Garrett Polhamus

Measurement and Prediction of Thermal Injury in the Retina of the Rhesus Monkey

The authors measured temperature rises with specially designed microthermocouples in over 60 retinae for various image sizes, wavelengths, and exposure durations. Measured temperatures varied with a standard error of 6 percent, and agreed well with a mathematical model for temperature-time response. Observed injury also compared favorably to that predicted by a rate process model for thermal injury. Suggested rate constants for the eye are A = 1.3 ×1099 1/s, and E = 150 000 cal/M. With these coefficients, predicted threshold injury agreed within a factor of two with experimentally determined injury from 10¿8 to 103 s. No difference in threshold temperatures was evident between either macular and paramacular exposures or between wavelengths of 488-647 nm. The model can be used to predict injury in the human eye by substituting absorption coefficients and thickness for the human PE and Ch in the thermal portion of the model.

Porcine skin visible lesion thresholds for near-infrared lasers including modeling at two pulse durations and spot sizes

With the advent of such systems as the airborne laser and advanced tactical laser, high-energy lasers that use 1315-nm wavelengths in the near-infrared band will soon present a new laser safety challenge to armed forces and civilian populations. Experiments in nonhuman primates using this wavelength have demonstrated a range of ocular injuries, including corneal, lenticular, and retinal lesions as a function of pulse duration. American National Standards Institute (ANSI) laser safety standards have traditionally been based on experimental data, and there is scant data for this wavelength. We are reporting minimum visible lesion (MVL) threshold measurements using a porcine skin model for two different pulse durations and spot sizes for this wavelength. We also compare our measurements to results from our model based on the heat transfer equation and rate process equation, together with actual temperature measurements on the skin surface using a high-speed infrared camera. Our MVL-ED50 thresholds for long pulses (350 micros) at 24-h postexposure are measured to be 99 and 83 J cm(-2) for spot sizes of 0.7 and 1.3 mm diam, respectively. Q-switched laser pulses of 50 ns have a lower threshold of 11 J cm(-2) for a 5-mm-diam top-hat laser pulse.

Simultaneous Exposure Using 532 and 860 nm lasers for visible lesion thresholds in the rhesus retina.

The growth of commercially available, simultaneous multi-wavelength laser systems has increased the likelihood of possible ocular hazard. For example, many systems utilize frequency multiplying methods to produce combinations of visible, near-infrared, and ultraviolet wavelengths. Unfortunately, very little data exists to substantiate the current methods for estimating hazards from simultaneous lasing. To properly assess the retinal hazards from these wavelengths, the retinal effects of 10-s laser irradiation from 532 and 860 nm were determined in non-human primates for four different relative dosage combinations of these wavelengths. This pair of wavelengths represents the typical problem of a visible-wavelength laser combined with an in-band, infrared wavelength that is not as well focused at the retina—a situation difficult to address. To add confidence to the experimental results obtained, a theoretical thermodynamic model was developed to predict the minimal damage threshold for simultaneous wavelengths at 1 h post exposure. The new model calculations and the data obtained are compared with results from one currently accepted method of predicting relative exposure limits from multi-wavelength systems. In addition, the current ANSI-Z136-2000 standard was used to compute the combined MPEs for comparison with measured visible lesion thresholds. A total of 12 eyes were exposed using four different ratios of power levels (532/860 power rations) to determine the contribution to the damage levels from each wavelength. The experimental data were analyzed using probit analysis at both 1-h and 24-h post exposure to determine the minimum-visible-lesion (MVL) thresholds at ED50 values, and these thresholds at 24 h varied from 5.6 mW to 17 mW total intraocular power.

Model predictions of ocular injury from 1315-nm laser light

With the advent of future weapons systems that employ high energy lasers, the 1315 nm wavelength will present a new laser safety hazard to the armed forces. Experiments in non-human primates using this wavelength have demonstrated a range of ocular injuries, including corneal, lenticular and retinal lesions, as a function of pulse duration and spot size at the cornea. To improve our understanding of this phenomena, there is a need for a mathematical model that properly predicts these injuries and their dependence on appropriate exposure parameters. This paper describes the use of a finite difference model of laser thermal injury in the cornea and retina. The model was originally developed for use with shorter wavelength laser irradiation, and as such, requires estimation of several key parameters used in the computations. The predictions from the model are compared to the experimental data, and conclusions are drawn regarding the ability of the model to properly follow the published observations at this wavelength.

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.

13.4: The Realism of Reflection Holographic Stereograms

The realism of reflection holographic stereograms was evaluated to determine how acceptable they are to a non-technical audience. The stereograms were rated for realism by comparing them to photographic images and real objects. Although the ratings were favorable, they did not quite achieve the realism of photographic images, but 3D cues of depth and parallax were important contributors to realism.

Retinal injury from simultaneous exposure to 532-nm and 860-nm laser irradiation

To properly assess the retinal hazards from several lasers using multiple wavelengths, the retinal effects of 10-second laser irradiation from 532 and 860 nm were determined in non-human primates for several different power combinations of these wavelengths. A total of 12 eyes were exposed using four different ratios of power levels to determine the contribution to the damage levels from each wavelength. The data are compared to the calculations resulting from use of the currently accepted method of predicting hazards from simultaneous laser. The ANSI-Z136 - 2000 standard was used to calculate the combined maximum permissible exposure (MPE) and for comparison with the measured visible lesion thresholds, i.e., ED50s.

Validation and verification of the laser range safety tool (LRST)

The U.S. Dept. of Defense (DOD) is currently developing and testing a number of High Energy Laser (HEL) weapons systems. DOD range safety officers now face the challenge of designing safe methods of testing HELs on DOD ranges. In particular, safety officers need to ensure that diffuse and specular reflections from HEL system targets, as well as direct beam paths, are contained within DOD boundaries. If both the laser source and the target are moving, as they are for the Airborne Laser (ABL), a complex series of calculations is required and manual calculations are impractical. Over the past 5 years, the Optical Radiation Branch of the Air Force Research Laboratory (AFRL/HEDO), the ABL System Program Office, Logicon-RDA, and Northrup-Grumman, have worked together to develop a computer model called teh Laser Range Safety Tool (LRST), specifically designed for HEL reflection hazard analyses. The code, which is still under development, is currently tailored to support the ABL program. AFRL/HEDO has led an LRST Validation and Verification (V&V) effort since 1998, in order to determine if code predictions are accurate. This paper summarizes LRST V&V efforts to date including: i) comparison of code results with laboratory measurements of reflected laser energy and with reflection measurements made during actual HEL field tests, and ii) validation of LRSTs hazard zone computations.