KEYWORDS: 3D modeling, Sensors, Minimum resolvable temperature difference, Bolometers, Performance modeling, Atmospheric modeling, Modulation transfer functions, Image sensors, Signal to noise ratio, Thermal modeling
Presented is a comprehensive, physics-based model for microbolometer detector and sensor performance prediction. The
model combines equations found in the literature and various standard models that generate NETD, MRTD, 3-D noise
statistics and atmosphere characteristics (MODTRAN-based), with a comprehensive microbolometer model and HgCdTe
model developed by the author to provide an end-to-end detector/FPA/sensor analysis and design tool, as well as a
realistic image sequence generation tool. The model characterizes the individual pixel element based on the structure
used, the various layer thicknesses, the electrical and thermal characteristics of the bolometer material and the biasing
and readout circuit, and uses these results to calculate response and noise, NEP and NETD. The NETD, MTF and
MRTD are predicted from the optics, detector and readout. Predicted NETD has been compared and verified with values
found in literature, results from other models, and to uncooled camera measurements. The MRTD prediction has been
verified with camera measurements and with standard industry MRTD model outputs. The model also calculates
atmospheric path radiance and transmittance for horizontal paths based on MODTRAN outputs for the LWIR band at
altitudes from 0 to 10km and ranges from 1 to 50km for assessments of air-to-air engagement SNR's. The model in
matlab utililizes a 3-D noise model to provide accurate realistic imagery used to present realistic sensor images and to
further validate the NETD and MRTD routines.(1) Images at 30Hz and 60Hz have been generated for visual assessment
by the user and have mirrored industry model results and real-time camera images for MRTD's for the temporal noise
case. The model's 3-D noise generation feature allows the prediction of MRTD vs. frequency under any 3-D noise
combination. This model provides an end-to-end performance prediction tool useful in bolometer element design,
readout design and for system level trade studies.
A unique polarization camera has been fabricated out of a wire grid polarizer attached to the surface of a
InGaAs FPA. The wire grid was configured as a Stokes polarimeter. Data has been collected for both space
and earthbound applications using both active and passive illumination. A mini-range and scaled targets of
representative materials were constructed to simulate space based distances for both resolved and
unresolved targets. For the purpose of providing advanced warning for rotorcraft, data has been collected on
power lines to test the feasibility and appropriateness of this type of technology to aid in their detection.
Spectral results including reflectance, transmittance, rTIS, and tTIS are presented for diffractively structured GaAs using the Automated Rasterable Integrated Spectrometric and Total Integrated Scatter Measurement System (ARISTMS). The data is for the bandwidth of 10&mgr;m to 12&mgr;m over a range of incidence angles between 0° to 75°. A description of the diffractively structured GaAs and the operation of the ARISTMS are given.
A new capability to acquire large amounts of spectrally determined optical data for a wide range of materials has been designed and developed from commercial off the shelf equipment. The software control system was written using LABVIEW 7.0. The Automated Rasterable Integrated Spectrometric and Total Integrated Scatter Measurement System (ARISTMS) represents a fusion of state-of-the-art technology and systems software to facilitate automated data acquisition to determine a material's spectral characteristics, surface roughness, and absorptance. It was developed as part of an ongoing Phase II SBIR effort to develop diffractively structured gallium arsenide infrared windows that are 100 mm in diameter transmitting between 1 and 10 microns. It was necessary to develop a capability that could scan or raster across the entire surface area of the window, vary the incident spot size, step size, and angle of incidence over the infrared spectrum of interest. The system offers a cost effective capability to screen many samples against preset thresholds for reflectance, transmittance, absorptance, and total integrated scatter for any number of measurement scenarios and sample classes.
A process to diffractively structure GaAs for enhanced optical performance is described. The benefits of diffractively structuring an EOIR window material include improved FOR/FOV, consistent broadband performance, the ability to design and implement hyper-spectral characteristics directly into the substrate without incorporating a complex anti-reflective coating. Progress to date will be discussed including design evolution, process implementation, and optical characterization using the Automated Rasterable Integrated Spectrometer and TIS Measurement System (ARISTMS). Results will be presented on 100mm diameter samples.
The goal of this effort was to design, develop, and demonstrate diffractive anti-reflection structures (DARS) on gallium arsenide (GaAs). Structures were designed and fabricated in GaAs intended to reduce the reflectance to infrared radiation from 1-10 microns wavelength. Design trade studies were performed to determine the optimum overall depth and period of the structure. The wafers were coated with UV sensitive photoresist and exposed in our interferometric stepper and our reduction stepper. Patterned areas were approximately 1cm x 1cm square. The wafers were then developed and measured to determine that the appropriate size and shape had been achieved prior to etching the pattern into the substrate. The wafer was etched in a plasma reactor to transfer the developed pattern into the GaAs. The depth and period of the surface was characterized using an atomic force microscope and a Scanning Electron Microscope. Reflectance spectra were measured for several angles of incidence.
Germanium witness samples were impacted with the NAWCADWAR modified Cambridge liquid jet device introducing varying levels of damage about the center of each sample. Surface damage statistics were collected, scatter measurements were made at 0.67 micrometers and the samples were failed in tension using a bi-axial flexure test setup. The level and character of the damage was correlated with the reflected scatter measurements as a function of local stress and flaw size distribution. Bi-axial flexure data was analyzed to predict fracture stress and the probability of failure of the germanium samples. The mechanical data were then correlated with the scatter data in order to correlate the BRDF with the material failure. The BRDF measurements were taken in several different orientations in order to study the differences in scatter character for the in-plane and out-of-plane conditions.
The erosion resistance of thin film epitaxial phosphide coatings has been shown to be affected by substrate crystal orientation when deposited on Ge substrates. GaP was systematically studied using optical microscopy, FTIR spectrophotometry, image analysis and x-ray diffraction. Microstructural aspects of the surface fractures indicated a clear crystallographic preference for erosion. X-ray diffraction identified the (111) plane as having the preferred crystal orientation for erosion resistance and variations of the (100) as the erosion prone planes. Removal of the hard carbon coating was observed in a large number of samples resulting in measurable damage to the GaP layer. Crack morphology was distinctly different in (111) and {100} crystals. (111) planes displayed erosion as discrete isolated pits and {100} planes developed very intricate orthogonal crack networks with pits at the intersections. Depth of damage varied as a function of crystal orientation and non (111) coating sections suffered some substrate exposure. Subsections of (111) protected Ge had an average depth of damage (measured vertically) of 0.47 mm and non (111) subsections had an average depth of damage of 2.3 mm. In most specimens, complex crack networks developed within 10 minutes of exposure to the simulated rain environment. The resultant GaP coating loss averaged 18 percent.
Pilkington Optronics (Barr & Stroud Ltd) has an ongoing development and pre-production activity for ultra-durable coatings. Such coatings provide enhancement of environmental durability of infrared transmissive windows and domes on airborne platforms. This activity places particular emphasis on providing protection against rain and solid particle impact at airborne velocities. This program has produced a very effective rain and sand erosion protective, anti-reflective multilayer, based on boron phosphide overcoated with diamond like carbon (DLC/BP). This coating has been demonstrated on a range of infra-red (IR) materials: germanium, FLIR ZnS, TUFTRAN, silicon and gallium arsenide. This paper describes a pre- production program of work to coat the external, convex face of the F14 aircraft IR search and track dome with DLC/BP. This is a 9 inch diameter, extended hemispherical germanium dome. Results are presented of coating uniformity, optical performance, UDRI whirling arm rain erosion, single water jet impact assessment and fracture toughness enhancement.
Texas Instruments (TI) has an ongoing development effort for protective coatings to enhance the environmental durability, particularly against water and particle impact, of infrared (IR) (8 to 12 micrometers ) transmissive windows and domes on airborne platforms. This program has produced a very effective, rain-erosion-resistant coating consisting of polycrystalline or epitaxial gallium phosphide (GaP). The GaP coating has been grown on germanium (Ge), gallium arsenide (GaAs), and zinc sulfide (ZnS) window/dome materials using a metal-organic chemical vapor deposition (MOCVD) process. The coatings have low IR absorption coefficients of 2 cm-1 at 10.6-micrometers wavelength, as measured by laser calorimetry. At a thickness of 20 micrometers , these GaP coatings degrade the transmission of the window/dome materials by only 1 percent. These high-transmission coatings have been shown to be very effective in protecting the window/dome materials from rain impact damage, as evidenced in testing by single-waterdrop impact, multiple-impact jet apparatus and whirling arm rain erosion. The details of the properties of these GaP IR protective coatings are presented and discussed.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.