An active interference projector for the electro-optical test facility

InJrmd Physic.\ Vol. 20, pp. 299 to 308 Pergamon Press Ltd 1980. Printed in Great Bntain

AN ACTIVE INTERFERENCE ELECTRO-OPTICAL

PROJECTOR FOR THE TEST FACILITY

DEVON G. CROWE Bell Technical Operations Textron, 1050 East Valencia Road, Tucson, AZ 85706, U.S.A.

and THOMAS M. NOWAK Bell Aerospace

Textron,

P.O. Box 1, Buffalo, NY 14240, U.S.A.

(Received 25 February 1980) Abstract-A projection system is described which can simulate emissions from flares, muzzleflashes, shellbursts, and other emissive agents which may degrade the performance of electrooptical systems in the 0.5-15 pm spectral range. The simulation capability obtained will allow the apparent radiance and temporal characteristics of muzzleflashes and shellbursts to be mimicked at simulated ranges as close as 23 m within the Electra-Optical Test Facility. This demonstrates that tests of electro-optical system performance in the presence of interferers can be performed under laboratory conditions with higher repeatability and lower cost than field tests.

INTRODUCTION

The performance of electro-optical (EO) sensor systems such as image intensifiers, lowlight-level television, and thermal images can be degraded by sources of radiation which are present within the sensor’s field of view. In a military environment, these radiation sources could be flares, shellbursts, or weapon muzzleflashes. An Electra-Optical Test Facility (EOTF) is presently being built which will have the capability of providing empirical data on the degree of EO sensor performance degradation when subjected to such interfering sources of radiation. This is a government owned facility which is being developed by the U.S. Army Electronic Proving Ground at Fort Huachuca, Arizona. Within the EOTF various types of targets are physically simulated under controlled conditions, and performance degradation is measured for interfering sources having various radiance levels, sizes, time durations, and positions related to the target. This paper describes the design of an ‘interferer’ projector system which is being incorporated into the EOTF. To the authors’ knowledge, there has been no previous publication of the requirements to simulate this type of interferer combined with an easily duplicated system design which uses catalog components. Figure 1 presents a drawing of the EOTF which consists, in part, of an environmentally controlled tunnel 50 m long. Sensors under test are located at one end of the tunnel and view the simulated targets and interferers at the other end. The parametric sets of data obtained in the EOTF are used as inputs to a computer model which predicts the field performance of the sensor in scenarios of interest. Each radiation source which can be encountered in the field will have unique temporal and spectral characteristics. Figure 2 is an artist’s conceptual extrapolation of the temporal and spectral characteristics of a shellburst derived from the data in Ref. (1). The double peak exhibited by the time waveform has been observed in airbursts ranging from small diameter exploding wires to nuclear blasts. While it would be desirable to provide an exact spectral and temporal match for each interferer being simulated, this is not required. What is required is that the total integrated effects be the same. This requirement can be expressed as

(1) 299

300

DEVON G. CROWE and THOMASM. NOWAK

Fig. I. Electra-Optical

Test Facility.

where t, - tr = effective integration time of the sensor under test L!(t) = spectral radiance of the simulated source L”,(t) = spectral radiance of the actual source R(A) = normalized spectral response of the sensor r,(2) = spectral transmission of the atmosphere in the test tunnel r,(n) = spectral transmission of the atmosphere in the real world. A mechanical shutter system is used in the EOTF to simulate the temporal characteristics as shown in Fig. 3. PROJECTOR

HARDWARE

The projector design represents an effort to achieve the highest possible performance using easily available catalog components to keep costs at a minimum. Other workers should be able to duplicate the hardware for their own applications. The design has been verified extensively, using GOAD (Geometric Optics Analysis and Design), a program developed at Bell Aerospace Textron, which can perform ray tracing and aberration analysis through third order. The GOAD study confirmed that the design should perform as described in this paper. No modifications to the design were indicated by the GOAD analysis. The hardware for building the projector has been procured but not yet installed, so that experimental verification of the design has not yet been performed.

Fig. 2. Spectral

history

of gunflash.

Active interference

projector

for the electro-optical

test facility

301

A (pm) /

TIME

Fig. 3. Simutation

of gunflash

WAVEFORM

spectral

A? 4 p-n

history.

The general layout of the interferer projector is presented in Fig. 4. A Unislide assembly is used to position a flat mirror anywhere in one quadrant of the field of view. It is therefore possible to study the effect of angular separation of interferer and target on device degradation. This assembly uses two motor-driven screws synchronized by a timing chain for the vertical motion and a single motor-driven screw for the horizontal motion. This unit has digital position readouts and motion control loops. Figure 4 shows a projector system for illustrative purposes only. The actual projector optical configurations are shown in Fig. 6. The optical scheme used in both the visible and infrared projector sections is presented in Fig. 5. Lens A collimates the source with a finite divergence. Lens B forms an image of the source at the entrance pupil of C. The upright arrow between A and B is the location of the aperture stop. Lens C images the aperture stop on the collimator D. Varying the size of the aperture stop will therefore determine the fraction of the area of D which is illuminated. This defines the apparent size of the interfering source. The collimator has a diameter of 15 cm. A common collimating mirror is used with two projector systems to cover the spectral range of 0.4 to 14 pm, as shown in Fig. 6. A flat mirror will be used to direct the visible and near-infrared (0.4-2.5 pm) projector energy to the collimating mirror. W4ien the infrared (2-15 pm) projector is to be used, the flat mirror will be removed and Fhe i.r. energy will impinge directly on the collimator. VERTICAL

HOR T M

TO SENSOR

Fig. 4. Physical

configuration

of interference

simulator.

DEVON G. CROWE and THOMAS M. NOWAK

302

Fig. 5. Optics

schematic.

Figure 6 shows the i.r. projector components. Lens A of Fig. 5 has been replaced by the two-element assembly to reduce spherical aberration. The laser source alignment mirror is inserted only during alignment. All lenses are zinc selenide which will transmit the helium-neon laser energy for visual alignment. The laser may be detached from the fiber optic bundle when not being used for alignment. For control of the interferer radiance, germanium neutral density filters are used. Germanium has an index of refraction of about 4.0, which yields a transmittance of 0.47 at normal incidence. The filters will be at a slightly non-normal incidence to prevent transmission of reflected energy. A maximum of eight filters has been provided which yields a transmission range of 1.0 down to (0.47)8 = 0.0024. The source is a 1600 K graybody. A programmable shutter is used to provide exposure times from 5 msec to 99 set or may be held open continuously. The shutter has gold-plated blades facing the source to reduce heating which would cause the shutter itself to be an interfering source. Figure 6 also depicts the visible projector configuration. Lens A is a 4-element design in this case because the Abbe numbers of the optical materials involved in a visible projector system require that four lens elements be used to correct spherical aberration to approximately the same extent as a two-element mid-infrared system. This degree of correction is required to assure a highly collimated beam with known spot irradiance

i

INFRAFiED LIGHT SOURCE

Fig. 6. Interference

projector

optics.

Active interference projector for the electro-optical test facility

303

distribution. These characteristics provide for ease of calibration as well as the large scaling advantage calculated in a later section of this paper. The rest of the visible system is similar to the i.r. system except that the optics are glass and the source is a tungsten projector lamp with an internal mirror which images the filament beside itself to fill in spatial gaps in the source. Radiance control is provided by standard neutral density filters. Due to the finite size of the visible and infrared sources, the projected beam has a finite divergence full angle of 3.5” in both cases. This implies that the projector collimator mirror appears to be at its optical path distance of 50m rather than at an infinite distance as would be the case for zero divergence angle. The possible differences in interferer and target distances which can be simulated can therefore be found. For a given sensor system focal length fT the image distance Si is related to the object distance So by 1 1 s=s,-7=

1

1 - f/S, s

(2)

The focus error due to differing object distance for the target and background must be duplicated for an accurate simulation. This can be done by matching the differences in reciprocal image distances if the sensor is focused on the target in both cases 1 1 ---=2: Si S[

1 - f/So f--f

1 -f/X

1 = L $;So

(3)

The condition for accurate simulation is to match the differences in reciprocal object distances between simulation interferer and target with the actual interferer and target. For example, if the maximum focus error to be simulated is represented by an interferer at 30 km and a target at 200 m, a projector distance of 50 m requires a target distance of 40 m within the EOTF 1 ---_-~= 200

1 30,000

1 1 5 x 1o-3 = 40 - 50

In fact, the target distance can be varied from about 7-55 m so that no restriction is imposed upon EOTF simulation capability. The size of the projected spot with this divergence is large enough to allow fixed alignment of the moving mirror shown in Fig. 4. Figures 7 and 8 illustrate that the sensor will be illuminated by the projector from any position of the moving mirror provided that alignment was performed in the center of the region of travel. An intensity map must be found ex~rimentally, using a fixed position radiometer at the sensor window to measure irradiance while the mirror is varied in position. Other workers may prefer to

REGION OF UNIFORM INTERFERER SIZE

SENSOR WALL

Fig. 7. Spot projection geometry.

/

DEVON G. CROWE and THOMAS M. NOWAK

304

REGION OF UNIFORM INTERFERER SIZE WITH MIRROR AT END OF \ TRAVEL ,I 05 METERS FROM CENTER)

REGION OF UNIFORM APPARENT INTERFERER SlZE WITH MIRROR CENTERED

SENSOR ,

, I

/

I I \ \

\ \

\

/

\

1

\ -----‘\

,G,,. OFUNIFORM INTERFERER SIZE WITH MIRROR AT CORNER OF TRAVEL (I.47 METERS FROM CENTER)

SENSOR WALL

Fig. 8. Projected

spot movement.

simplify data analysis by installing two motor-driven axis motions on the final moving flat mirror to maintain centering of the projected spot. However, Fig. 9 demonstrates that an interfering source of constant size will be seen from the sensor window without the controlled alignment feature over nearly the entire range of motion. INTERFERER

SIMULATION

In order to simulate the actual interferers, it is necessary to provide power on the detector due to the simulator which is equal to that which would be present in the real-world case. The power which must be emitted by the interference projector source can be related to the power emitted by the actual source being simulated (whether that source is apparently extended or a point) by the simple relation (4) where P, = the simulator source power required P, = the actual interferer source power emitted R, = the solid angle into which the simulator radiates R, = the solid angle into which the interferer radiates R, = the distance to the simulator R, = the distance to the actual interferer.

Equation (4) implies a scaling advantage for the simulator. For example, in the EOTF the distance to the simulator is 50 m. If the projector emits into a cone subtending a full angle of 3.5”, the simulator need only emit the fraction 1.5 x 10m6 as much power as a real interferer 1 km away. AVAILABLE POSITIONS OF MOVING MIRROR TO PROVIDE UNIFORM ;;;ARENT INTERFERER

MIRROR POSITIONS POSSIBLE UNIFORM INTERFERER SIZE (2.75METER DIA CIRCLE1

‘MIRROR TRAVEL MECHANICALLY

Fig. 9. Available

mirror

positions

Active interference

projector

for the electro-optical

test facility

305

The following data from Ref. (2) are used as an example of the simulation capability of the interferer design. These data are the spectral radiant intensities of muzzleflashes integrated over 4-l 2.5 pm : 81 mm mortar: I = 89 W/sr ; diameter = 90 cm; t = 0.01 sec. 42 in. mortar: I = 250 W/sr ; diameter = 150 cm; t = 0.05 sec. 105 mm howitzer: I = 1300 W/sr ; diameter = 360 cm; t = 0.15 sec. Because the shutter is programmable from a minimum exposure time of 5.5 msec up to 99 set or continuously open, the interference projector can simulate any of these interferers temporally. The minimum scale range Rminat which the size of the interferer DI can be simulated by the 15 cm dia collimator at 50 m is R,i, = 50 = 333 DI 0.15 For the interferers listed, this represents: Rmin (81 mm mortar) = 300 m Rmin (4.2 in. mortar) = 500 m Rmin (105 mm howitzer) = 1200 m

The 105 mm howitzer may not be simulated using this system at a simulated scale range of closer than 1200 m to the sensor. It should be noted, however, that this is by far the largest interfering source in the data examined, and the probability of operating an i.r. sensor near a large howitzer may be low. Also non-imaging wide-field-of-view systems do not require that the size be simulated. For these devices, the simulation is radiance limited. The final simulation constraint is that sufficient source power be provided to simulate the power radiated by the real interferer. Assuming that these three interferers emit into 471 steradians and the simulator has a divergence angle of 3.5”, the simulator powers required in the three cases are: P, (81 mm mortar, 300 m) = 14.5 mW P, (4.2 inch mortar, 500 m) = 13.5 mW

P, (105 mm howitzer, 1200 m) = 13.5 mW It now remains to calculate the power available from the simulator source. The expression for calculating the power is P, = e,A,t02nhc2

12 dl 1, ;15[echlak’ - l]

I

where t0 = the transmission of the optical system. E, = the emissivity of the source. A, = the area of the source. The infrared projector uses a source with T = 1600 K, A, = 1.0 cm’, E, = 0.8. Assuming ~~ = 0.65, equation (6) implies P, = 4.2 W. Power is therefore not a limiting consideration in this simulation. The visible interferer simulator is able to exceed the radiant intensity of the i.r. projector because its brightness temperature is 3000 K compared to 1600 K for the i.r. simulator. After allowing for the difference in source areas, the visible source emits 7.7 times the power of the i.r. source. For the simulation of most interferers, then, the i.r. projector limits R,i,.

DEVON G. CROWE and THOMAS M. NOWAK

306 Table

1. Range limitations

of interferer

simulation

capability

Muzzleflashes Size limited Xl mm mortar: 4.2 in. mortar: 105 mm howitzer:

Intensity t3OOm ~5OOm 2 1200 m

Shellbursts intensity limited

limited

XI mm mortar: 4.2 in. mortar: 105 mm howitzer:

20 mm rifle: 66mm rocket: 40mm rifle: 4.2 inmortar:

223m 232m 219m

244 m 2 111 m 276m 2 108 m

A final example of simulation capability can be presented based on data from Ref. (1). No apparent size information was included, so the Rmin data is calculated assuming that radiant intensity is the limiting consideration. Shellbursts R,i, (20 mm rifle) Rmin (66 mm rocket) Rmin (40 mm rifle) R,i, (4.2 in. mortar)

= = = =

44 m 111 m 76 m 108 m

Table 1 summarizes these two examples and additionally includes a calculation of the ranges at which muzzleflashes could be simulated if apparent size were not the limiting factor. This is the extent to which estimates of the interferer projector capability can be estimated with the available data. CONCLUSION

It has been shown that the simulation capability summarized in table 1 can be obtained with an interference projector designed around catalog components. This level of performance is achieved at a projector distance of 50 m. Higher levels of performance (simulation of closer interferer ranges) can be achieved in installations using a shorter optical path length. The combination of simulation requirements and achievable simulation capability presented here demonstrates that interference degradation tests of visible and infrared systems can be performed under laboratory conditions. This implies improvement in repeatability and reduction in cost compared to field testing. Acknowledgenlent-The authors wish to thank preparation of this article for publication.

W. L. Wolfe for his valuable

comments

and suggestions

in the

REFERENCES I. DAVIS R. M., Sonar lt$w-ed Mea.wrrnw~r.s of Weupon Shellhursrs. Avco Corporation, Electronics Division. Cincinnati. Report No. 2708, 1969. Firing.s. 2. NANEVICZ J. E. & R. C. HONEY, Thermal Imaging Cumeru Ohsercutions of Morrur trnd Arrillery Stanford Research Institute, Menlo Park, California. Meeting of IRIS Specialty Group on Infrared Imaging, February 1976. APPENDIX-DERIVATION

OF

THE

POWER

SCALING

ADVANTAGE

In order to simulate the real-world interferers, it is necessary to provide equal incident power on the detector as would be present when viewing the real-world interferer. Two cases must be considered: Point sources and extended sources. Point sources are defined as any source whose apparent size subtends less than one resolution element at the sensor. The radiant intensity I of a point source is defined as I=

L, dA

(Al)

where L, = the radiance of the source in watts per steradian per square of the source in square meters I = the radiant intensity of the source in watts per steradian.

meter

A, = the area

The radiant

intensity

required

in the EOTF

to simulate

a real-world

source

is now derivaed

Active interference

projector

for the electro-optical

307

test facility

642) (A3) where +n. &_ I, I, R,, R,, A, R, R,

= = = = = = = = =

the the the the the the the the the

radiant flux received by the sensor from the simulator in watts radiant flux received by the sensor from the actual interferer in watts radiant intensity of the simulator in watts per steradian radiant intensity of the real world interferers in watts per steradian solid angle subtended by the collecting optics of the sensor at the simulator in steradians solid angle of the sensor at the actual interferer to be simulated in steradians collecting area of the sensor optics in square meters distance between the simulator and the sensor in meters distance between the actual interferer and the sensor in meters.

For an accurate

simulation,

we desire that C$s, = dR,. Setting equation

(AZ) equal to equation

(A3) yields

RI R%

I, = I,

(A4)

~

Equation (A4) defines the range scaling advantage. Another scaling advantage can be gained over the real-world case by emitting power into a smaller angle with the simulator than would be the case with an actual interferer. The relevant relations are

combining

P, P, R, Q,

solid

P, = I$,

WI

P, = I,R,

W)

(A5) and (A6)

= = = =

the the the the

power emitted by the power emitted by the solid angle into which solid angle into which

simulator in watts interferer in watts the simulator radiates the interferer radiates

in steradians in steradians.

Equation (A7) implies that less power can be used by the simulator if it radiates into a smaller divergence angle. The power advantages described above can be summarized in one equation by substituting equation (A4) into equation (A7) R, = the solid angle into which the simulator radiates in steradians

$

p, = p, ; Y

For an extended

source whose apparent

648)

‘7

size is larger than a resolution

element of the system under test

where R, = the solid angle subtended by the simulator at the sensor Q,, = the subtense of the actual interferer at the sensor. For an accurate simulation it is necessary to have a simulator sensor as would be the case with the interferer. Setting equation L, = L, *f SC

which causes the same power to irradiate (A9) equal to equation (AlO) yields

the

(All)

A further requirement for the simulation of an extended source is that the apparent angular size of the simulator be the same as the apparent size of the interferer. This condition reduces equation (Al 1) to L, = L.

6412)

The goal of the simulation projector is to emit radiance equal to that of the source being simulated while maintaining the same apparent solid angle at the sensor. The radiance constraint of equation (A12) can be rewritten (A13)

I.P. 2015-e

DEVON G. CROWE and THOMAS M. NOWAK

308 The quantity

A, is a function

of A, because

of the solid angle constraint (A14)

Substituting

(A14) into (A13) (Al51

Simplifying (A161 This is identical

to equation

(A8) derived

for the point source

case, and is equation

(4) in the text.