Joseph A. Zuclich

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.

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...

Ophthalmoscopic and pathologic description of ocular damage induced by infrared laser radiation

This study compares ocular damage effects induced by infrared (IR) lasers in the “eye-safe” wavelength range to those observed following visible wavlength laser exposures. In addition to routine fundus camera and slit-lamp observations, exposed subjects were examined via confocal scanning laser ophthalmoscopy. Histopathologic evaluation was conducted on eyes with both visible and IR laser induced focal lesions. IR laser exposure parameters which resulted in corneal, lenticular and/or retinal damage will be detailed. Unique aspects of the IR laser–tissue interaction will be discussed, including concurrent lesion formation in several tissues and secondary responses such as delayed inflammatory reactions. This latter effect may involve tissue not directly irradiated during the laser insult. Finally, implications for laser safety standards will be suggested.

New data on the variation of laser induced retinal-damage threshold with retinal image size

In earlier studies, we examined the dependence of the laser induced retinal damage threshold on retinal image size for extended-source ocular exposures. We reported the spot-size dependence of the retinal threshold (based on ophthalmic observations at 24 h postexposure) for two pulsewidth regimes: nanosecond-duration (Q-switched) pulses from a doubled Nd:yttrium–aluminum–garnet laser and microsecond-duration pulses from a flashlamp-pumped dye laser at 590 nm. In either case, the retinal threshold was shown to vary with the area (i.e., diameter squared) for image diameters >5 mrad. More recently, we have collected additional data for the intermediate spot-size range (1.5–10 mrad) and have compared both the absolute values and the spot-size trend of retinal thresholds determined via ophthalmoscopic observation at 1 h postexposure to the analogous threshold data collected with observations at 24 h postexposure. These additional data and analyses reinforce our earlier conclusions regarding the threshold vs. s...

Review of exposure limits and experimental data for corneal and lenticular damage from short pulsed UV and IR laser radiation

Laser exposure limits as promulgated by the International Commission on Non-Ionizing Radiation Protection are compared to relevant experimental animal injury data for cornea and lens exposure in the nanosecond to microsecond pulse duration regime in both the ultraviolet (UV) and infrared spectral ranges. In the UV spectral range, thermal and photochemical damage mechanisms compete and thresholds must be carefully distinguished as a function of wavelength and pulse duration. The thermal UV damage data are compared with levels inferred from CO2 radiation thresholds and it is shown that the reduction factors between experimental data for thermal injury and the corresponding exposure limits appear to be unnecessarily high. The lack of data for nanosecond exposures for wavelengths below 355 nm is identified. Available experimental data for infrared radiation (1.4–4 μm) can be fitted well with an inverse-absorption curve for saline. The exposure limits roughly follow the absorption curve with a varying degree of safety scaling factor. A lack of experimental threshold data is identified for wavelengths around the 3 μm absorption peak for water absorption. The inverse curve for the spectral absorption of water would suggest a rather low threshold for a biological effect at 3 μm.Laser exposure limits as promulgated by the International Commission on Non-Ionizing Radiation Protection are compared to relevant experimental animal injury data for cornea and lens exposure in the nanosecond to microsecond pulse duration regime in both the ultraviolet (UV) and infrared spectral ranges. In the UV spectral range, thermal and photochemical damage mechanisms compete and thresholds must be carefully distinguished as a function of wavelength and pulse duration. The thermal UV damage data are compared with levels inferred from CO2 radiation thresholds and it is shown that the reduction factors between experimental data for thermal injury and the corresponding exposure limits appear to be unnecessarily high. The lack of data for nanosecond exposures for wavelengths below 355 nm is identified. Available experimental data for infrared radiation (1.4–4 μm) can be fitted well with an inverse-absorption curve for saline. The exposure limits roughly follow the absorption curve with a varying degree o...

Laser-induced retinal damage thresholds for annular retinal beam profiles

The dependence of retinal damage thresholds on laser spot size, for annular retinal beam profiles, was measured in vivo for 3 μs, 590 nm pulses from a flashlamp-pumped dye laser. Minimum Visible Lesion (MVL)ED50 thresholds in rhesus were measured for annular retinal beam profiles covering 5, 10, and 20 mrad of visual field; which correspond to outer beam diameters of roughly 70, 160, and 300 μm, respectively, on the primate retina. Annular beam profiles at the retinal plane were achieved using a telescopic imaging system, with the focal properties of the eye represented as an equivalent thin lens, and all annular beam profiles had a 37% central obscuration. As a check on experimental data, theoretical MVL-ED50 thresholds for annular beam exposures were calculated using the Thompson-Gerstman granular model of laser-induced thermal damage to the retina. Threshold calculations were performed for the three experimental beam diameters and for an intermediate case with an outer beam diameter of 230 μm. Results indicate that the threshold vs. spot size trends, for annular beams, are similar to the trends for top hat beams determined in a previous study; i.e., the threshold dose varies with the retinal image area for larger image sizes. The model correctly predicts the threshold vs. spot size trends seen in the biological data, for both annular and top hat retinal beam profiles.

Laser-induced retinal damage threshold as a function of retinal image size

The dependence of retinal damage threshold on laser spot size was examined for two pulsewidth regimes; nanosecond- duration Q-switched pulses from a doubled Nd:YAG laser and microsecond-duration pulses from a flashlamp-pumped dye laser. Threshold determinations were conducted for nominal retinal image sizes ranging from 1.5 mrad to 100 mrad of visual field, corresponding to image diameters of approximately 22 μm to 1.4 mm on the primate retina. In addition, baseline 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. The results are compared to previously published extended source damage thresholds and to the ANSI Z136.1 laser safety standard maximum permissible exposure levels for diffuse reflections.

Modeling of laser-induced threshold damage in the peripheral retina

Though allowable (safe) energy doses of pulsed laser radiation have been determined in the central retina, the sensitivity of the peripheral retina to damage must also be assessed. We used results from ray-tracing in an eye model to estimate laser spot size at the retina and recent thermal model computations of damage thresholds to predict off-axis retinal injury from laser irradiation. The predictions were made for threshold exposures with a 532-nm, 10-ns, Nd:YAG laser beam that filled the dilated pupil (7-mm diameter). Results were compared to previously published measured energy doses at the cornea needed to produce a minimally visible lesion (MVL) in the peripheral retina of rhesus subjects. We predicted the threshold for injury at the macula, and at selected portions of peripheral retina out to 60 degree(s) from the fovea. Both predictions and measured data were normalized to their respective macula values. Normalized predicted thresholds in the peripheral retina increased as a function of angular distance from the macula. This varied from the measured data which, on the other hand, were relatively insensitive to angular position in the peripheral retina. The difference is likely due to improvements in methods of assessing retinal injury that have been incorporated into the model.

Wavelength dependence of ocular damage thresholds in the near-IR to far-IR transition region (proposed revisions to MPEs)

We have previously reported on unique aspects of the laser-tissue interactions associated with penetrating laser wavelengths in the near-IR to far-IR transition region. Retinal, lens, and corneal effects have been described following 1.3-1.4 μm laser exposures with exposure durations ranging from cw to sub-millisecond. The puslewidth range studied has now been extended down to the nanosecond regime using a Q-switched, 1.319-μm, Nd:YAG laser. This paper presents the Q-switched data and discusses the wavelength dependence of the IR thresholds across all pulsewidth regimes. The corneal and retinal thresholds are contrasted to laser safety standard maximum permissible exposure (MPE) limits and changes suggested to the existing MPEs with the objectives of: (1) relaxing the standards over wavelength ranges where unusually high safety margins may unnecessarily hinder applications of recently developed military and telecommunications laser systems and; (2) replacing step-function discontinuities in the MPEs by more gradually changing functions of wavelength which follow the trends of the retinal and corneal ED50 threshold data. Based on these considerations, revisions to laser safety standard MPEs are proposed over the wavelength range from 1.15 μm to 2.6 μm.We have previously reported on unique aspects of the laser-tissue interactions associated with penetrating laser wavelengths in the near-IR to far-IR transition region. Retinal, lens, and corneal effects have been described following 1.3-1.4 μm laser exposures with exposure durations ranging from cw to sub-millisecond. The puslewidth range studied has now been extended down to the nanosecond regime using a Q-switched, 1.319-μm, Nd:YAG laser. This paper presents the Q-switched data and discusses the wavelength dependence of the IR thresholds across all pulsewidth regimes. The corneal and retinal thresholds are contrasted to laser safety standard maximum permissible exposure (MPE) limits and changes suggested to the existing MPEs with the objectives of: (1) relaxing the standards over wavelength ranges where unusually high safety margins may unnecessarily hinder applications of recently developed military and telecommunications laser systems and; (2) replacing step-function discontinuities in the MPEs by mo...

Variation of laser-induced retinal damage threshold with retinal image size (Abstract Only)

The dependence of retinal damage threshold on laser spot size was examined for two pulsewidth regimes; nanosecond- duration Q-switched pluses from a doubled Nd:YAG laser and microsecond-duration pulses from a flashlamp-pumped dye laser. Threshold determination were conducted for nominal retinal image sizes ranging form 1.5 mrad to 100 mrad of visual field, corresponding to image diameters of approximately 22 micrometers to 1.4 mm on the primate retina. Together, this set of retinal damage threshold 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. The experimental results were compared to the predictions of the Thompson-Gerstman granular model of laser-induced retinal damage. The experimental and theoretical trends of threshold variation with retinal spot size were essentially the same, with both data sets showing threshold dose proportional to image area for spot sizes >= 150 micrometers . The absolute values predicted by the model, however, were significantly higher than experimental values, possibly because of uncertainty in various biological input parameters, such as the melanosome absorption coefficient and the number of melanosomes per RPE cell.