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Kitnery

INTERIM REPORT-PART II LIGHT SOURCE EVALUATION

February 1970

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1. FACTORS AFFECTING THE PERFORMANCE OF PYROTECHNICMIXTURES

The object of this study is to investigate the feasibility of utilizing high intense light emitted from rapidly burning pyrotechnic mixtures for optical incapacitation. Several mixtures have been selected for initial study. The compositions ofthese mixtures were itemized in a previous progress report.1

It is useful to review the mechanisms of pyrotechnic burning and the important factors, relevant to light emission, so as to fully understand the utility of this approach.

1.1DESCRIPTION OF BURNING PROCESS

The basic ingredients of most pyrotechnic compositions consist of a fuel and an oxidizer. These ingredients should be stable under normal shelf conditions, yet they shouid be easy to ignite. Further, once ignition occurs, the heat released during burning should be sufficient.to.sustain the. buming process.

It is useful to consider the following model to explain the burning behaviorofapyroteehicmixture.

Three thermal zones are established when an illuminating composition is ignited and burns propagatively (see Figure 1).

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Zone A is essentially the "burning surface". Both exothermic and endothermic reactions take place resulting in the formation of gaseous fuel and oxidizer intermediates. These intcrmediates react exothermically in the flame zone. Usually the pyrotechnic is designed to be fuel rich and the excess fuel reacts with oxygen from the atmosphere. In these cases the flame size and intensity are somewhat dependcnt upon the required environmental oxygen and its availability.

The energy required to form the reactive intermediates are generated by the exothermic rèactions which occur in the flame zoñe and in Zone A. The dominant heat transport mechanisms are radiation feedback from the flame and conductive transport from condensed phase reactions which occur in Zone A. Energy from Zone A is also transferred to Zone B which may be considered the pre-ignition zone. Directly below Zone B is the remainder of the unreacted pyrotechnic composition, ar Zone C.

The radiative heat transport from the flame to Zone A can be described mathematically by the Stefan-Boitzmann equation

$$
\texttt { I } = \texttt { e o r } \{ \mathbb { T } _ { \Sigma } ^ { 4 } - \mathbb { T } _ { \Omega } ^ { 4 } \}\tag{1}
$$

where I is in terms of energy per unit area per unit time (e.g., cals/ cm²-sec),e-ts the emissivity -of the flame, ots-the-Stefair-Boltzmann constant, F is a flame shape factor, $\pmb { \tau } _ { \pmb { \updownarrow } }$ is the flame temperature and $\mathtt { T } _ { \circ }$ is the temperature of the surface receiving the radiation.

The temperature in Zone A is further affected by the local endothermic and exothermic transitions which occur in forming the reactive intermediates. The important reactions which limit the burning rate of the pyrotechnic are the endothermic processes. Typically, these processes involve

a. phase changes, and

b.pyrolytic decomposition

to generate gaseous combustion reactants.

Under steady-state conditions the thin pre-ignition zone (i.e. Zone B) can be assumed to have a uniform temperature. For simple systems this zone has thicknesses in the molecular size range, and its temperature is governed almost completely by the endothermic processes taken place.

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The temporature profile within the body of the pyrotechnic (i.e., Zone C) can be approximated by the Rosenthal equation2

$$
\mathbb { T } _ { \pmb { x } } = \mathbb { T } _ { \pmb { \bar { \alpha } } } + \{ \mathbb { T } _ { \pmb { s } } - \mathbb { T } _ { \ 0 } \} \exp \ ( - \mathrm { \bf { v } } \mathrm { \bf { x } } / \alpha )\tag{2}
$$

where $\pmb { \tau _ { x } }$ is the temperature at distance x below the reacting surface (Zone B), $\pmb { \tau } _ { \overrightarrow { \mathbf { a } } }$ is the ambient temperature, $\pmb { \tau } _ { \pmb { \ s } }$ is the temperature within Zone B, v is the buring rate, and α is the thermal diffusivity of the mixture: It should be noted that thermal diffusivity is related to more conventional thermal properties1.e.

$$
a = k / p c , \cos ^ { 2 } / s e c\tag{3}
$$

where k is the thermal conductivity, p is density andc is the specific heat.

1.2 FACTORS AFFECTING LIGHT OUTPUT

The distribution of radiation in any spectral region is determined by the chemical nature and physical state of the products which emit in that region, and the temperature reached by these emitting species. The rate at which a pyrotechnic mixture bums depends on the amount and rate at which heat is evolved. Sufficient heat must be produced to raise the temperature.of the ingredients.to:a pointat which.an exothermic.... reaction will be initiated and the reaction rate must be sufficient to more than compensate for heat losses in order for the burning to be sustained. Mathematically, the buring rate, $\mathbf { v } _ { \pmb { \mathscr { s } } }$ can be related to the energy fedback from the exothermic processes and the rate determining endothermic process which must occur to produce the reactive intermediates

$$
v = \Sigma ( I ) / \rho ( \Delta \mathbb { H } + \mathbb { C } \Delta \bar { \mathbb { T } } )\tag{4}
$$

where Σ(I) is the radiative, convective and conductive heat flux fedback to the pre-ignition zone, ΔH is the heat absorbed in the pre-ignition zone by the endothermic processes and C ΔT is the heat required to elevate the temperature of the pyrotechnic (i.e., at the boundary between Zones B and C) to the reaction temperature. The latter (pC ΔT), can be best described as a heat loss term.

The rate of burning, the products formed, and the flame temperature are affected markedly by the composition-of the mixture, as well as by the physical condition of the materials and the ambient conditions under which it is burned. Some of the more important factors which affect the performance of light producing pyrotechnics which were considered in our initial selection compositions are as follows,

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1. heat of reaction

2. composition of emitters

3. particle size

4. consolidation

5. pyrotechnic diameter

6. container design

1.2.1Heat of Reaction

The heat of a reaction is defined thermochemically as the difference between the thermodynamic heats of formation of the reactants and products of the reaction. For example, referring to the energy diagram shown in Figure 2, one selects a pyrotechnic mixture having substituents which have a higher heat of formation than the reaction products and one which requires a minimal amount of input energy, $\Delta \sqrt { 2 } a$ , to initiate burning.

One of the important factors in determining the luminous intensity of a light-producing pyrotechnic device is the temperature reached by the emitting species in the flame and produced by the burning of the pyrotechnic mixture. The temperature reached depends, in turn, on the amount and rate at which energy is released by the reaction. Therefore, the energy released during combustion should be high and the products formed should be stable at the high temperatures necessary to produce the • desired luminous intensity.

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1.2.2 Desired Output Spuctra

The ultimate pyrotechnic composition must emit intense light in regions most sensitive to the eye. It can be seen from the "standard observer curve", shown in Figure 3 that the eye is only sensitive to a very narrow region of electromagnetic radiation.

This region extends between approximately 400 to 700 mu (or 4000 to 7000 Angstroms). In Figure 4, the visual response curve of the human eye 1s shown. In this figure the effective radiation, in terms of photometric units (lumens) as a function of wavelength is shown. The absolute photopic luminosity is defined as the ratio of the electromagnetic flux sensed by the eye (in units of lumens) to the total radiant flux (in terms of watts). The most sensitive region in this narrow spectrum lies between 500 and 560 mu. Therefore, for effects related to visibility of the light source, the pyrotechnic mixtures should be designed to emit strongly in this wavelength region. A separate question arises as to whether optical incapacitation effects are similarly correlated over the visible range. This answer is not known at present but will be assumed . to be positive for the present.

Special design features must be included in a pyrotechnic to insure that a significant fraction of the total radiation emitted by the flame is in the visible region. The emission from a pyrotechnic flame is composed of line spectra, band spectra and continuum. The latter is directly dependent on the temperature of the flame. The continuum is essentially blackbody or greybody radiation. The distribution of the radiant energy versus wavelength can be estimated from Planck's equation

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$$
I _ { \lambda } = \frac { 2 - c ^ { 2 } \hbar c } { \lambda ^ { 5 } ( e x p ( \hbar c / k \lambda \hat { \lambda } ) - 1 ) }\tag{(5}
$$

where $t _ { \lambda }$ is the radiant flux, in terms of energy per unit area per unit time at wavelength λ, emitted by a hot source at temperature T, h is Planck's constant, c is the speed of light, k is Boltzmann's constant and e is the emissivity of the flame. Typical flux-wavelength distributions calculated from this equation are shown in Figure 5.

It can be seen from this figure that very little of the energy emitted by blackbody radiation is distributed in the wavelength band most sensitive to the eye. Further, in order to generate an intense source in the visible spectrum, a very high flame temperature will be required and even under these conditions the efficiency of the source in terms of the fraction of visible light energy generated out of the total radiation energy would be • very low.

Fr

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1.2.3 Desirable Pyrotechnic Ingrcdients

This observation leads one to recognize that pyrotechnics composed of organic fuels which have the highest heat of reaction cannot be considered since these mixtures produce flames having characteristics similar to blackbody emitters. This is particularly true for fuel rich compositions which have a tendency to generate significant amounts of carhon and aromatic soot particles.

The radiantemission- in the-visible-canbe-improved-by-inchuding chemicals in the pyrotechnic mixtures that will form thermally excited reaction products capable of emitting radiation at desired wavelengths. This process can be described by the following equation,

$$
{ \begin{array} { r l r l r l r l } { { \dot { \mathbf { A } } } + \mathbf { B } } & { \to \ } & & { \left\{ { \boldsymbol { \mathsf { C } } } + \mathbf { \Pi } { \mathsf { h e a t } } \right\} \ } & { \to \ } & & { \left\{ { \boldsymbol { \mathsf { C } } } ^ { \star } \right\} \ } & { \to \ } & & { \mathbf { { \dot { C } } } \ + \ \mathsf { \Pi } \mathsf { h v } } \\ { { \mathsf { P y r o t e c h n i c } } \ } & { \quad } & & { \operatorname { \dot { R e a c t i o n } } \ } & & & { \operatorname { \dot { T r u e r m a l l y } } \ } & & { \qquad { \mathsf { L i g h t } } } \\ { { \mathsf { C o m p o n e n t s } } \ } & { \qquad } & & { \operatorname { P r o d u c t \ a n } \ } & & { \qquad { \dot { \operatorname { E x c i t e d } } } } & & & { \qquad { \dot { \operatorname { E m i s } } } \le \mathsf { i o n } } \\ & { \qquad } & & { \qquad \operatorname { F l o m e } \ } & & & { \qquad \operatorname { P r o d u c t } } \\ & { \qquad } & & & { \qquad \operatorname { S p e c i e s } } & & & { } \end{array} }\tag{6}
$$

Many inorganic salts exhibit this phenomena. Several elements which react in pyrotechnic flames forming oxides, hydroxides and chiorides have been used to "color" flames. These include, strontium which produces a.red.color,barium.(green)sodium.(yeHow),calctum(yellowgreen and orange) and copper (blue to green), Lithium (red), boron (green), thallium (green), rubidium (red) and cesium (blue) are also strong color producers but their use is not as practical becausè of cost, toxicity or the nature of their compounds.

The actual emitting species of these metals are known to be the di- and tri-atomic spècies which can exist at high temperatures in the flame. For example,

a. . the red light produced by flares containing strontium and a source of chlorine is a result of SrCl emission (strong emission near 640 mu). In the absence of chlorine, emission has been attributed to SrO.

b. BaCl2 emits in the 505-535 mμ region (green)..

c. BaO emits over a broad spectrum, 400 to 800mμ.

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d. Thc hydroxides of these mctals also emit in the respective wavclength bands.

e. MgO emits at apprcximately 500 mμ.4

1.2.4Color Intensifiers

Where possible, chlorides are added to the pyrotechnix mixtures to enhance the color of the flame. Perchlorate oxidizers contribute the minimal requirements without reducing the efficiency of the energy output. In some cases it has been found that the addition of chloroorganics significantly increase the coior intensity. Substances such as hexachlorethane, hexachlorobenzene, polyvinylchloride are sometimes employed for this purpose.

2. CRITERIA USED TO SELECT CANDIDATE PYROTECHNIC MIXTURES

The following considerations were made in selecting candidate pyrotechnic formulationsl for the initial experimental investigations. Ba sed on thepremised. desirability of producing flames which emit radiation in the visible wavelength region,primary consideration has been given to inorganic compositions.

2.1 OXIDIZER

Comprehensive literature surveys by Shock Hydrodynamics and other investigators have shown that perchlorates are the most desirable oxidizers. They contain a relatively high ratio of oxygen to total molecular weight, their heats or reaction with metals such as aluminum and magnesium are significantly better than other oxidizers and they generate only oxygen as a gaseous product. Perchlorates are generally stable, yet they are easily ignited, unlike oxidizers such as nitrates and metallic oxides. Nitrates suffer additional disadvantages in that they are not as exothermic and thus tend to have a slower burning rate with fuels such as aluminum and magnesium (see Equation 4), and they generate a nonreactive gaseous product, nitrogen.

Of the alkali perchlorates, lithium and sodium perchlorates have the highest rates of oxygen to total molecular weight, viz., 64/106.4 and 64/127.45, respectively. However, these compounds are extremely -hygroscopic. This characteristic decreases the storage life of a pyrotechnic system. The absorption of water by these oxidizers is an exothermic process. Because of this factor one must also be concerned with the design of special safety precautions.

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Potassium perchlorate was selccted as a primary oxidizer for the initial candidate mixtures. This oxidizer has an oxygen-total weight ratio of $6 4 / 1 3 8 . 5 5$ , which is somewhat lower than $\mathtt { I i C i O _ { 4 } }$ and $\tt N a C l O _ { 4 }$ however, it is a stable compound and its heat of reaction with aluminum, for example, is slightly greater than the reactions between $\tt L i C l O _ { 4 }$ or $N a C l O _ { 4 }$ and aluminum.

2.2FUEL COMPONENTS

It has been found that aluminum and magnesium are the best fuels for use-in photoflash mixtures. The heat of reaction and peak light intensities resulting from $A 1 / \tt R C l O _ { 4 }$ are much higher than equivalent $\mathbb { M } \mathbb { g } / \mathbb { K } \mathbb { C } 1 \mathbb { O } _ { 4 }$ compositions. The radiation emitted follows generally what would be expected for continuum radiation. Peak light intensities for stoichiometric mixtures of $A 1 / \ K C 1 0 _ { 4 }$ and $\mathtt { M g / K C 1 0 _ { 4 } }$ have been measured to be approximately 40 and 18 million candles; respectively.

2.3CONSOLIDATION OF MIXTURE

It has been shown, that the manner in which the fuel and oxidizer are incorporated in the pyrotechnic device.will greatly influence its performance.5 Consolidated compositions contain binders, usually organic polymerswhich.form.a rigid or semi-rigid body.The.consolidated composition however is a burning system which has a relatively large spatial separation between fuel and oxidizer. Thus, the burning rates are relatively slower than a comparative non-consolidated system. Non-consolidated systems under confinement usually have higher deflagration rates than consolidated systems and the intensity of the emitted light is greater.Most photoflashsystems are thus nonconsolidated, and this type of system was selected for these studies.

2.4OUTPUT IMPROVEMENT (SELECTION OF STANDARD MIXTURE)

A standard photoflash composition, III-A, was selected as a reference. The composition of this mixture is shown in Table I. The addition of barium nitrate to this mixture increases the radiation output in the visible spectrum over that of the basic $A l / \tt K C l O _ { 4 }$ mixture. As discussed in a previous section, the BaO and $B a C l _ { 2 }$ formed in the burning processes emits strongly in the wavelength region most sensitive to the eye.

2.4.1 Mixture "D"

During the Korean conflict5 an experimental photoflash mixture having a very high fuel to oxidant ratio was developed having a peak

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light output twicc that of the Type III mixture. This mixture consisted of 70% $\tt A i / 3 0 \% \ K O 1 0 _ { 4 }$ . A further improvement in performance is anticipated by replacing a portion of the $\tt N C 1 0 _ { 4 }$ oxidizer with Ba $( N O _ { 3 } ) _ { 2 }$ which will act as an oxidizer and color enhancer (see Table II for compositions of candidate pyrotechnic mixtures).

\*All mixtures prepared in accordance with PA-PD-267.

TYPE A Follows the formulation given in PA-PD-267 for Type III Class A (Fine Oxidizers) .

TYPES B, C, & D follows the same guidelines including particle size given in PA-PD-267.

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2.4.2 Mixture "B"

The addition of calcium silicide should have two effects on the $\mathtt { I a s i c a l \mathrm { A l / K C i O _ { 4 } } }$ reaction:

1. Because of the exothermic nature of CaSi in an oxidizing environment, the heat of reaction of this mix should be greater than the selected reference (i,e. mixture A).. This. increase in. the heat of reaction will raise the flame temperature and the radiation intensity of the flame.

2. The flame emission of calcium is at 550 and 620 mu , This is in the yellow-green and orange areas of the spectrum and as can be seen from Figure 4 should improve the flame luminosity in the most sensitive wavelength regions.

A smaller percentage of calcium silicide than barium nitrate is required because, of molecular weight differences. Calcium has an atomic weight of less than a third that of barium. Thus, on a weight basis calcium should be more efficient.

2.4.3 Mixture "C"

It has been observed that the use of magnesium-aluminum fuel mixturesresults in flashes of longer duration. This fuel combination is also easier to ignite as compared with aluminum. Mixture "C" was therefore formulated for purposes of determining the differences in potential effectiveness with flash duration.

2.5 PARTICLE AND SHELL SIZE

The same quidelines of particle size suggested for the reference photoflash mixture (desianation "A") are being used for the other mixtures. This control is necessary so as to minimize the number of unknown experimental parameters.

A representative array of sheli sizes have been included in the experimental plan, These shells were described in Reference l. All of the casings are composed of aluminum.

2.6 METHOD OF INITIATION

An additional variable which was included in the experimental plan was the method of initiation, Simple central burster initiation as well as an imploding initiation system are included in the test plan for comparison.

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3. R. L. Tischer and K. Scheller, "A Gaseous System of High Luminous Efficiency", (Presented at the Western States Combustion Institute Meeting, 1967), Report WSCI-67-12, Aerospace Research Labs.

6. Engineering Design Handbook, "Military Pyrotechnics Series", AMCP 706-185, U. S. Army Materiel Command (April 1967).

REFERENCES

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FINAL SUMMARY REPORT PART 2 LIGHT SOURCE EVALUATION

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TABLE OF CONTENTS

1. INTRODUCTION. 1
2. DESCRIPTION OF EXPÉRIMENTAL
PHOTOFLASH DEVICES 2
2.1 Photoflash Cartridge, Type S 2222
2.2 Photoflash Cartridge, Type SDE
2.3 Photoflash Shell, Type SS .
2.4 Photoflash Shell, Type SSDE
3. DESCRIPTION OF EXPERIMENTS. 5
3.2 3.3 Experimental Errors 556
4. EXPERIMENTAL RESULTS 9
4.1 Comparison Between Reported and
Measured Output of Type III Mixture 9
4.2 Comparison Between Charges 9
5. DISCUSSION OF POTENTIAL FLASH
BLINDNESS EFFECTS 17
5.1 Permanent Eye Damage 17
5.2 Recovery Time to Light Flashes 19
5.2.1 Review of Flash Blindness Experiments . 19
5.2.2 Additional Observations 22
5.3 Interpretation of Results 24
5.3.1 Illumination of Photoflash 24
5.3.2 Estimated Eye Effects 24
6. CONCLUSIONS 31
6.1 Cartridge Design 31
6.2 Photoflash Mixture 31
6.3 Data Intcrprctations 31
REFERENCES 32
APPENDIX I 33

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1.INTRODUCTION

The objective of this invcstigation of pyrotechnic light sources was confined primarily to the cvaluation of output illumination levels, the factors controlling them and comparisons with the existing criteria for optical incapacitation, since these considerations determine the suitability of such a system to a variety of possible weapon applications.

The effort was directed primarily at comparisons of existing threshold energies for incapàcitation and damage, pyrotechnic output energies, and the fabrication of test devices with controlled parameters, followed by estimation of their output energies by means of appropriate instrumentation. The approach permitted a comparison with existing data on energy levels corresponding to optical incapacitation and damage. This comparison was used to obtain first order estimates of the effective. ranges at which these pyrotechnic illumination devices would either incapacitate or permanently damage exposed personnel.

Thc four pyrotechnic light mixtures shown in Table I were evaluated during this investigation. In the following sections there are presented the methods and techniques employed, results obtained, and the conclusions regarding the feasibility of this approach.

TABLEI..COMPOSITION OF EXPERIMENTAL MIXTURES

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2.DESCRIPTION OF EXPERIMENTAL PHOTOPLASH DEVICES

A brief description of the charges was included in thc Final Summary Report - Part I. Two basic photoflash cartridges werc employed. Design drawings of these cartridges are presented in Figures 1 and 2. The performance of each of the four selected pyrotechnic mixtures was evaluated in each cartridge design. The specifications of each of the mixtures were previously described (see Reference 1).

2.1 PHOTOFLASH CARTRIDGE, TYPE S

The outer diameter of the Type S photoflash cartridge is 2.7 in. and the length is 6.6 in. The outer shell is made of aluminum. A scale drawing of the cartridge is shown in Figure 1. A linear charge of DuPont PETN primacord, .22 in. diameter and 5.25 in. long, was surrounded by the photoflash mix. This central bursting charge was used to break the casing, and to ignite and disperse the pyrotechnic mixture.

2.2 PHOTOFLASH CARTRIDGE, TYPE SDE

This cartridge was identical to the type S with the exception that DuPont Deta sheet explosive.was wrapped around the exterior cylindrical wall of the Type S cartridge. These sheets were cut, as shown in Figure 1, to permit the complete coverage of the outer wall. The purpose of this design was to compact the photeflashmixture first beforeitwas"dissemi nated, in an attempt to improve the light output (i.e., as discussed in the last progress report, the rate of burning and light output should increase with the degree of charge compaction).. The ignition train was designed so that the Deta sheet would be detonated before the central bursting charge.

2.3 PHOTOFLASH SHELL, TYPE SS

The Type SS shotgun shell has an O.D..of 0.853 and a length of 3.20 in. It is also made of aluminum. Except for dimensions, this photoflash shell design is similar to the 2.7 in. photoflash cartridge (see Figure 2).

2.4 PHOTOFILASH SHELL, TYPE SSDE

This shell is identical to the Type SS, with the exception that Dupont Deta sheet is wrapped around the cylindrical body in a similar manner as the SDE:

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3. DESCRIPTION OP EXPERIMENTS

The experimental tochniques employed and the specific experiments performed arc outlined in this section.

3.1 EXPERIMENTAL ARRANGEMENT

The intonsity of the light output as a function of time wàs measured and recorded using a silicon photodiode (United Detcctor Technology Inc., PIN-10) and a Tektronix 535 oscilloscopc with a Polaroid film attachment. The PIN-10 photodiode has a half-value response between 3800 and 10500 Å. The photodiode, PIN-10, was protocted from fragments generated from the explosive dissemination by a metal shield. The light from the flash was indirectly focused onto the PIN-10 sensor using a mirror.

A Corning 1-56 filter was placed in front of the PIN-10. This filter has transmission charácteristics similar to the eye response. It has a near gaussian trans mission curve between 3600 and 7000 Å with a peak transmission at 5200 A. The ultraviolet is cut-off at 3600 A and less than 10% of the 1-R radiation (i.e., between 1 and 4.5 microns) can be transmitted through this filter. Depending on the expected output of each test device, neutral density filters were also used so that the PIN-10 would operate within its linear response regime (i.e., so that light intensity versus output voltage would remain linear).

The output signal from the sensor was fed into a Tektronix 535 oscilloscope with a 50 ohm termination. The signals were permanently recorded using a Polaroid camera attachment. The oscilloscope and event were triggered using a 5 KV firing panel.

The events were also monitored photographically. A speed graphic single exposure camera and a.Beckman & Whitley Dynafax motion picture camera were used with Polaroid filters. The Dynafax camera was operated at a framing rate of 3000 frames per second.. Both cameras were protected from the blast. The Dynafax camera was located behind a baricade and received the light from the photoflash via a front surface mirror. The speed graphic camera was located behind thick glass.

3.2 DETECTOR CALIBRATION

The PIN-10 photodetector was calibrated using a National Bureau of Standards certified light source. The response of the detector was measured as a function of distance away from the standard source using the following equation

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where

Iis the intensity of the light source in candle-power,

K is the calibration factor, 2640 foot-candles pcr 38.07 millivolts response,

F is the factor which compensates for the transmission of the neutral density filters,

V is the output signal, volts, generated by the photodetector, and

is the opticał distance between the PIN-10 photodetector and the light source.

The spectral response characteristics of the photodetector are shown in Fiqure 3, The relative spectral sensitivity of the PIN-10 with the Corning 1-56 filter was calculated. These results are presentedin Figure 4.

The optical distance between thc light sources and the photodetector was 23.5 feet in éach experiment. The intensity of the source expressed in candle-power was calculated using Eq. (1). Thevalues of candle-power estimated can be interpreted directly in terms of the luminous flux traveling through a normal plane which intersects the light path 1 foot away from the source (i.e., the density of luminous flux incident on a normal surface one foot away from a 1 candle-power source is 1 lumen/ft²).

3.3 EXPERIMENTAL ERRORS

An experiment was performed to determine the amount of light received by the photo-detectors via wall reflection. A General Electric No. 22 photoflash bulb was placed 23.5 ft. away from the photo-detector in a similar manner as were the photoflash pyrotechnic test devices. Black paper was placed directly in front of the flash bulb to prevent direct light transmission to the photo-detector. When compared with a control , test in which the light barrier was not used, it was calculated that less than 5% of the light receivod by the photo-detector was due to light reficctions from the walls of the test chamber. As a precaution, in subsequent tests, all light colored objects were rémoved from the test chamber before each experiment. The walls of the chamber were washed down after each test, also, to remove any dcbris which might increase the light reflection.

I=K·F·V·D²

(1)

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4. EXPERIMENTAL RESULTS

The measured outputs versus time for each of the exporimental photoflash devices are shown in Figurcs 5 through 8. These figures were drawn from the oscilloscope traces obtained in each experiment.The results of cach photoflash cartridge design wore combined so that a ready comparison could be made regarding the relative períormance of the light output from each of the four pyrotechnic mixtures. The peak light intensities, the half width of intensity-time and the total integrated area under each curve (i.e., total light output in candle-power-seconds) are reported in Table II. The maximum cloud size generated by each device is also included in this table. The latter data was obtained irom the . film records taken.

4.1 COMPARISON BETWEEN REPORTED AND MEASURED OUTPUT OF TYPE III MIXTURE

Mixture A has a composition identical to the standard TypeII (i.e., mixture A) photoflash mix. This Type III photoflash mixture has previously been used in a variety.of standard photoflash cartridges. Light outputdata was obtained for this mixture,² from other sources, and is summarized in Figure 9.. The peak intensities and integrated candle-power-seconds total output are plotted as a function of charge size in pounds. Approximations of best-fit curves were drawn through each set of data as shown. The arrow on the abscissa represents the charge weight of the pyrotechnic mixtures in the Type S caftridge, 1.4 lbs.

The predicted peak intensity and total output for this charge are, respectively, 2.3x108 c. p. and values obtained durinġ the tests. on this program were 1. $4 . 4 \times 1 0 ^ { 6 } 0 . 0 . 5 .$ The experimental $2 . 0 \times 1 0 ^ { 6 } . 0 . 9 . 5$ $9 6 \times 1 0 ^ { 8 } c$ .p. and . (see Table II). These data are considered to be in reasonable agreement and provide an additional check on the accuracy of the measuring techniques and the charge preparations.

4.2 COMPARISON BETWEEN CHARGES

Based on the peak intensity data, the overall performance of the Type A mixtures in the various shells was the best. In order of decreasing performance it was found that

$$
A > C > B > D
$$

The peak intensity of the output however is not the only critcrion that should be used in evaluating the performancé of a mixture for this application. The duration of the light pulse is of equal importance. From

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\*S and SDE refer to the type of photoflash cartridge used. A, B, C and D refer to the pyrotechnic mixture used (see Table I).

TABLE II. SUMMARY OF TEST RESULTS

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the plots of intensity versus time it was slown that the light pulses from the $^ { \mathfrak { u } } C ^ { \mathfrak { n } }$ mixture were in most cases of longer duration. Based on the total integrated light output the $\because$ and $" \hbar ^ { \mathfrak { n } }$ mixtures were equally eficctive.

$$
i , e , , C \approx \tt A > B > D
$$

It could be concluded that of the mixtures studied, the $" \Delta ^ { \prime \prime }$ and $" 0 "$ mixtures provided the most intense and total light output. The "B" and "D" mixtures generated, significantly, less light in the visihle spectrum.

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5. DISCUSSION OI POTENTIAL FIASH BLINDNESS EFFECTS

Several important questions had to be answcred before these data could be interpreted in terms of potential flash blindness effects:

1. What is the levcl of flash luminosity thaț will produce permanent eye injury?

2. What is the level of flash luminosity that will produce transient flash blindness?

3. If the measure of the transient flash blindness is expressed in terms of functional requirements calling for viewing and resolution of specific objects, how does the luminosity of the objects to be viewed by the observer affect recovery time from non-injurious flash blindness?

5.1 PERMANENT EYE DAMAGE

It has been reported that the threshold energy level for permanent eye damage is between 0.2 and 1.6 cals/cm2.a-s

Zaret³ expresses the requirements for permanent eye injury in terms of the fraction of the photopigments which are bleached by the light flash. This bleaching process involves the photochemical transformation of ll-monocis retinene to.trans-retinene. The ll-monocis retinene complexes with the opsin enzymes to form the active photosensitive pigments. Upon light excitation the il-monocis olefin is transformed to the more chemically stable trans isomer via an electronically or vibrationally excited state.-9. The trans-retinene, a yellow pigment, which is formed is not compatible with the opsin enzyme and, thus, does not form a photosensitive pigment. Recovery of the bleached pigment is dependent upon à chemical transformation back to the 1l-monocis isomer catalysed by retinene isomerizase.

The concentration of visual pigment only begins to bé significantly affected by light intensities of the order of 105 troland-seconds\* and decreases rapidly with further increase in intensity. The concentration of bleached pigment as a function of light intensity can be expressed as ,follows³

\*The troland is the unit of rctinal illumination. It is equal to the product of the luminance of the object viewed in candles/(meter)² and the area of the pupil in mm².

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$$
\mathsf { C } _ { \mathrm { b } } / \mathsf { C } _ { \mathrm { o } } = \mathrm { \Omega } _ { 0 } = \mathrm { \Omega } _ { \mathrm { - } } \mathrm { e x p } \mathrm { \Omega } \left( \mathrm { - } \mathrm { a } \gamma \mathrm { \Omega } \mathrm { I } \mathrm { t } \right)\tag{2}
$$

where

$c _ { \mathrm { b } }$ is the concentration blcachcd at exposure It,

$c _ { 0 }$ is the original pigment concentration,

It is the retinal total irradiance in troland-seconds, and

aγ is the photosensitivity expressed in $( 1 , 1 , 3 , 5 , 5 ) = 1$

In the case of the human pigments the value of aγ is approximately $1 0 ^ { - 7 } ( t d - s e c ) ^ { - 1 }$ . Zaret estimates that as the fraction of pigment bleaching approachcs unity permanent retinal damage occurs. Within the time frame of the flashes which were produced in the experiments reported here, the threshold damage irradianceș are approximately 0.4 ca $1 5 / { \tt c m } ^ { 2 } \ \left( 4 \times 1 0 ^ { 9 } \thinspace \mathrm { t d } - \tt s e c \right)$ and $\mathtt { 1 . 6 ~ c a l s / c m ^ { 2 } } \mathtt { ( 1 . 6 \times 1 0 ^ { 1 0 } ~ t d - s e c ) } ^ { \mathtt { 1 0 - 1 2 } }$ for exposures of 1 and l00 msec, respectively. According to Brown,\* these irradiance levels must be delivered at flux levels of at least 0.7 ca $1 s / \tt c m ^ { 2 } - s$ sec or the rate of heat dissipation in the eye tissue will be sufficient to prevent an elevation of temperature to the degree where thermal burn will occur.

At 5550 angstroms the wavelength of maximum sensitivity to the eye, one watt of radiant energy corresponds to 672.1 lumens. Assuming that all of the light emitted from the pyrotechnic flash devices tested during the program is at this wavelength, the light intensities required to affect thermal damage to the retina would be 1.75 and $6 . 5 9 \times 1 0 ^ { 4 }$ lumen-sec/ft2 for exposures of 1 and l00 msec, respectively. The above estimates take into account the fact that the light received at the cornea is intensified when it arrives at the retina (i.e., the imago size is reduced): Hams noted that an irradiance received at the cornea of a rabbit eye is intensified by a factor of 60 times when it reaches the retina.

With respect to the experiments performed in this investigation, these values are considered to be low. Àssuming that the light emitted from the flash units have the same spectral characteristics as the sensor, than the luminous efficiency of the light output is only 28 percent that of 5550 angstrom light. The threshold light exposures would then be $6 . 2 5 \times 1 0 ^ { 4 }$ and $2 . 4 8 \times 1 0 ^ { 5 }$ lumen-sec/ft² for exposures between 1 and 100 msec, respectively.

Based on the luminous intensity data, shown in Table II, $\smash { \bar { \mathbb { I } } \bar { \mathbb { I } } , }$ it can be seen that the small photoflash charges (i.e., the SS and SSDE series) are

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not capable of delivering damaging light flashes. The larger charges (i.e., the S and SDé serics) can produce permanent eye damage. Further discussions rcgarding the possibility of permanent eye injury are presented clsewhcre in this report (sce Section 5.3.2).

5.2 RECOVERY TIME TO LIGHT FLASHES

Before one can estimate the recovery period after light exposure it is important that the imporiant recovery measurcment conditions be defined. Clearly, the intensity of the light which the obscrver is exposed to will determine recovery time. An additional consideration is the illuminance of the object which the observer needs to detect after exposure for funç- tional reasons.

A: review of the literature was made in order to define the.dependence of recovery on the two factors noted above. The results of this. search and the subsequent analyses are shown in Figure 10.

5.2.1 Review of Flash Blindness Experiments

Metcalí and Horns ,is conducted flash blindness experiments using the high intensity flashes from a carbon arc. The experiment was designed to determine the effect of light exposures likely to be encountered during nuclear operations. Each of the four subjects had their pupils dilated prior to exposure. A 6mm artificial pupil was used in order to maintain constant pupil size. The subjects were exposed for 100 msec to illuminà- tion ranging from 70 to 12,000 lumens per square foot. Following this exposure, the subjects were required to detect the flashing of a 17 minute visual angle circular patch. The luminance of the test patch was varied between ,07 and 71 foot-Lamberts. A summary of a complete set of this $5 \times 1 0 ^ { 5 }$ $\mathrm { i } \mathrm { u m e n } - \mathrm { s e c } / \mathrm { f } \pm 2$ is shown in Figure 10. data at a flash luminosity·of

The time required.to recover visual sensitivity following exposure to high intensity, short duration adapting flashes also has been investiğated by Chisum and Hill.6, 14 Adapting flashes of 33 to 165 μsec and 9.8 msec in duration with luminances from $1 \times 1 0 ^ { 4 }$ to $5 \times 1 0 ^ { 8 }$ lume $\cos ^ { 2 } / \tan ^ { 2 }$ were used. Visual sensitivity was determincd by the resolution of gratings requiring acuities\* of 0.13 and 0.33 at display luminances between approximately ,004 to 200 millilamberts. The 0.33 acuity level requires the function of cones whilc the 0.13 acuity level can be resolved by rod vision. The light puises used by Chisum and Hill which best represent the flashes

\*Acuity is defined as the relative ability of the visual organ to resolve detail. It is usually expressed as the reciprocal of the minimum angular separation in minutes of two lines just resolvable as separate.

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from the pyrotcchnic devices tested during this program were selected for the comparisons made in Figure 10. Also the data for the acuity level 0.33 was used, since the effects to cone vision are the most critical to this study. It should be noted that rocovery from rod saturation is a much faster process than rccovery from cone saturation. The latter also rcquires more energy for saturation.

It was observed that the recovery times for the light illuminations of $5 \times 1 0 ^ { 5 } \mathrm { ; } 3 \mathrm { u m e n s - s e c / f t } ^ { 2 }$ , reported by Metcalf and Horn, and $5 \times 1 0 ^ { 4 }$ lumens-$s \in C / I t ^ { 2 }$ , reported by Chisum and Hill were almost identical. This is not too surprising after one reviews the discussions by $B r o w n ^ { 4 }$ and Zaret.³ Namely, both postulate that the relation between the energy of an adapting flash and recovery time for a specific visual task is similar in natureto that shown·in Figure 1l.

Figure 1l. A hypothetical curve iliustrating the relation between energy of a blinding flash and time required for detection of information in a visual display. The minimum detection time at low flash energy corresponds to visual reaction time. Detection time approaches infinity as flash energy approaches a value which will cause irreversible injury.

For very low adapting flash energies thcre is very little, if any, effect on the visual capability, and recovery time is minimal. As energy is increased, there is an increase in recovery time at an increasing rate. The form of this function depends on the nature of the visual task. As the energy of the adapting flash reaches a level which corresponds to a maximum possibie bleaching of the photosensitive pigments of the retina, the rate of increase of recovery time may be expected to decrease. It is postulated as shown in Figurc ll that recovery time may actually assume a constant value over . some range of adapting flash energles beyond that at which maximum bleaching occurs.

\*Adapting flash energy usually rcfers to the total energy to which the subject is exposed and from which the subject must recover normal vision.

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It appears thut the data of Chisum and Hilí at $\mathfrak { s } \propto 1 0 ^ { 4 }$ lumens-scc $\sin ^ { 2 }$ and Metcalf and Horn at ${ \tt S x 1 0 ^ { 5 } }$ lumer $\scriptstyle 1 5 - \operatorname { s c c } / 5 + 2$ can be explaincd by this qualitative argument. Although there are some diflerences bctwcen the individual expcriments, the very close correlation in measured rccovery times, together with thc fact that the saturated bleach levels are being approached or perhaps exçeeded in both, indicate that the intensity plateau is in the range of $\mathtt { i } \mathtt { 0 } ^ { \mathtt { A } }$ to $1 0 ^ { 6 } \mathrm { \ : 1 u m e n - s e c / f t } ^ { 2 }$

The data reported by Chisum and Hili at the illumination level of $1 \times 1 0 ^ { 3 }$ lumen-sec/ft² was extended as shown in Figure 10 using data reported by Sevcrin.s Severin reported results in which the display luminance of test patches were between 0.06 and 0.013 foot-Lamberts. The results obtained at an adapting flash intensity of $8 . 9 \times 1 0 ^ { 2 }$ lumen-$\tt s e c / f r ^ { \tt d }$ , very close to ${ \tt 1 } \times { \tt 1 0 } ^ { 3 }$ $\sin e n - s e c / \mathrm { f t } ^ { 2 }$ , were used to extend the Chisum and Hill data. One reservation about Severin's data is that the duration of the adapting flashes was 150 msec. This is slightly greater than the normal blink time of 100 msec. Nonetheless, Severin's data does appear to agree with the general trend at high display luminances found by Chisum and Hill.

5.2.2Additional Observations

An experiment was performed by Brown\* to determine the dependence of recovery time from flash blindness on the luminance of objects viewed. The test subjects were exposed to light intensities of $3 \times 1 0 ^ { - 2 }$ to $3 \times 1 0 ^ { 2 }$ ft-Lamberts for durations of 0.9 seconds. The display consisted of a grating pattern and observers were required to identify its orientation. A timer was started and the grating was illuminated with presentation of the flash. As soon as the grating orientation was detected, the observer depressed a switch that turned off the timer. Detection times were recorded only for correct identification of grating orientation.

The results of this study are reproduced in Figure 12. The families of curves in the upper part of the figure represent results with a grating, the individual lines of which subtended a visual angle of 3.8 min. According to Brown, the detection of the orientation of this target display depended on cone vision. The curves in the lower two graphs were obtained \*for a grating which represented a visual angle of 12.5 min. Rods may serve in detection of the orientation of this coarser grid.

It is clearly shown, again, that recovery times are dependent on the luminance of the object which the eye attempts to detect. There is one major uncertainty, regarding the quantitative nature of Brown's data, howevcr. This experiment was performed by exposing the test subjects to a 0.9 sec light pulse duration. Since the blink time is of the order of 0.1 sec it is possible that the observers were not exposed to the full duration

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of the light pulse. Thcreforc therc is an uncertainty in the total cxposure duration of cach subjcct to the light. Therefore these data were not used to correlate results obtained inthe presont study.

S.3 INTERPRETATION OF RESULTS

The measurcd light intensities and durations were interpreted in terms of possible irreversible and revcrsible eye effects.

5.3.1Illumination of Photoflash

The experimentally measured illuminance of each pyrotechnic test device as reported in Table I was estimated as a function of distance from the flash origin using the inverse square law. These estimates are shown in Figures 13 and 14. The expected illuminance which observers would received from exposure to a G.E. No. 50 flash bulb are also shown in these figures. The value for the total output of this flash bulb, $1 \times 1 0 ^ { 5 }$ lumensec/ft², was obtained from General Electric specifications.

5.3.2Estimated Eye Effects

Some of the iso-illuminance curves shown in Figure 1l were extrapolated to a display illuminance level of 0.1 millilamberts (or 0.093 lumens/ft²). The recovery time for each of the reported adapting flash energies shown in Figure 1l were estimated for each of four display luminances; 0.1, 0.2, 0.5 and 1.0 millilamberts. These estimates are tabulated in Table II.

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These cstimates are also indicated by tho vertical dashos crossing each of the curves in Figurc 11.

For each pyrotechnic device tested, the recovery times for a subject exposcd to thc light flash werc estimated using thc data in Figures 11, 13, and 14 and Table III. These estimates were made as a function of distance away from the flash and the luminance levels of objects which the observer might attempt to detect after exposure. These estimates are reported in Figures 15 and 16.

As expected the large photofiash devices (i.e., the S and SDE series) should be the most effective as far as separation distance is concerned. An observer separated from the flash by 50 feet can be affected if an SA charge is employed. At separation distances less than 7 feet there would be the possibility that irreversible eye damage could be affected using the SA charge. Significant flash blindness effects are expècted within this distance range for all of the charges. Recovery times of as long as 60 seconds are predicted for the detection of objects which are very dimly illuminated.

The effects of exposure to a G.E. 50 flash bulb were also predicted., It can be seen that the estimated effects are not as great as for the S and SDE series photoflash units. By further comparison with a recent report by Tiller et al.15 (ARPA Contract DAAK02-69-C-0338) the estimates made for the G.E. 50 flash bulb appear to be reasonable. Tiller et al. evaluated the effects of exposure to this flash bulb to subjects performing military tasks. The subjects were exposed to a flash at distances between 6 and 19 feet. After exposure the subjects were required to detect ground emplaced mines or detect and fire upon a test target. All of these tests were performed under various night time conditions to which the subjects had adapted before being exposed tothe light flash. It was found that the subjects, all trained Marines, were able to resume their assigned task with the same efficiency after an average reċovery time of 5 to 20 seconds. No indication of reflected luminances of the objects detected were made. It is felt, however, that the predictions of recovery times for dimly lit displays (viz., 0.1 and 0.2 millilamberts) agree with the results obtained by Tiller et al. Between 6 and 10 feet it is predicted that exposure to the G.E. 50 flash should take approximately 8 to 18 seconds. No predictions beyond 10 feet for the G.E. 50 were made because of lack of data. However there is much indication to suggest that at longer distances (i.e., lower flash energies) the recovery times versus distance decreases at a very small rate.

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The estimated rccovcry timcs after exposure to the smaller photoflash sources are shown in Figure 16. It is not anticipated that eye. damage could be affected by thcse charges cven at short separation distances. Again the "À" and "C" mixtures are expected to be the most efficient.

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6. CONCLUSIONS

Sixteen pyrotechnic flash mixtures - fragmenting container combinations were tested. It wes shown that significant flash blindness cffects can be expected to result from the exposure to these flashes, all of which occur within 50 mscc. These effccts can result by exposure to these charges at distances within a range of 50 feet depending on the pyrotechnic mixture and quantity, container design, and method of initiation.

6.1 CARTRIDGE DESIGN

The series S and SDE charges (i.e., the 2.7 in. photoflash cartridge) produced the most intense light and are expected to be effective at distances as far as 50 feet. The external explosive burster attached to the outside of the "S" cartridge was expected to increase the light intensity by compacting the mix before ignition., However, for the larger cartridge this does not appear to have been successful. For the smaller 0.83 in. photoflash cartridge the expected trend resulted. The effective compaction by this imploding mechanism probably increases with decreasing cross-sectional area.

6.2 PHOTOFLASH MIXTURE

The type "A" and "C" mixtures in all cases generated the most light output. In some cases the "C" mixture produced light pulses of longer duration as previously anticipated. The "B" and "D" mixtures were not as effective. In fact the performance of the "D" mixture was relatively poor.

6.3 DATA INTERPRETATIONS

In order to estimate.the flash blindness effects,correlations between reported data had to be made. The results of the analyses appear to be consistent with expectation, namely that reċovery time is dependent not only on the flash energy but also on the luminance of objects which are visually sought during the recovery period. Also the relatively insensitive change of recovery time at flash energies which produce 90 to 100 percent pigment bleach was shown in this analysis.

It is useful to note that the large light sources used in our experiments (i.e., the S and SDE series), produced more intense illumination than the source employed by the Vertex Corporation. Correspondingly, longer incapacitation times are predicted for the S and SDE photoflash units as compared with the G.E. 50 photoflash used in the Vertex studies. In addition, on the basis of our independent experimental data, we could predict the shorter incapacitation times reported by Vertex for their weaker light source.

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REPERENCES

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APPENDIX 1

In order to clarify the qucstion of potential applications of bright light sources, a series of simple scenarios have becn developed, which illustrate possible suitable situations.

The use of a detonating pyrotechnic permits the generation of casing fragments as well as intense light, if the material is enclosed in a metal casing. On the other hand, packing the pyrotechnic into a nonmetallic (c.g., cardboard) casing, essentially eliminates any significant fragment hazard. These two modes of operation find separate regimes of possible application.

I. PERIMETER DEFENSE

Given a situation in which a village or a group of men wish to provide a very distinct indication of an attempt at perimeter penetration, by the enemy, and in addition wish to either inflict temporary optical incapacitation alone, permanent optical incapacitation alone, or fragment damage in addition to the optical incapacitation, these pyrotechnic light sources can play a useful role.

Thus, cased in metal and triggered by sensors (or trip wires) within the effective fragment range, they provide direct fragment damage capability with a good possibility of severe permanent optical impairment at such relatively short ranges.

Triggered by sensors deployed outside the effective fragment range, the effects would be primarily temporary optical incapacitation and disorientation with a low probability of fragment damage.

In specific situations, calling for no fragmentation effects, such as one in which friendly personnel may inadvertantly trigger the charge, the sensors can be deployed far enough away to assure only temporary optical incapacitation and fragmentation can be completely eliminated with a cardboard casing for the pyrotechnic.

II. VEHICLE PROTECTION AGAINST KIDNAP ATTEMPT

Given the premise that abductors (e.g., of South American diplomatic representatives) do not wish to kill the hostage during the kidnap attempt, a system for providing even 5 - 10 seconds of optical incapacitation in a 360°field around the car in which the hostage is driving, provides an opportunity for escape, while the abductors are optically disoriented. This system would be more effective at night than in the daytime. The light source could be either pyrotechnic or electric discharge. It could be made safe against accidental discharge causing permanent damage to innocent bystanders.

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HI. TEMPORARY DISRUPTION OF VEHICLE CONVOY BY CAUSING OPTICAL INGAPACITATION OF LEAD DRIVER

The sccnario herc is rclatively simple in that the lead driver can be optically incapacitated as he's rounding a turn, or caused to blcck the road by his inability to see it for a sufficient time to cause a wreck.

IV. ESCAPE FROM AN ENCLOSURE WITH NO PERMANENT DAMAGE TO OTHERS

In some situations, where the presence of innocent bystanders, e.g. women and children prevents the use of more damaging techniques, the use of temporary optical incapacitation is of potential interest.

V. PRELIMINARY TO INDIVIDUAL CAPTURE

Where a single individual is to be captured alive, the use of optical incapacitation may provide useful assistance. Thus, a bright light source generated near him by impact functioning of a device fired from a shotgun can provide sufficient temporary optical incapacitation to permit other capture techniques to be employed more reliably.

VI. SUMMARY

While these scenarios do not provide a complete list of potential applications, they should be useful in examining the value of a system which combines the capability for fragmentation damage, severe permanent optical incapacitation and transient optical incapacitation with.the choice fairly easy to control.

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