Edward Early

Dynamic bidirectional reflectance distribution functions: Measurement and representation

With high-energy lasers, not only the direct laser beam can pose significant eye and skin hazards, but also light reflecting off material illuminated by the beam. Proper hazard analysis for a material irradiated by a laser relies upon the reflecting properties of the material surface, as these properties determine the magnitude and direction of the reflected laser energy commonly characterized by the bidirectional reflectance distribution function (BRDF). However, a high-energy laser heating and possibly melting a material can change the reflecting properties of that material, so these changes must be included in the hazard analysis. Traditional methods for measuring the BRDFs of materials are not practical for measurement of materials with rapidly-changing surface properties. However, BRDF measurement by imagery of a witness screen allows for practical measurements of the dynamically-changing BRDFs of materials under high-energy laser irradiation. Using this technique, the dynamic BRDFs of stainless stee...

Probabilistic retinal exposure model

Exposure of retinal tissue to a laser source requires two conditions: the person must be exposed to the laser beam and the laser must be in his/her optical field-of-view. The authors have developed a probabilistic model for the second condition. This model is based on random eye movements, known as saccades, about a look direction. This direction is an input to the model and is known either from head orientation or a task. The model calculates the instantaneous probability that the laser is in the optical field-of-view of the retinal tissues of fovea, macula, and retina. Because of saccades, the location of exposure will change several times a second. The duration of laser exposure for the look direction is then used to calculate the probability that the retinal tissue is exposed at least once, based on independent exposures from the saccades. The concepts and equations of the model are presented, along with example calculations from applying the model to a specific exposure scenario. For a given retinal ...

Automatic construction of probabilistic dynamic bidirectional reflectance distribution functions from reflection screen images

The reflections of high energy laser off surfaces can present hazards to persons and instruments at significant distances. The heating from these lasers cause changes in the reflection characteristics of surfaces they impact. As such, the reflections from these surfaces cannot be properly modeled with static bidirectional reflectance distribution functions (BRDFs), but require time-dynamic BRDFs. Moreover, the time-evolution of the surface reflections is not deterministic, but can vary even when the materials and irradiance conditions are nearly identical, such that only probabilistic characterization is realistic. Due to the swiftly changing nature of the reflections, traditional BRDF measurements with goniometric instruments is impossible, and BRDFs must be deduced from images of the reflected light incident on a screen which intercepts a portion of the reflection solid angle. A model has been constructed to describe these complex probabilistic dynamic BRDFs with only a moderate number of intuitive parameters, where these parameters have central values and statistical variances. These simple parametric representations are appropriate for use in predictive modeling codes and are also easily adjustable to allow facile exploration of the sensitivity of hazards to laser, material, and model uncertainties. An automated procedure has been created for determining appropriate parameter values and variances from captured screen images, without the need for case-by-case human judgment. Examples of the parameter determination procedure are presented.

The specular methodology for reflected laser hazard analyses

Reflection of a laser beam from a surface depends on the optical properties of the surface, and can have components that are diffuse (in all directions) or specular (in one direction). Hazard distances tend to be short and easily calculated for diffuse reflections. However, specular reflections have longer hazard distances and their calculation poses challenges due to the spatial and temporal characteristics of the reflected beam. We have developed and refined a methodology to calculate hazard distances from specular reflections. This methodology uses analytical expressions to determine the irradiance and exposure time of the reflected beam, from which hazard distances are calculated. The method accounts for the properties of the incident beam, the material reflecting properties, and the shape of the surface.The motivations and concepts of the specular methodology are presented, along with the equations resulting from this approach. The spatial and temporal characteristics of the reflected beam for a specific laser and surface are calculated from the application of these equations. These characteristics are then used to determine either localization of hazardous regions in the surrounding space or a description of possible hazards at specific locations. Examples of the application of the specular methodology are presented with their corresponding hazardous conditions.Reflection of a laser beam from a surface depends on the optical properties of the surface, and can have components that are diffuse (in all directions) or specular (in one direction). Hazard distances tend to be short and easily calculated for diffuse reflections. However, specular reflections have longer hazard distances and their calculation poses challenges due to the spatial and temporal characteristics of the reflected beam. We have developed and refined a methodology to calculate hazard distances from specular reflections. This methodology uses analytical expressions to determine the irradiance and exposure time of the reflected beam, from which hazard distances are calculated. The method accounts for the properties of the incident beam, the material reflecting properties, and the shape of the surface.The motivations and concepts of the specular methodology are presented, along with the equations resulting from this approach. The spatial and temporal characteristics of the reflected beam for a spec...

Simulations of high energy laser reflections

Reflections of high energy lasers from targets present safety challenges, as the resulting hazard distances can be significant. We have developed two different but complementary simulations to determine hazard zones resulting from high energy laser reflections. One, the High Energy Laser Collateral Assessment Tool (HELCAT), is a high-fidelity successor to the Laser Range Safety Tool (LRST), which had been developed specifically for distant targets. HELCAT has an improved user interface and uses first-principle physics calculations to obtain reflected irradiances at observer locations for each time step in a simulation. The time histories of the irradiances are used to determine the hazard zone. It supports a variety of simple and complex targets, trajectories, reflecting properties of materials, and observer locations. The other, termed the Specular Methodology, considers only specular reflections to derive analytical expressions for the irradiance and exposure time at observer locations. The hazard zone ...

An approach to high energy laser safety in open environments

High energy lasers present new challenges for safety evaluation. Not only is the direct beam hazardous, but the reflected beam may also cause injury. Two important issues with high energy laser safety have been addressed in simulations used to model reflected-beam hazards: the reflecting properties of materials and the time history of exposure. The reflecting properties are quantified by the Bi-directional Reflectance Distribution Function (BRDF), and a technique has been developed to extract parameters for BRDF models, primarily the Maxwell-Beard model, from measurement data and to calculate the reflected irradiance at an observer. In addition, the reflecting properties of materials often change upon heating, so a procedure using a camera and screen is being used to measure the time-varying BRDF of materials, which in turn is modeled empirically with spherical harmonics. Because the use of high energy lasers is often dynamic in nature, involving a moving laser, target, or observer, the time-history of exposure to the observer is calculated in the simulations. A sliding-window algorithm compares the time-history of exposure to maximum permissible exposure limits for a range of exposure times in order to determine if a hazardous condition exists for the observer.High energy lasers present new challenges for safety evaluation. Not only is the direct beam hazardous, but the reflected beam may also cause injury. Two important issues with high energy laser safety have been addressed in simulations used to model reflected-beam hazards: the reflecting properties of materials and the time history of exposure. The reflecting properties are quantified by the Bi-directional Reflectance Distribution Function (BRDF), and a technique has been developed to extract parameters for BRDF models, primarily the Maxwell-Beard model, from measurement data and to calculate the reflected irradiance at an observer. In addition, the reflecting properties of materials often change upon heating, so a procedure using a camera and screen is being used to measure the time-varying BRDF of materials, which in turn is modeled empirically with spherical harmonics. Because the use of high energy lasers is often dynamic in nature, involving a moving laser, target, or observer, the time-history of ex...