Long Range Hand Held Thermal Imager

Transcription

1 Long Range Hand Held Thermal Imager Edward Seibel, Andrew Struckhoff, and Robert McDaniel, Kollsman Inc., Merrimack NH Shlomo Shamai, Electro Optical Industries, Ltd., Rehovot Israel ABSTRACT Today s war fighter requires a lightweight, high performance thermal imager for use in night and reduced visibility conditions. The operational requirements dictate that the system be lightweight, but still have significant range capabilities and extended operating time on a single battery load. Electro-Optics Industries, Ltd. (ElOp) with its US partner Kollsman, Inc., responded to this need with development of a hand-held binocular FLIR using a staring focal plane array imager. This paper will discuss the resulting system design and performance, a successful result of tradeoffs made in the areas of infrared characteristics, size, weight and power (SWaP) to ensure a lightweight, but high performance thermal imager. Keyword: thermal imager, detector-dewar-cooler (DDC), medium wave IR (MWIR), hand-held, binocular 1. INTRODUCTION Modern day ground forces operating throughout the world need for their personnel to be equipped with a personal thermal imaging camera to provide capability for detection, recognition and acquisition of tactical targets at day and night, and under adverse weather conditions. This imager would give combat and reconnaissance units a decided edge in today s combat environment. The desired system was to have been fully developed with state of the art core technology. The system was also identified to give the user other potential detection capabilities. Specific requirements included: Detection of Man-Sized Targets at 3500 meters (minimum) Recognition of Man-Sized Targets at 2200 meters (minimum) Detection of Vehicle-Sized Targets at 7500 meters (minimum) Weight of under 6 pounds MIL-STD-810F Qualified To address the problem posed by this application, Electro-Optics Industries, Ltd. (ElOp), teamed with its affiliate company, Kollsman Inc., to develop the system shown in Figure 1; a state-of-the-art, hand held, battery operated thermal imager operating at the medium-wave infrared band (MWIR 3-5 µm). The primary components are shown in Figure 2. IR radiation from a remote scene is imaged by a dual Field of View (FOV) optical configuration onto a staring, InSb detector array. Each detector element converts the incident radiation into a corresponding time-varying electrical signal. The detector outputs are multiplexed, digitized, and stored in processor memory. The stored image is reformatted into a standard video image for display on a high-resolution organic light-emitting diode (OLED). In addition to a comprehensive, permanent factory Non-Uniformity Correction (NUC), a fast field-nuc algorithm allows the user to easily correct for image non-uniformities during normal operation. The system uses standard military rechargeable or disposable batteries (NSN). An external communications protocol and video output allow for remote control and imagery viewing. Optional accessories include a 2X optical telescope, laser rangefinder, integral GPS receiver, and integral digital magnetic compass (DMC). The system was intended to be used primarily for surveillance and reconnaissance; however, with the options, it can also be used as part of a fire control solution. Infrared Technology and Applications XXXII, edited by Bjørn F. Andresen, Gabor F. Fulop, Paul R. Norton, Proc. of SPIE Vol. 6206, 62062V, (2006) X/06/$15 doi: / Proc. of SPIE Vol V-1

2 FOCUS NEAR! FAR FOV- Binocur Viewer GPS Antenna Enter - Polarity Objectwe / Len5 / Neck Strap Battery / Main Connector Auto / Man + Menu Figure 1 Long Range Hand Held Thermal Imager Components Thermal Camera JR Radiation Video 1/ Figure 2 System Block Diagram Proc. of SPIE Vol V-2

3 2. INFRARED BAND SELECTION The two main thermal imaging systems in use for hand held ground applications today are the uncooled longwavelength infrared (UCIR) and the cooled mid-wavelength infrared (CMIR). Other system types exist such as scanning and staring cooled long-wavelength infrared, but most of the hand held applications are trending toward UCIR versus CMIR. Each type of system has inherent advantages and disadvantages. Advantages of the UCIR system include: lower cost (as much as ½ less than a typical cooled system), no cool-down time, and quieter operation. Disadvantages include: lower sensitivity, requiring large optics to achieve longer-range performance, and slower response time (mostly critical in high-speed applications). Advantages of the CMIR system include higher sensitivity (and higher performance) and smaller optics; while disadvantages include higher cost and weight (due to the cooler and additional battery power required), as well as a required detector cool-down period and some acoustic noise from the cooler during operation. Another key difference is the inherent characteristics of the two infrared bands used in the devices. The typical theatre of operation for the system includes humid environments such as coastal areas, tropical areas and at sea. At short ranges, there is no significant difference in atmospheric attenuation of the IR radiation between 8-12µm and 3-5µm spectral bands. Therefore an 8-12µm (LWIR) system, such as the commonly available un-cooled systems, may be acceptable. For medium to long range applications, especially in humid environments however, there is a significant advantage for 3-5µm systems over 8-12µm systems, since the accumulated atmospheric attenuation at the 8-12µm band is much higher. This clear advantage of 3-5µm systems is a well known phenomenon that has been demonstrated in numerous field tests and demonstrations. For comparison we have calculated range performance of cooled mid-wavelength infrared (CMIR) and un-cooled long-wavelength infrared (UCIR) systems, with various typical optical configurations, at US standard (dry) atmosphere as well as at tropical atmosphere (~70% Rh). Table 1 shows that a CMIR system with similar (F/1.0) optics and focal plane array format compared to the UCIR has longer Detection, Recognition and Identification (DRI, per Johnson s Criteria) ranges. The table also shows that for a standard Tropical atmosphere compared to a US Standard atmosphere, the CMIR ranges are degraded only by 5% to 6%; while the UCIR system range performance degrades by 7% to 34%. Table 1. Range performance change with atmosphere UCIR CMIR Range, KM US Standard Tropical % Change US Standard Tropical % Change Man D % % R % % I % % Vehicle D % % R % % I % % This relative range degradation relationship with humidity will hold for all long-wavelength infrared systems, regardless of whether they are cooled or uncooled, given similar optical and focal plane dimensions. The range performance of the 8-12 µm system will be further degraded (versus the 3-5µm system) for non-standard (but very common) humid tropical atmospheres (90% Rh or more), which are typical to sea and near-equator conditions. Therefore the CMIR system meets almost all objective range performance requirements in its basic configuration. Range performance can be further enhanced by using a field-installable X2 add-on telescope or by using a higher Proc. of SPIE Vol V-3

4 resolution detector (available growth potential). Extension of range performance in the uncooled system is very difficult due to the already large optics that would be required to meet the predicted performance for this application. The NETD of the CMIR system is very low (20mK) while the NETD of UCIR is significantly higher (approximately 110 mk). This would yield a very noisy picture of a UCIR system versus a very quiet and high sensitivity image of the CMIR. When observing a remote object, the UCIR system will typically show the object floating on a milky background, and almost no background scenery details will be distinctive. The CMIR image, on the other hand, will be very detailed and emphasizing many background scenery details. Finally, the magnification of a CMIR system in the narrow FOV (basic configuration) is X8 (versus only X5 of the typical UCIR), yields a much more detailed image even at the outer boundaries of the range performance envelope. Finally, UCIR systems usually use a shutter mechanism which may be noisy and causes periodic image blinks. Therefore, while UCIR is a good choice for cost-driven, low- to moderate-performance applications, the CMIR is still the best choice for high-performance applications, especially in high-humidity environments. 3. SIZE, WEIGHT AND POWER CONSIDERATIONS A key design requirement for the system was to be small and lightweight to support ground force activities. However, as system designs evolve, designers will look for ways to shrink packaging, share system resources, and fit more components into less space. As such, the level of interconnectivity and required interaction between the various components increases. This includes the need for more integration of electrical functionality and increasingly tight packaging of the system components. This design philosophy can be counter to requirements for ease of system maintenance in the field. For system maintenance in the field, technicians must be able to easily identify and then repair or swap out failed components with a minimum of required re-alignment and re-calibration. Leveraging of existing test equipment resources is also desired. As such, the design must achieve a balance between maximizing ease of maintenance while still minimizing the overall package dimensions. The thermal imager is constructed of very few modules an integral detector / dewar / cooler (DDC), modular focus and field of view (FOV) assemblies, an independent display assembly, and a small number of compact circuit card assemblies (CCAs). Each module is independently tested and verified resulting in an increased reliability at the system level. The modules have been arranged within the unit in order of decreasing life, so that the components most likely to fail are the easiest to access and repair or replace. An internal view of the system is shown in Figure 3. The heart of the imager is the compact DDC assembly. A critical decision in the size, weight and power (SWaP) tradeoff, a cooled detector was selected over an uncooled detector in order to achieve superior range performance. The detector is an InSb focal plane manufactured by SCD, in Israel. The detector is integrated with a closed-cycle cooler with integrated electronics from Ricor, Ltd to form the DDC package. The integrated DDC package is highly reliable and is a primary contributor toward increasing the system MTBF to over 2000 operating hours. Detector parameters are shown in Table 2. Table 2 - Detector Parameters Parameter Value Spectral Band 3 5 microns Pixel Pitch 30 microns Detector Size 24 microns Pixel Count 320 x 240 Peak D* > 6 e10 cm Hz 1/2 / W NEDT < 40 mk Proc. of SPIE Vol V-4

5 Contributing factors to the cooled versus un-cooled detector decision include the availability of high-power Lithium Ion batteries from the available stock of US military equipment, as well as Ricor s work towards lowering the power consumption of the cooler. The imager can use either rechargeable (BB-2847) or non-rechargeable (BA-5347) batteries. Normal operating time on a single battery exceeds 4 hours at 23 C. Using the appropriate adapters, the imager can also be powered from wall outlets or vehicles. Figure 3 Internal Component Layout The design of the optical system was another critical part of the SWaP tradeoff. Here, a balance between system performance (increased optics size) and system weight had to be achieved. Trade factors include system focal length, f#, and number of elements. The choice of the system FOV had to provide the ability for the user to scan a field of regard for general situational awareness (wide FOV), while providing the ability for increased detection and identification range (narrow FOV) all in as small a footprint as possible. The system s optical parameters were chosen to reflect the imager s use as a handheld (i.e. not stabilized) system. The general parameters for the optical system are shown in Table 3. The resulting optical path from the objective to the DDC was also designed to be folded to increase the overall compactness of the system. Proc. of SPIE Vol V-5

6 Table 3 - Optical Parameters Parameter Value Wide Field Of View 8.25 x 6.1 Narrow Field of View 2.75 x 2 Entrance Aperture 50 mm The Long Range Hand Held Thermal Imager s concept of operations showed that there would be situations where the war fighter would be able to make use of increased range detection capability. Rather than incorporate larger optics that would contribute to the overall weight of the system, the resulting design includes an X2 telescope to provide increased detection ranges, but allows the user the flexibility to not use the telescope when reduced weight is desired. Once the design team had established the primary performance parameters and components for the system, the remainder of the components had to be packaged in order to reduce the volume of the system while maximizing the ease of system maintenance. Figure 1 depicts some of this packaging. Design of the system enclosure had to satisfy the requirements to reduce weight while still allowing the imager to withstand the conditions required to meet the specified environmental conditions. The environment had been specified from -30 C to +50 C and included exposure to sand, rain, and salt exposure, tested to MIL-STD-810F. The final design is environmentally sealed and has a user-friendly weight of 5.7 pounds (excluding the battery) with the standard aluminum case. If additional weight reduction is desired, the imager can be manufactured with an optional Magnesium case, reducing the system weight to 5.1 pounds (excluding the battery). 4. HUMAN FACTORS DESIGN The hallmark of the resulting imager design is its simplicity and ease of use, with the typical training time for most operators being less than 1 hour. In addition to the SWaP trades that were performed for the system design, the imager incorporates other important features to make the system more useable by the war-fighter. These features begin at the primary interface to the system the binocular display. The display is an organic light emitting diode OLED that is integrated into the top cover assembly of the system. The display provides the user with a high intensity, high resolution (VGA), and monochrome image of the thermal scene. To achieve the VGA (640 x 480) resolution, each pixel of the 320 x 240 is replicated to a 4 x 4 pixel area, along with some image processing. Display symbology (Figure 4) provides the user with an intuitive interface for system control. A 5-mil reticle is also provided. The user views the display through the binocular optics. The binocular assembly includes both a diopter adjustment (from -6 to +2 diopters) and an interpupilary distance adjustment. The display and binocular components are incorporated into the top cover assembly of the imager. This design adds to the ease of system maintenance, as it allows the display portion of the system to be placed out of the way as the user accesses the other internal components. The imager top cover assembly is shown in Figure 5. The user controls the imager through a series of buttons that are incorporated into the top cover assembly. Six buttons on the top of the assembly provide the user easy access to the most-used functions of the system FOV selection, focus control, polarity selection (black hot / white hot), and gain selection (manual / auto). A 4-way Proc. of SPIE Vol V-6

7 thumb switch provides the user with further control to navigate through the imager menu system in order to perform other system control functions. Figure 4 Imager Display Figure 5 Top Cover Assembly Proc. of SPIE Vol V-7

8 5. GROWTH CAPABILITY The imager can be modified to accomplish specialized missions through the installation of a recently developed image processor and a color OLED eyepiece display. This processor allows implementation of image processing algorithms for specialized applications including dynamic range stretching, contrast enhancement, edge detection and other common and useful functions. Optional accessories include a 2X optical telescope, laser rangefinder, integral GPS receiver, and integral digital magnetic compass (DMC). The system was intended to be used primarily for surveillance and reconnaissance; however, with the options, and by use of the included video output and external data interfaces, it can also be used as part of a fire control solution. The imager, therefore, is a standard platform in which is readily modified and upgraded depending upon specific user requirements. SUMMARY The El-OP-Kollsman team believes the above outlined Long Range Hand Held Thermal Imager system with the DDC technologies represent a significant combat advantage for thermal imaging requirements of the modern battlefield. What the system truly offers military forces is a light weight thermal imager that is easily operated, has superior operational performance, is compact and has a growth path for technology upgrade enhancements. Proc. of SPIE Vol V-8