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Design Considerations for an InfraredFlying-Spot Telescope

A. D. Shulman

This device combines the image-forming capability of a passive-ir line-scanning radiometer with motionsensing. The main purpose is to enhance image interpretability by adding motion information to thepicture. Selection of wavelength and other salient features is discussed. The 10.6-,a wavelength hasunique advantages and is the preferred choice, mainly due to the existence of high power, high efficiencyow lasers at this wavelength, coherent receiver techniques, and haze/fog penetration capability. Asimplified method of estimating laser power is given, based on area scan rate as the dominant factor.

Introduction

In the ir-flying-spot telescope, a narrow cw laser beamscans a given terrain or target area, and an ir-optical re-ceiver, optimized for the laser light, detects photonsscattered from the points of incidence. A video pictureof the scanned area can then be synthesized. Theprimary functions are obtaining interpretable imageryand moving target detection to supplement the imagery.

This sensor integrates capabilities that are otherwiseonly partially available, and then only independently, inother systems. The imagery is of useful resolution,such as that available in a passive-ir line-scanningradiometer, but is a reflectivity picture, complementingthe temperature and emissivity sensitive character ofthe radiometer.

It also offers doppler detection of moving targets,such as with cw doppler radar, but with the additionalcapability for presenting doppler shifted signals in con-junction with imagery. The purpose of the combinationis to improve interpretability. Compared to radar, thedoppler sensitivity and the spatial resolution are somuch greater due to the shorter wavelength that sub-clutter visibility is much improved. This makes it pos-sible to detect relatively small, slowly moving targets interrain.

The most important constraint is that the maximumrate of area scan attainable is limited by available laserpower. For example, it is estimated that about 200,000m2 sec-' is attainable with a 2.5-W laser, exclusive of at-

The author was with AIL, a Division of Cutler-Hammer, Inc.,when this work was done. He is now with the MITRE Corp.,Westgate Research Park, McLean, Virginia 22101

Received 23 December 1969.

mospheric losses. At long range, this amounts to aquite restricted field of view.

Discussion of Salient Features

The following summarizes the major issues involvedin selecting wavelength and other salient features, andgives a simplified method for estimating required laserpower.

The 10.6-,u ir wavelength has important advantagesfor this application:

(1) Relatively high power, high efficiency cw lasersare feasible. CO2 lasers, operating at 10.6 y, can bemade in capacities to at least hundreds of watts cw.Such lasers tend to excessive length, but can be greatlycompacted by folding the optical path.' Other en-couraging developments have been reported. 2 -4 It isprobably more a matter of engineering than funda-mental development to fit the application. It should benoted that at this wavelength, even with high power,there is negligible ocular hazard at the target. In-vestigations of hazard have been concerned only withthe risk of burning the cornea,5 a virtually impossiblecontingency in this kind of service.

(2) Since 10.6 A is near the center of the 8-14 Aatmospheric window, clear air propagation is known tobe excellent. It is better than visible light, particularlyin dry air.' In addition, 10-Au radiation is compara-tively very penetrating in haze and light fog-even inheavy fog if droplet size is small compared to wave-length. Chu and Hogg7 found experimentally thatvisible light attenuation can vary greatly in fog, fromabout 6 dB km-' to 17 dB km-' depending on dropletsize, while 10-A attenuation remains approximatelyconstant at about 1-2 dB km-.7(a) On the other hand,visible light offers greater penetration in rain. How-ever, this advantage is limited to about 4 dB km-' less

November 1970 / Vol. 9, No. 11 / APPLIED OPTICS 2505

attenuation, the differential being roughly constantover a wide range of rainfall.7 (b)

(3) Sensitive coherent 10.6-A receivers, adaptable todoppler detection service, are available. For example,Mocker8 describes a compact, rugged design. Thetransmitter and local oscillator are CO2 lasers that arefrequency locked by an AFC feedback loop to providestable i.f. The main additional requirement for theflying-spot application is a suitable beam-scanning de-vice, such as a rotating mirror.

The following uncertainties must also be carefullyweighed:

(1) Effect of particulate matter in the atmosphere,such as precipitation, smoke, or dust on attenuation,polarization, and beam broadening.

(2) Characteristics of backscatter from particulatematter, such as extent of doppler shift.

(3) Effect of clear atmosphere inhomogeneities,particularly turbulence, on beam propagation and di-vergence. Atmospheric effects are discussed further.

(4) Effect on image interpretability of the so-calledspeckle effect, an interference phenomenon which ischaracteristic of coherent illumination. It presents agranular appearance which is particularly marked indiffusion from smooth surfaces, and may degrade imagequality. Speckle has been observed at visible wave-lengths,9 and in radar imagery' 0 (for example, Skol-nick"). Similar effects are to be expected at 10.6 .

(5) The effect of deposits, such as moisture, snow,mud, etc., on the reflectivity of specular targets.

In addition to doppler detection, laser illuminationoffers other unique advantages:

(1) It complements imagery from a passive-ir line-scanning radiometer. Radiometer imagery primarilyshows emissivity variations but is also strongly tem-perature dependent and consequently interpretabilitysuffers. For example, terrain areas heated by sunlighttend to be accentuated. Poulin et al.'2 show particu-larly interesting examples of this effect. As a result theappearance of imagery tends to vary with time of day.Other heat sources can have a marked effect, as illus-trated in Slavecki. '3 The problem is always whether tointerpret a tonal variation as a structural feature ortemperature change. Laser illumination avoids thedifficulty, since the imagery is independent of tempera-ture but dependent on reflectivity-that is, comple-mentary to emissivity.

(2) Target ranging and target designating functionsmay be provided in the same system with appropriatebeam modulation.

(3) Laser illumination permits reduction of atmo-spheric backscatter by limiting the effective range in-terval over which the transmitter and receiver beamscoalesce. This improves contrast, quite apart frompenetration ability. For example, contrast improve-ment has been demonstrated with pulse gated laser il-lumination. 1 4

With a flying-spot system, the effective range intervalis automatically limited by angular displacement be-

tween transmitted and received beams. This helps toavoid the more serious backscatter from the near field.Displacement results from: (a) time delay in theround trip to the target, particularly at long range orhigh scan rate; and (b) physical separation, if any,between receiver and transmitter apertures.

(4) Polarization analysis may prove useful for con-trast enhancement. At visible and near ir wavelengths,there is known to be much depolarization in backscatterof plane polarized radiation, up to 100% depolarizationdepending on the nature of the surface."," Compari-son of parallel and cross polarized returns may allowcontrast enhancement, since the ratio varies with targetor terrain; this has been demonstrated at K band,'7 Lband,18 and visible light. '9 In particular, specularreflectors may be enhanced relative to background.

Effective moving target detection requires sensing ofdoppler shift. For this it is necessary that the receiverbe an ir-optical heterodyne. This would be true even ifdoppler detection were not required, since there is in-sufficient photon energy at 10.6 ,u for incoherent photonnoise limited detection without impracticably low de-tector temperature. Incoherent receivers are practicalfor ir radiometry because by accepting thermally gen-erated radiation over the entire 8-14-,u atmosphericwindow, they can be made background noise limited at areasonable detector temperature. Obviously, this willnot help with laser generated radiation.

Doppler detection of 10.6-u4 laser light scattered froma moving diffuse surface, has been demonstrated.2 0

For practical purposes, doppler filtering can be used asin cw doppler radar.2 ' Filtering is simplified comparedto radar because of the very much greater dopplershift-about 52 kHz for a 1 km h-' radial speed. Verywide i.f. bandwidths, up to at least 1 GHz, are achiev-able.2 2 Thus large doppler shifts can be accommodatedif need be.

Other points of interest favoring heterodyne receptionat 10.6 u are:

(1) Extraneous lO.6 -,u background radiation is easilyexcluded because of the relatively narrow acceptanceband of the coherent receiver. Excluding backgroundlight is more difficult at visible wavelengths, since anincoherent receiver, which would be the logical choice[see the discussion of Eq. (3) ], is limited by the narrow-ness of available spectral filters.23(a) This difficulty willobviously be serious if there is daylight to contend with.

(2) For a coherent receiver, the receiving apertureas well as the transmitting aperture is diffractionlimited,2 4 as in the case of radar. For this reason anarrow receiver beam width is attainable with a rela-tively modest aperture. For example, with a 100-mmaperture, the beam width will be theoretically about0.12 mrad.

Resolution can be good even with a wide receiverbeam, provided the transmitter beam is narrow; how-ever, ideally, for power efficiency, both transmitting andreceiving apertures and beam widths should be equal.If the receiver beam is wider than the transmitted beam,its aperture is unnecessarily small, which loses sensi-

2506 APPLIED OPTICS / Vol. 9, No. 11 / November 1970

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tivity; while if the transmitter beam is wider, trans-mitter power is lost. The desirable equality relation-ship between receiver and transmitter beams is dis-cussed by Kroeger.2

1

(3) Because the aperture is diffraction limited, acoherent receiver will be more subject to speckle than anincoherent receiver.26 27 There may be some disad-vantage from the point of view of image interpretability,but also possibly some advantage for moving target de-tection of highly reflective targets, since the effectiveS/N may be increased.

Estimating Laser Power

The following gives a method of estimating requiredlaser power for a given rate of area scan.

Let

A = area of the receiver or transmitter aperture;PT = transmitted laser power;PR = power collected by the receiving aperture;a = area of the spot subtended by the beam at the target (for

simplicity, the spot is assumed square shaped with uni-formly distributed power);

R = target range;K = rate of area scan in ml sec-';X = wavelength.

Assuming lambertian backscatter, P = PTA17rR2.For a diffraction limited beam, the beam width is ap-proximately 1.2X/Al, which leads to A/R 2 = (1.2X)2 /a.Hence,

PR PT- (1.2X)2 /ra. (1)

For an ideal photon noise limited receiver,28 the noisepower is

PN = (hc/X)B, (2)

where B is the receiver bandwidth, which is made equalto information bandwidth, B 0.35 K/a. Then forthreshold detection, where PR = PN, Eqs. (1) and (2)give for the required laser power,

PT 0.764(hc/X')K. (3)

At 10.6 A,

Pr/K -98 dBW m-2 sec-, (4)

where the unit dBW inditates dB above a 1-W refer-ence level.

This shows that, ideally, required power is directlydependent only on the rate of area coverage, and notdirectly on aperture or range. The gain in collectionefficiency due to an increased aperture is exactly offsetby increased bandwidth, since the same area is scannedin the same time period with a smaller spot. Thusresolution can be varied with no change in sensitivity,provided the scan and bandwidth are adjusted to suit.Similarly, for a given rate of area coverage, the gain dueto decreased range is exactly offset by increased band-width due to the resulting smaller spot.

There is a large advantage at longer wavelengthsbecause of the (1/X)3 factor. For example, the sensi-tivity advantage of 10.6 , over visible light (0.63 ,u) with

coherent detection is about 37 dB. For this reason co-herent detection is not attractive in the visible range.A similar conclusion applies also to the communicationsapplication.2 9

The following points favor incoherent detection in thevisible range:

(1) Direct detection can be photon noise limitedwithout need for cryogenic detector temperature.23(b)30

(2) The diffraction limitation is avoided23(c) andtherefore for a given beamwidth, a much larger aperturewith concomitantly greater sensitivity can be used.

Thus where the application requires doppler detec-tion, or rigorous exclusion of background radiation, forwhich a heterodyne receiver is necessary, the 10.6-4wavelength is much to be preferred.

Relative to radar wavelengths, on the other hand, ir isat a great disadvantage with respect to power require-ment. Wavelength is involved as (1/X)2 rather than as(1/X)3, since radar sensitivity is limited by Johnsonnoise rather than photon noise. In place of Eq. (2), let

PN = RTB.

Then proceeding in the same way, the result is

PT 0.763(kT/X2 )K.

At X band, for example, where X - 0.03 m,

PT/K -175 dBW m - sec-1.

(5)

(6)

(7)

Thus, comparing with Eq. (4), there is about a 77-dBpower advantage over 10.6-u ir for a given area scanrate.

This illustrates that radar has enormously greaterarea coverage capability. For example, Rouse andMoore3 calculate that with reasonable transmitterpower, of the order of 25-dBW average, an X-band ra-dar could map a 35-km swath from an orbiting satellite,amounting to about 84-dB area scanning rate. Inprinciple, the required power could be as much as 30 dBless.

Radar area rate coverage capability is so great that itis difficult to find applications where the full potential isneeded. Even though the ir coverage rate is so muchless, it is still enough to be usable.

The above comparison is based on the terrain map-ping application, without assuming any fixed target size.If there is in fact such a target to be detected, of sizemuch smaller than beam width, the advantage of theradar wavelength diminishes greatly, due to the nar-rower beam width at ir. For example, assume that apractical X-band antenna has 1000 times greater aper-ture area; then for a given area scanning rate, the radarsensitivity advantage is reduced relative to ir, by afactor of about 103 (XA-bad/Xft)2, the ratio of the spotareas. This reduces the comparative advantage of ra-dar from 77 dB to about 37 dB.

Carrying the comparison further, ir has still greaterinherent sensitivity for detecting retroreflectors. Forexample, there are plans for putting a 10.6-ug retroreflec-tor into synchronous orbit.32 Consider the problem ofdetecting such a retroreflector from the ground with a

November 1970 / Vol. 9, No. 11 / APPLIED OPTICS 2507

given zone scanning rate. Again, assume that the X-band receiving-transmitting aperture is 1000 times lar-ger in area, and that in both cases the retroreflector hasthe same cross section, which is smaller than the beam-width. In this case, the 77-dB radar sensitivity advan-tage is reduced relative to ir by the square of the abovefactor, amounting to about a 79-dB reduction. Thus inspite of the postulated smaller aperture, the ir systemhas, theoretically, comparable sensitivity, due largely tothe much narrower beamwidth of the retroreflection.

The doppler threshold, which is the minimum de-tectable doppler shift, is an important parameter formoving target detection. It is limited mainly by in-formation bandwidth. In the following example, it isequivalent to about 10 km h minimum detectablevelocity, which would be reasonable for detection ofmoving vehicles. This threshold cannot be reduced byincreasing laser transmitter power, since the variationsof background reflectivity set the ultimate limitation,like the ground clutter limitation in radar.

The comparatively high doppler sensitivity at 10.6 upermits very much greater information bandwidththan with radar for a given minimum detectable veloc-ity-about 3000 times greater than X band. Thus, it isfeasible to scan the terrain at a reasonable rate, withsimultaneous moving target detection based on dopplerfilters.

The doppler sensitivity which is a disadvantage forcommunications, or space tracking,3 2 is therefore signi-ficant for the flying-spot application. It also offers apotential for detecting useful doppler signature informa-tion, as has been demonstrated with radar,3 3 but ap-plicable to slower moving targets.

Also, at 10.6 u, the spot would normally be smallerthan the target, while with radar it is likely to be muchlarger. The small spot contributes further to subcluttervisibility, since the doppler-shifted return is receivedseparately from ground clutter, which facilitates filter-ing.

Example

The following example illustrates use of Eq. (3) toestimate required laser power. Let the area to bescanned be 100 m X 100 m, at a rate of two frames persecond, equivalent to 43-dB area rate (in units of m2

sec-'). For simplicity, the aperture is taken to be 100mm, and the range is assumed to be 1 km at a constant100-m altitude; these parameters are of concern chieflyin estimating atmospheric effects. Power requirementsand loss estimates are shown in Table I.

Notes

(1) Typical reflectivities, at 10.6 I, can be estimatedfrom data in Wolfe et al.3 4 Values range from about98% for aluminum foil, to about 3% for cotton herring-bone uniform fabric. Road asphalt is about 8%, anddry meadow gross about 15%. For example, taking 3dB as the minimum reliably discriminable increase, todistinguish the uniform fabric from road asphalt re-quires about 13-dB S/N from a perfectly reflective

Table I. Loss Budget

Paragraphunder section

Power level Description Notes

-98 dBW Basic power requirementm- 2 sec-1 for unity S/N

+43 dB Rate of area coverage+20 dB S/N (includes reflectivity (1)

factors)+7 dB Quantum-noise factor (2)+4 dB Loss in transmitter optics)+5 dB Loss in receiver optics (+6 dB Beam-tracking loss (4)+2 dB Spot and line corrections (5)+2 dB Duty-ratio loss (6)+3 dB Depolarization loss (7)

Atmospheric losses: (8)

+6 dB Beam divergence (9)0 dB Decoherence (10)0 dB Beam quivering (11)

+4 dB Attenuation (12)Total +4 dBW Required laser power (13)

target. To accord with common radar practice, a 20-dB S/N is specified.

(2) The quantum noise factor, defined by Aramset al.,2 2 is the ratio of noise equivalent power (NEP) ofthe detector to the ideal NEP for which Eq. (3) isderived. A quantum-noise factor somewhat betterthan 7 dB was reported for photoconductive Ge:Cudetectors.22 More recently, this has also been achievedwith photovoltaic HgCdTe detectors,35 which have theadvantage of permitting operation at liquid nitrogenrather than liquid helium temperatures,36 though at theexpense of considerably greater laser oscillator power.

It should be noted that both photoconductive andphotovoltaic detectors are theoretically limited to aquantum noise factor of 3 dB.35 Thus 7 dB is a reason-able approach to the theoretical limit.

(3) In addition to scanning and focusing, the 4-dBloss in the transmitter allows for some inefficiency in thebeam spreading optics [see Note (4) ]. The 5 dBquoted for the receiver makes allowance for loss in themixer optics.

(4) Beam-tracking loss results from error in theangular displacement that is required between receivedand transmitted beams to compensate for round-triptime delay. The loss is a function of range and scanrate. It depends on the method used to determinerange, and on range variation over the scan. To allowfor the angular error, the transmitted beam may bestretched in the direction of high speed scan, but withloss of effective power. In this example, it is stretchedto 4: 1 aspect ratio. The resulting power loss is about6 dB, since the receiver sees only one-quarter of the il-luminated spot.

This loss can be reduced by use of multiple-detectorchannels fed by a linear detector array. Such a configura-tion allows the linear scan rate to be reduced, for a given

2508 APPLIED OPTICS / Vol. 9, No. 11 / November 1970

area rate coverage, and thus eases the beam tracking prob-lem. Use of multiple detectors does not improve sensi-tivity, since for a given aperture, the total signal powercollected remains the same; also, since the decreasednoise bandwidth per channel is exactly offset by theincreased number of channels, the total noise power istheoretically unchanged. In this respect, it is quitedifferent from the ir scanning radiometer, where use ofmultiple detectors does improve sensitivity.34b).

(5) This includes 1-dB line-overlap loss to accountfor excess laser power needed due to overlap of succes-sive lines of the scan pattern. There is also a 1-dB cor-rection to account for nonsquare spot shape.

(6) Duty ratio loss accounts for about 50% of thescan period which it is expected will be wasted due tooverscanning.

(7) Depolarization loss accounts for signal returnsthat have other than the polarization for which thereceiver is designed. Egan and Hallock,' 5" 6 previouslycited, found complete depolarization for some terrainsamples, equivalent to a 3-dB depolarization loss. Forthis estimate, 3 dB is quoted conservatively.

(8) Atmospheric effects are most important. Thepresent data background is quite inadequate consideringthe great range of possible variations. Most work hasbeen oriented toward the communications application.

Of the clear atmosphere effects, we say that only (a)beam divergence (broadening), (b) decoherence, result-ing from small turbulons (inner scale), and (c) randomdirectional variation (quivering) resulting from largeinhomogeneities (outer scale), seem to be significant.

(9) Even in clear weather, atmospheric turbulencelimits resolution. Turbulons much smaller than thebeam diameter are the chief contributors to beamdivergence. According to Subramanian,3 7 the effect ismore marked at 10.6 1i than in the visual range, beingproportional to wavelength. The data show less than0.2 mrad of turbulent beam divergence under worst con-ditions, according to recent measurements. Of course,0.2 mrad is a large divergence when dealing with aper-tures of the order of 100 mm.

Aperture and resolution are not directly involved inthe laser power estimate of Eq. (3), and therefore inprinciple, beam divergence need not be accounted for.However, this is true only to the extent that the scan andbandwidth are adjusted to conform to the degradedresolution.

As a practical matter, the brute force approach is pre-ferable; that is, to supply enough laser power for theworst atmospheric condition to be designed for, ratherthan attempt to adjust for varying conditions. Theeffect of 0.2 mrad of turbulent beam divergence on a100-mm aperture system is equivalent to about 6 dB ofworst-case power loss, as quoted in the budget.

(10) Decoherence reduces apparent reflectivity.The effect is quite uncertain. The resulting loss de-creases about as the inverse square of wavelength.According to Chase,3 8 the decoherence loss due toturbulence is negligible with a 100-mm aperture, sincethe coherence length at 1-km range is of the order of 1 mat this wavelength.

(11) Beam quivering is random directional variationdue to atmospheric inhomogeneities that are largecompared to beam breadth. From Hodara39 it isestimated, assuming an anisotropic medium, that therms quiver at 100-m altitude is small enough to be ne-glected.

(12) A 4-dB attenuation is quoted as the two-wayloss for the assumed 1-km range. The atmosphericcondition is assumed to be fog with 2-dB km-' attenua-tion, as given in Chu and Hogg.7(a)

(13) The conditions assumed for the example resultin +4 dBW (about 2.5 W) as the required laser power.This is reasonably conservative and certainly feasible.However, at slow 2-frames sec-' display rate, real timeimage display requires some form of auxiliary imagestorage, such as a direct-view storage tube, or scan con-version. Increasing the frame rate could avoid this, butwould increase the required laser power and the dopplerthreshold in the same proportion. Multiple detectorchannels would be helpful here since they allow higherframe rate without increasing information bandwidthper channel [see Note (4) above]. Thus the dop-pler threshold could be preserved, at the cost of equip-ment complexity.

References1. D. R. L. Whitehouse, Microwaves 6, A6 (1967).2. B. M. Elson, Aviat. Week Space Technol. 91, 77,81, 82 (1969).3. A. Crocker and M. S. Wills, Electron. Lett. 5, 63 (1969).4. G. J. Desenberg and J. A. Merritt, Appl. Opt. 6, 1541

(1967).5. N. A. Peppers, A. Vassiliadis, K. G. Dedrick, H. Chang,

R. R. Peabody, H. Rose, and H. C. Zweng, Appl. Opt. 8, 377(1969).

6. P. W. Kruse, L. D. McGlauchlin, and R. B. McQuistan,Elements of Infrared Technology (Wiley, New York, 1962),Fig. 5.4.

7. T. S. Chu and D. C. Hogg, Bell System Tech. J. 47, 723(1968); (a) Fig. 13(c); (b) Fig. 19.

8. H. W. Mocker, Appl. Opt. 8, 677 (1969).9. J. D. Rigden and E. J. Gordon, Proc. IRE 50, 2367 (1962).

10. R. B. Innes, in Proceedings 6th Symposium Remote Sensing ofthe Environment (April 1968), (Infrared Physics Lab., Univ.Mich., Ann Arbor, 1968), p. 107.

11. M. I. Skolnik, Introduction to Radar Systems (McGraw-Hill,New York, 1962), Figs. 12.4, 12.5, 13.1.

12. A. 0. Poulin and T. A. Harwood, in Proceedings 4th Sym-posium Remote Sensing of the Environment (April 1966)(Infrared Physics Lab., Univ. Mich., Ann Arbor, 1966),p. 231.

13. R. J. Slavecki, in Proceedings 3rd Symposium Remote Sensingof the Environment (April 1966), (Infrared Physics Lab.,Univ. Mich., Ann Arbor, 1965), p. 537.

14. D. B. Neumann, J. Soc. Motion Picture Television Engrs.74, 313 (1965).

15. W. G. Egan and H. B. Hallock, Appl. Opt. 7, 1529 (1968).16. W. G. Egan and H. B. Hallock, Proc. IEEE 57, 621 (1969).17. R. D. Ellermaier, A. K. Fung, and D. S. Simonett, in Ref.

12, p. 657.18. W. E. Brown, Jr., Proc. IEEE 57, 612 (1969).19. C. J. Rudder and R. L. Carpenter, Appl. Opt. 8, (1969).20. M. C. Teich, Proc. IEEE 57, 786 (1969).21. D. J. Povejsil, R. S. Raven, and P. Waterman, Eds. Air-

borne Radar (Van Nostrand, Princeton, 1961), pp. 311-313.

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22. F. R. Arams, E. W. Sard, B. J. Peyton, and F. P. Pace, IEEEJ. Quantum Electron. QE-3, 484 (1967).

23. M. Ross, Laser Receivers (Wiley, New York, 1966); (a)p. 100; (b) Chap. 4; (c) Chap. 2.

24. A. E. Siegman, Appl. Opt. 5, 1588 (1966).25. R. D. Kroeger, Proc. IEEE 53, 211 (1965).26. L. H. Enloe, Bell System Tech J. 46, 1479 (1967).27. L. H. Enloe, Bell System Tech. J. 48, 817 (1969).28. B. M. Oliver, Proc. IEEE 53, 436 (1965).29. J. L. Randall, in Aerospace Electronic Systems Technology

(National Aeronautics and Space Administration, NASASP-154, 1967), p. 184.

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Electron. GE-7, 48 (1969).

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34. W. L. Wolfe, Ed., Handbook of Military Infrared Technology(Office of Naval Research, Washington, D. C., 1965); (a)Sec. 4.5; (b) Sec. 18.8.2.3.

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vention Record (Sept. 1968) (IEEE, New York, 1968), p. 125.38. D. M. Chase, J. Opt. Soc. Amer. 56, 33 (1966).39. H. Hodara, Proc. IEEE 54, 368 (1966).

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2510 APPLIED OPTICS / Vol. 9, No. 11 / November 1970