Current technologies for optical gas imaging use one of the following methods: (1) single band-pass filter, which dictate a “man in the loop” for detection; (2) bi-channel sensors that need allocation of half of the detecting area for each band; (3) low rate and/or highly expensive multi/hyperspectral sensors. We suggest a low-cost, fast electrically-switchable notch-filter, based on liquid crystal (LC) in an infrared (IR) transparent cell which enables dynamic dual band sensing on the entire detector area.
We utilize fundamental properties of the LC: its molecular IR absorption bands (rovibrational modes) are overlapping those of gaseous (hydrocarbons, carbon dioxide , and more), and the transmittance of these bands depends on the orientation of the LC molecules relative to the polarization of the incident light (ordinary or extraordinary), Applying voltage on the LC cell affects the molecules' orientation, and therefore significantly changes the band transmittance. The high frequency response of LC cells to the electrical field enable detecting dynamic gas plumes.
We proved this concept experimentally using custom made polyamide-coated Germanium cells and a commercial LC, E7. The spectral response, measured by a fast spectro-radiometer, for wavelengths between 2 to 14 micrometer, shows several absorption lines overlapping those of hydro-carbonates hazardous and greenhouse gaseous that require monitoring, and we demonstrated the detection of the refrigerant gas R-134a. We are currently preforming an experimental imaging setup for gas detection based on this method, and the results will be presented in this talk.
Spectral imagers in the Long Wave IR spectral range (8 to 12 microns) suffer from the problem of high production costs because the existing commercial cooled array detectors are expensive, and in fact they are prohibitively expensive for many applications. As a result, the drive to lower the cost of Long Wave IR spectral imagers is strong: this is the main motivation for CI to investigate a new design that allows these spectral imagers to be more affordable. One area of possible cost reduction without relinquishing the advantages of a cryogenically cooled detector is the method used to provide the spectral information. CI Systems has developed a long wave IR (7.7 to 12.3 micron) spectral imager concept using a Circular Variable Filter (CVF), (a proprietary component based on multiple layer interference filter technology) which has advantages over the interferometric Fourier Transform method commonly used in this spectral range. The CVF method has its own development challenges; however, once proven, this concept may be more suitable and affordable for applications in which a spectral resolution of 0.5% of the wavelength (or 50 nm at 10 μ) is required. The design of the optical system must minimize background signals without being cooled to cryogenic temperatures, so we called it VIrtually COld (or VICO). CI is in the final stages of prototype building and characterization. Present initial calibration results and measurement examples are given in this paper.
In a previous paper we have presented the design and the expected performance of the latest generation of visible/infrared spectroradiometric system for field use, called SR 5000N. Examples of significantly advanced performance are expected in compactness, field of view response uniformity, measurement speed, ease of pointing the Line of Sight (LOS) on the object to be measured, and interchangeability of configuration modules, such as fields of view size, wavelength range and detectors. One unique and advanced feature of this system is that a Visible/Near IR spectrometer is incorporated in the system to provide, together with the IR optical channel, a simultaneous measurement of the same object in the whole 0.2 to 14.5 microns spectral range. In this paper we present the actual system after being built and some performance results on field of view uniformity and symmetry. Unfortunately, due to delays in R and D schedule, not all features have been completed so far and therefore not all results can be shown here as we initially planned.
Additional performance results will be reported in a future paper, as soon as the system development and characterization will be completed.
Since the early '90's CI has been involved in the development of FTIR hyperspectral imagers based on a Sagnac or
similar type of interferometer. CI also pioneered the commercialization of such hyperspectral imagers in those years.
After having developed a visible version based on a CCD in the early '90's (taken on by a spin-off company for
biomedical applications) and a 3 to 5 micron infrared version based on a cooled InSb camera in 2008, it is now
developing an LWIR version based on an uncooled camera for the 8 to 14 microns range. In this paper we will present
design features and expected performance of the system. The instrument is designed to be rugged for field use, yield a
relatively high spectral resolution of 8 cm-1, an IFOV of 0.5 mrad., a 640x480 pixel spectral cube in less than a minute
and a noise equivalent spectral radiance of 40 nW/cm2/sr/cm-1 at 10μ. The actually measured performance will be
presented in a future paper.
As far as we know, CI has been the only manufacturer in the world of commercial visible/infrared spectroradiometers for
remote sensing applications for many years. In this paper we describe the new design and some performance
improvements that we are developing to renew and modernize the system. Simultaneous visible and infrared
spectroradiometry, field of view flatness of response and scan speed are only some aspects of the system which have
undergone significant improvement. The challenge is to achieve these functional improvements without losing any of the
advantages of the traditional system as far as ruggedness for field use, interchangeability of spectral range and field of
view and aiming and image recording facility. Important factors in the success of this endeavor are: i) the development
of a new electronic signal processing package, ii) a modular optical concept that mechanically separates modules for
small, medium and large field of view ranges, and iii) a compact overall shape for convenience of use. Actual instrument
performance results will be reported in a future paper.
A digital FLIR (Forward Looking Infrared camera) in the 1.5 to 5 micron range can measure the contrast of selfemission
of objects with respect to their background in real time: however, when such a measurement is carried out in a
narrow spectral range through an interference filter the amount of unfiltered stray photons from the environment
reaching the detector contributes a significant part of the total signal with respect to the filtered photons from the object
of interest. The result is a significant reduction of dynamic range of the measurement and of the ability of the FLIR to
measure large variation of signal in real time.
A somewhat more advantageous instrument to measure the contrast of an object against its background in real time is
proposed here: it is a non-imaging single-detector, 1.5 to 5 micron radiometer (BDR). A classical example of application
is measuring the radiant intensity contrast of an airplane or missile in the background of sky during its flight. The
instrument is built so that it can measure this contrast in one narrow or wide wavelength range as function of time or in
successive wavelength ranges to provide contrast information in absolute units of irradiance (Watts/cm2) in different
regions of the spectrum. Several filters can be accommodated in a wheel to provide the spectral capability. The reason
for the ability of such configuration to avoid the dynamic range problem of the FLIR is the fact that in this detection
method the detector is AC coupled and the electronic amplification acts only on the difference between the source and
background signals.
We present here the instrument's design and its calibration algorithm1.
KEYWORDS: Transmittance, Sensors, Calibration, Projection systems, Atmospheric modeling, Signal detection, Signal to noise ratio, Infrared radiation, Asynchronous transfer mode, Mirrors
During the second half of the '70's Ben-Shalom et al.1 developed a system for research of the spectral transmittance of
the atmosphere in the infrared range on nearly horizontal atmospheric paths between 2.5 and 14 microns. The system
configuration is bi-static: a source (of known emission) and a measuring sensor station linked by line of sight.
Comparison of the measured radiation with the known emission gives the fraction of transmitted radiation. The longest
path published there was a respectable 44 Km. and provided significant data. However, the system required a very high
temperature (2400K) short lived (~3 hours) emitter element,
home-built projection optics, and a cumbersome closed
cycle water cooling system for the 6 KWatt source. The sensor end of the system was a Circular Variable Filter (CVF)
based spectroradiometer using liquid nitrogen (LN2) cooled detectors. The signal processing electronics was based on a
synchronous detection method using a chopper at the source and a radio-transmitted reference signal in phase with the
chopper.
In this paper we describe the Atmospheric Transmittance Measurement System (ATMS) recently built by CI and
presently undergoing reliability and accuracy tests. Its main advantages over the old system are: it is built of only
commercial off-the-shelf items (COTS), it can measure in both the IR and visible ranges simultaneously, it is cost
effective and easy to use and maintain. The calibration method and transmittance measurement algorithm are also
described. A transmittance measurement of a nearly horizontal sea level path of 6.5 Km. is shown here as an example for
the whole spectral range of 0.4 to 14 microns in less than very clear and dry weather conditions. The result shows
agreement with the MODTRAN model on the spectral behavior but at the time of publication we are still investigating
the accuracy of the actual transmittance values. We believe that the ATMS can be used for longer paths, based on the
signal to noise ratio encountered at 6.5 Km. In addition and in contrast to the old '70's system, which could be used only
for measurements above 1 Km. paths, the ATMS is built and calibrated so that it can measure transmittance through
short paths of few tens of meters.
Proliferation and technological progress of Mid Wave Infrared (MWIR) sensors for Missile Warning Systems (MWS)1,2
and increased sophistication of countermeasures require more demanding in-flight testing. Spectral discrimination is
being introduced for higher specificity and lower false alarms. As a result, testing such spectrally more capable systems
requires a more spectrally capable stimulator. In a previous paper3 we have described a system we developed to test
missile warning systems mounted on an aircraft. The system is placed in the field and projects a time dependent infrared
beam towards the flying aircraft, simulating the infrared emittance of an approaching missile in the 3 to 5 micron
spectral range as sensed by an MWS system. It can be used also as a trainer for the pilot himself to practice his/her
reaction to being targeted. Now we have developed a new system based on the above concept but allowing the user to
synchronously produce time profiles of two different infrared ranges independently within the 3 to 5 micron range (3.5
to 4 and 4.5 to 4.8 μ). This new dual color system (the DCIRTS) can now be used stationary or mounted on a vehicle
while traveling, for even more realistic simulation. In this paper we describe the DCIRTS and its capability. The system
design and preliminary test data were presented in two previous papers (references 4 and 5), but now after having done
additional work, we present here additional performance results: expected and measured angular dependent intensity,
whose behavior is important in the design of the simulation experiment.
Proliferation and technological progress of Mid Wave Infrared (MWIR) sensors for Missile Warning Systems (MWS)1,2
and increased sophistication of countermeasures require more demanding in-flight testing. Spectral discrimination is
being introduced for higher specificity and lower false alarms. As a result, testing such spectrally more capable systems
requires a more spectrally capable stimulator. In a previous paper3 we have described a system we developed to test
missile warning systems mounted on an aircraft. The system is placed in the field and projects a time dependent infrared
beam towards the flying aircraft, simulating the infrared emittance of an approaching missile in the 3 to 5 micron
spectral range as sensed by an MWS system. It can be used also as a trainer for the pilot himself to practice his/her
reaction to being targeted. Now we have developed a new system based on the above concept but allowing the user to
synchronously produce time profiles of two different infrared ranges independently within the 3 to 5 micron range (3.5
to 4 and 4.5 to 4.8 μ). This new dual color system (the DCIRTS) can now be used stationary or mounted on a vehicle
while traveling, for even more realistic simulation. In this paper we describe the DCIRTS and its capability. The system
design was presented in a previous paper (reference 4), but now after assembly and preliminary testing, we show the
actual system performance and most important physical characteristics.
Proliferation and technological progress of Mid Wave Infrared (MWIR) sensors for Missile Warning Systems (MWS)1,2
and increased sophistication of countermeasures require demanding in-flight testing. The IR sensors are becoming more
sensitive for longer range of detection, the spatial resolution is improving for better target detection and identification,
spectral discrimination is being introduced for lower False Alarm Rate (FAR), and the imaging frame rate is increasing
for faster defensive reaction. As a result, testing a complex MWS/countermeasure system performance before
deployment requires ever more realistic simulation of the threats in their natural backgrounds, and faster measurement of
the radiometric output, directionality and time response of the countermeasures. In a previous paper3 we have described
a system (IRTS or Infrared Threat Stimulator) we developed to test missile warning systems (MWS) mounted on an
aircraft. The IRTS is placed in the field and projects a time dependent infrared beam toward the flying aircraft. The time
dependent intensity of the beam simulates the infrared emittance of an approaching missile in the 3 to 5 micron spectral
range as sensed by an MWS system.
Now we have developed a new system based on the IRTS concept allowing the user to separately control the time
profiles of two different infrared ranges independently within the 3 to 5 micron range. This is important because MWS
instrumentation now has higher spectral discrimination capability in order to be more missile-specific and less prone to
be confused by clutter and background signals. In this paper we describe the new dual band IRTS system and its
capability (or Dual Color IRTS, DCIRTS).
As more and more spectral ranges are used by different threat detecting sensors, the effectiveness of a countermeasure is becoming more and more dependent on how similar its emitted spectrum is to the object that it is supposed to simulate. As a result, the need to model and test the countermeasure radiometric output (in radiance units) and contrast (in radiant intensity units) or effective temperature at different wavelengths simultaneously increases in importance during both R&D and production for both the producer of countermeasures (to confuse the seekers) and the producer of missile seekers (to prevent seeker confusion). We have developed a family of multi-spectral radiometers (ColoRad) specifically designed to quantitatively measure countermeasure spectral signatures dynamically for precise characterization. In this paper we describe the design of such instrumentation, including the various modes of operation and highlighting the important instrument features for the present application. In addition an example of measurement is given here to demonstrate its usefulness. The ColoRad performance parameter values are also given in this paper.
Understanding of the temporal and spectral behavior of the radiation emitted from fast transients such as gun shots, explosions, missile launches and kinetic ammunition is very important for the development of IRST, MWS and IRCM systems. The spectral-temporal behavior of the signature of these events is an essential factor for their detection and for the filtering of false alarms. Munitions flashes are fast transient phenomena with time duration that range from the sub-millisecond to a fraction of a second. A full characterization of the infrared signature of these events involves measurement of the evolution of its spectral distribution in time where the temporal resolution required is of the order of microseconds. We describe here a method for utilizing a four-channel radiometer to extract the above-mentioned data from these events. We show that we can derive the temporal evolution of the temperature of an explosion on time scale of 20&mgr;sec and separate energy releasing processes. Several practical examples will be given.
Spectral imagers rely mainly on two techniques for collection of spectral information: gratings and interferometers. The
former type needs cooling of the optics to avoid background signals which significantly limit the dynamic range of the
measurement. The latter type, in its present commercial configurations, is not suitable for pushbroom operation in an
airborne situation. A recent spectral imager configuration based on a shearing interferometer has been shown to be
suitable for pushbroom operation without the need for cooling the optics.
In this paper we describe the planned implementation of such a spectral imager for the 3-5 μ range, where the
interferometer is a specially designed single prism. The advantages of this interferometer configuration are: i) compact
optics, ii) high S/N ratio in the 3-5 μ range with small optical collection diameter, and iii) enhanced mechanical stability.
The instrument yields a spectrum for 320x240 pixels of the image with a spectral resolution of better than 50 cm-1. The
spectrum is calibrated in units of Watt/(steradian.cm2.cm-1). If used in an airborne pushbroom mode it provides a swath
width of 240 pixels in a ~6.9 degree transverse field of view. If used in a horizon scanning configuration, it has a vertical
field of ~6.9° and a horizontal field up to 300 degrees. The IFOV is 0.5 milliradians.
In this paper the major instrument design considerations are presented. The instrument is being constructed and we will
give more details on actual performance and examples of measurement results in a future paper, as we gain more
experience. An 8-12 μ range version is also planned for the near future.
Proliferation and technological progress of Mid Wave Infrared (MWIR) sensors for Missile Warning Systems (MWS)1,2 and increased sophistication of countermeasures require demanding in-flight testing. The IR sensors are becoming more sensitive for longer range of detection, the spatial resolution is improving for better target detection and identification, spectral discrimination is being introduced for lower False Alarm Rate (FAR), and the imaging frame rate is increasing for faster defensive reaction. As a result, testing a complex MWS/countermeasure system performance before deployment requires more realistic simulation of the threats in their natural backgrounds, and more accurate measurement of the radiometric output, directionality and time response of the countermeasures. Existing stimulator systems for MWS testing during R&D and production cannot reproduce the field conditions faithfully enough, so that it is possible to rely on them for the most sophisticated MWS' testing.
CI has developed a unique integrated MWS/countermeasure test system for field use, composed of: i) high intensity dynamic Infrared Threat Stimulator (IRTS), based on large optics and high speed shutter for time dependent scenario construction and projection to several kilometers; ii) sensitive IR Jam Beam Radiometer (JBR) for countermeasure testing. The IRTS/JBR system tests the MWS/countermeasure combination: efficiency range, probability of detection, reaction time, and overall well functioning2 can be determined in-flight through projection of threat profiles prepared in advance by the user, and through measurement of the countermeasure IR radiation output as function of time.
Design, performance, and example of operation of the IRTS/JBR are described here.
In the late eighties CI Systems pioneered the radiometric calibration and testing of electro-optical infrared test equipment1,2 by using its advanced in-house developed infrared spectroradiometer (the SR 5000), applied to measurements of signatures of military objects and long path atmospheric spectral transmission. Technological advances of frame rates, temperature resolution, spatial resolution, widened spectral ranges and other performance parameters of Forward Looking Infrared imaging systems (FLIR's) and other electro-optical (EO) devices require more advanced test and calibration equipment. The projected infrared radiation of such equipment must be controlled with better radiance resolution and accuracy.
CI has carried out a number of optical modifications of the SR 5000, together with especially dedicated calibration algorithms, to significantly improve the blackbody radiance measurements at temperatures close to room temperature, resulting in: i) a factor of 5 improvement in sensitivity, measured by Noise Equivalent Temperature Difference (NEΔT), and ii) a factor of 5 to 10 improvement in accuracy (in the 3-5 micron and 8-12 microns spectral regions respectively). The most important modifications are the use of a higher D* and smaller detector, a different detector alignment procedure in which the signal to noise ratio is traded off with field of view uniformity of response, and a calibration procedure based on the division of the blackbody temperature range into several independent sub-ranges.
As a result, the new spectroradiometer (the SR 5000WNV) has advanced infrared spectroradiometry so that it now allows the EO device manufacturers to characterize the most modern and future test equipment, and insure its being suitable to test the new advanced infrared imaging systems.
The SR 5000 Infrared spectroradiometer, developed in the mid '80's, is a robust research tool for the electro-optics system development laboratory. It has been the top-of-the-line IR spectroradiometer since then, with high sensitivity and useful software packages for data analysis, but its user interface became outdated, because of the enormous advances that personal computers underwent in the 90's. Recently, after being on the verge of disappearance, CVF based spectroradiometry has been revived.
Here we present some important new features of the system:
1. Synchronized imaging.
A CCD camera is boresighted with the line of sight of the SR 5000, to digitally record the image of the measured object and its background, in synchronization with the spectral measurement. This feature is useful in the field to avoid mishaps, and sometimes for later analysis of the results.
2. Windows Operating System.
The new system control, storage and analysis software package has been developed to take advantage of modern PC's, generally accepted user interface modalities, and a powerful database for file management.
A common path interferometric element introduced in the optical path of an imaging device is a well documented method to perform multidimensional spectroscopy. Recent design modifications however have provided significant improvements including enhanced spectral resolution and optical throughput, reduced acquisition time, as well as reduced instrument weight and volume. The new design will be reviewed in addition to its impact on three applications: spectral karyotyping, spectral imaging of the human ocular fundus and remote sensing of water reservoirs.
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