Franco Docchio

Study of the temporal and spatial dynamics of plasmas induced in liquids by nanosecond Nd:YAG laser pulses. 1: Analysis of the plasma starting times.

We report on a theoretical and experimental study of the temporal and spatial dynamics of plasmas produced in liquids by single Nd:YAG laser pulses of nanosecond duration. This study was motivated by the increasing attention paid to the phenomenon of optical breakdown and to its related effects on tissues and media in connection with microsurgical techniques developed for ophthalmology and urology. Streak camera recordings of the emission from laser-induced plasmas were taken in distilled and tap water in controlled irradiation conditions. From streak recordings, plasma starting times as a function of the axial distance from focus, the overall length of the plasma column, plasma lifetimes, and plasma absorption were derived and analyzed. In this first paper we analyze the curves of plasma starting time, as a function of the irradiation parameters and of the properties of the medium. We show that a model obtained by upgrading the theory of the moving breakdown allows accurate interpretation of the experimental observations.

Three-dimensional imaging based on Gray-code light projection: characterization of the measuring algorithm and development of a measuring system for industrial applications

A three-dimensional (3-D) imaging system based on Gray-code projection is described; it is thought to be used as an integration to the already developed profilometer based on the projection of multifrequency gratings. The Gray-code method allows us to evaluate the 3-D profile of objects that present even marked discontinuities of the surface, thus increasing the flexibility of the measuring system as to the topology of the objects that can be measured. The basic aspects of Gray-code projection for 3-D imaging and profiling are discussed, with particular emphasis devoted to the study of the resolution of the method and to the analysis of the systematic errors. The results of this study allow us to determine the optimal setting of the parameters of the measurement and to develop a suitable calibration procedure. The procedures for implementing the Gray-code method are presented, and some interesting experimental results are reported. Calibration of the system reveals an accuracy of 0.2 mm, corresponding to 0.1% of the field of view.

Relationship between media-to-lumen ratio of subcutaneous small arteries and wall-to-lumen ratio of retinal arterioles evaluated noninvasively by scanning laser Doppler flowmetry.

Background: Structural alterations of subcutaneous small resistance arteries, as indicated by an increased media-to-lumen ratio, are frequently present in hypertensive and/or diabetic patients, and may represent the earliest alteration observed. Furthermore, media-to-lumen ratio of small arteries evaluated by micromyography has a strong prognostic significance; however, its extensive evaluation is limited by the invasivity of the assessment, since a biopsy of subcutaneous fat is needed. Noninvasive measurement of wall-to-lumen of retinal arterioles using scanning laser Doppler flowmetry (SLDF) has recently been introduced. However, this new technique has not yet been compared to micromyographic measurement, generally considered the gold standard approach. Methods and results: We investigated 40 individuals and patients, 24 of them were hypertensive patients and 16 normotensive individuals. All patients underwent a biopsy of subcutaneous fat during an elective surgical intervention. Subcutaneous small resistance arteries were dissected and mounted on a wire myograph, and media-to-lumen ratio was measured. In addition, an evaluation of wall-to-lumen ratio of retinal arterioles by SLDF was performed (Heidelberg Retina Flowmeter, Heidelberg Engineering). A close correlation was observed between media-to-lumen ratio of subcutaneous small arteries and wall-to-lumen ratio of retinal arterioles (r = 0.76, P < 0.001; P < 0.001, r2 = 0.57). Conclusion: A noninvasive and easily repeatable procedure (intraobserver and interobserver variation coefficient <13%) such as an evaluation of the arterioles in the fundus oculi by SLDF may provide similar information regarding microvascular morphology compared with an invasive, accurate and prognostically relevant micromyographic measurement of media-to-lumen ratio of subcutaneous small arteries.

A novel, adaptive system for 3-D optical profilometry using a liquid crystal light projector

A 3-D optical whole-field profilometer based on adaptive projection of structured light is presented. The system is based on the projection of gratings by means of an LCD unit. The gratings can be varied both in contrast and in period, to adapt to the shape of the object under measurement. A video camera acquires at a different angle the object-deformed pattern. Suitable pre-elaboration is performed, to decrease dependence on background illumination and nonuniform reflectivity of the surface. Adaptive demodulation of the pattern allows the object profile to be evaluated. The performance of the instrument has been evaluated by means of a system calibration against a traceable high-precision commercial Contact Measuring Machine (CMM). The overall accuracy of the system is equal to 0.15 mm with a precision of 0.2 mm. In this article, the theoretical aspects of the technique are discussed, and the description of the complete system is presented. Profile reconstruction, calibration, and certification of the system are also covered. The accuracy of the system is discussed, and experimental results are presented. >

Study of the temporal and spatial dynamics of plasmas induced in liquids by nanosecond Nd:YAG laser pulses. 2: Plasma luminescence and shielding

We describe and analyze the temporally and spatially resolved luminescence and attenuation characteristics of plasmas induced in liquids commonly accepted as models for ocular media by high-irradiance Nd:YAG laser pulses of nanosecond duration. Measurements of plasma generated in different irradiation conditions, performed with the aid of a streak camera, show that, when expansion of the plasma column toward the incoming beam occurs, it coincides with quenching of the luminescence exhibited by plasma regions located closer to the focus, where breakdown has first occurred. Evidence for this effect is given both with impurityfree media, where plasma expansion occurs in a continuous and regular way, and with impurity-rich media, where plasma columns are composed either of unevenly distributed plasma regions or of single plasmas with irregular breakdown starting patterns. Experimental findings have been analyzed using a model of spatially distributed plasma shielding, in combination with a previously developed model of moving breakdown, that well interprets, in liquids, the spatially dependent breakdown starting times.

Tunable, double-wavelength heterodyne detection interferometer for absolute-distance measurements.

We present a novel absolute-distance meter based on double heterodyne interferometry. The synthetic wavelength is obtained with a pair of tunable Nd:YAG lasers operating at 1.064 μm. The nonambiguity range of the measurement can be arbitrarily extended by subsequent measurements with different synthetic wavelengths. Experimental results from a prototype breadboard prove the feasibility of a large-nonambiguity-range, high-resolution absolute-distance meter.

OPL-3D: A novel, portable optical digitizer for fast acquisition of free-form surfaces

The paper presents OPL-3D, a novel, powerful three-dimensional optical digitizer based on structured, incoherent light illumination, developed in our laboratory for noncontact acquisition and digitization of free forms in space. The system exploits active stereovision using time multiplexing based on a combination of Gray code and phase-shifting techniques. The system has been specifically designed for applications to reverse engineering and rapid prototyping of free-form shapes, as well as for applications to measurement and quality control. Suitable estimation of the measurement parameters, as well as specifically developed two-dimensional image analysis, allows us to retrieve very dense point clouds in a few seconds. Special care has been devoted to the implementation of easy-to-use fast calibration procedures, and to enhancement of the flexibility of the system to the measuring problem as well as its portability. A kernel for elaboration of the views and for their alignment is provided. OPL-3D exhibits low-measurement uncertainty (120 μm) over large measurement areas (450 mm×340 mm), linearly scalable in the case of smaller areas. The output formats of the data files are fully compatible with the formats commonly used by elaboration environments dedicated to the production of polygonal models and to computer-aided design models of the shapes. The performance of OPL-3D has been tested in a number of applications, ranging from industry to biomedicine and virtual reality.The paper presents OPL-3D, a novel, powerful three-dimensional optical digitizer based on structured, incoherent light illumination, developed in our laboratory for noncontact acquisition and digitization of free forms in space. The system exploits active stereovision using time multiplexing based on a combination of Gray code and phase-shifting techniques. The system has been specifically designed for applications to reverse engineering and rapid prototyping of free-form shapes, as well as for applications to measurement and quality control. Suitable estimation of the measurement parameters, as well as specifically developed two-dimensional image analysis, allows us to retrieve very dense point clouds in a few seconds. Special care has been devoted to the implementation of easy-to-use fast calibration procedures, and to enhancement of the flexibility of the system to the measuring problem as well as its portability. A kernel for elaboration of the views and for their alignment is provided. OPL-3D exhibit...

A simple and reliable system for measuring the refractive index of liquids using a position-sensitive detector

A simple and yet efficient apparatus for the measurement of the refractive index of liquids has been developed, with the purpose of controlling adulteration in liquids of everyday use. In the system, the index of refraction of the liquid under test is determined by measuring the lateral displacement of a laser beam, that passes obliquely through a rectangular cell filled with the liquid under characterization. The lateral displacement is accurately determined by a position-sensitive detector. A careful analysis of the errors that can be obtained with the system has been carried out, that yielded calibration curves for easy on-line correction. To characterize the performance of the system, a series of tests has been performed on simple liquids, under controlled conditions. The residual errors have proven to be within one part over 10/sup 3/. The proposed system is suitable for easy automation, obtaining simple and accurate measurements that can be performed also for volatile compounds. >

Autofluorescence methods in ophthalmology

An overview of the fluorophores of the eye, the methods used to measure ocular fluorescence, and the existing or ready-to-market instrumentation for the early diagnosis and monitoring of ophthalmic pathologies is presented. Emphasis is given to the impact that a more detailed knowledge of the fluorophores of the eye, their age and pathology relationship, and the considerable technology-driven progress in optical devices and components has had on the development of new compact yet powerful instruments for population screening and for patient follow-ups, using fluorometry alone or in combination with other optical techniques such as light scattering.

Dispersive white light combined with a frequency-modulated continuous-wave interferometer for high-resolution absolute measurements of distance

A nonincremental interferometer for the absolute measurement of distances is presented. The measuring technique is based on both dispersive white-light (DWL) interferometry and frequency-modulated continuous-wave (FMCW) interferometry. The proposed configuration integrates both techniques in the same interferometer by use of a single laser diode. This solution enables the results from the coarse measurements from the FMCW interferometer to be combined with the fine readouts from the DWL interferometer. Preliminary experimental results confirm the capability of the system to combine the advantages of the two techniques.