Additivity of Retinal Damage for Multiple-Pulse Laser Exposures PDF Download
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Category :
Languages : en
Pages : 24
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Book Description
This study has examined the cumulative effects of multiple-pulse laser exposures in contributing to retinal damage via a thermal mechanism. Three sets of experiments have been conducted to determine the variation of multiple- pulse thresholds: With the interval between pulses; with the number of pulses in the pulse train when the interpulse interval is held constant; and with the retinal image size when both number of pulses and interpulse interval are invariant. In addition, thermal model calculations were carried out to compare model predictions with the experimental data. Conclusions from the experimental data are: (1) The threshold for a multiple-pulse train is related to that for an individual pulse in the train by a function of the number of pulses, but is independent of pulse-repetition frequency and pulse train length. This observation is in general accordance both with the thermal model predictions and with previously empirical models. (2) The additivity of multiple pulses is quantitatively similar for collimated and expanded laser beams incident at the eye. (3) The repair or recovery of laser-induced reversible retinal damage (i.e., sub-threshold with respect to the ophthalmoscopic lesion endpoint) is slow, having a time constant of the order of days. Thus, the cumulative nature of multiple-pulse or repeated exposures within a 24-hr period is relatively unaffected by the ongoing repair process; and only when repeated daily exposures are of concern does the repair process become a factor. Keywords: Laser damage; Retina.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 24
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Book Description
This study has examined the cumulative effects of multiple-pulse laser exposures in contributing to retinal damage via a thermal mechanism. Three sets of experiments have been conducted to determine the variation of multiple- pulse thresholds: With the interval between pulses; with the number of pulses in the pulse train when the interpulse interval is held constant; and with the retinal image size when both number of pulses and interpulse interval are invariant. In addition, thermal model calculations were carried out to compare model predictions with the experimental data. Conclusions from the experimental data are: (1) The threshold for a multiple-pulse train is related to that for an individual pulse in the train by a function of the number of pulses, but is independent of pulse-repetition frequency and pulse train length. This observation is in general accordance both with the thermal model predictions and with previously empirical models. (2) The additivity of multiple pulses is quantitatively similar for collimated and expanded laser beams incident at the eye. (3) The repair or recovery of laser-induced reversible retinal damage (i.e., sub-threshold with respect to the ophthalmoscopic lesion endpoint) is slow, having a time constant of the order of days. Thus, the cumulative nature of multiple-pulse or repeated exposures within a 24-hr period is relatively unaffected by the ongoing repair process; and only when repeated daily exposures are of concern does the repair process become a factor. Keywords: Laser damage; Retina.
Author:
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ISBN:
Category : Laser materials
Languages : en
Pages : 522
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Book Description
Author: Carmen A. Puliafito
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ISBN:
Category :
Languages : en
Pages : 9
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Book Description
We investigate laser-induced shock waves from melanin particles as a possible cause of retinal injury from ultrashort pulse laser exposures. Melanin granules were isolated from calf retina and suspended in gels. The melanin particles were irradiated with 100 psec laser pulses from an amplified, mode- locked Nd:YAG laser and shock waves were observed under a microscope using a time-delayed strobe pulse. Spherical shock fronts were observed at incident laser fluences 4J/sq cm and were imaged as close as 20 micrometers from the melanin particles. Multiple shock fronts were resolved when several melanin particles were irradiated simultaneously. Shock front radii were measured as a function of photographic delay time and laser fluence. Average shock from velocities during the first 10 nsec ranged from 2500 to 4000 m/sec. A velocity of 3000 m/sec corresponds to a shock pressure of 11 kbars. These results indicate that shock wave emission from melanin particles in the retinal pigment epithelium is a potential cause of retinal injury from ultrashort laser pulses.
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Category : Aeronautics
Languages : en
Pages : 456
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Book Description
Lists citations with abstracts for aerospace related reports obtained from world wide sources and announces documents that have recently been entered into the NASA Scientific and Technical Information Database.
Author:
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ISBN:
Category :
Languages : en
Pages : 38
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Book Description
Exposure of the fovea to single or multiple pulses of coherent light (532 nm) can produce both transient and permanent changes in the eye's ability to resolve fine spatial detail. The immediate effects for extended duration exposures (>50 msec) are often large, producing acuity deficits as great as 90% of its pre-exposure level. The size of these deficits often reflect a total loss of foveal functioning although, depending upon the energy and duration of the exposure, these changes are reversible. Permanent changes in acuity can be noted in the absence of gross morphological damage and at power densities below the ED50 level provided the area of involvement is large. On the other hand, at power densities above the ED50 level, little if any permanent or consistent visual deficits are noted if the damage is restricted to relatively isolated areas within either the foveal or parafovea. Multiple pulses which increase the area of total involvement are more effective in permanently shifting postexposure acuity than are the single pulse conditions. Cumulative effects of repetitive exposures separate in time by as much as several al days are possible. The exact parameters of any observed loss in visual performance of course is dependent upon the discrimination task and its ability to depict subtle changes in the retina mosiac. Laser safety (MPE), Visual acuity, Nd/YAG, Rhesus monkey, Flash effects, Long-term visual deficits, Light-induced damage, RA 3.
Author:
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ISBN:
Category : High power lasers
Languages : en
Pages : 508
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Book Description
Author: Gordon L. M. Gibson
Publisher:
ISBN:
Category :
Languages : en
Pages : 15
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Book Description
A ruby laser photocoagulator was used to deliver single and multiple subthreshold exposures to the retina of Macaca mulatta. The subexposures parameters were 0.5-msec. pulse, 1.3-mm.-diameter focal spot, and energy density of about 375 mJ/sq cm. Single exposures produced no microscopic changes on sacrifice at 1 to 30 days following exposure. Repetitive exposures (7 to 17 exposures) at the same energy level invariably caused characteristic damage in the outer retinal layers. These cumulative effects are similar to and more widespread than the minimal-damage lesions produced by single suprathreshold exposures using higher energy density (900 to 1000 mJ/sq cm). Current safety criteria are based on the assumption that laser damage occurs on an all-or-none basis so that damage is not cumulative if a single exposure causes no visible lesion. This work indicates that single subthreshold exposures which are less than half of a threshold dose are cumulative and therefore each 'subthreshold' exposure must damage or in some manner increase the retinal susceptibility to subsequent exposures. (Author).
Author:
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ISBN:
Category : Power resources
Languages : en
Pages : 848
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Book Description
Author: United States. Congress. House. Committee on Armed Services. Subcommittee on Research and Development
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ISBN:
Category : History
Languages : en
Pages : 400
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Author: Wesley J. Marshall
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ISBN:
Category :
Languages : en
Pages : 11
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Book Description
The present method for evaluating repetitive pulse lasers in based primarily on data taken for a fixed exposure time (0.5 s). The biological effect from these studies was interpreted to be a function of the pulse repetition frequency when in fact this same effect could just as well have been plotted against the total number of pulses in the pulse train. It is the opinion of this author that the reason several pulses cause retinal injury when a retinal lesion is not observed from a single pulse of the same energy is that microscopic cell changes partially combine together to cause visible injury. The additivity method offers a more logical approach for evaluating repeated exposures from short pulsed laser devices. A uniform safely margin may be maintained for multiple pulse and single pulses from the same laser device. Permissible exposures to a very few pulses or to coded pulses may be easily calculated by this method. Most importantly, an adequate margin of safety may be maintained for all repetitively-pulsed lasers. Keywords: Laser damage; Laser hazards; Radiation protection; Standards: Exposure limits. (AW).
