Focal Plane Array Camera Research Articles

Infrared Emitted Intensity Measurements from Burning Aluminum Droplets in Solid Propellants

Infrared emitted intensities (absolute) were measured from burning aluminum droplets in aluminized composite solid propellants near the burning surface of the propellant from 1 to 50 atm. A high-speed, focal-plane array camera was used for mid-infrared imaging at a wavelength of 3.42 μm. At 1–5 atm high-resolution images were able to resolve three potentially important regions of infrared emission: molten metal, molten oxide cap on the droplet surface, and oxide smoke in the detached flame envelope. At this wavelength, which is of particular importance due to the strong influence of gas composition on oxide stoichiometry and emissivity, spectral intensities ranged from 0.2–3.6 W/m2 μm sr at 1 atm increasing to 2.0–5.5 W/cm2 μm sr at 50 atm. The large variation in emitted intensity at each pressure was attributed to variations in oxide cap emission and oxide smoke emission. Molten metal emission, which was relatively uniform over the droplet surface, corresponded well with fundamental predictions for an opaque, optically smooth surface if the droplet temperature was assumed to be several hundred degrees Celsius below the metal boiling point. Oxide cap emission, on the other hand, was quite non-uniform over the droplet surface, exhibiting significant volumetric (path length-dependent) effects. Oxide caps were optically thick enough over most slant paths to augment metal emission significantly. Smoke envelope emission was less than 10% below 5 atm.

TERAHERTZ QUANTUM CASCADE LASERS AND REAL-TIME T-RAYS IMAGING AT VIDEO RATE

We report our development of terahertz (THz) quantum-cascade lasers with record performance. Using those high-power lasers as the illumination sources and a focal-plane array camera, we are able to perform real-time THz imaging at video rate.

Laser dazzling of infrared focal plane array cameras

Modern weapons systems often use infrared imaging for acquisition and tracking of targets. In the acquisition phase, the operator uses the system to detect the target and aim the weapon. In the tracking phase, the images from the camera are used to guide the weapon to the target. One possible method of protection against such imaging systems is laser dazzling. The intended effect is to deny the imager a clear view of the target by saturating the detector and consequently preventing the weapon from making a hit. The problem of locating the threat and pointing the laser at it is closely connected to the countermeasure effectiveness. This has been investigated particularly for specific applications such as directed infrared countermeasure (DIRCM) systems for aircraft self-protection. Another application is sniper detection, which uses the ‘cat’s-eye’ or retro-reflection effect, which occurs when a searching laser beam is directed at the visor of the rifle. We currently aim to use the retro-reflection effect in a wider range of applications for locating optics.1 The aim of our dazzling work is to quantify the effects of laser countermeasures on mid-wave infrared (MWIR) imaging to predict the decrease in the detection and pointing capabilities of weapons systems. Dazzling with a continuous or quasicontinuous pulsed laser causes a reversible saturation effect on the detector array. Most of the current MWIR lasers provide output power levels that do not exceed saturation levels for practical applications. Permanent damage effects require much higher irradiance levels at the camera front optics. This might be within reach at shorter ranges with these lasers, but is not addressed in our current research. In our experiments we used a periodically poled lithium niobate optical parametric oscillator (PPLN OPO) laser pumped by an Nd:YVO4 (yttrium orthovanadate) laser to generate a 3.9μm laser wavelength.2 Figure 1 presents images of a standard 256×320 indium antimonide (InSb) camera irradiated by Figure 1. Impact of increasing laser irradiance on the InSb camera image.

Evaluation of photothermal effects in cartilage using FT-IR spectroscopy

Photothermal effects after laser irradiation of cartilage are investigated using an infrared focal plane array (IR-FPA) camera and a Fourier transform infrared (FT-IR) spectrometer. The IR-FPA camera records radiometric temperature profile, while the local laser heating is applied to the sample; whereas the FT-IR spectrometer analyzes absorption peaks of cartilage constituents. As the major effect of photothermal heating in cartilage is water evaporation, spectral changes because of dehydration between control and laser-irradiated cartilage are recorded by FT-IR spectrometer measurements. Additionally, another interest was the observation of the spectral changes from macromolecules such as collagen and proteoglycans because of phase transformation and/or conformational changes after laser irradiation. The methodology may be useful for quantitative investigation of the relationship between the clinically important phenomenon of accelerated stress relaxation and the kinetics of macromolecular denaturation.

Thermographic Analysis of Fatigue Dissipation Properties of Steel Sheets

ABSTRACT: This paper presents several properties of heat source fields accompanying the high‐cycle fatigue of a dual‐phase steel grade. Heat sources were derived from thermal data provided by an infrared focal plane array camera. An especially developed image processing estimates separately the thermoelastic coupling source amplitude and the mean dissipation per cycle. Our experiments underline that dissipation sources are heterogeneous and this forms the beginning of the fatigue test. They also point out a linear evolution of the mean dissipation per cycle as a function of the loading frequency for a given stress range and a given loading ratio.

Diffraction‐limited Polarimetry from the Infrared Imaging Magnetograph at Big Bear Solar Observatory

ABSTRACTThe Infrared Imaging Magnetograph (IRIM) system developed by Big Bear Solar Observatory (BBSO) has been put into preliminary operation. It is one of the first imaging spectropolarimeters working at 1565 nm and is used for the observations of the Sun at its opacity minimum, exposing the deepest photospheric layers. The tandem system, which includes a 4.2 nm interference filter, a unique 0.25 nm birefringent Lyot filter, and a Fabry‐Pérot etalon, is capable of providing a bandpass as low as 0.01 nm in a telecentric configuration. A fixed quarter‐wave plate and a nematic liquid crystal variable retarder are employed for analyzing the circular polarization of the Zeeman components. The longitudinal magnetic field is measured for the highly Zeeman‐sensitive Fe i line at 1564.85 nm (Landé factor g = 3). The polarimetric data were taken through a field of view of ∼145′′ × 145′′ and were recorded by a 1024 × 1024 pixel, 14 bit HgCdTe CMOS focal plane array camera. Benefiting from the correlation tracking system and a newly developed adaptive optics system, the first imaging polarimetric observations at 1565 nm were made at the diffraction limit on 2005 July 1 using BBSO’s 65 cm telescope. After comparing the magnetograms from IRIM with those taken by the Michelson Doppler Imager on board SOHO, it was found that all the magnetic features matched very well in both sets of magnetograms. In addition, Stokes V profiles obtained from the Fabry‐Pérot etalon scan data provide access to both the true magnetic field strength and the filling factor of the small‐scale magnetic flux elements. In this paper, we present the design, fabrication, and calibration of IRIM, as well as the results of the first scientific observations.

Growth of InAs/GaSb short-period superlattices for high-resolution mid-wavelength infrared focal plane array detectors

InAs/GaSb short-period superlattices (SLs) with a broken gap type-II band alignment are investigated for the fabrication of photovoltaic pin-photodetectors on GaSb substrates. The structures were grown by molecular beam epitaxy using valved cracker cells for arsenic and antimony. Effective bandgap and strain in the SL were adjusted by varying the thickness of the InAs and GaSb layers in the SL and the controlled formation of InSb-like or GaAs-like bonds at the interfaces. MBE growth conditions were investigated and optimized in order to achieve good morphological, electrical and optical properties. IR-photodiodes with a cut-off wavelength of 5.4 μm reveal quantum efficiencies around 30% and detectivity values exceeding 10 13 Jones at 77 K. A focal plane array camera with 256×256 detector elements and 40 μm pitch based on InAs/GaSb short-period SLs was fabricated for the first time. The camera system reveals an excellent thermal resolution with a noise equivalent temperature difference below 12 mK for an integration time of 5 ms using f/2 optics. The detector performance, comparable with state of the art mercury–cadmium–telluride IR detectors, makes this material system very interesting for the fabrication of advanced thermal imaging systems.

Focus on Analytical Instruments

Focus on Analytical Instruments

Focal plane array infrared camera transfer function calculation and image restoration

Infrared images often present distortions induced by the measurement system. Image processing is thus an essential part of infrared measurements. A distortion model based on a convolution product is presented. The analytical form of the convolution kernel has been obtained from an image formation theory, along with an analysis of the sampling of the focal plane array camera detector's matrix. Image restoration is an ill-posed problem, and its solution can be obtained using regularization methods. In this work, image restoration is performed using a variation of Tikhonov regularization that makes use of the particular form of the convolution kernel matrix, which is built as a block-circulant matrix that admits a diagonal form in the 2-D Fourier space. The restoration procedure is used to restore a knife-edge infrared source image.

Measurement of Internal Stress in Glass Articles

We have developed a method for measurement of internal stress in glass articles. The method uses Rayleigh‐scattered light from a properly polarized laser beam propagating through glass at an oblique angle. This light is imaged with an electronic focal plane array camera. The method is similar to earlier published methods except for the inclusion of an externally controlled phase retarder. The phase retarder allows for the success of the method. The method is suitable for use on flat or curved glass and is applicable over a broad range of residual stresses. Experimental results are provided showing the in‐plane stress throughout the thickness of a television glass sample.

Lock-in thermography with a focal plane array

A focal plane array is used in a lock-in infrared thermography set-up. Twoprocedures are used to verify the validity of the phase lags given by the numericallock-in. The first one makes use of a body with a periodically modulated emissionin near and middle infrared. The second one uses an ARMCO iron samplewhose thermal properties are well known. This experimental set-up is usedto estimate the thermal conductivity of epoxy polymer coatings on analuminium substrate. An example of a phase lag map is shown for sucha sample and the use of this focal plane array camera is discussed.

Alternative measurement techniques for infrared sensor performance

This paper deals with measurement and characterization of IR sensor performance. Alternative methods have been investigated in order to determine the performance more reproducibly and accurately. Furthermore, the new methodologies reduce the subjectivity of the performance parameters such as the standard minimum resolvable temperature difference (MRTD) and tend to reduce the time (and thus cost) of measurement. The three methods described concern the use of (1) and IR zoom collimator, (2) a test device with line targets, and (3) an extended target combined with a TV camera instead of a human observer and a double-line source, resolving the ambiguity beyond the Ny-quist frequency in staring focal plane array cameras. In the IR zoom collimator a variety of targets are used in four different orientations; the observer has to report not only the recognition, but also the orientation of the target. Instead of variation of the temperature difference of the target, its range is varied by zooming in and out. In the rapid objective MRTD tester, the performance of the eye is simulated with a simple formula. The complete MRTD is obtained in about two minutes. In the double-slit method the best phase of the detector matrix is taken in comparison with the two line sources; instead of the MRTD curve, a new type of performance curve is measured: The minimum resolvable intensity difference (MRID), from which the range performance is obtained similarly to the classical MRTD method. The paper contains examples of measurements with each of the methods. ©2003 Society of Photo-Optical Instrumentation Engineers.

Infrared imaging of 2-D temperature distribution during cryogen spray cooling.

Cryogen spray cooling (CSC) is used in conjunction with pulsed laser irradiation for treatment of dermatologic indications. The main goal of this study was to determine the radial temperature distribution created by CSC and evaluate the importance of radial temperature gradients upon the subsequent analysis of tissue cooling throughout the skin. Since direct measurement of surface temperatures during CSC are hindered by the formation of a liquid cryogen layer, temperature distributions were estimated using a thin, black aluminum sheet. An infrared focal plane array camera was used to determine the 2-D backside temperature distribution during a cryogen spurt, which preliminary measurements have shown is a good indicator of the front-side temperature distribution. The measured temperature distribution was approximately gaussian in shape. Next, the transient temperature distributions in skin were calculated for two cases: 1) the standard 1-D solution which assumes a uniform cooling temperature distribution, and 2) a 2-D solution using a nonuniform surface cooling temperature distribution based upon the back-side infrared temperature measurements. At the end of a 100-ms cryogen spurt, calculations showed that, for the two cases, large discrepancies in temperatures at the surface and at a 60-micron depth were found at radii greater than 2.5 mm. These results suggest that it is necessary to consider radial temperature gradients during cryogen spray cooling of tissue.

Decoding of invisible laser markings using infrared technology

Due to the drastic increase in the field of product imitation and the request for plagiarism protection, there is a high interest in new protection methods providing new security features. Invisible laser markings within or below paint layers embrace these requirements, as they can be detected and decoded using an IR-reflection technique in conjunction with a focal plane array camera.

Long-Wavelength Quantum Well Infrared Photodetectors

A majority of IR sensors used for imaging arrays operating in the long-wavelength IR region between 8 µm-12 µm are based on mercury cadmium telluride (HgCdTe). This material system is unable to satisfy all the requirements imposed by modem applications. Structural difficulties due to poor uniformity, high defect densities, and weak bond strengths cause difficulties in manufacturing large IR focal plane array cameras. As an alternative, quantum well infrared photodetectors (QWIPs) utilising intersubband absorption between gallium arsenide (GaAs) wells and aluminium gallium arsenide (AIGaAs) barriers were perfected. These QWIPs possess better uniformity in comparison to HgCdTe detectors, and QWIP imaging arrays have recently become commercially available. However, the responsivity of GaAs/AlGaAs QWIPs is still lower than HgCdTe detectors. To further improve the responsivity of QWIP detectors, QWIPs with wells or barriers of GaInAsP instead of AlGaAs have been developed. Results of QWIPs made from the material systems GaAs/GaInP, GaInAs(P)/InP, (Al)GaInAs/InP, and GaInAs/AllnAs have been discussed.

640×486 long-wavelength two-color GaAs/AlGaAs quantum well infrared photodetector (QWIP) focal plane array camera

We have designed and fabricated an optimized long-wavelength/very-long wavelength two-color quantum well infrared photodetector (QWIP) device structure. The device structure was grown on a 3-in semi-insulating GaAs substrate by molecular beam epitaxy (MBE). The wafer was processed into several 640/spl times/486 format monolithically integrated 8-9 and 14-15 /spl mu/m two-color (or dual wavelength) QWIP focal plane arrays (FPAs). These FPAs were then hybridized to 640/spl times/486 silicon CMOS readout multiplexers. A thinned (i.e., substrate removed) FPA hybrid was integrated into a liquid helium cooled dewar for electrical and optical characterization and to demonstrate simultaneous two-color imagery. The 8-9 /spl mu/m detectors in the FPA have shown background limited performance (BLIP) at 70 K operating temperature for 300 K background with f/2 cold stop. The 14-15 /spl mu/m detectors of the FPA reaches BLIP at 40 K operating temperature under the same background conditions. In this paper we discuss the performance of this long-wavelength dualband QWIP FPA in terms of quantum efficiency, detectivity, noise equivalent temperature difference (NE/spl Delta/T), uniformity, and operability.

QWIP FPAs for high-performance thermal imaging

System properties of focal plane array (FPA) cameras based on GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) operating in the long-wavelength infrared are analyzed. Due to the limited charge storage capacity of available readout circuits, a small photoconductive gain improves the noise-equivalent temperature difference (NEΔT) of the sensor. We have demonstrated several camera systems based on conventional photoconductive QWIPs with NEΔT<10 mK and NEΔT<20 mK for FPAs with 256×256 and 640×512 pixels, respectively. A small photoconductive gain is obtained by operating the QWIP at very small electric fields ( <3 kV/ cm ). Finally, we discuss the possibility of reducing the NEΔT even further by using QWIP structures in which the photoexcited carrier mean free path is controlled by the incorporation of additional epitaxial layers. We thus obtain a high detectivity at very small signal and noise currents, which is useful in particular for large arrays with reduced pixel sizes. In addition, these detectors are shot-noise limited since the recombination noise is suppressed. The performance of FPA sensors based on these detectors will be addressed.

Spectral and radiometric calibration of midwave and longwave infrared cameras

Accurate spectral and radiometric calibrations of IR cameras are needed to interpret infrared imagery properly, to monitor camera performance over time, and to evaluate new imaging radiometers. In this paper, the physical basis for calibrating IR cameras is derived from first principles, and a laboratory setup used to perform both radiometric and spectral calibrations is described. The calibration procedures used with this setup are then demonstrated on a midwave IR focal plane array camera, a midwave IR scanned camera, and a longwave IR scanned camera. The system spectral response and radiometric response of each camera is given and analyzed. An error analysis of approximating a camera's spectral response by an equivalent top-hat responsivity is also given. © 2000 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(00)01702-5)

Estimation of local thermophysical properties by means of front-face photothermal infrared thermography: application to mechanical stress analysis

Mechanical stresses, applied or residual, are identified by means of photothermal infra- red thermography. Stresses are detected via their influence on thermophysical properties and more particularly on the thermal conductivity tensor. Because this influence remains weak, a sufficiently accurate identification procedure is needed as well as a significant enhancement of the signal-to-noise ratio, by means of a focal plane array camera.

Detection of buried objects using infrared imaging of impulse laser heated soils

In this work we examine the time-dependent aspects of thermal imaging of buried metallic and non-metallic objects, when impulse heating of the soil surface is performed. A 1500 W carbon dioxide laser is used to provide the thermal impulse. Time-dependent thermal images are obtained using an infrared focal plane array camera and a gated data acquisition/analysis system. Experimental studies are conducted as a function of object material, size, and burial depth, and incident laser fluence. Quantitative evaluation of the surface temperature differentials as a function of time are presented.