The current data strongly indicates that there is no photochemical effect of in vivo exposure to 1090 nm near IRR
radiation within the pupil. Four groups of 20 Sprague-Dawley rats were unilaterally exposed in vivo to 96 W·cm-2
centered inside the pupil for 10, 18, 33 and 60 min, respectively depending on group belonging. This resulted in radiant
exposure doses of 57, 103, 198 and 344 kJ·cm-2. Temperature evolution at the limbus during the exposure and difference
of intensity of forward light scattering between the exposed and the contralateral not exposed eye was measured at
1 week after exposure. The temperature at the limbus was found to increase exponentially towards an asymptote with an
asymptote temperature of around 7 °C and a time constant (1/k) of around 15 s. No increase of light scattering was found
despite that the cumulated radiant exposure dose was [80;250] times the threshold for photochemically induced cataract
suggested by previous empirical data. It is concluded that at 1090 nm near IRR there is no photochemical effect.
KEYWORDS: Eye, Temperature metrology, In vivo imaging, Light scattering, Cornea, Lenses, Scatter measurement, Eye models, Infrared radiation, Data modeling
The damage mechanism for near-infrared radiation (IRR) induced cataract is unclear. Both a photochemical and a thermal mechanism were suggested. The current paper aims to elucidate a photochemical effect based on investigation of irradiance-exposure time reciprocity. Groups of 20 rats were unilaterally exposed to 96-W/cm2 IRR at 1090 nm within the dilated pupil accumulating 57, 103, 198, and 344 kJ/cm2, respectively. Temperature was recorded at the limbus of the exposed eye. Seven days after exposure, the lenses were macroscopically imaged and light scattering was quantitatively measured. The average maximum temperature increases for exposure times of 10, 18, 33, and 60 min were expressed as 7.0±1.1, 6.8±1.1, 7.6±1.3, and 7.4±1.1°C [CI (0.95)] at the limbus of the exposed eye. The difference of light scattering in the lenses between exposed and contralateral not-exposed eyes was 0.00±0.02, 0.01±0.03, −0.01±0.02, and −0.01±0.03 transformed equivalent diazepam concentration (tEDC), respectively, and no apparent morphological changes in the lens were observed. An exposure to 96-W/cm2 1090-nm IRR projected on the cornea within the dilated pupil accumulating radiant exposures up to 344 kJ/cm2 does not induce cataract if the temperature rise at the limbus is <8°C. This is consistent with a thermal damage mechanism for IRR-induced cataract.
An in vivo exposure to 197 W/cm2 1090-nm infrared radiation (IRR) requires a minimum 8 s for cataract induction. The present study aims to determine the ocular temperature evolution and the associated heat flow at the same exposure conditions. Two groups of 12 rats were unilaterally exposed within the dilated pupil with a close to collimated beam between lens and retina. Temperature was recorded with thermocouples. Within 5 min after exposure, the lens light scattering was measured. In one group, the temperature rise in the exposed eye, expressed as a confidence interval (0.95), was 11±3°C at the limbus, 16±6°C in the vitreous behind lens, and 16±7°C on the sclera next to the optic nerve, respectively. In the other group, the temperature rise in the exposed eye was 9±1°C at the limbus and 26±11°C on the sclera next to the optic nerve, respectively. The difference of forward light scattering between exposed and contralateral not exposed eye was 0.01±0.09 tEDC. An exposure to 197 W/cm2 1090-nm IRR for 8 s induces a temperature increase of 10°C at the limbus and 26°C close to the retina. IRR cataract is probably of thermal origin.
Conclusion: The current data are consistent with a potential photochemical effect of in vivo exposure of the crystalline lens to near infrared radiation since the onset of cataract after in just above threshold dose was at least 18 hrs delayed after the exposure. Materials and methods: The eyes of 6 weeks old Sprague-Dawley rats were exposed unilaterally in vivo to 1090 nm, 6.2 W quasi-top hat spatial distribution with a 3 mm spot on the anterior lens surface within the dilated pupil. First, four exposure time groups of rats were exposed to increasing exposure times. At 24 hrs after exposure, the difference of light scattering between the lenses from the same animal was measured. Then, based on the first experiment, four post-exposure time groups were exposed unilaterally in vivo to 8 s of 1090 nm, 6.2 W quasi-top hat spatial distribution with a 3 mm spot on the anterior lens surface within the dilated pupil. After, the intended post-exposure time, the difference of light scattering between the lenses from the same animal was measured. Results: A 3 mm spot of 6.2 W induces light scattering in the lens with exposures of at least 8 s. Further, after 8 s of 6.2 W within a 3 mm spot on the lens surface, the light scattering increase in the lens was delayed at least 18 hrs after the exposure.
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 µs 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 µs 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.
A computer model was used to predict thresholds for 532 nm scanned retinal exposure and exposure for different retinal beam profile geometries, including rectangles and ring shaped profiles. The image analysis method described in IEC 60825-1 Edition 1.2--maximizing the ratio of power within a rectangle over the value of &agr; for this rectangle--was applied to the different profiles to determine &agr; and the fractional power that would be compared to the MPE value. The predicted thresholds for these special types of retinal exposure were compared with the predicted damage threshold for top hat profiles for the value of &agr; that resulted from the image analysis method. The comparison shows that the most restrictive power/&agr; ratio method produces appropriate results, provided that a time dependent &agr;max is used, as was proposed at BIOS 2006.
Excised bovine retinas were used as an explant model for threshold determination of laser induced thermal damage for
multiple pulse exposures for the laser wavelength of 532 nm. The thresholds as determined by fluorescence viability
staining compare very well with the prediction of thermal damage computer model that is based on the Arrhenius
damage integral. Comparison of the experimental data with the thermal damage computer model that additivity of
multiple pulses can be understood on the basis of partial thermal damage induced by the individual pulses. Both models
were previously (BIOS 2006) validated against non-human primate threshold data. The multiple pulse thresholds for a
given series of pulses were compared against the MPE evaluation method for multiple pulses, referred to as N-1/4 or
Total on Time (TOT) rule. Variation of the pulse duration, retinal spot size and the spacing between pulses shows that
the TOT rule either reflects the damage threshold trend for multiple pulses very well or errs on the conservative side.
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 µm, 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 ED50 (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 µm. 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.
Excised bovine retinas were used as model for threshold determination of laser induced thermal damage in the pulse
regime of 1 ms to 655 ms for a range of laser spot size diameters. The thresholds as determined by fluorescence
viability staining compare very well with the prediction of thermal damage models. Both models compare well with
published and new Rhesus monkey threshold data. A distinctive dependence of the threshold on laser spot size diameter
for different pulse duration was found which indicates that current (ICNIRP, ANSI and IEC) laser exposure limits for
large spots can be increased in this pulse duration regime. A time dependent αmax is proposed which only for the case of
long exposure durations has the current value of 100 mrad, but decreases to smaller angles for short exposure durations,
effectively increasing the permissible exposure level. An explanation based on intra-retinal scattering is offered for the
unexpected spot size dependence for spot diameters less than about 80 µm. The time dependence and nature of damage
is discussed for pulse durations shorter than 1 ms where bubble induced damage seems to lead to a threshold a factor of
10 lower than the thermally induced threshold, resulting in the need to lower the MPE values for this condition.
Possible changes of the MPE values are offered and discussed.
To assign a diameter to a beam profile which does not feature an actual perimeter presents a formidable problem. Traditionally, different beam diameter criteria are used which, however, depending on the beam profile, may yield drastically different results. The second moment method has been standardised by ISO, however, it also has some limitations. While a certain diameter criterion may have advantageous properties for instance for describing the propagating beam, it may not be appropriate for other 'applications' such as scaling the thermal damage threshold or for calculating the power that passes through an aperture. We discuss the latter two on the basis of some example profiles which show that the second moment diameter is in these cases not an appropriate parameter.
For laser safety, where the exposure limit for the eye (MPE) is stated as a function of what could be referred to as the 'thermal beam diameter', a diameter criterion is needed that yields a correct value for this parameter for any arbitrary size and shape of the beam profile at the retina. A procedure is proposed that can be applied to determine a 'thermal damage parameter' for a given beam profile. This procedure is checked against thermal model calculations of the damage threshold. It is also shown that the power that passes through an aperture (such as the pupil of the eye) can be seriously underestimated when using the second moment diameter for non-Gaussian beams.
The retinal injury threshold for exposure to a laser source varies as a function of the irradiated area on the retina. Currently accepted guidelines for the safe use of lasers provide that the MPE will increase as the diameter of the irradiated area for retinal diameters between 25 mm and 1700 mm, based on the ED50 data available in the late 1970s. Recent studies by Zuclich and Lund produced data showing that the ED50 for ns-duration exposures at 532 nm and ms duration exposures at 590 nm varied as the square of the diameter of the irradiated area on the retina. This paper will discuss efforts to resolve the disagreement between the new data and the earlier data though an analysis of all accessible data relating the retinal injury threshold to the diameter of the incident beam on the retina and through simulations using computer models of laser-induced injury. The results show that the retinal radiant exposure required to produce retinal injury is a function of both exposure duration and retinal irradiance diameter and that the current guidelines for irradiance diameter dependence do not accurately reflect the variation of the threshold data.
For a given power entering the eye, the level of retinal thermal hazard depends on the retinal image size over which that power is distributed. Maximum permissible exposure limits are given in terms of the angular subtense of the apparent source, which describes the diameter of the retinal image. Based on a simple beam propagation model for a laser beam being transformed by the eye, it will be discussed that both the location as well as the angular subtense of the apparent source depend on the exposure position of the eye in the beam. For a given position, it is important to consider different accommodation conditions of the eye to determine the location and angular subtense of the apparent source. Only when the eye is fixed in the relaxed condition is the angular subtense of the apparent source equivalent with the far field divergence for any exposure position. For a Gaussian beam, when the eye is located in the far field, the beam waist can be considered as the apparent source, while when the eye is located at or close to the beam waist, the apparent source is located in infinity and the angular subtense of the apparent source becomes equivalent with the far field beam divergence.
For a given power entering the eye, the level of retinal thermal hazard depends on the retinal image size over which that power is distributed. Maximum permissible exposure limits are given in terms of the angular subtense of the apparent source α, which describes the diameter of the retinal image. Since this parameter scales the retinal thermal exposure limits (MPE), it is a direct measure of the relative thermal hazard of different retinal images, and thus should be seen as 'thermally effective' rather than 'optical' diameter of the retinal image. From the method given in IEC 60825-1 for the analysis of non-uniform sources, a general method for the analysis of non-top hat profiles was derived and is suggested as general analysis method for the angular subtense of the apparent source for a given image. This and other criteria are compared with the results of thermal models.
An indication of the level of uncertainty in laser injury studies relates to the slope of the transformed dose-response curve, or the "probit plot" of the data. The most cited threshold in a laser injury experiment is the point on the probit plot that represents a 50 % probability of injury: the ED-50. This value is frequently referred to as the "threshold," even though some experimental damage points exist below this "threshold." An analysis of any number of example data sets reveals that the slope in most experiments could not be explained by biological variation alone. The optical, thermophysical and biological factors influencing the probit plot are critically analyzed. By theoretically modelling an experiment, small errors in focus are shown to produce a substantial change in the ED-50 and the slope of the probit plot. The implications of plotting spot-size dependence with ED-50 values are shown to be significant, and can lead to erroneous conclusions regarding the apparent spot-size dependence.
Concepts in dosimetry pertinent to hazard evaluation of optical radiation and specifically laser radiation are discussed. The basic units of power, energy, irradiance, exposure and radiance will be reviewed, as well as the relation of retinal exposures and experimental data given as intra-ocular energy to exposure limits specified in exposure at the cornea or time integrated radiance. Averaging apertures and field of views are specified with the exposure limits to be used when exposure values are measured or calculated which in turn are compared to exposure limits for laser radiation or broadband optical radiation. The size of the averaging aperture for irradiance measurements or the size of the averaging field of view for radiance measurements is closely linked to biophysical effects and dimensions such as the diameter of the pupil of the eye or the angular extent of eye movements. In some cases, the specified size of the averaging aperture and FOV result in measured irradiance and radiance values, which are much smaller than the real physical values. In the latest revision of the international laser safety standard, IEC 60825-1, and in the revised ICNIRP laser limits, blue light limits are split from the thermal limits and are given in irradiance, specifying corresponding measurement criteria for the measurement FOV. The derivation of the irradiance limit from the basic radiance limit as it is specified for the broadband blue light hazard (for instance by ACGIH and ICNIRP) can be understood on the basis of the specification for the measurement FOV.
In laser safety, dose-response curves describe the probability for ocular injury as a function of ocular energy, and are often used to quantify the risk for ocular injury given a certain level of exposure to laser radiation. In principal, a dose-response curve describes the biological variation of the individual thresholds in a population. In laser safety, a log-normal cumulative distribution is generally assumed for the dose-response curve, for instance when Probit analysis is performed. The lognormal distribution is defined by two parameters, the median, called ED50, and the slope. When animal experiments are performed to obtain dose-response curves for laser induced injury, experimental uncertainty such as focussing errors as well as variability within the group of experimental animals, such as inter-individual variability of absorption of the ocular media, can influence the shape of the dose-response curve. We present simulations of uncertainties and variabilities that show that the log-normal dose-response curve as obtained in a animal experiments can grossly overestimate the probability for ocular damage for small doses. It is argued that the intrinsic slope for an individual’s dose-response curve is rather steep, even for retinal injury, however, the dose-response curve for a group or population can be broader when there is inter-individual variability of parameters which influence the threshold. The quantitative results of the simulation of the grouping of individual dose-response curves can serve as basis to correct potentially biased dose-response curves as well as to characterize the uncertainty associated with the ED50 and the slope of the dose-response curve. A probabilistic risk analysis model, which accounts for these uncertainties by using Monte-Carlo simulation, was developed for retinal laser injuries from pulsed lasers with wavelengths from 200 nm to 20 µm, and the interpretation of the results are discussed on the basis of example calculations.
Laser safety training at present is based primarily on conventional lectures. We present a multimedia concept which allows for adjusting of the training to laser users' specific applications and needs. Applying a blend of written and spoken text, drawings, photographs, animations and videos, computer based multimedia training is capable of conveying information very efficiently. Preferences regarding medical fields and modes of presentations are taken into account. This concept meets the recommendations of international standards and guidelines concerning contents and is capable of presenting information in variable quantity, depth and speed thus increasing the training efficiency for the individual.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.