U.S. Air Force T&E Days Conference

2 - 5 Feb 2010 Nashville, Tennessee

Collateral Damage Effects of Directed Energy Weapons

Michael C. Chesterman �, Richard E. Huffman, Jr.†

AFIT, Dayton OH

Directed Energy Weapons are highly sought new weapons, whose potential to change the battle fieldis great, but their unintended consequences are poorly understood. This research uses atmospheric andmathematic models to capture and quantify potential collateral damage. The atmospheric models cap-ture humidity, particulate, turbulence and blooming effects to the target and capture humidity, particu-late and blooming effects leaving the target. The mathematic models capture Gaussian divergence andcombine all the models with a specific frequency and power source to determine specular energy quan-tities leaving a target as potential collateral damage. The specular energy is compared with AmericanNational Standards Institute (ANSI) ocular damage and skin burn data to determine collateral damageranges.

I. Introduction

THIS research analyzes current data on Directed Energy Weapon (DEW) testing, to quantify the collateraldamage effects from scattered, reflected and refracted energy from different targets collectively called

specular energy. Harnessing the power of light and energy into weapons has been rooted in the consciencesince humans told stories around cooking fires. From all corners of the world, mythology has captured thedesire for controlling directed energy. In 214-212 BC Archimedes is credited with slowing the Roman fleetby focusing sunlight to start fires.1 With Archimedes story, the idea of energy weapons moved from thehands of ’gods’ to the minds of mortals. Through the decades, many scientists claimed to have inventedDEWs, including Nikola Tesla, who claimed to have developed a ‘teleforce weapon.’ Nikola Tesla claimedit would send concentrated beams of particles powerful enough to down aircraft.2 In 1959, Gordon Gouldpublished a conference paper, “The LASER, Light Amplification by Simulated Emission of Radiation.”This led to the first working laser demonstrated a year later by Theodore Maiman at the Hughes ResearchLaboratories.3 Now that human desires to field directed energy weapons are becoming a reality, we needto understand both the benefits and pitfalls of its use.

A. Background

DEWs are categorized by their source of directed energy or wavelength and how much power they gener-ally produce. Sources of directed energy (by wavelength) include: microwave energy, radio frequency orhigh-energy radio-frequency (HERF), particle beam, plasma, pulsed energy projectile and of course lightenergy or laser. As this area of study expands, this list of sources is expected to grow. A short overviewof the sources of directed energy places into perspective how the high power laser compares to similartechnology.

Microwave energy has been named Microwave Amplification by Stimulated Emission of Radiation(MASER). MASERs are used for precise measurements in atomic clocks and radio telescopes. There is cur-rently work to turn MASERs into weapons, mostly for human deterrent or non lethal applications. CurrentMASER systems have relatively short ranges and negligible permanent damage effects. They are therefore

�Masters student, Dept. of Aeronautical and Astronautical Engineering.†Assistant Professor, Ph.D. , Dept. of Aeronautical and Astronautical Engineering. AIAA Senior Member.This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. The

views expressed in this article are those of the author and do not reflect the official policy or position of the Air Force, the Navy, theDepartment of Defense or the U.S. Government.

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not investigated in this research. HERF weapons operate similarly to microwave weapons and collateraldamage effects similarly limited, therefore are also excluded from this research.

Particle beam weapons use a high energy beam of atoms or electrons to impact their target. When theseparticles hit a material, due to their subatomic size, they penetrate easily not just heating the material’s sur-face like a laser.4 The Office of Naval Research in conjunction with the U.S. Department of Energy’s ThomasJefferson National Accelerator Facility is pursuing a Free Electron Laser (FEL) for ship board defense.5

Plasma weapons use the high energy ionized gas (plasma) most usually created from superheatinglasers. The weapon focuses the plasma at a target. Although this is an interesting area of DEW study, thesmaller quantity of systems being developed using this source of energy led this research to other moreimmanent systems.

Pulsed Energy Projectile (PEP) is a specific use of laser technology. The PEP was originally called thepulsed impulsive kill laser. The current version is a non lethal weapon that sends out a pulsed laser whichcreates a small amount of exploding plasma upon contact producing a shock wave intended to stun thetarget and knock them down. PEP produces electromagnetic radiation effects that cause pain to nerve cells.The PEP is intended for crowd control and similar to MASERs, has no lasting direct damage effects, norcollateral damage effects, excluding it from this research.

Laser weapons are the most common and most studied of the energy weapons. Lasers are defined bytheir mode of operation, the type of laser and the power produced. The modes of operation are: continuousconstant-amplitude output known as continuous wave (CW), or pulsed. CW lasers give constant powerwith respect to time and are created by a steady pump source. Pulsed lasers are achieved by gain switching,Q-switching, mode locking or pulsed pumping. Pulsed lasers are able to achieve much higher peak power.Currently, the push in DEW systems is for more power on target. Since laser weapons are the most widelystudied and many DEW systems utilize lasers as the power source, this research focused on high poweredlasers.

Figure 1. History of Laser Intensity6

With the operating mode established, lasers are broken down further into types: gas, chemical, excimer,solid-state, semiconductor, dye and free electron. These names are representative of the laser’s active ma-terial. Once the active material is specified, the lasers are further categorized by power output or pulseintensity. The more power a laser can produce potentially yields more power on a target, therefore caus-ing more damage. The quantity of power produced by DEWs has been increasing and as can be seen inFigure 1, is now at high enough levels to produce viable weapons.

The operational environment for laser based DEWs is much more extreme than their laboratory counterparts. They have to operate in the outside world, with variations in temperature, density, humidity, trans-mission medium and distance. There are three main complications to using current DEW weapons: atmo-spheric losses, weight (including fuel) and line of sight limitations. Major atmospheric losses are blooming,

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turbulence and airborne particulates. Blooming occurs when high energy weapons like lasers break the airdown into plasma. This usually occurs at energy densities of a mega-joule per cubic centimeter. Bloomingcauses the laser to disperse energy into the atmosphere and defocus. Turbulence is caused by variationsin density (or refractive index) and is strongest below five kilometers of altitude. Turbulence also tends todefocus a laser and spreads the beam energy over a larger area, decreasing the power for a given area.7

Additionally an atmosphere with a large amount of particulate in it such as clouds, fog, dust, smog or hu-midity will absorb and scatter laser energy. These effects combine to reduce the amount of energy on thetarget.

The weight of current weapon systems is large, mostly comprising of the active lasing material. Theweight restriction is not as limiting to ground systems, but is detrimental to airborne systems. Currentlythe only airborne systems are mounted in large aircraft such as C-130s and jumbo jet airliners. As minia-turization and other advances reduce the size and weight of DEW systems, more airborne applications willcome to fruition.

Laser weapons are line of sight weapons which allow for only direct fire. Since a laser operates onlyalong the line of sight (LOS) to the target, it also places the shooter within the LOS of the target as well,leaving it vulnerable to counter attack. There are many advantages to laser systems, such as: speed ofdelivery, relatively undetectable, and low cost of multiple rounds. Since lasers travel at the speed of lightand are basically unaffected by gravity, there is no need to lead a target. The same speed advantage doesnot allow the target time to maneuver once the laser is fired, nor can an unintended target easily maneuverinto the way.

Figure 2. THEL8

Specific current and envisioned systems for both of-fensive and defensive capabilities are not mere specu-lation, several systems are in use. Some of the defen-sive DEWs include: The Airborne Laser (ABL), TacticalHigh Energy Laser (THEL), Mobile Tactical High EnergyLaser (MTHEL), and High Energy Liquid Laser Area De-fense System (HELLADS). ABL uses a MegaWatt classchemical oxygen iodine laser (COIL) in a Boeing 747-400. This platform was designed to use the laser to shootdown ballistic missiles during their boost phase.9 TheU.S. Army and Israel have developed the THEL, as seenin Figure 2. The THEL was developed for static loca-tions and a mobile version, known as MTHEL was alsoproduced. The system was designed to destroy rockets,artillery shells, mortar rounds and low flying aircraft.THEL was demonstrated between 2002 and 2004 show-ing it was capable of destroying targets, including a salvoattack by mortars.8, 10 The HELLADS program was de-

veloped to design a high-energy laser weapon system (150 kiloWatt), with very low weight compared tocurrent systems.11

The Advanced Tactical Laser is currently the only offensive DEW in the laser frequency range. The ATLis a system modeled after the ABL, but designed to be small enough to fit on an AC-130 Gunship or V-22Osprey. Like the ABL, the ATL is also a COIL system, however the ATL uses a closed cycle, to capturethe waste and keep the operators safe. The system is designed for both lethal and non lethal applicationsagainst ground targets and can engage cruise missiles as well. According to the web site, Global Security,“The Advanced Tactical Laser can place a 10-centimeter-wide beam with the heating power of a blowtorchon distant targets for up to 100 shots. The Advanced Tactical Laser can produce a four-inch-diameter beamof energy that can slice through metal from a distance of 9 miles.”12 The tremendous power the ATL canput on target is impressive, but it is also one of the beginning DEW systems, newer ones are only going toget more powerful.

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Photobiological Spectral Eye Effects Skin EffectsDomain

Ultraviolet C (0.200- 0.280 μm) Photokeratitis Erythma (sunburn) Skin CancerUltraviolet B (0.28 - 0.315 μm) Photokeratitis Accelerated Skin Aging

Increased PigmentationUltraviolet A (0.315 - 0.400 μm) Photochemical UV Pigment Darkening

Cataract Photosensensitive ReactionsSkin Burns

Visible (0.400 - 0.780 μm) Photochemical & Pigment DarkeningThermal Retinal Injury Photosensensitive Reactions

Skin BurnsInfrared A (0.780 - 1.400 μm) Cataract, Retinal Burns Skin BurnsInfrared B (1.400 - 3.000 μm) Corneal Burn Skin Burns

Aqueous FlareIR Cataract

Infrared C (3.000 - 1000 μm) Corneal Burn Only Skin Burns

Table 1. Laser Beam Hazards (Based on ANSI Z136.1-1993)

DEWs can easily hurt humans, as any laser safetyprogram can attest. The extent of damage a human in-curs depends on energy absorbed, wavelength of the en-ergy, duration of exposure, pulse repetition rate and ex-posed part of the anatomy. Many programs use Maxi-mum Permissible Exposure (MPE) which is, “the level oflaser radiation to which an unprotected person may beexposed without adverse biological changes in the eyeor skin.”13 MPEs are often defined as one tenth of thefifty percent level for minimum tissue reaction.14

To better understand the relationship between wave-length and damage, Table 1 shows eye and skin damagefor common wavelengths. Note that damage levels in-crease with wavelength.14 The Laser Institute of America calculates levels of MPE for both ocular damageand skin burns, using wavelength and exposure duration as inputs. The next section will detail the proce-dure used to determine which parameters are most influential in estimating collateral damage effects.15

II. Methodology

Using data from multiple modeling programs and conservative assumptions, this research evaluatesseveral energy outputs from an airborne source and models the specular energy leaving the target afterlaser impact as the source of potential collateral damage. This research assumes the laser beam is Gaussianin nature, focusing all of the energy at the target. This Gaussian beam assumption is very conservative, notsubtracting any power from the beam since a Gaussian beam is clean. Atmospheric losses are calculatedfrom altitude, humidity, range and frequency. From these parameters, the atmospheric losses of absorbtionand scattering can be predicted according to Beer-Lambert’s Law. From atmospheric data, the calculatedatmospheric losses are converted to a transmittance fraction in HELEEOS3 and is multiplied to the totalpower. Turbulence models are used to determine losses due to atmospheric turbulence, also converted inHELEEOS3 to a transmittance fraction and multiplied to the total power. All objects have have an absorp-tivity dependent upon wavelength and temperature. The energy absorbed by the target is subtracted fromthe total power by multiplying power by 1-absorbtivity. All power not accounted for in the calculationdisplayed in Equation 1 below, is potentially available to cause collateral damage effects.

Ptarget = Psource � Latmospheric � Lturbulence � Labsorbtion (1)

At this point, power is converted to energy by removing the time element; power = energy/time. Timeis removed by multiplying by 1 second, and time will return when calculating damage thresholds in Sub-section C. The quantity of remaining available energy leaving the target will be one of three types; scattered,reflected or refracted. It has been shown that laser energy leaving a target forms two regions around thetarget, one of diffuse energy with relatively short ranges and one of reflected specular energy with muchlarger ranges. For the purposes of this research, the diffuse region is assumed to be part of the intendedresults from firing a laser weapon, not a source of collateral damage.

Especular = ELeavingTarget � Ldiffuse � Latmospheric � LGaussiandivergence (2)

The specular energy however, appears to have the potential to harm humans at a significant range.The specular energy diverges from the target as it is past the focal point (assumed to be focused at thetarget). The divergence is modeled conservatively as a Gaussian beam and is applied as a de-focuser tothe laser energy leaving the target. The Gaussian divergence is normalized to create a fraction that can bemultiplied to the energy leaving the target to model the loss. The atmospheric losses are again calculatedand converted in HELEEOS3 to a transmittance fraction and multiplied to the energy leaving the target.Turbulence is not currently accounted for and is discussed further in the Propagation Subsection. Thecalculation of energy leaving the target at a specified range is depicted in Equation 2 above. The amount ofreflected specular energy is compared to known quantities and wavelengths of energy that cause damageto humans, specifically ocular damage and skin burns. A radius of each effect is calculated, includingatmospheric losses. Future investigations will include a Monte Carlo simulation based on geographical

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location, obstructions and population density to determine the probability of causing collateral damagefrom the reflected specular energy on realistic terrain and population densities.

A. Frequency, Power and Geography

Any laser study is highly dependent on the wavelength of energy observed. For this study 1.064mm waschosen for several reasons. First, it is a common laser used in industrial applications which makes it alikely candidate for a weapon system. Second, the wavelength is outside the absorption wavelengths ofwater, which is a major factor in a weapon application. Third, Ytterbium fiber lasers which operate at thiswavelength are extremely efficient due to their optically pumped design. The remarkably high gains froma relatively low power source coupled with their small, energy-efficient, air-cooled, and solid-state designmakes these lasers a likely candidate for future weapons applications.16 Future weapons will need to belower weight and have much higher power than current designs used and Ytterbium lasers are a strongcandidate for future weaponization. Therefore, 1.064um was chosen as the study wavelength for its futurepotential applicability.

The next major factor of a laser is power output. This research focused on high power lasers, becauseof the desire for increased stand off ranges for DEW applications. For disclosure purposes the three powerlevels investigated will be labeled A, B, and C. The approximate power that many DEW systems eitherwant to achieve or have achieved for a fielded system powerful enough to damage or destroy an aircraftor vehicle is over 50kW, therefore power level A was used as the lowest power limit.16 Power level A isnot 50kW. As power increases, blooming effects start to dominate, therefore power level C was chosen asa high limit to capture blooming effects. A middle value was needed for comparisons and to determine ifthere was a linear return for increased power. Power level B was chosen as an intermediate point and ishalf of power level C.

As varied as our earth is, atmospheric conditions change with geographic location, time of the year, timeof the day and slant range through the atmosphere. To capture these variations, three locations were chosen.Albuquerque, New Mexico was chosen for its low humidity and high altitude. Albuquerque is also close toKirtland Air Force Base which is a test site for some DEW programs, so hopefully data from this researchcan be directly applied by programs there. Hurlburt Field, Florida was also chosen due to DEW testingthat takes place there as well as capturing high humidity low altitude atmospheric models. Mexico City,Mexico was chosen due to its high pollution, which will increase the particulate levels in the atmosphere.This increase in particulate at high altitude and low humidity helps isolate the pollution particulate fromhumidity contributions found at lower altitude coastal cities.

35,000 ft MSL

25,000 ft MSL

10,000 ft MSL

10 Nautical Miles

Figure 3. Shot Line Profiles

To simplify annual and daily variances in atmo-spheric effects, all calculations in HELEEOS3 were com-pleted with summer, daily average and no clouds orrain. These assumptions are very conservative, provid-ing the greatest potential for maximum energy transmis-sion through the atmosphere to the target. Deviationfrom these chosen inputs will provide different results.

Coupled with geographic location and time, is theslant range. Slant range represents the slice of atmo-sphere that the laser energy passes through on the wayto the target. Three altitudes were chosen and applied atall three locations. All geometries place the laser sourceten Nautical Miles (NM) from the target. Since each lo-cation has very different altitudes, the MSL values wereconverted to Above Ground Level (AGL) for each loca-tion to calculate a slant range when combined with theten NM offset.

The three different shot line profiles are depicted inFigure 3 Ten thousand feet Mean Sea Level (MSL) was chosen as a lower altitude shot line, capturing boththe closest geometry to the target as well as being the geometry with the highest density atmosphere topass through overall. Twenty five thousand feet MSL was chosen as an intermediate firing altitude, gettingabove much of the denser atmosphere and giving contrast to the high and low geometries. Thirty five

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thousand feet MSL was chosen as most aircraft do not fly much higher than this altitude and this geometrycaptures the least time in dense atmosphere amongst the geometries.

The variables of three power settings, three locations and three geometries create twenty seven distinctscenarios. Twenty seven scenarios allow for useful comparisons to investigate the trends.

B. Propagation

To model propagation to the target, a MATLABr based program called HELEEOS3 was used. HELEEOS3was developed at the Air Force Institute of Technology. HELEEOS3 uses atmospheric data bases with worldwide coverage and allows the user to pick a turbulence model as well. This research used the Hufnagel-Valley (5/7) turbulence model because it is one of the most popular and reliable models. Each of the twentyseven scenarios were run through HELEEOS3 giving a transmitted power to the target.17

Targets for DEW systems will include a variety of structures, materials, surface types etc. All of thesevariations will change the absorptivity of the target. As the target is heated and ablation occurs, againabsorptivity will change. To simplify, it was assumed that most targets would be painted, the absorptivityof paint is approximately 0.9.18 The remaining energy will leave the target as diffuse and specular energy.It is very difficult to predict the exact ratio of diffuse to specular energy, again it is highly dependent on thetarget composition. To simplify, a conservative estimate of fifty percent diffuse, fifty percent specular wasassumed. These two factors mean that five percent of the energy impacting a target will become specularenergy leaving the target with the potential to cause collateral damage. The energy leaving the target againpasses through the atmosphere. HELEEOS3 was used to model the transmission of energy. Turbulence willhave a great impact on ground level laser energy energy transmission. Turbulence causes scintillation, orvariance over time of a laser beam. In order to reduce the variability introduced by turbulence, an averageeffect of turbulence over time is used. The turbulence models used by HELEEOS3 were not expected to beaccurate enough at very low altitude, (ground level), which the specular energy travels along, when leavingthe target. Therefore only atmospheric models and Gaussian divergence were used as sources for energyloss leaving the target. All results without the turbulence factored in will indicate much longer ranges thanshould actually be observed, but their relative distance will be useful for comparison.

C. Biological Hazard Thresholds

Once the distance specific quantities of energy travel are calculated, the energy needs to be evaluated incomparison to the energy needed to cause collateral damage to a person. This comparison will yield rangesfor specific collateral damage effects. This research used ANSI Z136-2007 standards to calculate ocular andskin burn damage thresholds. These MPEs are based off of wavelength and exposure time. Equations 3and 4 show the ANSI standard for calculating the MPEs for point source ocular exposure and skin exposureto laser energy.13

MPEOcular(J/cm2) = 9.0 � Cc � t(0.75) � 10�3 (3)

MPESkin(J/cm2) = 1.1Ca � t(0.25) (4)

These calculations are based on the 1.064mm wavelength and are valid for exposure times between fiftymicro seconds up to ten seconds. These thresholds are compared to the calculated energy leaving the targetand a specific range at which these levels are met is computed. Separate ranges for ocular damage andskin burn are determined. The ANSI standards for MPE require the minimum amount of energy to do anydamage. To actually cause blindness, a much higher amount of power or longer duration is required. Thismeans that ranges calculated in this research are a worst case scenario. To determine the range at whichmore significant collateral effects occur, this research can be repeated with higher MPE values. The higherMPE values will require more power per area, therefore generating much shorter ranges. Shorter rangesare expected to significantly reduce the calculated collateral damage effects.

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III. Results

Table 2. Power Transmitted to the Target for VaryingSource Power Levels and Slant Ranges Using a Humid At-mosphere Based on Hurlburt Field, FL Data

Following the methodology described in the previ-ous section, the energy received at the target is based onthe 1.064mm wavelength coupled with the power and at-mospheric path. Table 2 shows the energy at the targetfor all geometries and power levels at Hurlburt Field, FL.Similar results were analyzed for Mexico City, Mexicoand Albuquerque, NM. As can be seen, each MSL alti-tude was combined with the 10 NM standoff and con-verted to meters to generate a slant range as an inputinto HELEEOS3.

The first thing that this table indicates is that powerlevels at the target appear higher than the power at thesource. This apparent increase is caused by the focusingoptics. The whole point of a DEW system is to generate the maximum energy on a target, therefore thefocusing optics concentrate the laser power at the target into a smaller area. This effect causes the apparentincrease of power at the target when in reality it is just focusing. This table clearly shows as power increasesat the source, power at the target increases, as expected. However, this effect is not linear. Doubling of thepower at the source, e.g. power level B to power level C, only results in a 16% to 38% increase in powerat the target, depending on geometry. This result is most likely attributable to blooming, where significantpower is spent heating the atmosphere on the way to the target.

Figure 4. Hurlburt Field Ranges for 25,000ft Power Level B

The combination of the target absorption factor and the assumption that only half of the energy leav-ing the target is specular, reduces the specular energy leaving the target to five percent of the energyarriving at the target. This new energy level is used as an energy source for HELEEOS3, and from theHELEEOS3 program, a transmission curve is generated. The power available for collateral damage versusrange from the target for power level B transmitting through the atmosphere model of Hurlburt Field, FL isdepicted in Figure 4. Added to this transmission curve is the Gaussian divergence calculated by a separateMATLABrbased algorithm. The convolution of these two factors directly show how much power per areaexists as distance from the target.

There are many variables that dictate the maximum exposure time a target would be exposed to col-lateral specular energy. The aircraft platform motion, target motion and target surface’s change throughout the laser exposure are all major factors that are difficult to take into account. Due to these factors, it isuncertain how long a person would be exposed to specular energy leaving a target. This research thereforeused four exposure times: 1 second, 0.1 second, 0.01 second and 0.001 second exposure times. The MPE

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equations, Equations 3 and 4 were used to calculate the amount of energy needed to cause ocular or skindamage for each exposure time. Based on these exposure times and the laser wavelength of 1.064mm, theMPEs in Table 3 show how much power is needed to cause damage. The curve in Figure 4 also shows thatalthough power decreases quickly with distance, there is such a large quantity of power leaving the target,that small quantities of power can be found at long ranges. These small power quantities are large enoughto meet the thresholds from Table 3. A relatively small change in power leaving the target, or a larger MPEthreshold will result in a significant decrease in range. For example if the power was reduced by a factor often, or the MPE threshold increased by a factor of ten, the range in km would be more than halved.

MPE (W/m^2)Time Ocular Skin Burn

1 90 55,0000.1 160 309,288

0.01 284 1,913,1800.001 506 9,780,540

Table 3. MPE Thresholds for 1.064mm

Taking the MPE values from Table 3, an additionalMATLABr algorithm uses these inputs along with theMPE data shown in Table 3 to determine the range atwhich a human will be injured from collateral specu-lar energy. The ranges for Hurlburt Field, FL, for bothpower levels B and C at all geometries are displayed inTable 4. The very long ranges for ocular damage imme-diately garner attention.

These ranges are long for a variety of reasons. First,throughout this research conservative estimates have al-lowed for a best case scenario in quantity of power on

target. In all likelihood this would not occur, however these same conservative estimates lead to a worstcase scenario in regards to collateral damage as depicted by the longer than anticipated ranges. Second,turbulence, which is a major factor close to ground level was not accounted for, also allowing for longranges. Finally, the very low amount of power needed to cause an ocular hazard for a one second exposure,90W/m2, also leads to very long collateral damage ranges.

Table 4. Hurlburt Field Ranges for Power Levels B and C, Based on MPE Values

Table 5. Ranges for All 3 Locations, Power Level B, Basedon MPE Values

Table 4 also shows that doubling the power at thesource does not appreciably affect collateral damagerange. The ranges for ocular MPE thresholds only in-crease between 1.84% and 12.14% with an average in-crease of about 5%. The ranges for skin burn MPE thresh-olds increase more, but not significantly so, between7.57% and 23%.

Table 5 allows comparison between all three locationsfor the same power setting, power level B and similar ge-ometric profile, 25,000 ft MSL. The slant ranges are dif-ferent due to the altitude of each location being different.The ranges for Albuquerque, NM and Mexico City, Mex-ico are similar for all MPE values. This similarity is dueto similar geometries, both cities have similar altitudes. However Mexico City, Mexico ranges are shorterthan Albuquerque, NM’s ranges, most likely attributable to increased pollution particulate in the atmo-sphere above Mexico City, Mexico.

An unexpected result from Table 5 is that although Hurlburt Field, FL has the longest slant range of all

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three sites, the longest collateral damage ranges are recorded at Hurlburt Field, FL. It would be expectedthat a longer slant range would decrease power on the target with the follow on effect of having less powerleaving the target resulting in shorter collateral damage ranges. The difference in geometries betweenHurlburt Field, FL and the other sites is what drives the difference, but in an unexpected way. All altitudesare in MSL, but turbulence and extinction profiles are strongest closer to the ground level regardless ofMSL altitude. Since Albuquerque, NM and Mexico City, Mexico have significant altitude themselves theyflatten the lasers trajectory keeping the laser in the higher extinction zone longer. This reduces the amountof power that will arrive at the target. Since more power gets to the target there is more power leaving thetarget resulting in longer collateral damage ranges. Specifically the ocular MPE threshold ranges increasedbetween 13.7% and 15.5% for Hulburt Field, FL over Albuquerque, NM. The ocular MPE threshold rangesincreased between 14.48% and 18% for Hurlburt Field, FL over Mexico City, Mexico. Also noteworthy isthat these percent increases scaled down for Albuquerque, NM as exposure time decreased, while MexicoCity, Mexico increases scaled up as exposure time decreased. This may also be attributable to the increasedparticulate at Mexico City, Mexico, but further research is needed to confirm that hypothesis.

IV. Conclusion

These collateral ranges by themselves are very large compared to conventional weapons. There arethree points to consider before using them at face value. First, no turbulence model has been used for theenergy leaving the target. Second, the thresholds for collateral damage from conventional kinetic weaponsare based on a piece of metal penetrating a human, which is much more injurious to a human than a slightskin burn. Third the thresholds for laser damage in this research are based on MPE values which only startto cause “...adverse biological changes...” .13 The MPE values from this research are very conservative forcollateral damage calculations.

Turbulence close to the earth’s surface is expected to be very strong and will quickly diverge the beam,decreasing the collateral damage ranges significantly. The next step for this research is to model mathemat-ically the turbulence close to the ground to predict more accurate ranges.

Conventional kinetic weapon collateral damage effects are based on very traumatic effects to the humanbody which are much more violent than the kind of damage that laser energy, at the low thresholds ofMPE this research used, would cause. To meaningfully provide context for DEW collateral damage effects,some larger MPE threshold will be required to compare conventional weapons with DEW systems. Beingable to compare conventional weapons with DEWs is very important. The comparison allows militarystrategists to know what outcomes their decisions will have. From projected collateral damage effects ofeither conventional weapons or DEWs, they can make better decisions with the desired political, strategicand tactical results they expect to receive from a given weapon.

Along with modeling turbulence mathematically, the next step for this research is to model terrain uti-lizing Digital Terrain Elevation Data (DTED) data imported into MATLABr. The collateral specular energywill be modeled with the terrain to determine the limits of exposure to a randomly dispersed population.Along with the DTED and specular energy model, a Monte Carlo simulation will be run with varying popu-lation densities to determine mathematically the probability of collateral energy impacting the population.This added stochastic modeling is expected to provide the needed realism to compare collateral damageeffects between directed energy weapons and conventional explosives. This modeling will also add a newdimension for collateral damage determination. When a conventional kinetic weapon hits, there is a prob-ability of hitting a person at different ranges, however a kinetic weapon spreads out in all directions atonce, basically creating a sphere of damage. A DEW weapon is sending a single beam in a finite area. Thislimited geography would be similar to a narrow cone projecting the base outward from the target. Thecone could conceivably change directions throughout the lase time. Changing direction has the adverseeffect of potentially placing more people at risk of exposure, but the faster it changes direction the shorterexposure time people experience. The shorter the exposure, the greater amount of power is need to harmthem. This leads to either having a stationary cone lasing in one direction for a longer time allowing forlonger collateral damage ranges in a limited direction or a moving cone who’s shorter lase times per direc-tion decrease the collateral damage range but potentially increases more people to exposure. Determiningthe probabilities of injuring people at different ranges and power levels and relating these collateral effectsto conventional collateral effects will be critical to provide meaningful collateral damage comparisons formilitary strategists.

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Many conservative estimates, assumptions and simplifications were utilized sequentially in this re-search creating the worst case scenario for long collateral damage ranges. These estimates, assumptionsand simplifications point to many areas in which further research is needed. One of the areas needing re-finement is laser propagation for other than Gaussian beams. Many DEW systems do not produce Gaussianbeams, therefore determining a way to relate their quality to a Gaussian beam is needed. Target absorptionfor materials at differing temperatures is an area wide open for research. As a laser impacts a target, it isheating the target and may be changing absorption properties, directly effecting laser energy leaving a tar-get as potential collateral damage. Determining the percent diffuse and specular energy leaving a target forwavelength and power levels would allow for much more accurate modeling of collateral effects. Turbu-lence at ground level seems to be one of the hardest pieces to model. It is time and density based, affectedin ways that are extremely hard to model. Improving ground level turbulence models would immediatelyimprove collateral damage effects modeling. The last piece that needs to be determined is what biologicaleffect threshold MPEs the military will use for collateral damage effects. Will the conservative values usedin this research be the standard, or would an MPE value that causes total or partial blindness be used? Thisresearch does not begin to suggest one standard over another, it only points out that the current MPE valuesare restrictive and consensus is needed on what future values should be.

Directed Energy Weapons exist and are the future of warfare. Knowing both the capabilities and collat-eral effects of DEWs is not just a good idea, it is necessary. The goal of this modeling effort is to provide atool that relates DEW collateral damage to conventional weapons effects to better describe DEW’s potentialpitfalls and enhancements on the battlefield. Knowing the collateral effects of DEWs gives the weaponsdesigners, military strategists and politicians the facts they need to be wise in designing DEWs that areuseable, choosing the DEWs for the appropriate applications and responses to comply with the rules ofwarfare and international treaties.

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http://www.navy.mil/search/display.asp?story id=14520.6History of Laser Intensity, 19 September 2007, URL http://en.wikipedia.org/wiki/File:History of laser intensity.svg.7Smith, F. G., Atmospheric Propagation of Radiation Volume II, Infrafred Information Analysis Center, Environmental Research

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Air Force Institute of Technology, March 2008.12Pike, J., “Advanced Tactical Laser (ATL),” 9 July 2005, URL http://www.globalsecurity.org/military/systems/aircraft/systems/atl.htm.13American National Standards Institute Inc., “ANSI Z136.1-2007,” 16 March 2007.14American National Standards Institute, Inc., “ANSI Z136.1-1993,” 1993.15Laser Institute of America, “Laser Safety Officer Training,” 17-21 September 2007.16Hayes, O., “Marking with Fiber Lasers,” 01 May 2004, URL http://www.industrial-

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