A novel optical apparatus for the study of rolling contact wear/fatigue based on a high-speed camera and multiple-source laser illumination.

Rolling contact wear/fatigue tests on wheel/rail specimens are important to produce wheels and rails of new materials for improved lifetime and performance, which are able to operate in harsh environments and at high rolling speeds. This paper presents a novel non-invasive, all-optical system, based on a high-speed video camera and multiple laser illumination sources, which is able to continuously monitor the dynamics of the specimens used to test wheel and rail materials, in a laboratory test bench. 3D macro-topography and angular position of the specimen are simultaneously performed, together with the acquisition of surface micro-topography, at speeds up to 500 rpm, making use of a fast camera and image processing algorithms. Synthetic indexes for surface micro-topography classification are defined, the 3D macro-topography is measured with a standard uncertainty down to 0.019 mm, and the angular position is measured on a purposely developed analog encoder with a standard uncertainty of 2.9°. The very small camera exposure time enables to obtain blur-free images with excellent definition. The system will be described with the aid of end-cycle specimens, as well as of in-test specimens.

Simple, low-cost, portable corneal fluorometer for detection of the level of diabetic retinopathy.

A simple, low-cost, portable instrument for measurement of the autofluorescence of the human cornea is presented. Corneal autofluorescence has proved to be strongly correlated with the grade of retinopathy in diabetic patients. It is therefore a reliable parameter for detection of different levels of diabetic retinopathy, thus permitting timely intervention by ophthalmologists. The instrument contains custom optics and electronics and exhibits excellent linearity and repeatability both in vitro and in vivo. Preliminary tests on volunteers show promise for its use in clinical practice.

Diabetic retinopathy assessed by dynamic light scattering and corneal autofluorescence.

Autofluorescence of the cornea within specific wavelength region and dynamic light scattering measurements of ocular tissue have both been used for early-stage detection of the presence of diabetic retinopathy. In the present study, autofluorescence of the cornea and dynamic light scattering in the cornea and vitreous have been measured and compared using two innovative research instruments in twenty-two insulin-dependent diabetes mellitus patients, for the diagnosis of diabetic retinopathy. Corneal autofluorescence mean values in each diabetic retinopathy grade significantly correlated with dynamic light scattering measurements in the vitreous, and in the cornea, thus confirming that changes in the natural fluorescence is strictly correlated with molecular changes of ocular tissues. © 1998 Society of Photo-Optical Instrumentation Engineers.

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.

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.

Vitreous laser absorption following fluorescein angiography in diabetic patients.

BACKGROUND: Sodium fluorescein staining of the vitreous following fluorescein angiography may interact with laser photocoagulation. METHODS: We evaluated the laser absorption by fluorescein in the vitreous when photocoagulation is performed following fluorescein angiography in 15 eyes of nine diabetic patients. Axial fluorescein concentration in the vitreous was measured by a scanning vitreal fluorophotometer. The amount of light absorbed by the fluorescein within the vitreous was calculated according to the Lambert-Beer law. RESULTS: The mean fluorescein concentration ranged from 2.93 ng cm-3 to 105.16 ng cm-3 at 1 h after injection of fluorescein and from 8.03 to 188.56 ng cm-3 after 4 h. Maximum laser absorption at 488 nm ranged from 6.79% (after 1 h) to 14.53% (after 4 h); at 514.5 nm it ranged from 0.96% to 2.14%; at 532 nm it ranged from 0.03% to 0.07%. At lambda > 550 nm, laser absorption was found to be negligible. CONCLUSIONS: In order to optimize the effect of photocoagulation, especially during long photocoagulation sessions, argon blue laser (488 nm) should be avoided following fluorescein angiography. Argon green laser (514.5 nm) should be used within 1 h after fluorescein injection. Frequency-doubled Nd:YAG laser (532 nm), krypton laser (647 nm) or semiconductor diode laser (810 nm) may be used at any time.

Corneal autofluorescence in diabetic and normal eyes.

Corneal autofluorescence has been lately studied as a predictor of retinopathy severity in diabetic patients. We measured corneal autofluorescence in 138 eyes of 69 diabetic patients and 64 eyes of 32 healthy controls. Diabetic patients were subdivided by the severity of retinopathy according to the Modified Airlie House Classification (stage 1: no or minimal retinopathy; stage 2: minimal background retinopathy; stage 3: background retinopathy; stage 4: (pre-) proliferative retinopathy. The fluorescence peak value and the area underlying the corneal autofluorescence curve were measured with a scanning fluorophotometer (Fluorotron Master, Coherent Radiation Palo Alto CA) Healthy controls' values of corneal autofluorescence (peak value: mean 11.03 +/- 3.77 ng. equivalent fluorescein/ml; area: mean 163.85 +/- 61.40 scan-point x ng. equivalent fluorescein/ml) resulted similar (peak value: p = 0.83; area: p = 0.61) to those of diabetic patients without retinopathy (peak value: mean 11.2 +/- 3.4 ng.eq/ml; area: 170.07 +/- 57.23 scan-pnt.ng.eq/ml). A statistically significant difference was found between diabetic patients without retinopathy and patients with stage 2, 3, 4 retinopathy. No statistically significant difference was found both for the peak value (p = 0.50) and for the area (p = 0.61) between stage 3 and stage 4 retinopathy. The sensitivity and specificity of corneal autofluorescence as a screening test for diabetic retinopathy were 82% and 62% for the peak value, 87% and 60% for the area; the positive predictive value for the presence of diabetic retinopathy was 65% for the peak and 63% for the area.

Refractive hazards of intraoperative retinal photocoagulation.

The refractions of a laser beam during transpupillary and endo-ocular intraoperative retinal photocoagulation as it passes through vitreous substitutes, with varying refractive indexes, were investigated with the use of mathematical algorithms and ray tracers. The following modifications and potential hazards were noted: 1) deviations of the laser beam (unlikely, however, to cause target error when surgery is properly conducted); 2) variations in laser-beam diameter and power density (up to 60%), easily leading to retinal underexposure or overexposure; and 3) asymmetry in beam distribution at the target site, leading to non-uniform retinal photocoagulation. Proper surgical techniques must be adopted before and during intraoperative photocoagulation to minimize retinal hazards. The relatively short distance between the endolaser probe and the retina means that fewer undesirable refractions occur with endo-ocular photocoagulation than with transpupillary indirect intraoperative photocoagulation.

Interactions between light and vitreous fluid substitutes.

To determine the interactions between light and vitreous fluid substitutes, we studied the absorption and fluorescence properties of the following fluids that are commonly used in vitreoretinal surgery: Ringer's solution, balanced salt citrate-buffered solution, balanced salt bicarbonate-buffered solution, hydroxypropyl methylcellulose ophthalmic solution, hyaluronate sodium, perfluorocarbons, silicone oil, and fluorosilicone oil. The absorption spectra for all the fluids peaked in the UV-C (reference range <280 nm) and UV-B (reference range from 315 to 280 nm) regions of the spectrum, with little or no absorption in the visible region of the spectrum (from 400 to 700 nm). Emission of almost all of the fluids occurred mainly in the 300- to 360-nm region, with fairly low-quantum efficiency. The limited light absorption properties of the fluids calls for caution during transpupillary and intraocular laser photocoagulation to avoid excessive retinal damage, mainly when the laser power is increased during treatment. Transmission of incoherent light (environmental and ophthalmic artificial light) through highly transparent vitreous fluid substitutes may lead in time to dangerous light exposure, particularly in aphakic eyes. The emitted fluorescence in these fluids generates a minimal risk of long-term damage.

Study of laser damage to injection-molded diffractive intraocular lenses.

A study of the probability of damage induced in two implant quality injection-molded diffractive intraocular lenses by a commercial Q-switched Nd:YAG laser is presented. The damage thresholds of the diffractive intraocular lenses (0.10 mJ at 10% probability, 0.24 mJ at 50% probability, and 0.90 mJ at 99% probability) did not differ significantly from those of other poly(methyl methacrylate) lenses manufactured with the same technology. These low values (compared to those of other lens types) suggest that care should be taken during laser capsulotomy in eyes implanted with refractive lenses, with special attention given to power losses at the target site due to diffraction effects.

Age-related changes in the fluorescence of melanin and lipofuscin granules of the retinal pigment epithelium: a time-resolved fluorescence spectroscopy study.

The photophysical properties of purified populations of melanin and lipofuscin granules from human retinal pigment epithelium, and their changes with donor age, have been investigated using high-sensitivity time-resolved fluorescence spectroscopy techniques with picosecond gating capabilities. The overall fluorescence intensity of both melanin and lipofuscin granules clearly increased with increasing donor age, the increase being most marked for melanin. In all granule populations the fluorescence decays were multiexponential with subnanosecond and nanosecond decay components. The resultant time-integrated and time-gated spectra also exhibited marked age-variations for each type of granule. Young melanin showed spectral patterns similar to those of bovine melanin, while a yellow-orange fluorescence band appeared in melanin samples from older age groups. Lipofuscin granules exhibited a blue, a yellow and an orange band whose relative amounts were age-related. The results demonstrate the potential of time-resolved techniques for discriminating fluorophores in vitro and in situ, and have confirmed results previously obtained using extraction techniques. Furthermore, the ability of this technique to identify and quantify individual fluorophores within granules may provide an important insight into the origin and development of lipofuscin within the retinal pigment epithelium and ultimately into the mechanisms of age-related retinal diseases.

Comprehensive study of damage to intraocular lenses by single and multiple nanosecond neodymium: YAG laser pulses.

The results of an extensive study of the probability of damage induced in implant quality intraocular lenses (IOLs) by a commercially available nanosecond Nd:YAG photodisruptor are presented. These results were compared with the morphology of damage seen on scanning electron microscopy. The differences in the shape of the damage probability curves derived for the different lens groups demonstrate that a single threshold value does not characterize the threshold for IOL damage. We suggest that a standard method using a clinical photodisruptor be used by all workers to derive these curves for all IOLs in clinical use. The clinical relevance of our findings is emphasized.

Age-related changes in the morphology, absorption and fluorescence of melanosomes and lipofuscin granules of the retinal pigment epithelium.

The morphological and spectral characteristics of purified populations of melanosomes and lipofuscin granules from the human retinal pigment epithelium (RPE) were studied with respect to donor age. All melanosome and lipofuscin fractions exhibited the typical ultrastructural appearance associated with these granules. Absorption profiles of both melanin and lipofuscin granules demonstrated an increased optical density of the granules with increasing age. The former was associated with an overall increase of melanin within the granules. Melanosomes were weakly fluorescent; emission in the blue decreased with increasing age while emission in the red increased. The fluorescent intensity of lipofuscin granules increased with age. These results provide support for the concept that melanogenesis is occurring within the human RPE throughout life and that pigment granules within the RPE undergo age-related modifications during life.

Time-gated fluorescence spectroscopy of the tumor localizing fraction of HpD in the presence of cationic surfactant.

Time-gated fluorescence spectroscopy was performed on the tumor localizing fraction (TLF) of HpD in buffer at different concentrations of cationic surfactant. This technique obtains emission spectra with programmable delay relative to the excitation pulse. According to the measured fluorescence decay-time constants (approximately 0.7, approximately 3 and approximately 15 ns) three gates were considered, delayed by 0, 5 and 18 ns, respectively, to evaluate the contribution of the emitting molecular species to the spectra. Simultaneous to these measurements, fluorescence decay waveforms and time-integrated spectra were also detected. In buffer and in detergent micelles the fluorescence spectra are given by the superposition of the emission of the different molecular species present in the solution, and no appreciable interaction among the chromophores is observed. On the contrary, in the pre-micellar range of the surfactant, evidence for the existence of an energy transfer mechanism was found. This effect seems to be related to the configurational state of the TLF polymeric chains and depends on the relative TLF/surfactant concentration.

Ocular fluorometry: principles, fluorophores, instrumentation, and clinical applications.

Ocular fluorometry is rapidly evolving as a versatile technique for research and diagnosis in ophthalmology. The main reasons for this increasing success are 1) the ideal characteristics of the eye as an optical device for excitation of tissue fluorescence and for the detection of the fluorescent emission; 2) the development of novel fluorometric techniques, including differential and time-resolved fluorescence spectroscopy; and 3) the increasing use of coupling geometries with high-resolution and high spatial selectivity. Both endogenous and exogenous fluorophores are of interest to ocular fluorometry. The most significant among endogenous fluorophores are the fluorescing pigments of the lens and of the retinal pigment epithelium (RPE). The nature, topography, and fluorescence properties of such pigments depend on age and pathology and on the level of light exposure. Exogenous fluorophores of interest are both intentionally induced and unintentionally accumulated drugs (some of which are phototoxic). Laser-based fluorometric techniques play a leading role in ocular fluorometry. The peculiar properties of the laser for the excitation of fluorescence make this source a favorite candidate for ocular fluorometry both in vitro and in vivo.

Intraocular lens damage from Nd:YAG laser pulses focused in the vitreous. Part I: Q-switched lasers.

The damage resulting from laser-induced optical breakdown and plasma formation on or near an intraocular lens (IOL) has been well described, but there is another form of damage, not associated with optical breakdown, that can harm an IOL. The parameters that lead to this damage were investigated. Polymethylmethacrylate IOLs were irradiated by a Q-switched Nd:YAG laser beam focused so that plasmas were formed well clear of the IOL. Damage was induced in the volume of plastic through which the laser beam passed and was cumulative, not being visible until several laser pulses had passed through the lens. The number of pulses required to produce damage varied inversely with the distance between the IOL and the site of plasma formation. This damage is most likely to occur clinically when vitreous structures are targeted in the pseudophakic eye.

Shielding properties of laser-induced plasmas in ocular media irradiated by single Nd:YAG pulses of different durations.

Shielding of laser pulses by plasmas generated in ocular media has been experimentally investigated using single Nd:YAG laser pulses of different durations, ranging from several nanoseconds (ns) to a few tens of picoseconds (ps), both in saline solution and extracted calf vitreous. At the respective threshold radiant exposures for breakdown in saline (259 mJ/cm2 for 7 ns, 20.5 mJ/cm2 for 220 ps and 4.7 mJ/cm2 for 30 ps pulses) the single pulse energy transmission is found to be about 74% for 7 ns, 55% for 220 ps and 50% for 30 ps, thus showing that shielding of laser-induced plasmas is more effective for shorter pulses than for longer pulses. Moreover, the decrease in transmission with increasing radiant exposure is faster in the picosecond case than in the nanosecond case. These results show that direct irradiation of the retina is, to some extent, present especially near threshold. In the case of picosecond pulses, a comparison with published results indicates that shielding between subsequent pulses in a train is likely to occur only to a very limited extent.

Nd:YAG laser photodisruption: an experimental investigation on shielding and multiple plasma formation.

In an ocular model plasmas were induced by Q-switched, commercially available Nd:YAG lasers that operated in low order or fundamental mode. Plasma shielding was measured and analysed in relation to energy input, number of plasmas and plasma evolution. Near breakdown threshold, attenuation or shielding of the laser beam resulting from absorption and scattering by the plasma is low and is characterized by high variability from shot to shot. More importantly, shielding does not increase when multiple plasmas are formed, possibly due to a competitive mechanism between these plasmas. Further evidence of this mechanism was obtained from measurement performed using a streak camera. The effectiveness of shielding is scarcely changed by an increase in the cone angle of the incident radiation. In the light of these experiments and other published data, safety guidelines for the clinical use of the Nd:YAG laser have been formulated.

A new form of damage to PMMA intraocular lenses by Nd:YAG laser photodisruptors.

The damage that Nd:YAG laser radiation can cause to plastic (PMMA) IOLs during pseudophakic capsulotomy is well documented in the literature. This damage is the result of direct plasma action on the lens material. We report here another form of damage to PMMA IOLs which is more subtle and does not result from plasma action in the plastic. Even when the irradiance within the IOL is well below the threshold for optical breakdown, the PMMA can be damaged if irradiated by many laser pulses. This subthreshold damage is therefore cumulative. Its importance, likelihood of occurrence and ways to minimise this are considered.