Design Considerations of IRST System

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Design considerations of IRST system

Venkateswarlu, Ronda, Er, Meng, Tam, Siu Chung, Chan, Choong, Choo, Lay

Ronda Venkateswarlu, Meng Hwa Er, Siu Chung Tam, Choong Wah Chan, Lay Cheng Choo, "Design considerations of IRST system," Proc. SPIE 3061, Infrared Technology and Applications XXIII, (13 August 1997); doi: 10.1117/12.280379 Event: AeroSense '97, 1997, Orlando, FL, United States Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 05 Aug 2020 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Design considerations of IRST system. R.Venkateswarlu, M. H. Er, S.C.Tam, C. W. Chan, L.C.Choo. Nanyang Technological University, School of EEE, Singapore 639798, Singapore ABSTRACT InfraRed Search and Track (IRST) system is a wide field of view surveillance system, meant for autonomous search, detection, acquisition, and cue of potential targets. The first and second generation IRSTs utilized detectors with multiple elements followed by discrete preamplifiers for signal read-out. They have many performance limitations. With the advent of infrared focal plane array (IRFPA) sensors, the present trend is to build IRSTs based on line FPA sensors to achieve higher

sensitivity and resolution. However, due to system limitations of line IRFPA sensors, scanning mode of IRST cannot be stopped at any desired position to scan a small sector of interest. They also suffer from more false alarms in target detection. In future, it may be desirable to reduce false alarms, and also to use an IRST system for closed-loop-tracking of a potential target, in addition to its surveillance mode. IRST based on area array sensors may be a better option for this purpose, but it may pose some problems when used in a surveillance mode. This paper addresses this issue. Design considerations of all sub-systems of an IRST based on line/area array sensors, such as scanner assembly, interface electronics with the sensor, nonuniformity correction, signal processor, and the display methodology to cover 3600 are also discussed.

Keywords: IRST, scanner assembly, nonuniformity, display, interface electronics, closed-loop-tracking.

1. INTRODUCTION InfraRed Search and Track(IRST) system is similar to a surveillance radar with a wide field-of view(FOV) but operates in a passive mode. Being passive, it would not give away its position to the enemy and hence it is resistant to jamming. The technologies involved are similar to those used in thermal imagers(TI) and forward looking infrared (FUR) systems but with a difference. TIs are meant for obtaining high resolution imagery within specified FOVs in order to detect, recognize, and

track the target at maximum possible range with human intervention. It uses line FPAs (LFPA) with mirrors to scan in azimuth and elevation. FUR is also an imaging system , whose output is usually an JR image of the forward scene. It is used as a navigation and landing aid. The present FURs are also being used for target cueing. JRST, strictly speaking, particularly first generation IRST, is not an imaging system, characterized by radar like scan patterns, covering a wide FOV. With the advent of high resolution line sensors , the current IRSTs are able to form images and cover 3600. The first and second generation JRSTs utilized detectors with multiple elements followed by discrete preamplifiers for signal

read-out. They have many performance limitations. With the advent of infrared focal plane array sensors, the present trend is to build IRSTs based on LFPA sensors to achieve higher sensitivity and resolution. However, due to system limitations of LFPA sensors, scanning mode of IRST cannot be stopped at any desired position to scan a small sector of interest. In future, it may be desirable to use an 1RST systemfor closed-loop-tracking ofa potential target, in addition to its surveillance mode.

IRST based on area FPA (AFPA) may a better option for this purpose, but it may pose some problems when used in a surveillance mode. This paper addresses this issue, including design considerations of all sub-systems of an IRST based on line/area array sensors, such as scanner assembly, interface electronics with the sensor, nonuniformity correction, signal processor, and the display methodology to cover 360°. JRST comprises of (i) scanner assembly with detector, optics and necessary electro-mechanical assembly, (ii) interface electronics to supply the necessary bias voltages and different clock signals, (iii) 2-D signal processing to read-out the signals from JRFPA in real time, apply nonuniformity compensation, and reformat the signal data, (iv) signal processor for target cueing, (v) display system to display 360° image in a user friendly format. The block schematic of a typical third generation IRST is shown in figure 1. Further author information R.V.: Email: [email protected]

SPIE Vol. 3061 • 0277-786X/971$100O

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If we forget the sensitivity aspect for a while, AFPAs can offer lot advantages in respect of target detectabilty with less false alarm rates, variable scan rate, and closed-loop tracking of the target. These aspects are discussed in section 5.

Figure 1. IRST system block diagram

The following sections briefly discuss various subsystems of IRST. Criteria for selecting an IRFPA sensor is discussed in section 2. A brief account of optical system, particularly to cover 360° is given in section 3. The interface electronics and nonuniformity compensation are mentioned in section 4. The advantages of using AFPA sensors are discussed in section 5. The display methodology is given in section 6. The conclusions and future scope of work are given in section 7.

2. SELECTION OF AN INFRARED SENSOR FOR IRST The focal plane array sensor is the key component in IRST, and it must provide adequate sensitivity, and should support compact optical configuration. There are three generic detector types , namely, (a) LFPA, (b) LFPA with TDI, and (c) fully staring area FPA (AFPA). These are illustrated in figure 2.

2

11 SCAN LINE ARRAY

/

-,

.)

L

,-

LINE

/

SCAN

AREA ARRAY (STARING)

WITII ID!

ARP.AY

Figure 2. Three generic types of detectors suitable to IRST

The applicability of these detectors to IRST has been extensively studied by various users, particularly by Pilkington Optronics. In terms of better sensitivity and high resolution, the LFPA with TDI shown in figure 2(b) is the best choice for IRST. So far no one has used area arrays for IRST application due to some problems which are addressed in this paper. The advantages and disadvantages of line and area array FPAs are given in table 1.

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Table 1. Applicability of line /area array IRFPAs to IRST

Area array IRFPA

SINo.

parameter

1.

sensitivity

linear array IRFPA about 1 51 W1 Hz 1/2 cm

2.

size

@0.25 sr 288x4 & 480x4 available

3. 4. 5.

resolution spectral region TDI mode

high resolution possible 3-5 & 8-12 jim established

256x256 & 320x 240 in 3-5 jim 256 x 256 in 8-12 jim with frame rates of 400 frames/sec currently not possible 3-5 pm ok, 8-12 yet to be field worthy only possible with futuristic architectures with

6. 7.

excellent target cueing possible

may be feasible with innovative methods may be possible with new techniques

8.

applicability target detection and tracking FOV

wide FOV feasible

9. 10.

track record variable scan rate

systems available very difficult

limited to array size. WFOV, can be obtained by mechanically scanning/panning - but results in severe image smear no system using area arrays for IRST possible

about 1 .5x 101 1 W1Hz"2cm

-

sweep_and_read-out rates_are_matched

If we forget the sensitivity aspect for a while, area array FPAs can offer lot advantages in respect of target delectability with less false alarm rates, variable scan rate, and closed-loop tracking of the target. These aspects are discussed in section 5.

3. SCANNER-ASSEMBLY DESIGN CONSIDERATIONS

dome

,

dewar and

adjustable supports

Slip-ring

direct drive motor Figure 3. Sketch of experimental IRST prototype

The main issue in this paper is to explore the possibilities as to how to use area FPAs for IRST to achieve some advantages mentioned above. Hence, a prototype IRST(experimental platform) is under development using 3-5 jim sensors, to try out some new concepts for signal processing. The basic configuration of IRST with a lot of flexibility to align optics and FPA in

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all three axes, change of sensors, optics, etc. is shown in figure 3. A steering mirror provides the elevation control. The size of the mirror decides the instantaneous vertical FOV. Scanning the mirror in azimuth, one may get a maximum of 600 coverage1. One way to achieve 3600 coverage is to move the whole assembly to a new position and get another 600 and so on. This demands a derotation optics and slip-rings, in addition to mechanical scanning in azimuth. For experimental prototypes, it

may be better to opt for only mechanical scanning, thus doing away with derotation requirement. However this scheme requires a slip-ring to take out the signals from FPA which is on-board rotating platform.

3.1 Optics design considerations

The design of the 3 - 5 im optics is fundamentally not very different from the design of visible optics. The major design considerations are F/number, field-of-view focal length, first-order layout, and the availability of optical materials. In addition, however, the optical designer has to cater for the stringent requirements of the cold shield efficiency, narcissus effects, and methods of athermalization2 . The designer may also have to decide on the use of refractive or reflective optics, scanners3 , aspherics4 , and/or diffractive optics.

3.2 Optical materials There are many types of optical materials5 that could be used for the MWIR waveband. These include semiconductor materials, chalcognide glasses, alkali halides, and dielectrics. Of these materials, four types are most common, viz. silicon (Si), germanium (Ge), zinc selenide (ZnSe), and zinc suiphide. This issue of selecting an optical material for a particular application are well discussed6. In general, the desired properties for field applications are: high refractive index, low dispersion, low absorption, compatibility with anti-reflection coatings, low thermal coefficient of refractive index, high surface hardness, high mechanical strength, and insolubility in water. 3.3 Scan mirror

Depending on the search field of view in azimuth and elevation, this mirror has to be scanned with required speed6 .This can be static if the complete assembly is rotated in as shown in figure 3.

3.4 Optical derotator Image rotation occurs when a beam is steered in azimuth by a 45°mirror. This means that the vertical lines in object space do not remain in a constant angular relationship to the detector. Generally it is compensated by means of optical derotator, which

is difficult to realise and implement. However, for the purpose of experimental platform, it is avoided by rotating the complete assembly. 3.5 Narcissus effect

The narcissus effect is a phenomenon in which the cooled detector senses its own cold surface as a result of internal retroreflections from the refractive surfaces of the system. It arises at any time during a scan, when part of the cold detector focal plane is reflected by a refracting surface within the system, such that this reflected image is focused back onto the detector array. This cold image will be superimposed onto the signal field and part of the warm background, resulting in systematic pattern noise. The shape of narcissus depends on the shape of the cold focal plane area and the extent to which it is focused. Design methods with which narcissus may be reduced include: (i) reduce the effective radiating cold area of the focal plane by warm baffling, (ii) reduce the surface reflections of lenses using high-efficiency antireflection coatings, (iii) defocus the cold return by designing the optical system so that no confocal surfaces are present, and (iv) slant all flat windows at an angle to increase the angle of incidence. Practical measures to cut down narcissus can be effected through the use of a thermal source or by electronic video signal compensation7.

3.6 Athermalization Thermal effects are inherent in infrared imaging systems as most of the useful infrared materials, especially germanium, have a significant value of nJT, or thermal coefficient of refractive index. In addition, most infrared imaging systems are used in

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environments where extreme operating temperatures of —20°C to +40°C are often encountered. Hence, the effects on the imaging systems due to changes in refractive index with temperature must be considered. The thermal defocus caused to a single thin lens in air, Af, is given by8

f where a is the coefficient of thermal expansion of the refracting medium, f is the focal length, cc,1, is the coefficient of thermal expansion of the mount, L is the overall length of the system, n is the refractive index of the lens, and AT is the temperature change. However, the thermal effects of multiple lens groups are more complicated in optical designs that are more complex, such as the telephoto design, the Petzval design, and the inverse telephoto design.

If Af is greater than the depth of focus of the system, compensation schemes must be implemented. Active and passive athermalization may be attempted. Some possible solutions are: (i) To provide a manual focus adjustment, (ii) To provide an automatic electromechanical focus adjustment, (iii) To athermalize the lens by a combination of lens materials having an effective an/T of zero, (iv) To athermalize the lens using lens mountings of different coefficients of thermal expansion so

that the optics will move passively to compensate the defocus, and (v) To use diffractive optics to attain passive athermalization.

3.7 Optical system

Dome

Objective Lens

1.0 .9 .8 U- .7 I0 .6 I-

h Cl) 3

.3 .2

-1. Relay

FPA —4

.0

jLens

Figure 4.Optical lay-out for an F/1.6 MWJR lens

SPATiAL FREQUENCY IN CYCLES PER MIWMETER

Figure 5. Polychromatic Modulation Transfer Function for optical system shown in fig.4

An optical system was designed around a line array sensor 288x4 from Sofradir, with a focal length of 92.35mm, F-number of 1.6, and cold efficiency of 100%. The optical layout is shown in figure 4. A dome is used for the protection of the lens from the field environment. A steering mirror provides the elevation control. The objective is made of a standard triplet having near-diffraction-limited performance. A relay lens transfers the intermediate image onto the MWIR focal plane array.

Cold shield efficiency is 100%. The MTF plots are given in figure 5. A total of three aspherical surfaces are used for aberrational control and the spherical surfaces are test-plate fitted to the vendor's stock. To minimize costs, only Si and Ge are chosen for the refractive elements. Diffractive optics were not used because of resource constraints. In an earlier design, diffractive optics were designed for aberrational control as well as a means for athermalization9 , but not opted for this application.

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Narcissus may be controlled in the optical design through the use of the paraxial quantity, yni, which is a product of the marginal ray height (y), refractive index (n), and the incidence angle at the refracting surface (i). The absolute value of this quantity measures the severity of narcissus signal. The smaller this quantity is, the greater the narcissus effect will be.

4. INTERFACE ELECTRONICS AND NONUNIFORMITY COMPENSATION As per the commercial data sheets available from the vendors, line array FPAs with TDI architecture require a good number of well regulated bias voltages, clock signals, parallel channels to read-out signals, and complicated address sequence. In contrast, the interface signals required for area array FPAs are very few and less complicated. Signal processing is straight forward.

4.1 Nonuniformity compensation FPAs offer higher sensitivity, amenability to signal processing and mechanical simplicity. However these sensors contain large detector-to-detector dark current (offset) and responsivity (gain) variations. These variations result in nonuniformity in the detector array. A typical solution for NUC of IRFPA sensors is to calibrate the sensor by presenting uniform sources of constant intensity over the detector field of view. The outputs are used to calculate the correction coefficients i.e. gain and offset terms. Low and high temperature references have to be set around the mean temperature of the scene. If the individual pixel outputs are

perfectly linear within the calibration range and stable in time, then it is possible to completely correct the pixel nonuniformities. However, 1/f noise and system instabilities create the need for recalibration .This calls for precise variable temperature references to be generated on board the system. This in turn calls for a mechanical/electro-optical shutter to bring the references into the detector' s field of view for periodic re-calibration. This detracts from the mechanical simplicity of IRFPAs. It is better if nonuniformity compensation can be implemented by making use of the moving IR scene statistics which dispenses with the temperature references. The SBNUC concept makes use of the scene statistics for calibration and update of compensation parameters without masking the field of view. This scheme assumes a moving scene across the IRFPA sensor over a long period of time. It also assumes that each pixel in the array sees the same scene statistics. It may not be possible to incorporate temperature references on-board IRST if it has to cover 3600. in view of this, two-point as well as scene-based nonuniformity compensation algorithms were studied and analysed10. It was found that scene -based

algorithm was also performing well, but resulting in artifacts. It has been decided to develop a common hardware to implement either of the algorithm.

5. ADVANTAGES OF USING AFPAs Let the specifications of IRST are (i) cover 36O in azimuth and 5° in elevation, (ii) automatic target detection with low false alarm rate, (iii) if necessary observe for more time in the desired direction, and (iv) if possible track the target in autonomous

mode. Just to understand the concept, let us as consider 256x4 line array with TDI, and 256x256 area array for this application. The general scan pattern in both cases are shown in figure 6.

LFPA covers the 360° in continuous scanning mode. The scan rate has to be synchronised with integration time, signal readout time for obvious reasons. To vary these parameters in real-time may be very difficult and therefore it may not be feasible to vary the scan rate from the designed specification.

AFPA covers the scene in discrete steps as shown in the figure 6(b). If it covers 3600 in a continuous mode ,there will be serious objectionable smear in the image. This is the main reason why AFPAs are not used. Assuming 36000 pixels in 3600, 256 pixels correspond to 2.56°, say 2.5°. That is AFPA needs about 140 steps to cover 360° without any overlap. If 0.5° overlap is allowed as shown in figure 6(c), it may need 1 80 discrete steps. In case of AFPA, in order to see a contiguous picture it is necessary to register adjacent overlapping areas. It is reported that more than 400 frames per second can be expected , i.e. it needs about 0.45 seconds to cover 360°. LFPAs , due to their complicated read-out mechanism, it may take about two seconds to cover 360°. That means, LFPA takes 10 msecs to cover 2.5°, where as AFPA can stay at the same

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frame during that time getting four frames of data. Assuming overheads, the worst case can be two frames. If we can realize this, it is of much use to detect targets with low false alarm rate. 3600

Linear array

2.5°

2.5°

2.5°

___ (a)

9 Area array

Areaarray

2.5° 2.5° 2.5° ___________

25° •

i

___

25 4

(b)

2.5°

(c) H

..I.II...

.

overlapping image

Figure 6. Scan pattern with linear array and area array

5.1 Target detection with low false alarm rate

48km

Figure 7. Sketch representing relationship and object space and image plane

Let us assume an aircraft target, with physical dimensions of 3mX3m, crossing across at a range of 8 km. Considering an instantaneous sector of 2.5°, as shown in figure 7, the sector at 8 km covers an area of 350 m. That is 350 meters corresponds to 256 pixels in the image plane. Each pixel corresponds to about 1 .5 meters in space at 8 km. 3x3 meter target may occupy about 4 pixels in the image plane. If we consider that the target is moving across with a speed of one Mach i.e. approximately 300m/s, the target may move by about 1 .5 meters (corresponds to one pixel) during 5 msecs, i.e. two frame times of AFPA. In such cases it is easy to detect the target without false alarm. The above concept is tried out on some images as shown in the figures 8 and 9. Ideally when the two frames are identical except for target movement, the difference picture should give out the moving target. The frames 8(a) and (b) are taken with AFPA sensor. There was no sensor movement, but the target was moving. The difference of these two frames (figure 8c) results in detection of the target. The black and bright spots correspond to the previous and the present position of the target. Based on one frame, which should be the case in LFPA scanning, it is very difficult to detect point targets. The same exercise is repeated on other images as shown in figure 8 (d), (e) & (f). However, due to intensity fluctuations, moving tree leaves, and

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(a)

Figure

8.

(e)

(b)

Detection of moving target using a stationary staring array

(c)

. ... : 00

vi 0

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of moving target, with and without stabilisation of frames

Indication 9.

0

vi 0

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clouds, will result in some difference. But that can be removed by using adaptive-window-adaptive-threshold(AWAT) technique.

When there is significant shift between two frames due to sensor motion, AWAT alone will not work. This is presented in figure 9. The imaging sensor moved slightly during frames 9(a) & 9(b). The difference output after preliminary threshold is shown in figure 9(c). In spite of using comprehensive thresholding technique, the target could not be detected. Instead, a number of false targets appeared. This is shown in figure 9(d). The two frames are first registered to take care of sensor movement resulting in change in images. Then the first frame is subtracted from the second frame, and the resulting output is as shown in figure 9(g). The target came out very clearly as shown in figure 9(h), when 9(g) is subjected to comprehensive threshold. However, due to complexity of images, even after frame registration, sometimes false targets may appear. These can be removed by checking them in the next scan by using estimated track path. Accurate and reliable image registration technique" and AWAT( yet to be published) are the key techniques for good results.

5.2 Hardware implementation aspects

Figure 10. Block schematic: hardware implementation of IRST using AFPAs

A block schematic to implement the above scheme is shown in figure 10. It assumes that the sensor stares at a sector for two frames time and then moves to the next sector. Using the overlap area as the reference, image registration can be used to get continuous image covering 3600. Image registration is also necessary if the sensors moves during the two frame time. The subtracted image has to be properly thresholded using adaptive-window-adaptive-threshold (AWAT). Having some apriori knowledge about the targets' size, their range can be estimated using AWAT. If the threshold is not proper , some false targets may appear, which can be confirmed in the next scan and removed. Once the targets are confirmed within two scans, the IRST can be directed towards threat direction. Then IRST can go into track-while-scan mode, spending more time in the direction of threat. Once threat is confirmed, IRST can autonomously track the target, if necessary.

6. DISPLAY SYSTEM Display system is configured to accept the data stream from 288x4 IRFPA,(Sofradir) covering 360° , as well as the data stream from 256x256 area array. As per the preliminary design, the image format would be 288 pixels in elevation and 36000 pixels per line in azimuth. It is very difficult to present such an image of 288x 36000 on a display. The total image can be

600

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compressed and presented in 6 sectors. However the details will be lost. The display format is designed to support both options.

This display system consists of an image processing sub-system to interface with the display. The image processing unit contains the operator interface, remote control interface, main processor, frame grabber and image display. The functions of the image processor are: accept nonuniformity corrected imagery and auxiliary data, process the data into an exploitable image, format the data into a common output format and output the data to the display. The user interface, written in Windows environment, provides human-system interface. The software, called IRSTView, performs image quality control functions, support dynamic retasking function and dynamic assignmentof exploitation to the sensor module. It has on line record for replay of data of the image.

6.1 Types of data interface The data stream is fed from the image processing unit to the frame grabber through differential line drivers. The frame grabber process the data into an exploitable image and format the data into a common output format. Graphics formats such as .bmp, .pcx, .gif are supported. 6.2 Display format The display software runs under Microsoft environment. For 360° FOV, a total of 36000 x 288 (x 8 bits) pixels are piped from the signal processing unit to the display. The pixels data image stream is divided into six sectors. Each sector represents 600 horizontal scan. Each sector is formatted with the standard graphics display format. The formatted image is piped to display unit. The prototype has the advantage of providing both an interactive "feel" of screen snap shots and VCR-like control for play, fast forward, and rewind. Images are stored in hard disk. These techniques have been selected in the prototype as they allows user to analyses the scene at a later time. The Microsoft APIs shrieked each region to the respective displays sector windows. The compressed version of the sector and the whole image stream is shown in figure 11. Double clicking the mouse on the sector displays a full screen of the selected sector.

60 degree

—sector display Slider for

,/' active sector

'/4

/

Compressed image of each sector

Angular scale I

180

I

I

120

60

I

0

60

I

120

Figure 11. Proposed display

180

format

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It would be advantageous to display processed image , that is after target detection, which may give better clarity. Another option is to use PPI type of display to present the processed likely targets. The same display can be configured to serve this purpose also.

7. CONCLUSIONS AND FUTURE SCOPE OF WORK An IRST to cover 3600 with low false alarm rate in target detection is considered for design. The advantages and disadvantages of using line/area arrays sensors are mentioned. IRSTs using line FPAs have definite advantage of higher sensitivity and resolution. However they are not flexible to change the scan rate in real time. It is very difficult to detect point targets based on a single image. For verification, the next image comes only in the next scan. The time between scan to scan is significant and target position may change significantly making it difficult to detect and track record it. Therefore they also suffer from high false alarm rate in target detection. Scanner assembly with flexibility to use it as an experimental platform, optical configuration to suit 288x4 IRFPA, nonuniformity compensation aspects are discussed. The advantages of using area

array sensor are presented. Display format for presenting 360° image is also presented. It may be possible in future to demonstrate an JRST, using AFPAs to some advantage. Following are areas for future research

(i) Generate mathematical model to simulate and assess the suitability of different FPAs. (ii) Evaluation of target detection capabilities using LFPAs & AFPAs (iii) Study on computational complexities involved.

ACKNOWLEDGMENTS The authors would like to acknowledge Defence Science Organisation, Singapore, for lending an JR camera to Nanyang Technological University for collecting the required JR images for studies. The authors also would like to acknowledge Gan Yu-Hin for his help in preparing this paper, and Dr Tan Kah Chye, Centre for Signal Processing for his help in extending the computing facilities.

REFERENCES 1. Magne Norland,et.al, "Design Of a High-Performance JR Sensor", SPIE Vol 2269, Infrared Technology XX 1994, pp 462-47 1. 2. RE. Fischer, "Lens design for the infrared",* 3. R. B. Johnson and C. Feng, "History of infrared optics",* 4. P. Nory, "Key technologies for JR zoom lenses: aspherics and athermalization",* 5 W. L. Wolfe, "Optical materials for the infrared",* 6. G.R. Armstrong, P. J. Oakley, and B. M. Ranat, "Multimode IRSTIFLJR design issues",*

7. E. H. Ford and D. M. Hasenauer, "Narcissus in current generation FUR systems", *in R. Hartmann and W. J. Smith (eds.), Infrared Optical Design and Fabrication, Vol. CR38, SPIE, Bellingham, Washington, USA, pp.95-1 19 (1991). 8. J. M. Lloyd, Thermal Imaging Systems, Plenum Press, New York, 1982. 9. W. C. Goh, W. M. Shi and S. C. Tam, "Dual field mid-JR lens design incorporating diffractive optics and athermalization", Proc. Mindef-NTU Joint R&D Seminar, Singapore, pp.160-165 (12 January 1996). 10. R. Venkateswarlu, et al., "Nonuniformity compensation for JRFPA sensors", to be published in SPill Vol. 3061, 1997. 11. R. Venkateswarlu, et al., "Area-correlation tracker with improved reliability", to be published in SPJE Vol. 3086, 1997.

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