David J. Lund

Wavelength Dependence of Ocular Damage Thresholds in the Near-IR to Far-IR Transition Region: Proposed Revisions to MPEs

This report summarizes the results of a series of infrared (IR) laser-induced ocular damage studies conducted over the past decade. The studies examined retinal, lens, and corneal effects of laser exposures in the near-IR to far-IR transition region (wavelengths from 1.3–1.4 &mgr;m with exposure durations ranging from Q-switched to continuous wave). The corneal and retinal damage thresholds are tabulated for all pulsewidth regimes, and the wavelength dependence of the IR thresholds is discussed and contrasted to laser safety standard maximum permissible exposure limits. The analysis suggests that the current maximum permissible exposure limits could be beneficially revised to (1) relax the IR limits over wavelength ranges where unusually high safety margins may unintentionally hinder applications of recently developed military and telecommunications laser systems; (2) replace step-function discontinuities in the IR limits by continuously varying analytical functions of wavelength and pulsewidth which more closely follow the trends of the experimental retinal (for point-source laser exposures) and corneal ED50 threshold data; and (3) result in an overall simplification of the permissible exposure limits over the wavelength range from 1.2–2.6 &mgr;m. A specific proposal for amending the IR maximum permissible exposure limits over this wavelength range is presented.

Simultaneous irradiation and imaging of blood vessels during pulsed laser delivery

Simultaneous irradiation and viewing of 10–120 μm cutaneous blood vessels were performed to investigate the effects of 2‐μs 577‐nm dye laser pulses.

Variation of laser induced retinal-damage threshold with retinal image size

The dependence of retinal damage threshold on laser spot size was examined for two pulse width regimes; nanosecond-duration Q-switched pulses from a doubled Nd:Yttrium–aluminum–garnet laser and microsecond-duration pulses from a flashlamp-pumped dye laser. Threshold determinations were conducted for nominal retinal image sizes ranging from 1.5 to 100 mrad of visual field, corresponding to image diameters of ∼22 μm to 1.4 mm on the primate retina. In addition, base line collimated-beam damage thresholds were determined for comparison to the extended source data. Together, this set of retinal damage thresholds reveals the functional dependence of threshold on spot size. The threshold dose was found to vary with the area of the image for larger image sizes. This experimentally determined trend was shown to agree with the predictions of thermal model calculations of laser-induced retinal damage for spot sizes ≳150 μm. The results are compared to previously published extended source damage thresholds and to th...

Thermal lensing in ocular media exposed to continuous-wave near-infrared radiation: the 1150–1350-nm region

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.

Retinal injury thresholds for blue wavelength lasers.

The interaction mechanism leading to laser-induced retinal alteration can be thermal or non-thermal, depending upon the wavelength of the laser radiation and the duration of the exposure. To investigate the effect of exposure duration on the interaction mechanism, retinal injury thresholds in the rhesus monkey were experimentally measured for exposure to laser radiation at wavelengths of 441.6, 457.9, 476.5, and 496.5 nm. Exposure durations were 0.1, 1, 5, 16, and 100 s; and 1/e retinal irradiance diameters were 50, 125, and 327 &mgr;m. Tissue response was observed via ophthalmoscope 1 h and 48 h post exposure. Thermal and non-thermal damage thresholds were obtained depending upon the exposure duration. These threshold data are in agreement with data previously reported in the literature for 100-s duration exposures, but differences were noted for shorter exposures. The current study yielded an estimated injury threshold for 1-s duration, 327-&mgr;m retinal irradiance diameter exposures at 441.6 nm, which is an order of magnitude higher than that previously reported. This study provides evidence that laser-induced retinal damage is primarily induced via thermal mechanisms for exposures shorter than 5 s in duration. Arguments are presented that support an amendment of the thermal hazard function, R(&lgr;).

Ex vivo and computer model study on retinal thermal laser-induced damage in the visible wavelength range.

Excised bovine eyes are used as models for threshold determination of 532-nm laser-induced thermal damage of the retina in the pulse duration regime of 100 micros to 2 s for varying laser spot size diameters. The thresholds as determined by fluorescence viability staining compare well with the prediction of an extended Thompson-Gerstman computer model. Both models compare well with published Rhesus monkey threshold data. A previously unknown variation of the spot size dependence is seen for different pulse durations, which allows for a more complete understanding of the retinal thermal damage. Current International Commission on Nonionized Radiation Protection (ICNIRP), American National Standards Institute (ANS), and International Electromechanical Commission (IEC) laser and incoherent optical radiation exposure limits can be increased for extended sources for pulsed exposures. We conclude that the damage mechanism at threshold detected at 24 and 1 h for the nonhuman primate model is retinal pigment epithelium (RPE) cell damage and not thermal coagulation of the sensory retina. This work validates the bovine ex vivo and computer models for prediction of thresholds of thermally induced damage in the time domain of 10 micros to 2 s, which provides the basis for safety analysis of more complicated retinal exposure scenarios such as repetitive pulses, nonconstant retinal irradiance profiles, and scanned exposure.

Ocular Effects of Holmium (2.06 μm) and Erbium (1.54 μm) Laser Radiation

Abstract Ocular dose-response relationships were experimentally determined for selected exposure conditions at the erbium and holmium laser wavelengths of 1.54 and 2.06 μm. The ocular responses were observed in Rhesus monkey eyes and were confined to the cornea. The EDsos (effective dose for 0.5 probability of producing a biomicroscopically visible corneal lesion) determined for single corneal exposures were as follows: Wavelength Exposure Irradiance ED50 duration diameter 1.54 μm 930 μsec 1.0 mm 9.6 J/cm2 2.06 μm 100 μsec 1.8 mm 2.9 J/cm2 2.06 μm 42 nsec 0.32 mm 5.2 J/cm2 The depth and diameter of the corneal lesions were both dose and wavelength dependent. The wavelength dependence of the dose required to produce a corneal response is indicative of the relative absorption properties of the cornea. These results suggest that current permissible exposure limits be altered to reflect the relative absorption properties of the ocular media.

Variation of laser-induced retinal injury thresholds with retinal irradiated area: 0.1-s duration, 514-nm exposures

The retinal injury threshold dose for laser exposure varies as a function of the irradiated area on the retina. Zuclich reported thresholds for laser-induced retinal injury from 532 nm, nanosecond-duration laser exposures that varied as the square of the diameter of the irradiated area on the retina. We report data for 0.1-s-duration retinal exposures to 514-nm, argon laser irradiation. Thresholds for macular injury at 24 h are 1.05, 1.40, 1.77, 3.58, 8.60, and 18.6 mJ for retinal exposures at irradiance diameters of 20, 69, 136, 281, 562, and 1081 microm, respectively. These thresholds vary as the diameter of the irradiated retinal area. The relationship between the retinal injury threshold and retinal irradiance diameter is a function of the exposure duration. The 0.1-s-duration data of this experiment and the nanosecond-duration data of Zuclich show that the ED(50) (50% effective dose) for exposure to a highly collimated beam does not decrease relative to the value obtained for a retinal irradiance diameter of 100 microm. These results can form the basis to improve current laser safety guidelines in the nanosecond-duration regime. These results are relevant for ophthalmic devices incorporating both wavefront correction and retinal exposure to a collimated laser.

Accidental human laser retinal injuries from military laser systems

The time course of the ophthalmoscopic and functional consequences of eight human laser accident cases from military laser systems is described. All patients reported subjective vision loss with ophthalmoscopic evidence of retinal alteration ranging from vitreous hemorrhage to retinal burn. Five of the cases involved single or multiple exposures to Q-switched neodymium radiation at close range whereas the other three incidents occur over large ranges. Most exposures were within 5 degrees of the foveola, yet none directly in the foveola. High contrast visual activity improved with time except in the cases with progressive retinal fibrosis between lesion sites or retinal hole formation encroaching the fovea. In one patient the visual acuity recovered from 20/60 at one week to 20/25 in four months with minimal central visual field loss. Most cases showed suppression of high and low spatial frequency contrast sensitivity. Visual field measurements were enlarged relative to ophthalmoscopic lesion size observations. Deep retinal scar formation and retinal traction were evident in two of the three cases with vitreous hemorrhage. In one patient, nerve fiber layer damage to the papillo-macular bundle was clearly evident. Visual performance measured with a pursuit tracking task revealed significant performance loss relative to normal tracking observers even in cases where acuity returned to near normal levels. These functional and performance deficits may reflect secondary effects of parafoveal laser injury.

Near infrared laser ocular bioeffects.

Thresholds for laser chorioretinal injury in the red end of the visible spectrum and the near-infrared (IR-A) spectral regions are presented. An unpredicted wavelength dependence of the injury threshold for single Q-switched pulses is demonstrated. Four lasers were used to determine thresholds at 40 wavelengths between 532 nm and 1064 nm: a ruby laser, a neodymium:YAG-pumped dye laser, an erbium:YLF laser and an alexandrite laser. Despite many careful and repeated efforts to determine a cause for the variation due to possible variations in the lasers or other aspects of the experimental technique and due to biological absorption properties of the eye, there is no complete or obvious explanation for the significant variations of threshold with small changes in wavelength. The implications of these findings for laser safety standards are presented.