in environmental stability and reduced computational effort. This new approach utilizes a single-phase-step technique to extract data and requires no additional optical components to produce a rapid analysis of static and dynamic fringe patterns. Continuous-wave, stroboscopic, and pulsed laser illumination are all amenable to this technique. Comparisons are made with similar phase-reduction techniques involving two and three phase steps. It is concluded that there is not a significant difference among these and hence that the new technique offers important practical advantages. Results are presented from the analysis of static and dynamic targets.

WHOLE FIELD IN-PLANE VIBRATION ANALYSIS USING PULSED PHASE-STEPPED ESPI

Electronic speckle pattern interferometry has been used to study resonant in-plane vibrations of a thin square metal plate. An in-plane sensitive arrangement is used with dual-beam illumination from a pulsed laser. Fringe patterns are formed which show a cosinusoidal intensity profile. These fringe patterns inherently carry phase information, which is extracted using the single phase step technique and analyzed to determine the amplitude and phase for the horizontal and vertical components of in-plane vibration. These are automatically combined to yield the total in-plane vibration mode. The final result is displayed as vectors drawn over an image of the object.

Application of ESPI to three-dimensional vibration measurements

Abstract There are many situation in vibration analysis where it is necessary or desirable to make three-dimensional measurements. By using three different illumination geometries around a single imaging system, electronic speckle pattern interferometry (ESPI) can be used to measure the orthogonal components of vibration amplitude independently. These can be combined to determine the three-dimensional amplitude and mode shape. Examples of experimental results are presented for volume vibrations of a thick cylinder, identification of vibration modes of a turbocharger blade, and observation of in-plane modes in a thin plate.

Nonperturbing measurements of spatially distributed underwater acoustic fields using a scanning laser Doppler vibrometer

Localized changes in the density of water induced by the presence of an acoustic field cause perturbations in the localized refractive index. This relationship has given rise to a number of nonperturbing optical metrology techniques for recording measurement parameters from underwater acoustic fields. A method that has been recently developed involves the use of a Laser Doppler Vibrometer (LDV) targeted at a fixed, nonvibrating, plate through an underwater acoustic field. Measurements of the rate of change of optical pathlength along a line section enable the identification of the temporal and frequency characteristics of the acoustic wave front. This approach has been extended through the use of a scanning LDV, which facilitates the measurement of a range of spatially distributed parameters. A mathematical model is presented that relates the distribution of pressure amplitude and phase in a planar wave front with the rate of change of optical pathlength measured by the LDV along a specifically orientated laser line section. Measurements of a 1 MHz acoustic tone burst generated by a focused transducer are described and the results presented. Graphical depictions of the acoustic power and phase distribution recorded by the LDV are shown, together with images representing time history during the acoustic wave propagation.

Recent Developments and Applications in Electronic Speckle Pattern Interferometry

Abstract Electronic speckle pattern interferometry (ESPI) is a wholefield non-contact optical metrology technique for displacement measurement, based on the optical physics of surface-generated laser speckle. Since its inception during the early 1970s ESPI has gradually evolved into different optical designs and has been applied to a range of engineering and non-engineering applications. Development of ESPI has continued during the 1990s with the introduction of new laser and optical technology into the interferometers, allowing further optimization and extending the potential applicability of the technique. This review considers the most notable developments of interferometer design and application that have occurred and been widely published during the 1990s, and examines the current and near-future direction of research into the technique.