Justin J. Zohner

Infrared laser damage thresholds for skin at wavelengths from 0.810 to 1.54 microns for femtosecond to microsecond pulse durations

In this paper we report on our combined measurements of the visible lesion thresholds for porcine skin for wavelengths in the infrared from 810 nm at 44 fs to 1318 nm at pulse durations of 50 ns and 350&mgr;s to 1540 nm including pulse durations of 31 ns and 600 &mgr;s. We also measure thresholds for various spot sizes from less than 1 mm to 5 mm in diameter. All three wavelengths and five pulse durations are used extensively in research and the military. We compare these minimum visible lesion thresholds with ANSI standards set for maximum permissible exposures in the infrared wavelengths. We have measured non-linear effects at the laser-tissue interface for pulse durations below 1&mgr;s and determined that damage at these short pulse durations are usually not thermal effects. Damage at the skin surface may include acoustical effects, laser ablation and/or low-density plasma effects, depending on the wavelength and pulse duration. Also the damage effects may be short-lived and disappear within a few days or may last for much longer time periods including permanent discolorations. For femtosecond pulses at 810 nm, damage was almost instant and at 1 hour had an ED50 of 8.2 mJ of pulse energy. After 24 hours, most of the lesions disappeared and the ED50 increased by almost a factor of 3 to 21.3 mJ. There was a similar trend for the 1.318 &mgr; laser for spot sizes of 2 mm and 5 mm where the ED50 was larger after 24 hours. However, for the 1.54 &mgr; laser with a spot size of 5 mm, the ED50 actually decreased by a small amount; from 6.3 Jcm-2 to 6.1 Jcm-2 after 24 hours. Thresholds also decreased for the 1314 nm laser at 350 &mgr;s for spot sizes of 0.7 mm and 1.3 mm diameter after 24 hours. Different results were obtained for the 1540 nm laser at 600 &mgr;s pulse durations where the ED50 decreased for spot sizes 1 mm and below, but increased slightly for the 5 mm diameter spot size from 6.4 Jcm-2 to 7.4 Jcm-2

Dynamic bidirectional reflectance distribution functions: Measurement and representation

With high-energy lasers, not only the direct laser beam can pose significant eye and skin hazards, but also light reflecting off material illuminated by the beam. Proper hazard analysis for a material irradiated by a laser relies upon the reflecting properties of the material surface, as these properties determine the magnitude and direction of the reflected laser energy commonly characterized by the bidirectional reflectance distribution function (BRDF). However, a high-energy laser heating and possibly melting a material can change the reflecting properties of that material, so these changes must be included in the hazard analysis. Traditional methods for measuring the BRDFs of materials are not practical for measurement of materials with rapidly-changing surface properties. However, BRDF measurement by imagery of a witness screen allows for practical measurements of the dynamically-changing BRDFs of materials under high-energy laser irradiation. Using this technique, the dynamic BRDFs of stainless stee...

Visible Lesion Thresholds with Pulse Duration, Spot Size Dependency, and Model Predictions for 1.54-mum Near-Infrared Laser Pulses Penetrating Porcine Skin

Er:glass lasers have been in operation with both long pulses (hundreds of microseconds) and Q-switched pulses (50 to 100 ns) for more than 35 yr. The ocular hazards of this laser were reported early, and it was determined that damage to the eye from the 1.54-microm wavelength occurred mainly in the cornea where light from this wavelength is highly absorbed. Research on skin hazards has been reported only in the past few years because of limited pulse energies from these lasers. Currently, however, with pulse energies in the hundreds of joules, these lasers may be hazardous to the skin in addition to being eye hazards. We report our minimum visible lesion (MVL) threshold measurements for two different pulse durations and three different spot sizes for the 1.54-microm wavelength using porcine skin as an in vivo model. We also compare our measurements to results from our model, based on the heat transfer equation and the rate process equation. Our MVL-ED50 thresholds for the long pulse (600 micros) at 24 h postexposure were measured to be 20, 8.1, and 7.4 J cm(-2) for spot diameters of 0.7, 1.0, and 5 mm, respectively. Q-switched laser pulses of 31 ns had lower ED50 (estimated dose for a 50% probability of laser-induced damage) thresholds of 6.1 J cm(-2) for a 5-mm-diam, top-hat spatial profile laser pulse.

Visible lesion thresholds and model predictions for Q-switched 1318-nm and 1540-nm laser exposures to porcine skin

Skin damage thresholds were measured and compared with theoretical predictions using a skin thermal model for near-IR laser pulses at 1318 nm and 1540 nm. For the 1318-nm data, a Q-switched, 50-ns pulse with a spot size of 5 mm was applied to porcine skin and the damage thresholds were determined at 1 hour and 24 hours postexposure using Probit analysis. The same analysis was conducted for a Q-switched, 30-ns pulse at 1540 nm with a spot size of 5 mm. The Yucatan mini-pig was used as the skin model for human skin due to its similarity to pigmented human skin. The ED50 for these skin exposures at 24 hours postexposure was 10.5 J/cm2 for the 1318-nm exposures, and 6.1 J/cm2 for the 1540-nm exposures. These results were compared to thermal model predictions. We show that the thermal model fails to account for the ED50 values observed. A brief discussion of the possible causes of this discrepancy is presented. These thresholds are also compared with previously published skin minimum visible lesion (MVL) thresholds and with the ANSI Standards MPE for 1318-nm lasers at 50 ns and 1540-nm lasers at 30 ns.

An Alternative Method of Evaluating 1540NM Exposure Laser Damage using an Optical Tissue Phantom

An optical phantom was designed to physically and optically resemble human tissue, in an effort to provide an alternative for detecting visual damage resulting from inadvertent exposure to infrared lasers. The phantom was exposed to a 1540-nm, Erbium:Glass, Q-switched laser with a beam diameter of 5 mm for 30 ns at varying power levels. Various materials were tested for use in the phantom; including agar, ballistic media, and silicone rubber. The samples were analyzed for damage lesions immediately after exposure and the Minimum Visible Lesion - Estimated Dose 50% (MVL-ED50 ) thresholds were determined from the data. In addition, any visible damage was evaluated for similarity to human tissue damage to determine if the phantom tissue would be a suitable substitute for in vivo exposures.

Incorporation of refractive index gradients in the solution of the radiative transport equation

We have proposed a modified Monte Carlo approach to the solution of the radiative transport equation which has the unique feature of incorporating refractive index gradients within a multi-layer biological tissue model. In the approach, photon trajectories are computed using a solution of the Eikonal equation (ray-tracing methods) rather than linear trajectories. The method can be applied to the specific problem of incorporating thermal lensing and other non-linear effects in turbid media (biological tissues) by coupling the radiative transport solution into heat-transfer and damage models. In turn, the method can be applied in the establishment of laser exposure limits for tissue-penetrating wavelengths, as well as a number of additional applications in imaging, spectroscopy and vision science.

Comparative analysis of histological results of visible lesion thresholds for thermal and LIB induced skin damage at 1.3 μm and 1.5 μm

An assessment of skin damage caused by near-IR laser exposures is reported. The damage from two distinct laser-tissue temporal regimes is compared at two wavelengths (1.3 &mgr;m and 1.5 &mgr;m). Skin damage caused by thermal effects from single laser pulses is compared to damage caused by LIB (laser induced breakdown) using histological examinations. Modeling applications are explored to determine crossover points between thermal and photomechanical damage thresholds.

Simulations of high energy laser reflections

Reflections of high energy lasers from targets present safety challenges, as the resulting hazard distances can be significant. We have developed two different but complementary simulations to determine hazard zones resulting from high energy laser reflections. One, the High Energy Laser Collateral Assessment Tool (HELCAT), is a high-fidelity successor to the Laser Range Safety Tool (LRST), which had been developed specifically for distant targets. HELCAT has an improved user interface and uses first-principle physics calculations to obtain reflected irradiances at observer locations for each time step in a simulation. The time histories of the irradiances are used to determine the hazard zone. It supports a variety of simple and complex targets, trajectories, reflecting properties of materials, and observer locations. The other, termed the Specular Methodology, considers only specular reflections to derive analytical expressions for the irradiance and exposure time at observer locations. The hazard zone ...

An approach to high energy laser safety in open environments

High energy lasers present new challenges for safety evaluation. Not only is the direct beam hazardous, but the reflected beam may also cause injury. Two important issues with high energy laser safety have been addressed in simulations used to model reflected-beam hazards: the reflecting properties of materials and the time history of exposure. The reflecting properties are quantified by the Bi-directional Reflectance Distribution Function (BRDF), and a technique has been developed to extract parameters for BRDF models, primarily the Maxwell-Beard model, from measurement data and to calculate the reflected irradiance at an observer. In addition, the reflecting properties of materials often change upon heating, so a procedure using a camera and screen is being used to measure the time-varying BRDF of materials, which in turn is modeled empirically with spherical harmonics. Because the use of high energy lasers is often dynamic in nature, involving a moving laser, target, or observer, the time-history of exposure to the observer is calculated in the simulations. A sliding-window algorithm compares the time-history of exposure to maximum permissible exposure limits for a range of exposure times in order to determine if a hazardous condition exists for the observer.High energy lasers present new challenges for safety evaluation. Not only is the direct beam hazardous, but the reflected beam may also cause injury. Two important issues with high energy laser safety have been addressed in simulations used to model reflected-beam hazards: the reflecting properties of materials and the time history of exposure. The reflecting properties are quantified by the Bi-directional Reflectance Distribution Function (BRDF), and a technique has been developed to extract parameters for BRDF models, primarily the Maxwell-Beard model, from measurement data and to calculate the reflected irradiance at an observer. In addition, the reflecting properties of materials often change upon heating, so a procedure using a camera and screen is being used to measure the time-varying BRDF of materials, which in turn is modeled empirically with spherical harmonics. Because the use of high energy lasers is often dynamic in nature, involving a moving laser, target, or observer, the time-history of ex...

Injury thresholds for topical-cream-coated skin of hairless guinea pigs (cavia porcellus) in the near-infrared region

The reflectance and absorption of the skin plays a vital role in determining how much radiation will be absorbed by human tissue. Any substance covering the skin would change the way radiation is reflected and absorbed and thus the extent of thermal injury. Hairless guinea pigs (cavia porcellus) in vivo were used to evaluate how the minimum visible lesion threshold for single-pulse laser exposure is changed with a topical agent applied to the skin. The ED50 for visible lesions due to an Er: glass laser at 1540-nm with a pulse width of 50-ns was determined, and the results were compared with model predictions using a skin thermal model. The ED50 is compared with the damage threshold of skin coated with a highly absorbing topical cream at 1540 nm to determine its effect on damage pathology and threshold. The ED50 for the guinea pig was then compared to similar studies using Yucatan minipigs and Yorkshire pigs at 1540-nm and nanosecond pulse duration.1,2 The damage threshold at 24-hours of a Yorkshire pig for a 2.5-3.5-mm diameter beam for 100 ns was 3.2 Jcm-2; very similar to our ED50 of 3.00 Jcm-2 for the hairless guinea pigs.