Brian J. Lund

Laser-induced retinal damage threshold measurements with wavefront correction

An adaptive optics (AO) system was incorporated into a laser retinal exposure setup in order to correct for refractive error and higher-order aberrations of the nonhuman primate (NHP) eye during an in vivo retinal ED(50) measurement. Using this system, the ED(50) for a 100-ms, 532-nm small spot size exposure was measured to be 1.05 mJ total intraocular energy (TIE), a reduction of 22% from the value measured without aberration correction. The ED(50) for a 3.5-ns, 532-nm exposure was measured to be 0.51 microJ TIE, the lowest ED(50) reported for a ns-duration exposure. This is a reduction of 37% from the value measured without aberration correction and is a factor of only 2.6 higher than the maximum permissible exposure (MPE) for a 3.5-ns, visible wavelength small spot size exposure. The trend of in vitro measurements using retinal explants suggests that the in vivo ED(50) for small spot-size exposures could potentially be one order of magnitude smaller than the previously reported in vivo ED(50). Distortion of the incident laser beam by ocular aberrations cannot fully explain the discrepancy between the in vivo measurements with no aberration correction and the in vitro results.

Damage threshold from large retinal spot size repetitive-pulse laser exposures.

AbstractThe retinal damage thresholds for large spot size, multiple-pulse exposures to a Q-switched, frequency doubled Nd:YAG laser (532 nm wavelength, 7 ns pulses) have been measured for 100 &mgr;m and 500 &mgr;m retinal irradiance diameters. The ED50, expressed as energy per pulse, varies only weakly with the number of pulses, n, for these extended spot sizes. The previously reported threshold for a multiple-pulse exposure for a 900 &mgr;m retinal spot size also shows the same weak dependence on the number of pulses. The multiple-pulse ED50 for an extended spot-size exposure does not follow the n−1/4 dependence exhibited by small spot size exposures produced by a collimated beam. Curves derived by using probability-summation models provide a better fit to the data.

Effect of source intensity on ability to fixate: Implications for laser safety

Abstract— During long-term viewing of a continuous light source, head and eye movements affect the distribution of energy deposited in the retina. Previous studies of eye movements during a fixation task provided data used for revising the safety limits for long-term viewing of such sources. These studies have been continued to determine the effect of source brightness on the nature of fixational eye movements. Volunteers fixated for 50 s on a HeNe laser (&lgr; = 632.8 nm) masked by a small aperture to produce a target subtending ∼0.03 mrad in the visual field. The source was attenuated to yield corneal irradiance values in the range 0.6 pW cm−2 to 6 &mgr;W cm−2. Eye movements were recorded using a Dual Purkinje Image Eyetracker. The data were characterized by fixation ellipses that represent areas of the retina in which the image of the spot was located 68% of the time of each trial. Significant variation across subjects in the tightness of fixation was observed. Over the eight orders of magnitude of source brightness used in this experiment (10−13 to 10−6 W cm−2), no subject showed more than roughly a factor of two variation in the area of the fixation ellipse. No statistically significant trend in tightness of fixation as a function of source brightness was observed. There was no loss of ability to fixate, nor any drive to aversion, at the higher source intensities.

Laser-induced retinal damage threshold for repetitive-pulse exposure to 100-μs pulses

Abstract. The laser-induced retinal injury thresholds for repetitive-pulse exposures to 100-μs-duration pulses at a wavelength of 532 nm have been determined for exposures of up to 1000 pulses in an in vivo model. The ED50 was measured for pulse repetition frequencies of 50 and 1000 Hz. Exposures to collimated beams producing a minimal retinal beam spot and to divergent beams producing a 100-μm-diameter retinal beam spot were considered. The ED50 for a 100-μs exposure was measured to be 12.8  μJ total intraocular energy for a minimal retinal beam spot exposure and 18.1  μJ total intraocular energy for a 100-μm-diameter retinal beam spot. The threshold for exposures to N>1 pulse was found to be the same for both pulse repetition frequencies. The variation of the ED50 with the number of pulses is described well by the probability summation model, in which each pulse is considered an independent event. This is consistent with a threshold-level damage mechanism of microcavitation for single-pulse 100-μs-duration exposures. The data support the maximum permissible exposure levels for repetitive-pulse exposure promulgated in the most recent laser safety guidelines.

Laser-induced retinal injury studies with wavefront correction

The ability of a laser beam to damage the retina of the eye depends on the accuracy to which the optics of the eye focuses the beam onto the retina. Data acquired through retinal injury threshold studies indicate that the focus achieved by the eye of an anesthetized non-human primate (NHP) is worse than theoretical predictions, and therefore the measured injury threshold will decrease with decreasing retinal irradiance area until the beam diameter at the retina is less than 10 &mgr;m. However, a number of investigations over a range of wavelengths and exposure durations show that the incident energy required to produce a retinal injury in a NHP eye does not decrease for retinal irradiance diameters smaller than ~100 &mgr;m, but reaches a minimum at that diameter and remains nearly constant for smaller diameters. A possible explanation is that uncompensated aberrations of the eye of the anesthetized NHP are larger than predicted. Focus is a dynamic process which is purposely defeated while performing measurements of retinal injury thresholds. Optical wavefront correction systems have become available which have the capability to compensate for ocular aberrations. This paper will report on an injury threshold experiment which incorporates an adaptive optics system to compensate for the aberrations of a NHP eye during exposure to a collimated laser beam, therefore producing a near diffraction limited beam spot on the retina.

Contribution of eye movements to thermal damage thresholds for long-duration laser exposures

In an awake and alert individual, intrinsic eye movements will cause a laser beam spot to move about an extended area of the retina during a long-duration exposure. A single point on the retina will be heated when directly exposed to the laser beam, but will cool when the beam spot is moved to another location. The thermal damage threshold is therefore expected to be larger than the value estimated in standard damage models, in which the eye is treated as a stationary receiver. Experimentally measured eye movement data, recorded during deliberate fixation, were input into a computer program to calculate the increase in temperature occurring in the retina during a long-duration exposure to a continuous wave laser. A simple Arrhenius damage integral model was used to estimate the thermal damage thresholds, which were then compared to the threshold estimated for a stationary eye. The eye movements are found to increase the damage threshold by 18% for 2 second exposures, and 38% for 50 second exposures.