Gavin D. Buffington

Infrared skin damage thresholds from 1940-nm continuous-wave laser exposures

Abstract. A series of experiments were conducted in vivo using Yucatan miniature pigs (Sus scrofa domestica) to determine thermal damage thresholds to the skin from 1319-nm continuous-wave Nd:YAG laser irradiation. Experiments employed exposure durations of 0.25, 1.0, 2.5, and 10 s and beam diameters of ∼0.6 and 1 cm. Thermal imagery data provided a time-dependent surface temperature response from the laser. A damage endpoint of fifty percent probability of a minimally visible effect was used to determine threshold for damage at 1 and 24 h postexposure. Predicted thermal response and damage thresholds are compared with a numerical model of optical-thermal interaction. Resultant trends with respect to exposure duration and beam diameter are compared with current standardized exposure limits for laser safety. Mathematical modeling agreed well with experimental data, predicting that though laser safety standards are sufficient for exposures <10  s, they may become less safe for very long exposures.

Simultaneous Exposure Using 532 and 860 nm lasers for visible lesion thresholds in the rhesus retina.

The growth of commercially available, simultaneous multi-wavelength laser systems has increased the likelihood of possible ocular hazard. For example, many systems utilize frequency multiplying methods to produce combinations of visible, near-infrared, and ultraviolet wavelengths. Unfortunately, very little data exists to substantiate the current methods for estimating hazards from simultaneous lasing. To properly assess the retinal hazards from these wavelengths, the retinal effects of 10-s laser irradiation from 532 and 860 nm were determined in non-human primates for four different relative dosage combinations of these wavelengths. This pair of wavelengths represents the typical problem of a visible-wavelength laser combined with an in-band, infrared wavelength that is not as well focused at the retina—a situation difficult to address. To add confidence to the experimental results obtained, a theoretical thermodynamic model was developed to predict the minimal damage threshold for simultaneous wavelengths at 1 h post exposure. The new model calculations and the data obtained are compared with results from one currently accepted method of predicting relative exposure limits from multi-wavelength systems. In addition, the current ANSI-Z136-2000 standard was used to compute the combined MPEs for comparison with measured visible lesion thresholds. A total of 12 eyes were exposed using four different ratios of power levels (532/860 power rations) to determine the contribution to the damage levels from each wavelength. The experimental data were analyzed using probit analysis at both 1-h and 24-h post exposure to determine the minimum-visible-lesion (MVL) thresholds at ED50 values, and these thresholds at 24 h varied from 5.6 mW to 17 mW total intraocular power.

LATTICE, TIME-DEPENDENT APPROACH FOR ELECTRON-HYDROGEN SCATTERING

A time-dependent approach for treating electron-hydrogen scattering is reported that utilizes a fully correlated two-electron wavefunction represented on a three-dimensional lattice using the basis-spline collocation method. The lattice, time-dependent approach obviates the need for consideration of the three-body Coulomb boundary conditions, avoids the use of severe approximations such as those of perturbation theory for slow collisions, and provides a relatively dense representation of the one- and two-electron continua. Probabilities for excitation and ionization are computed by projection onto lattice eigenstates of the H atom. Partial cross sections for excitation and ionization are obtained and compared with results of other theoretical methods for the 1S and 3S channels.

Modeling of Surface Thermodynamics and Damage Thresholds in the IR and THz Regime

The Air Force Research Lab has developed a configurable, two-dimensional, thermal model to predict laser-tissue interactions, and to aid in predictive studies for safe exposure limits. The model employs a finite-difference, time-dependent method to solve the two-dimensional cylindrical heat equation (radial and axial) in a biological system construct. Tissues are represented as multi-layer structures, with optical and thermal properties defined for each layer, are homogeneous throughout the layer. Multiple methods for computing the source term for the heat equation have been implemented, including simple linear absorption definitions and full beam propagation through finite-difference methods. The model predicts the occurrence of thermal damage sustained by the tissue, and can also determine damage thresholds for total optical power delivered to the tissue. Currently, the surface boundary conditions incorporate energy loss through free convection, surface radiation, and evaporative cooling. Implementing these boundary conditions is critical for correctly calculating the surface temperature of the tissue, and, therefore, damage thresholds. We present an analysis of the interplay between surface boundary conditions, ambient conditions, and blood perfusion within tissues.

A first-order model of thermal lensing of laser propagation in the eye and implications for laser safety

Many medical studies have investigated the safety threshold of ocular media at various wavelengths. Understanding and determining the damage threshold of laser exposure to the retina is important in order to set safety guidelines. A phenomenon known as thermal lensing where a media’s index of refraction changes as the laser energy is absorbed causes the beam profile and subsequently the position of the focal point of the beam to change as the temperature changes within the media. These changes in ocular media such as vitreous and aqueous humor, the cornea, crystalline lens, and retina all result in a slight change in the beam profile making the prediction of the focal point and laser spot size at the retina difficult to accurately predict. This paper describes how the effects of thermal lensing in the eye can be investigated to aid in determining the damage threshold for pulse durations of a few nanoseconds and continuous wave, CW, laser irradiation delivered to the retina and the relationship of how the damage is related to multiple pulse exposures. We present a first-order modeling effort for the thermal lensing effect in the eye along with initial estimates of impact on damage thresholds as predicted through computational biophysics thermal damage models.Many medical studies have investigated the safety threshold of ocular media at various wavelengths. Understanding and determining the damage threshold of laser exposure to the retina is important in order to set safety guidelines. A phenomenon known as thermal lensing where a media’s index of refraction changes as the laser energy is absorbed causes the beam profile and subsequently the position of the focal point of the beam to change as the temperature changes within the media. These changes in ocular media such as vitreous and aqueous humor, the cornea, crystalline lens, and retina all result in a slight change in the beam profile making the prediction of the focal point and laser spot size at the retina difficult to accurately predict. This paper describes how the effects of thermal lensing in the eye can be investigated to aid in determining the damage threshold for pulse durations of a few nanoseconds and continuous wave, CW, laser irradiation delivered to the retina and the relationship of how the ...

Extension of thermal damage models of the retina to multi-wavelength sources

Recent laser and broadband bioeffects applications have extended the requirements of modeling damage to the retina beyond single-wavelength sources. Also, the creation and validation of safe exposure limits for arbitrary combinations of spectral content is extremely difficult due to the large parameter space available. We present recent updates to computational methods which provide predictions of damage thresholds under conditions of user-specified spectral and temporal content. The modeling results are compared with recent and historical multi-wavelength laser exposure and broadband emitter damage thresholds measured in the laboratory. In all cases, the computational biophysics model is able to predict damage thresholds and trends within the uncertainties of the laboratory measurements.Recent laser and broadband bioeffects applications have extended the requirements of modeling damage to the retina beyond single-wavelength sources. Also, the creation and validation of safe exposure limits for arbitrary combinations of spectral content is extremely difficult due to the large parameter space available. We present recent updates to computational methods which provide predictions of damage thresholds under conditions of user-specified spectral and temporal content. The modeling results are compared with recent and historical multi-wavelength laser exposure and broadband emitter damage thresholds measured in the laboratory. In all cases, the computational biophysics model is able to predict damage thresholds and trends within the uncertainties of the laboratory measurements.

Damage thresholds to the retina from elliptical and array exposure sites using CW lasers

A three-dimensional thermal model is used to investigate the damage threshold for hazard assessments of lasers projecting non-uniform or non-symmetric images on the retina. The two source types considered are an array of sources and an elliptical source. In the array of sources, the spacing and size ratios of the sources are varied to determine their effect on the damage threshold. Additionally, it is determined where the spacing and size of the sources can be approximated by a large uniform source. For the elliptical source, the damage threshold is found as a function of the ratio of minor and major diameters. Short and long exposure times are considered for both the array of sources and the elliptical source.A three-dimensional thermal model is used to investigate the damage threshold for hazard assessments of lasers projecting non-uniform or non-symmetric images on the retina. The two source types considered are an array of sources and an elliptical source. In the array of sources, the spacing and size ratios of the sources are varied to determine their effect on the damage threshold. Additionally, it is determined where the spacing and size of the sources can be approximated by a large uniform source. For the elliptical source, the damage threshold is found as a function of the ratio of minor and major diameters. Short and long exposure times are considered for both the array of sources and the elliptical source.

Retinal injury resulting from simultaneous exposure to radiation from two lasers with different wavelengths

To assess the retinal hazards related to simultaneous exposure from two lasers of separate wavelengths, the retinal effects of 5-second laser irradiation from 532 nm and 647 nm were determined in non-human primates. A total of six eyes were exposed using equal amounts of power to determine the damage levels. The results were combined with those of previous, two-wavelength studies done by our group and compared to damage models developed in our lab. The data were also compared to the calculations resulting from use of the currently accepted method of predicting hazards from simultaneous lasing.

Experimental and theoretical studies of broadband optical thermal damage to the retina

The evaluation of the safety of high-power light sources requires a broad understanding of both thermal and photochemical damage mechanisms in retinal tissue. A comprehensive model which can support complex spectral, temporal and spatial dependency of these effects is essential to evaluation of existing safe exposure limits across a broad parameter space. We present an initial implementation of a thermal damage model along with validating experiments. The model is capable of examining a wide parameter space and is highly extensible to the examination of a variety of damage mechanisms. Also presented is a recent study which examines the effects of a filtered Xenon arc lamp for an exposure duration of ten seconds. This data is examined in relation to the model and a number of historical data points. We also examine exposure limits from the American Council of Government Industrial Hygienists as they apply to these sources.

Laser damage threshold trends for sub-100-fs pulses in the retina

We have previously demonstrated that retinal damage thresholds can vary as a function of ultrashort laser pulse chirp. Computations and in-vivo experiments have both demonstrated such results. Here, we present a study of the computation of damage thresholds as functions of laser wavelength, pulse duration, and chirp. Damage threshold trends are explored and related to current and future laser safety exposure limits.