This presentation describes a personal computer-based Pascal code which allows evaluation of airborne imaging system installation parameters by calculating the sensor imaging performance and comparing the sensor's laboratory resolution and its installed resolution, and by calculating the installed sensor's field of view obscuration or vignetting. A variation of this code has been used to characterize Stratospheric Observatory for Infrared Astronomy (SOFIA) and Advanced Tactical Airborne Reconnaissance System (ATARS) sensors. Flowcharts and applications of the airborne imaging system performance model are presented. Selected results from the use of similar codes also are shown.
Motion of the image presented to an electro-optical imaging system blurs the incoming energy and degrades the system's ability to detect or recognize distant targets. The motion that occurs during a sensor integration period or frame generally produces the most significant effect, although, frame-to-frame motion can also be important depending on the application and the image processing involved. Thus, line-of-sight stabilization specifications that limit only the amplitude of image motion without regard for frequency may be conservative and over- constrain the design in some cases. This paper addresses the effects of image motion on the performance of electro-optical imaging systems and investigates alternate methods of specifying the allowable image motion for the stabilization system. Specifically, the effects of both the spatial and temporal frequency content of the motion spectrum on the system MTF (modulation transfer function) and on the system's ability to recognize and detect distant targets are discussed. Types of image stabilization specifications considered include RMS jitter amplitude, maximum allowable motion amplitude during an integration interval, allowable MTF degradation and allowable motion versus temporal frequency. These are then compared according to whether or not they can be readily measured, whether they can be easily understood and converted to meaningful design parameters, and whether they inadvertently over-constrain or under-constrain the design.
In almost all medium- or high-performance line-of-sight (LOS) imaging systems some method of stabilization is utilized. This paper briefly reviews the reasons why LOS stabilization is required and methods of achieving this system requirement. It describes a novel combination of various stabilization techniques in which ultra-stable platforms can be achieved at relatively low cost.
The effect of thermal treatment and nuclear irradiation on the far infrared spectrum of polycrystal CsI was studied using pure and cerium doped crystals. Samples were exposed to neutrons and gamma radiation from a neutron activation tube. Thermal treatment was conducted in a 5.5 kilowatt furnace. Ionizing radiation caused significant infrared transmission losses at doses less than or equal to 1 MRad and the threshold wavelength of transmittance shifted to longer wavelengths. Transmission losses in cerium doped crystals were less than in pure crystals. Heat treatment also caused a significant reduction in transmittance. These results indicate that high temperature annealing will not decrease radiation damage.
The double dove prism is a common method used to achieve large scan angles with a straight line configuration. The basic concept uses two cemented dove prisms rotating at one half the desired scan angle. In most cases, this double dove prism is used behind a flat window. This paper investigates the design considerations required to use the double dove scanner behind an optical element with power, in particular, a concentric dome. The limits to achievable resolution are explored, as well as methods to compensate for the aberrations introduced by the power element.
The precise infrared refractive index for polycrystal and single crystal germanium was measured using the prism minimum deviation method. Samples were obtained from three major suppliers. Values for single and poly are compared. Variation among suppliers is discussed.
Many commonly used infrared window materials, such as zinc sulfide and zinc selenide, are subject to structural failure due to stress-corrosion induced cracking. This failure mechanism is of critical importance in applications in which the window experiences high static pressure loading for prolonged periods in humid atmospheres, conditions typical of airborne optical windows. The most effective means of screening windows against failure due to this mechanism is by use of overpressure proof testing. In this paper, the design of overpressure proof tests for large airborne infrared windows is discussed. The underlying physical phenomena and governing mathematical relationships are presented. A hypothetical proof test design for a large infrared window to be employed in a man-rated aircraft is developed to illustrate the application of the analytical methodology; Practical considerations in the execution of large infrared window overpressure proof tests are also discussed.
The following describes a lightweight infrared system that is ideal for mounting on light aircraft for surveillance purposes. Use of a platinum silicide staring array detector helps to minimize mechanisms and use of the 3-5 micrometers infrared band enables the infrared optics to consume a smaller volume. Use was made of this combination to create an infrared sensor head that weighs less than 45 pounds and only protrudes 5.5' outside an aircraft skin.
The family of high spatial resolution (HSR) digital infrared imagers is described. The first generation, called the High Spatial Bandwidth (HSB) FLIR, was developed from the group of FLIRs known as the Class II Thermal Imaging Common Modules (TICM II), and provides improved horizontal resolution, a noninterlaced digital output suitable for interfacing to digital image processing, and a second output which can be tailored for most standard interlaced video formats. The HSB FLIR has been flown in several applications. The second generation of HSR FLIR, which is currently in production, provides expanded digital signal processing and improved sensitivity by incorporating a larger aperture scanner. Automatic control of temperature span and offset are provided, with implementation using field-proven algorithms suitable for accommodating very hot and very cold objects. The digital output conforms to the ATRWG 86-002 standard. Image processing features include zoom and pan, de-rotation, distortion removal, local image patch stabilization, overlays, convolutions, snapshot and frame storage, frame integration, analogue display of recorded digital data, and pseudo-color mapping.
Lasers were first integrated into fire control systems in the 1960s and continue to show up as a requirement for many new systems as well as in upgrades to existing systems. In many cases the laser is a critical factor in the performance of the primary mission of the platform. In certain applications, the laser is integrated with a forward-looking infrared (FLIR) detection system and tracker, which provides the means to sight and hold the laser on target. In order for both the laser and the FLIR to operate at peak performance, it is important to understand the specific needs of the two devices and how they may interplay, and address their integration as early as possible in the design process. Both Nd:YAG lasers, used for target designation and rangefinding, and CO2 lasers, used primarily for rangefinding, are currently integrated into fire control systems. FLIR system designers must understand these different laser types and their specific capabilities as they relate to FLIR performance. New weapon system requirements have placed additional utilization demands on the laser-sensor combination to produce an active/passive sensor fusion. For example, chemical warfare point- to-point detection is being addressed through a combined FLIR and differential LIDAR. Likewise, low level navigational hazards are identified via wire sensors for helicopters. Target detection is further complemented with laser-based vibration sensors. Sensor fusion is rapidly emerging to solve future air-to-ground, ground-to-air, and ground-to-ground mission requirements.
Segmented windows for military aircraft introduce several optical effects into airborne electro-optical sensor imagery. A diffraction-based computer code has been written to calculate these effects and predict impacts on installed sensor performance. Experimental validation of the code agrees to within 3 percent. The window segments are seen to effectively divide the sensor pupil into separate optical apertures sharing a common focal plane. Extremely tight optical wedge tolerances are indicated for high-resolution sensors. Performance predictions for various pupil splitting geometries are shown, both for polychromatic incoherent sensor imagery and coherent laser radiation. Effects of varying differential wedge on sensor imagery are shown.
A tilt-tolerant collimating objective lens for dual-band interferometry has been designed. The lens is a 6-inch aperture f/5 all-spherical design, and is parfocal at 0.6328 and 1.061 micron. Optical correction is better than 1/20 wavelength peak-to-valley OPD at each wavelength, over a 0.34-deg (0.178 arcsec) field. Fabrication tolerances for this lens are challenging but realistic.
Large aperture, high performance sensor windows for new tactical aerospace applications must satisfy a broad spectrum of functional performance requirements. Several design and manufacturing technologies require additional development to achieve low observability, laser hardening, abrasion resistance, environmental resistance, and shared aperture multispectral optical transmission and surface conformality at subsonic and supersonic velocities. The state of development of these technologies is discussed. Specific technology accomplishments are discussed in the areas of low observable window design implantation, diamond and hard carbon abrasion resistant coatings and bond materials, grid and electrically conductive coatings, optical substrate materials, and in-flight optical performance analytical design software. Areas requiring further technology development are also identified and discussed.
The optical properties of GaAs, GaP, and CVD diamond, which are being developed by Texas Instruments specifically for infrared optics, are continually being improved. The data reported here indicates the current usefulness and directions for further improvement of these materials. Bulk-grown GaAs has been developed that has near-intrinsic absorption coefficients for both high-resistivity (107 ohm-cm) and conductive (<1 ohm-cm) type material. Its transparency extends from 0.9 to 16.0 micrometers . The absorption coefficient at 1.06 micrometers is 0.3 cm-1 for high-resistivity GaAs and 0.5 cm-1 for conductive GaAs. The absorption coefficients for 2 to 12 micrometers are <0.01 cm-1 for high-resistivity GaAs up to nearly 400 degree(s)C and up to nearly 200 degree(s)C for the conductive GaAs. GaP is transparent from 0.6 to 12.0 micrometers . The absorption coefficient at 1.06 micrometers is 0.1 cm -1; from 2 to 7 micrometers , the coefficients are <0.01 cm-1; from 8 to 10 micrometers , they are <0.1 cm-1, except at 9.25, where the coefficient is 0.5 cm-1. Diamond is transparent from 0.2 to 4.0 micrometers and for all wavelengths beyond 5.5 micrometers . Current CVD diamond is limited by light scattering, hydrogen and other impurity absorption, and defect-induced one-phonon absorption. Absorption coefficient values at 8 to 12 micrometers are 1 cm -1 with some peak absorption of a few cm-1.
Infrared window materials for modern military aircraft must simultaneously provide adequate optical performance and maintain stringent shielding levels for electromagnetic interference (EMI). Microscopic metal grids deposited on the surface of dielectric windows can provide the conductivity necessary for proper radio frequency or RF shielding. This paper will examine the optical impact of three grid patterns designed for EMI suppression. Transmission blockage and beam diffraction in the far-field have been measured and compared to predicted values. A discussion of durability under severe thermal and rain erosion environments is also included.