Thresholds for microcavitation of bovine and porcine melanosomes were previously reported, using single nanosecond (ns) laser pulses in the visible (532 nm) and the near-infrared (NIR) from 1000 to 1319 nm. Here, we report average radiant exposure thresholds for bovine melanosome microcavitation at additional NIR wavelengths up to 1540 nm, which range from ∼0.159 J/cm2 at 800 nm to 4.5 J/cm2 at 1540 nm. Melanosome absorption coefficients were also estimated, and decreased with increasing wavelength. These values were compared to retinal pigment epithelium coefficients, and to water absorption, over the same wavelength range. Corneal total intraocular energy retinal damage threshold values were estimated and compared to the previous (2007) and recently changed (2014) maximum permissible exposure (MPE) safe levels. Results provide additional data that support the recent changes to the MPE levels, as well as the first microcavitation data at 1540 nm, a wavelength for which melanosome microcavitation may be an ns-pulse skin damage mechanism.
There is an increasing use of high-power fiber lasers in manufacturing and telecommunications industries operating in the infrared spectrum between 1000 and 2000 nm, which are advertised to provide as much as 10 kW continuous output power at 1070 nm. Safety standards have traditionally been based on experimental and modeling investigations with scant data available for these wavelengths. A series of studies using 1070-nm infrared lasers to determine the minimum visible lesion damage thresholds in skin using the Yucatan miniature pig (Sus scrofa domestica) for a range of beam diameters (0.6, 1.1, 1.9, 2.4, 4.7, and 9.5 cm) and a range of exposure durations (10 ms to 10 s) is presented. Experimental peak temperatures associated with each damage threshold were measured using thermal imaging. Peak temperatures at damage threshold for the 10-s exposures were ∼10°C lower than those at shorter exposures. The lowest and highest experimental minimum visible lesion damage thresholds were found to have peak radiant exposures of 19 and 432 J/cm 2 for the beam diameter-exposure duration pairs of 2.4 cm, 25 ms and 0.6 cm, 10 s, respectively. Thresholds for beam diameters >2.5 cm had a weak to no effect on threshold radiant exposure levels for exposure times ≤0.25 s , but may have a larger effect on thresholds for exposures ≥10 s .
Thresholds for microcavitation of bovine and porcine melanosomes were determined using nanosecond laser pulses in the near-infrared (1000 to 1319 nm) wavelength regime. Isolated melanosomes were irradiated by single pulses (10 or 50 ns) using a Q-switched Spectra Physics Nd:YAG laser coupled with an optical parametric oscillator (1000 to 1200 nm) or a continuum laser at 1319 nm. Time-resolved nanosecond strobe photography after the arrival of the irradiation beam allowed imaging of microcavitation events. Average fluence thresholds for microcavitation increased nonlinearly with increasing wavelength from ∼0.5 J/cm 2 at 1000 nm to 2.6 J/cm 2 at 1319 nm. Fluence thresholds were also measured for 10-ns pulses at 532 nm and found to be comparable to visible nanosecond pulse values published in previous reports. Calculated melanosome absorption coefficients decreased from 925 cm −1 at 1000 nm to 176 cm −1 at 1319 nm. This trend was found to be comparable to the decrease in retinal pigmented epithelial layer absorption coefficients reported over the same wavelength region. Estimated corneal total intraocular energy retinal damage threshold values were determined in order to compare to current and proposed maximum permissible exposure (MPE) safe levels. Results from this study support recently proposed changes to the MPE levels.
Thresholds for microcavitation of isolated bovine and porcine melanosomes were determined using single nanosecond (ns) laser pulses in the NIR (1000 – 1319 nm) wavelength regime. Average fluence thresholds for microcavitation increased non-linearly with increasing wavelength. Average fluence thresholds were also measured for 10-ns pulses at 532 nm, and found to be comparable to visible ns pulse values published in previous reports. Fluence thresholds were used to calculate melanosome absorption coefficients, which decreased with increasing wavelength. This trend was found to be comparable to the decrease in retinal pigmented epithelial (RPE) layer absorption coefficients reported over the same wavelength region. Estimated corneal total intraocular energy (TIE) values were determined and compared to the current and proposed maximum permissible exposure (MPE) safe exposure levels. Results from this study support the proposed changes to the MPE levels.
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.
Several international organizations establish minimum safety standards to ensure that workers and the general
population are protected against adverse health effects associated with electromagnetic radiation. Suitable standards are
typically defined using published experimental data. To date, few experimental studies have been conducted at Terahertz
(THz) frequencies, and as a result, current THz standards have been defined using extrapolated estimates from
neighboring spectral regions. In this study, we used computational modeling and experimental approaches to determine
tissue-damage thresholds at THz frequencies. For the computational modeling efforts, we used the Arrhenius damage
integral to predict damage-thresholds. We determined thresholds experimentally for both long (minutes) and short
(seconds) THz exposures. For the long exposure studies, we used an in-house molecular gas THz laser (υ= 1.89 THz,
189.92 mW/cm2, 10 minutes) and excised porcine skin. For the short exposure studies, we used the Free Electron Laser
(FEL) at Jefferson Laboratory (υ= 0.1-1.0 THz, 2.0-14.0 mW/cm2, 2 seconds) and wet chamois cloths. Thresholds were
determined using conventional damage score determination and probit analysis techniques, and tissue temperatures were
measured using infrared thermographic techniques. We found that the FEL was ideal for tissue damage studies, while
our in-house THz source was not suitable to determine tissue damage thresholds. Using experimental data, the tissue
damage threshold (ED50) was determined to be 7.16 W/cm2. This value was in well agreement with that predicted using
our computational models. We hope that knowledge of tissue-damage thresholds at THz frequencies helps to ensure the
safe use of THz radiation.
An adaptive optics imaging system was used to qualitatively observe the types of aberrations induced by an infrared laser
in a rhesus eye. Thermal lensing was induced with an infrared laser radiation wavelength of 1150-nm. The adaptive
optics system tracked the temporal response of the aberrations at a frequency of 30 Hz for continuous-wave exposures.
Results are compared against thermal lensing aberrations induced in an artificial eye.
In recent years, numerous security, military, and medical applications have been developed which use Terahertz
(THz) radiation. These developments have heightened concerns in regards to the potential health risks that are
associated with this type of radiation. To determine the cellular and molecular effects caused by THz radiation, we
exposed several human cell lines to high-power THz radiation, and then we determined death thresholds and gene
expression profiles. Necrotic and apoptotic death thresholds were determined for Jurkat cells using an optically-pumped
molecular gas THz source (υ = 2.52 THz, H = 227 mW/cm2), MTT viability assays, and flow cytometric techniques. In
addition, we used confocal microscopic techniques to demarcate lethal spatial regions in a monolayer of dermal
fibroblasts exposed to THz radiation. Then, to determine if cells exhibit a THz-specific gene expression signature, we
exposed dermal fibroblasts to THz radiation and analyzed their transcriptional response using microarray gene chips. We
found that 60% of the Jurkat cells survived the 30-minute THz exposure, whereas only 20% survived the 40-minute
exposure. The flow data confirmed these results and provided evidence that THz-induced cell death was mediated using
both nectrotic and apoptotic processes. The preliminary microscopy studies provided convincing evidence warranting
future efforts using these techniques. Lastly, we found that dermal fibroblasts up-regulated several genes when exposed
to THz radiation. Overall, these results provide evidence for the cellular and molecular effects associated with THz
radiation, and we speculate that the identified up-regulated genes may serve as excellent candidate biomarkers for THz
exposures.
A confocal imaging system mounted to a micrometer stage was used to image the thermal lens induced into a water filled
Cain-cell artificial eye. A dual-beam pump-probe geometry was used to quantify the 633-nm visible wavelength probe
beam's transient response when exposed to the near-infrared pump-beam source. The infrared laser radiation
wavelengths tested were 1110, 1130, 1150 and 1318 nm for 1-s exposures to 450-mW of power. Analysis of video data
revealed the amount of refractive shift, induced by the thermal lens, as a function of time. Data demonstrate how the
formation and dissipation of the thermal lens follow a logarithmic excitation and exponential decay in time respectively.
Confocal imaging showed that thermal lensing was strongest for the 1150-nm wavelength followed by 1130, 1318 and
1110-nm.
Near-infrared (NIR) laser exposures to the retina are affected by intraocular absorption, chromatic aberration and retinal
absorption. We present the latest results of retinal exposure to wavelengths between 1.0 to 1.319 micrometers and show
how the trends for long-pulse exposure are dramatically affected by intraocular absorption in the anterior portion of the
eye.
Near threshold retinal lesions were created in the eyes of non-human primate (NHP) subjects in the near infrared (NIR)
wavelength range of 1100 to 1319 nm, with 80 to 100 ms laser exposures. Two new in vivo imagining techniques,
Adaptive Optic enhanced-Spectral Domain Optical Coherence Tomography (AO-SDOCT) and Adaptive Optic enhanced
confocal Scanning Laser Ophthalmoscope imagery (AOcSLO) were utilized to pinpoint areas of chronic damage within
the retinal layers resulting from laser exposure. Advantages and limitations of each technology with regard to the study
of laser retinal tissue interaction are highlighted.
Ocular damage threshold data remain sparse in the continuous wave (CW), near-infrared (NIR) radiation region save for the 1300-nm area that has been investigated in the past several decades. The 1300-nm ocular damage data have yielded unusual characteristics where CW retinal damage was observed in rabbit models, but never in nonhuman primate models. This paper reviews the existing 1300-nm ocular damage threshold data in terms of the fundamental criteria of an action spectrum to assist in explaining laser-tissue effects from near-infrared radiation in the eye. Reviewing the action spectrum criteria and existing NIR retinal lesion data lend evidence toward the significant presence of thermal lensing in ocular media affecting damage, a relatively unexplored mechanism of laser-tissue interaction.
A series of experiments were conducted in vivo on porcine skin to determine the ED50 damage thresholds for 1214 nm continuous wave laser irradiation. These results provide new information for refinement of Maximum Permissible Exposure (MPE). The study employed exposure durations of 1 sec, 3 sec, and 10 seconds with nominal spot diameters of 6 mm, 8 mm and 10 mm and as a function of laser power. The effect of each irradiation was evaluated acutely, one hour after exposure, and 24 hours post exposure. Probit analysis was conducted to estimate the dose for 50% probability of laser-induced damage (ED50); Damage was defined as persistent redness at the site of irradiation for the pig skin after 24 hours. The results indicated that Maximum Permissible Exposure (MPE) limits should be lowered for the laser beam diameters larger than 6 mm.
Computational physics methods are described for the evaluation of the role of propagation with regard to laser
damage to tissues. Regions of the optical spectrum, where linear and non-linear propagation affects irradiance
distributions within tissues, are examined. Effects described include group-velocity dispersion, aberrations,
thermal lensing, and self-focusing. Implications to exposure limits within safety standards, incorporating these
irradiance-altering effects, are addressed such that inherent trends agree over wide temporal and spectral ranges,
with damage thresholds measured experimentally. We present current regions of interest to the standard-setting
community and recent works showing how propagation effects may be playing a key role in assessing damage
thresholds.
A tissue phantom of water with an absorbing dye, Allura Red, was used to observe the effects of thermal lensing in a thick sample exposed to a CW 532 nm Verdi laser. A collimated beam was sent through a sample 2.9 cm thick. Results from the collimated beam revealed qualitative information about thermal lensing in a liquid media. The studies presented here argue the relevance of incorporating oscillating factors such as convectional flow into higher order thermal lensing models in a fluid such as water.
The phenomenon of thermal lensing was investigated in water using a Z-scan method and corresponding first-order mathematical models. Data from first-order thermal lensing models and ABCD beam propagation methods were used to simulate the non-linear absorption of water held in a thin sample cuvette for a Z-scan optical set up of CW cases at 1313 nm. The single beam closed aperture Z-scan was then used to determine the non-linear absorption at 1313 nm for water in 10 mm and 2 mm cuvettes at 48.00, 16.80, 9.80 and 2.83 mW then compared to the first-order model data. The results from the closed aperture Z-scan were also used to back calculate the spot size in the far field for comparison to the model's prediction of the beam's temporal response. Experimental Z-scan data were found not to correlate strongly with our first-order model suggesting the need for higher order models to successfully predict spot size in absorbing media inside the Rayleigh range.
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