1.

Introduction

The number of lasers in the operational range of 1000 to 2000 nm continues to increase in medical, industrial, communications, and military applications, while relatively few safety studies for skin damage from continuous-wave (CW) exposures at these wavelengths can be found in the literature. High-power fiber lasers utilize advanced technologies to combine active optical fibers with semiconductor diodes. These active fibers allow for an extremely high-intensity light out of a very small core, making it possible to produce multikilowatt lasers. For these lasers, powers $>10\mathrm{kW}$ are routinely used in manufacturing processes and the telecommunications industry. CW outputs can be obtained with fiber lengths $>100\mathrm{m}$, with small beam divergence and high power conversion efficiencies. Experimental data used in setting the ANSI standards have not kept pace with the rapid advancement of these lasers. Furthermore, the maximum permissible exposure (MPE) recommended through the ANSI Z136.1-2007¹ is for a limiting, or measurement, aperture diameter of 0.35 cm in this wavelength region for exposures to skin with little empirical evidence that the standard recommendations are adequate for larger beam diameters, especially for highly penetrating wavelengths such as 1070 nm. Irradiances can easily exceed $10\mathrm{kW}/\mathrm{c}{\mathrm{m}}^2$ and little or no minimum damage threshold data are available for these high levels. Therefore, this study was initiated to answer critical questions regarding the appropriateness of the standards for MPE in terms of relative penetration depth (wavelength) and for CW lasers (10 ms to 10 s) with beam diameters greater than the ANSI limiting aperture of 0.35 cm in the near-infrared (IR) region at 1070 nm.

To determine if an MPE is sufficiently safe, a series of experiments are conducted to evaluate the damage threshold for a minimum visible lesion (MVL). An MVL is considered to be any subtle, slight, morphological, or coloration, change observed at the exposure site post laser radiation at various time points. The MPE is typically several times less than the MVL, to ensure that exposures at or below the MPE would have no damaging effects.

The Yucatan miniature pig has been established as a model for human skin damage studies due to the morphological and physiological similarities to human skin.² Skin threshold studies were conducted with Yucatan miniature pigs at wavelengths of 1070 and 1940 nm with varying spot sizes and exposure times. Results for the 1940-nm measurements have been reported³ while only portions of the 1070-nm data presented here have appeared previously.⁴ Also, MVL skin thresholds have been reported with these miniature pigs for various spot sizes and exposure times for wavelengths at 1318 and 1540 nm.^5–7

A summary of threshold data obtained from the literature for skin exposure at 1060 nm are listed in Table 1. No published data were found for skin exposure at 1070 nm, which was used in this study, but optical properties of the skin are remarkably similar between 1060 and 1070 nm, justifying direct comparison.^8,9 The time of inspection used to assess damage endpoints is assumed to be 24 h post exposure; however, the exact time of evaluation for many of the studies was left ambiguous in the text and could range from immediate response to 48 h post exposure.

Table 1

Literature summary of skin damage threshold data at the 1060-nm laser wavelength.

There are several chromophores in the skin related to the absorption, ${\mu }_a$, of laser radiation at near-IR wavelengths. These chromophores include melanin, water, fat, hair, and blood. The values for ${\mu }_a$ have been reported by numerous sources and are summarized in Fig. 1.

Fig. 1

Comparison of the absorption coefficient (${\mu }_a$) for various chromophores present in skin.^8,9,16–,20

The experiments reported in this study were designed to supplement and extend the body of knowledge summarized in Table 1 in the following ways: (1) To determine the MVL damage thresholds with beam spot size and exposure duration dependency, (2) to examine the thermal response on the surface of the skin over a variety of exposure conditions, and (3) to better understand the impact of the relative absorption coefficient (${\mu }_a$) of the skin on damage threshold for the deeply penetrating 1070-nm wavelength.

2.

Methods

3.

Results

A total of 1306 skin exposures were completed on both flanks of 22 Yucatan miniature pigs with the 1070-nm laser. Sixteen different configurations of laser parameters were used as listed in Tables 2 and 3 along with correlating $\mathrm{E}{\mathrm{D}}_{50}$’s, Probit slopes, average irradiance, and peak radiant exposure data. It should be noted that the beam diameter is listed as $4\sigma$ and not the usual $1/e$ or $2\sigma$ because the $1/e^2$ diameter must be used to evaluate the hazards of laser beams on an average irradiance basis. The radiant exposure is calculated using peak power, $1/e$ diameter, not the average which uses $1/e^2$ diameter.

Table 2

Probit data results from observations for lesions 1 h after exposure from the 1070-nm laser.

Table 3

Probit data results from observations for lesions 24 h after exposure from the 1070-nm laser. Note the ratio of the ED50 to maximum permissible exposure (MPE) is taken as Ef/MPE, where Ef is the effective irradiance computed from Eq. (1).

Lesions created from exposure durations $<100\mathrm{ms}$ were typically absent at 1 h and present at 24 h post exposure. Exposure durations $\ge 100\mathrm{ms}$ were typically observed at the 1-h read and persisted at 24 h. This general observation is supported by comparing the number of yes/no lesions (column 1 in Tables 2 and 3) at 1 and 24 h post exposure and the ratio of the 1-h $\mathrm{E}{\mathrm{D}}_{50}$ to the 24-h $\mathrm{E}{\mathrm{D}}_{50}$.

To calculate the ratio of $\mathrm{E}{\mathrm{D}}_{50}:\mathrm{MPE}$ reported in Table 3, the ANSI Z.136.1-2007 standard for the Safe Use of Lasers recommends that for a skin exposure, the limiting aperture should be referenced to a standard 0.35 cm. Thus to compare the experimentally determined $\mathrm{E}{\mathrm{D}}_{50}$’s to the MPE, the irradiance values were scaled to an effective irradiance, $E_f$, by

Fig. 2

Damage threshold data converted to the effective irradiance contrasted to the maximum permissible exposure (solid line).

For exposures very near or above the $\mathrm{E}{\mathrm{D}}_{50}$, a raised appearance known as a wheal (also spelled weal) or flare was commonly observed for lesions at all durations and spot sizes. The raised area on the skin appeared more subtle for exposures near the $\mathrm{E}{\mathrm{D}}_{50}$ and more pronounced for exposures significantly higher than the $\mathrm{E}{\mathrm{D}}_{50}$. The wheal and flare appeared to be proportional to the size of the beam, which was easily visualized by illuminating the lesion at a glancing angle to the skin. The wheal was not always easily visualized when illuminated under general overhead lighting conditions. An example of skin-lesion appearance over observational endpoints is shown in Fig. 3. Black marker grid lines drawn on the skin in Fig. 3 were $\sim 4\times 4\mathrm{cm}$ apart. Though there is no pictorial documentation available, the exposed area appeared red immediately after the laser exposure, but had resolved completely by 1 h. No annotations were made of when the wheal and flare began to appear in any exposures, just that it was observed at 24 h post exposure. Note that the wheal was not used as an indicator for an MVL, but was a general observation reserved for discussion.

Fig. 3

Example of skin lesions observed in this study before and after laser exposure (10 ms exposure, 2.4 cm beam diameter, $86\mathrm{J}\pm 5.5\%$, 1.8 times higher than the 24-h $\mathrm{E}{\mathrm{D}}_{50}$). Both the 1- and 24-h post laser exposure reads were positive for a lesion. The white arrows indicate a very slight brown discoloration on the epidermis; no erythema was observed at 1 h. The white bracket indicates the extent of the observed erythema at 24 h. The dashed, white circle approximates the footprint of the observed raised area on the skin 24 h post exposure.

IR camera data proved exceptionally difficult to process for this wavelength due to hot spots from the absorption of laser energy in the hair and steam rising from the skin due to phase changes in the water inside the skin. IR camera data for the 0.6- and 1.1-cm beam diameters with exposure durations $<10\mathrm{s}$ were processed, but had enormous variability during laser heating due to these superficial hot spots and were not suitable for rate-of-heating analysis.

IR camera data were processed by selecting a region of interest (ROI) (typically 9 to 20 pixel diameters) placed in the central area of exposure to examine the averaged temperature in the ROI at each time $t$ during the laser exposure ($0<t\le \text{exposure duration}$) and after the laser was turned off ($t>\text{exposure duration}$). Since hair burns very quickly at this wavelength, care was taken to avoid hot spots believed to be follicles, or in some cases, steam, sometimes requiring the ROI to be parsed into two smaller 3 by 3 pixel ROIs. The initial temperature, ${\mathrm{T}}_0$, of the exposed skin surface was determined from IR camera data acquired before the laser turned on ($t\le 0$). The change in temperature, $\mathrm{\Delta }\mathrm{T}$, was then determined by the difference between ${\mathrm{T}}_0$ and the peak temperature, ${\mathrm{T}}_{\text{peak}}$, at the end of the exposure. IR camera data were processed to give the linear relationships, with a forced zero intercept, for the peak temperature rise, $\mathrm{\Delta }\mathrm{T}$, as a function of power P. The $\mathrm{\Delta }\mathrm{T}$ versus P data for the 1.9-cm beam diameter are shown in Fig. 4. The slopes, $m_T$, were recorded from each fit to processed thermographic data for $\mathrm{\Delta }\mathrm{T}=m_{T}P$ and are reported in Table 4.

Fig. 4

The peak temperature rise from numerous exposure powers for the 1.9 cm beam diameter as tested at 0.05, 0.25, and 10 s exposure durations. Units on the slope fit parameter, $m_T$, are $°\mathrm{C}/\mathrm{W}$.

Table 4

The linear slopes, mT, (°C/W) from the fitted thermal camera data P(ΔT) and calculated ΔTMVL to achieve the minimum visible lesion at 24-h ED50’s.

The average and standard deviation for ${\mathrm{T}}_0$ at each exposure condition shown in Fig. 4 are also reported in Table 4. The slopes in Table 4 are also corrected for radiant exposure by $m_T\times (\pi d^2/4t)$ $[°\mathrm{C}/(\mathrm{J}/\mathrm{c}{\mathrm{m}}^2)]$ for later discussion.

Time-temperature history for each of the three beam diameters in the 10-s exposures near $\mathrm{E}{\mathrm{D}}_{50}$ are shown in Fig. 5(a). The periodic ripples in the 10-s exposures stem from the subject’s breathing; care was taken to keep the ROI in the central area of the beam to minimize the variations induced by the breath (i.e., hair follicles/hot spots moving periodically through the ROI).

Fig. 5

Time-temperature history for cases nearest $\mathrm{E}{\mathrm{D}}_{50}$ resulting in a lesion at 24 h for (a) 10-s exposures, various beam diameters, and (b) largest beam diameter (9.5 cm) contrasted to the shortest exposure (10 ms) condition tested.

Time-temperature history for tested cases nearest $\mathrm{E}{\mathrm{D}}_{50}$ for the largest beam diameter (250 ms, 9.5 cm) and shortest exposure (10 ms, 2.4 cm) are shown in Fig. 5(b). Notice that the experimentally determined $m_T(°\mathrm{C}/\mathrm{W})$ in Table 4 for these two exposure duration-diameter pairs [depicted in Fig. 5(b)] are the same in $°\mathrm{C}/\mathrm{W}$, but very different in $°\mathrm{C}/(\mathrm{J}/\mathrm{c}{\mathrm{m}}^2)$. All time-temperature curves presented in Fig. 5 were positive for a lesion at 24 h. It should be noted that the uncertainty of the energy measurements was $\pm 5.5\%$ for all beam diameters $<9\mathrm{cm}$ and $\pm 7.2\%$ for the 9.5-cm beam reported in Figs. 5 and 6.

Fig. 6

The thermal image captured at the end of laser exposure of cases nearest 24-h $\mathrm{E}{\mathrm{D}}_{50}$ for (a) 2.4 cm diameter, 10 ms, $46.5\pm 2.6\mathrm{J}$ (Video 1, MP4, 43 kB) [URL: http://dx.doi.org/10.1117/1.JBO.19.3.035007.1] and (b) 9.5 cm diameter, 250 ms, $1067\pm 77\mathrm{J}$ (Video 2, MP4, 137 kB) [URL: http://dx.doi.org/10.1117/1.JBO.19.3.035007.2]. The laser comes on at $t=0$. Temperatures are absolute T (°C). The 1-cm scale bar applies to both (a) and (b).

The thermal video files corresponding to the time-temperature histories depicted in Fig. 5(b) for the 2.4- and 9.5-cm cases are shown in Video 1 and Video 2. Figure 6 shows still frames of the supplemental video files for the thermal images captured nearest to the laser’s cutoff time.

Analyzed thermal data for the 2.4-cm beam at the 10-ms pulse did not necessarily capture the peak temperature (200 fps, 1.5 ms integration time); thus these data were analyzed differently than all other thermal data by using the two frames where the laser was on, selecting an ROI in frame 1 to find the temperature rise $\mathrm{\Delta }{\mathrm{T}}_1$ at $t_1$ and then the same ROI in fame 2 to find $\mathrm{\Delta }{\mathrm{T}}_2$ at $t_2$. A linear fit was then applied to the thermal data to identify $\mathrm{\Delta }\mathrm{T}$ as function of time and solving the linear fit for an assumed $\mathrm{\Delta }{\mathrm{T}}_{\max }$ at $t=10\mathrm{ms}$ (having assumed $\mathrm{\Delta }\mathrm{T}=0$ at $t=0$). Most of these 10-ms data only had two frames showing increasing temperature, but one 2.4-cm, 10-ms thermal video captured three frames showing increasing temperature, giving some experimental evidence of the peak temperatures achieved nearest the end of this laser pulse. This uniquely captured event is presented as supplemental data Video 1.mp4 (43 kB); the time stamp in the video was created with the assumption the peak temperature profile occurred at $t=10\mathrm{ms}$. Discussion about when the peak temperature may occur from a 10-ms pulse is reserved for later.

Histology sections of Yucatan miniature pig skin punch biopsy samples were prepared with haematoxylin and eosin staining. Punch biopsies were collected from the central grid area to coincide with the center of the laser exposure site. Biopsies taken from unexposed areas of skin on the flank were used to measure skin thicknesses. Thicknesses of the stratum corneum (SC), epidermis (not including SC), and dermis ranged from 14 to 47, 52 to 82, and 2350 to 3860 μm with averages and standard deviations of $29\pm 10$, $59\pm 17$, and $2953\pm 153\mu \text{m}$, respectively. Histological evaluations of the damage lesions are reserved for a future paper.

4.

Discussion

MVLs are considered to be any subtle, slight, morphological or coloration change observed at the exposure site post laser radiation at 1- and 24-h time points. These changes include the faint brown discoloration or reddening (depicted in Fig. 3), or more obvious changes such as blistering or charring. Blistering and/or charring of the skin were not observed at the $\mathrm{E}{\mathrm{D}}_{50}$ level for any exposure condition in this study. The MVL does not necessarily describe persistent, irreparable damage to the tissue, but rather provides evidence for radiant energy levels that will begin to cause noticeable changes to the tissue, with some changes being transient in nature.

Laser-induced erythema and wheal and flare appear to have a transient, and perhaps related, nature that is not well understood. It was common to observe immediate erythema, which disappeared at 1 h, but reappeared at 24 h post exposure for the 10-ms cases (Fig. 3). This is in contrast to 250-ms cases of the same beam diameter where disappearing erythema at 1 h usually remained resolved at 24 h. Foster et al.²² reported the curious ability of erythema caused by $1320/1440\text{-}\mathrm{nm}$ laser radiation to resolve after 24 h, then reappear upon subtle thermal stimulation (i.e., hot shower) 48 h after exposure, and suggested this recall ability may be related to the wheal and flare response. Wheal and flare was observed at exposure levels at or above damage threshold for all laser beam diameters and exposure times in this study. The observation of the wheal and flare from exposed areas at or above damage threshold is not uncommon for laser exposures to skin in the visible or IR.^23–26 The wheal and flare is believed to be a neurogenic response caused by the stimulation of a neuropeptide protein called Substance P (SP).^23,24 The exact mechanism of how and why the laser stimulates SP is not known, though one hypothesis proposed by Algermissen et al.²⁶ suggests this involves the thermal destruction of the nerve endings in the dermis and/or epidermis from the laser exposure. The wheal and flare has not been used as an indicator of the MVL, though it should be included in future studies as part of the observational evidence lending insight into the tissue response at damage threshold exposure levels.

Comparing the MPE to the effective irradiances, $E_f$, used in this study found that the margin for safety between the $E_f$ and MPE ranges from a ratio of 6.2 to $>32.3$ (last column in Table 3). The data trends in Fig. 2 indicate that the damage threshold may have a weak dependence on beam diameter much larger than the 0.35-cm limiting aperture used in the ANSI.¹ For the 0.25-s exposures, the 1.9-cm beam had a statistically significant higher threshold ($51.26\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$) than the larger beam diameters, indicating that the threshold is dependent on beam diameter, at least for $d<2\mathrm{cm}$ (Fig. 2). The 2.4-, 4.7-, and 9.5-cm beam diameters converged toward a peak radiant exposure threshold level of $\sim 30\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$ as indicated by their significantly overlapping 95% confidence intervals. This suggests that for 0.25-s exposures at 1070 nm, the peak radiant exposure threshold converges to $\sim 30\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$, settling to a safety margin (ratio of $E_f:\mathrm{MPE}$) of $\sim 8$, for $d>2\mathrm{cm}$. The 0.1-s did not have overlapping 95% confidence intervals for peak radiant exposure thresholds of 52.2 and $30.65\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$ for the 1.1- and 4.7-cm beam diameters, respectively. If the evidence for convergence to $\sim 30\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$ in the 0.25-s data is considered, it is likely that the threshold for the 4.7-cm case represents a value for convergence between threshold and increasing beam diameters for 0.1-s exposures, giving a safety margin of $\sim 10$ between threshold $E_f$ and MPE for very large beam diameters. The 10-s exposure data, however, were 432.24, 189.4, and $106.38\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$ for the 0.6-, 1.1-, and 1.9-cm beam diameters, respectively, and exhibit no overlap in 95% confidence intervals, suggesting that convergence of the threshold with respect to beam diameter was not achieved. The larger spread in damage thresholds for the 10-s data also suggest that damage threshold is more dependent on beam diameter for very long exposures compared to exposures $<10\mathrm{s}$. The 0.01-s exposure thresholds for the 0.6- and 1.1-cm diameters have overlapping confidence intervals, but the larger 2.4-cm beam diameter had a peak-radiant exposure damage threshold 33% less than the smaller beams, suggesting some dependency on beam diameter even for this very short exposure. These trends in beam diameter dependency for MVL threshold data are suspected to be caused by different damage mechanisms for very short exposures ($<0.05\mathrm{s}$) compared to the longer exposure durations.

The nature of skin lesions generated with 1070-nm laser radiation is, in general, markedly different from those generated with longer IR wavelengths, such as 1940 nm. This difference is readily observed at shorter-duration exposures. Lesions generated from the 1070-nm wavelength for short (10-ms) exposure durations and smaller diameters (0.6 and 1.1 cm) seemed to be multifocal in nature and involved a desiccation process as evidenced by a white, flaky appearance to the lesion, whereas MVL studies of lesions from the 1940-nm wavelength do not report this characteristic.^3,27 The 1070- and 1940-nm laser radiation wavelengths have significantly different damage thresholds in skin at comparable conditions. For example, Oliver et al.³ reported a 1940-nm wavelength with a 1.8-cm, 10-s exposure had a 24-h $\mathrm{E}{\mathrm{D}}_{50}$ of $\sim 13.3\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$ on Yucatan mini-pig skin, as compared to the 1070-nm wavelength using a 1.9-cm, 10-s exposure, which had an $\mathrm{E}{\mathrm{D}}_{50}$ of $106.3\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$, eight times that of a comparable setting at the longer 1940-nm wavelength. These differences in lesion appearance and damage thresholds at comparable conditions between the 1940- and 1070-nm laser radiations can be explained by differences in optical absorption and scattering of melanin and water in the skin.

Based upon the literature cited in Table 1, damage thresholds at 1070 nm should be highly dependent upon pigmentation. The absorption of pigment should be considered up to wavelengths of $\sim 1400\mathrm{nm}$, where absorption in bulk tissue should become substantially less significant than water absorption due to the low fractional content of pigment as compared to the surrounding water content. The consistency of pigmentation between Yucatan miniature pigs used in this aspect of the study likely helped minimize variance of data. Rockwell and Goldman¹⁵ and Baozhang et al.¹⁰ have reported thresholds for darker skin to be as much as 20 to 25% lower than lighter-skin subjects when using a 1064-nm wavelength. This is in stark contrast to studies at the 1940-nm wavelength by Chen et al.,²⁸ which do not report significant differences in thresholds between light- and dark-pigmented skin. If the 24-h $\mathrm{E}{\mathrm{D}}_{50}$ MVL is $\sim 25\%$ lower for darker skin exposed to 1070-nm laser radiation, then the safety margin between $E_f$ and MPE would reduce significantly to a range of 1.5 up to 8.1. Adequacy of the animal model should be considered when applying these findings to all pigmentation levels expected across the human race and sufficient safety margins should take this variation into account.

The 1070-nm laser radiation wavelength also penetrates deeper into tissue, heating a larger volume, as compared to the 1940-nm wavelength. When comparing the thermal burn created by a 1064-nm laser radiation wavelength (1 cm beam diameter, 40 W delivered for 8 s) to a copper-brass iron, Zhang et al.²⁹ reported tissue damage through the skin into internal organs of a Wistar rat model where lesions had a coagulative burn appearance on the skin surface. Though the dorsal skin of the Wistar rat was significantly thinner and had far less subcutaneous tissue than the Yucatan mini-pig model, the observations reported in Zhang et al.²⁹ demonstrated the deeply penetrating nature of this laser radiation wavelength region. It is this very nature of these deeply penetrating wavelengths that is believed to give rise to the MVL threshold dependency on beam diameter. This wavelength heats a larger volume of tissue, compared to less penetrating wavelengths, enabling radial conduction to dominate over axial thermal diffusion.

Vapor or smoke was observed for some of the higher-energy exposures near or above threshold, particularly for the 10-ms exposures, providing evidence for water phase transitions occurring in deeper layers of the skin. On occasion, dry skin debris would explode off the surface (popcorn effect) when exposed to short exposures ($<100\mathrm{ms}$). However, these seemingly violent interactions did not always result in grossly observable damage to the skin surface. Due to better penetration at 1070 nm, the heat source generated by this laser within the tissue has a significantly larger volume than that generated at longer wavelengths where water absorption dominates. It is possible that the peak temperature of the skin surface occurs a few milliseconds after a 10-ms, 1070-nm laser exposure has concluded. This is feasible because the tissue beneath the surface heats rapidly, and potentially, to a higher temperature than the surface due to multiple scattering events and elevated subsurface fluence levels. Under such conditions, the thermal diffusion time required for heat to propagate to the surface would cause the peak temperature observed at the surface of the skin to occur after a sufficiently short exposure has ceased. The thermal confinement time at this wavelength can be quite long, especially for large-diameter beams. As a result, the build-up of pressure from steam generated in the volume of tissue can cause superficial dry skin to explode off the surface. This observation may reveal that there is a transition in surface damage processes with lasers operating near 1070 nm for exposure durations much longer (milliseconds versus microseconds) than typically associated with photomechanical damage.

Analysis of thermal video data revealed the thermal slopes, ${\mathrm{m}}_{\mathrm{T}}$ (Table 4), which described the linear relationship between the temperature rise, $\mathrm{\Delta }\mathrm{T}$, per Watt delivered to the exposure cite (Fig. 4) for a given exposure duration and beam diameter. Variance in the thermal data can be attributed to the angle of incidence from laser exposure (i.e., curved surface of flank), and water and melanin content in the skin. This linear relationship assists in understanding the tissue response. For instance, the $m_T$ for the 10-ms, 2.4-cm group and 250-ms, 9.5-cm group were both $0.0065°\mathrm{C}/\mathrm{W}$ (Table 4) and had similar peak temperatures at damage threshold ($\mathrm{\Delta }{\mathrm{T}}_{\mathrm{MVL}}\sim 30°\mathrm{C}$), but when the thermal slope was corrected for radiant exposure, the two exposure conditions were 2.99 and $1.82°\mathrm{C}/(\mathrm{J}/\mathrm{c}{\mathrm{m}}^2)$, respectively. Furthermore, the thermal slopes for the 10-s exposures revealed that the $\mathrm{\Delta }{\mathrm{T}}_{\mathrm{MVL}}$ was $\sim 20°\mathrm{C}$, significantly lower than the $\sim 30°\mathrm{C}$ temperatures found for most other exposures $<10\mathrm{s}$. This is consistent with thermal data analysis from a 1319-nm laser radiation skin damage study, which also found $\mathrm{\Delta }{\mathrm{T}}_{\mathrm{MVL}}$ for 10-s exposures to be $\sim 20°\mathrm{C}$.⁷ These thermography data may provide evidence for the rate of reaction as a function of peak temperature when combined with mathematical modeling.

Computational models of laser-tissue interaction traditionally use the Arrhenius equation to simulate the reaction rate of the processes with the model forced to estimate damage at time points at or very near the end of the laser exposure.³⁰ However, the experimental damage threshold is assessed at 1 and 24 h, much beyond the exposure duration of the laser. The appearance of lesions at the 24-h end point is complicated by numerous poorly understood pathways resulting in such sequelae as neurogenic inflammation and disappearing/reappearing erythema. When combined with computational models of laser-tissue interaction, these thermographic data (Table 4) will help to better understand the processes involved in lesion formation. Computational models of the data presented here are in progress.

Thermography data showed that regardless of exposure duration and beam diameter, hair follicles always reached higher temperatures than surrounding skin, but observation of lesions did not center around, nor seem directly related to, the follicle cites or the observation of vapor or smoke. The Yucatan mini-pigs used in this study had black colored hair, which always appeared darker in contrast to subjects’ skin. Hair that had not been effectively removed by clipping appeared to burn upon exposure with high irradiance levels. For example, consider the time-temperature histories presented in Fig. 5(b) where thermal camera images (Video 1 and Video 2) showed follicular cites reaching absolute temperatures in excess of $130+°\mathrm{C}$ (camera saturation) while the skin in between these hot follicles peaked near 70°C. For the 9.5-cm, 250-ms case, the lowest energy tested was about half the $\mathrm{E}{\mathrm{D}}_{50}$ (thermal data not shown) and still caused the hair follicles to reach $120+°\mathrm{C}$, but no lesion was observed. For a 2.4-cm, 10-ms case using a power at 980 W (a factor of 4.7 times less than the $\mathrm{E}{\mathrm{D}}_{50}$ of 4660 W), slight follicular hot spots were observed to reach $\sim 41°\mathrm{C}$, only $\sim 4°\mathrm{C}$ higher than the skin peak temperature. However, a 2.4-cm, 10-ms case at 1236 W (a factor of 3.7 times less than the $\mathrm{E}{\mathrm{D}}_{50}$) showed follicular hot spots beginning to appear with a follicle peaking at $\sim 54°\mathrm{C}$ while the skin peaked at $\sim 40.0°\mathrm{C}$. When the 10-ms, 2.4-cm exposure was about half of the $\mathrm{E}{\mathrm{D}}_{50}$, some follicles peaked above 90°C while the skin peaked at $\sim 49°\mathrm{C}$. Thermal data from these lower-power 10-ms, 2.4-cm cases indicate the hair residing in the follicle had a thermal slope, $m_T$, between three and five times greater than the $m_T$ for skin in this study.

5.

Conclusion

For 1070-nm skin exposures, beam diameters $>2.5\mathrm{cm}$ seem to have a weak to no effect on threshold radiant exposure levels for exposure times between 0.01 and 0.25 s, but may have a larger effect on thresholds for exposures $\ge 10\mathrm{s}$. The MVL thresholds beam diameter dependency for 0.01 s is not well understood, but is suspected to involve photomechanical damage mechanisms confounded by the large volume, especially for larger beam diameters, of tissue heated very quickly by this wavelength. At 1070 nm, damage threshold values determined in this study indicate that a buffer of at least a factor of six in the MPE as defined by the ANSI Standard 2007 (Ref. 1) exists, with higher levels of protection afforded by the standard at smaller beam diameters. These margins of safety are consistent with those reported in the literature for the pigmented pig skin at other near-IR wavelength bands between 1000 and 2000 nm.