Patent Application
20120199689
Modern warfare is based on the projection of lethal ordinance, with high precision that minimizes collateral damage. Various types of imaging systems are used to guide a projectile to its intended target. A passive infrared (IR) seeker detects the thermal signatures of targets, providing video used by the guidance system to track and impact the intended targets. Thermal IR radiation is emitted by all objects in a portion of the electromagnetic spectrum at frequencies less than that of visible light. Therefore, such IR guidance systems are also referred to as heat seeking guidance systems. Projectiles or missiles using such systems are often referred to as “heat-seekers.”
Heat-seekers typically contain IR thermal array sensors (i.e., detectors) which are sensitive to radiation in the IR band. Non-uniformities in the responsitivity of such sensors result in different outputs from picture elements (pixels) in the array in spite of receiving similar input information, which appear as non-uniformities in the video image. Such non-uniformities interfere with proper operation of the guidance system and need to be corrected.
The inventor is the first to recognize the need for an improved guidance system capable of enabling real-time, in situ, non-mechanical, scene-based non-uniformity correction (NUC) in IR thermal array sensors (hereinafter “IR sensor”). Accordingly, in one embodiment, a system for navigating an object having a flight path comprising: an IR sensor having an optical path (i.e., the path light takes in traversing an optical system); a NUC IR spatial modulator (hereinafter “IR modulator”) in the optical path of the IR sensor, wherein the IR modulator is configured to allow the IR sensor to perform in situ, scene-based NUC; and a guidance system within the object, wherein the guidance system can adjust the flight path of the object based on the NUC provided. The IR modulator is capable of clearly transmitting and scattering variable intensities of IR radiation from a target imaged to the focal plane array (FPA) of a thermal IR sensor. By activating and deactivating the IR modulator in this manner, the scene-based NUC can be performed rapidly, i.e., in less than one second. In one embodiment, the IR modulator is made from a smart glass, such as a liquid crystal or suspended particle device, such as an electrochromic device.
In one embodiment, a method of navigating an object comprising with an onboard infrared modulator, applying a non-uniform correction to an infrared sensor, the non-uniform correction comprising obtaining a first video image of a first translucent scattered image of a scene having a target equivalent temperature, the scene containing a target blackbody object, a first equivalent blackbody object, and a second equivalent blackbody object, wherein the first equivalent blackbody object has a first intensity associated with a first equivalent temperature; processing the first video image in the infrared sensor, wherein the infrared sensor provides a first intensity signal to a calculating device; obtaining a second video image of a second translucent scattered image of the scene, wherein the second equivalent blackbody object has a second intensity associated with a second equivalent temperature; processing the second video image in the infrared sensor, wherein the infrared sensor provides a second intensity signal to the calculating device; calculating non-uniform correction terms from information in the first and second intensity signals; with the infrared modulator operating in a transparent state, obtaining a third video image of the target blackbody object, the target blackbody object having a target blackbody intensity associated with the target equivalent temperature; processing the third video image in the infrared sensor, wherein the infrared sensor provides a scene intensity signal to the calculating device; and performing signal processing using the scene intensity signal and the non-uniform correction terms to produce a spatially corrected uniform image is provided.
As is explained in further detail herein, in addition to information in the first and second intensity signals, information on the relative difference of intensity levels of transmission states, using pre-calibration information is also used to calculate the non-uniform correction terms. This information, i.e., a calibrated transmissions difference, is applied to the data received in-situ to provide AT information used in the non-uniformity correction calculation.
The first and second translucent scattered images are from energy (photons) from the whole (entire) scene, not just a single blackbody object within the scene. Thus, the target equivalent temperature can be considered the “average temperature” of all the objects in the scene. (As is described herein, the first and second equivalent temperatures are associated with first and second equivalent blackbody radiances, respectively, emitted from the first and second equivalent blackbody objects, respectively).
The novel IR modulators described herein may be useful in a variety of applications, other than military, such as civilian or medical applications. Other features and advantages will become apparent from the following description of the embodiments, which description should be taken in conjunction with the accompanying drawings.
The following description and the drawings sufficiently illustrate specific embodiments to enable those skilled in the art to practice them. Other embodiments may incorporate structural, logical, electrical, process, and other changes. Portions and features of some embodiments may be included in, or substituted for, those of other embodiments. Embodiments set forth in the claims encompass all available equivalents of those claims.
The process of determining NUC gain and offset terms during a flight path is referred as Adaptive NUC (ADNUC). The novel system described herein allows for real-time, scene-based NUC, through utilization of an IR modulator in the optical path of an imaging IR focal plane array (FPA), which is capable of modifying the thermal image of a target by diffusely scattering the wave front, thus providing uniform illumination across the FPA. In one embodiment, at least two different levels of intensity are displayed by varying the degree of scattering projected upon the IR FPA, after which the IR modulator is activated, allowing a transition to clear transmission for unobstructed imaging by the IR FPA.
As used herein, projectile refers to missiles, interceptors, guided projectiles (i.e., seekers, which can include directional pointing via gimbal assemblies), bombs and sub-munitions. Technically, a blackbody or blackbody source refers to an idealized physical body that absorbs and transmits all incident electromagnetic radiation at a rate associated with the temperature of the blackbody. However, as used herein, the term “blackbody” or “blackbody source” may refer to any type of target (building, tank, people, etc.) which ideally absorbs and emits electromagnetic radiation of varying intensities that can be measured in the IR spectrum. In practice, most objects outside of the laboratory do not emit and absorb radiation at an equivalent rate; these objects are referred to as gray bodies. Targets for IR seekers are gray bodies, but are, for practical purposes, treated and referred to as blackbody sources. These blackbodies or blackbody sources reside in a larger environment, also emitting IR radiation, imaged in the field of view of the projectile. It is the collective irradiance of the scene in the field of view that is attenuated by scattering with the IR modulator providing a uniform image. The intensity of this scattered image is proportional to the intensity of the collective scene in the field of view and to which can be ascribed an “effective” temperature of the scene.
Several types of detection equipment are used to detect infrared light. A micro-bolometer, for example, is used as a thermal detector when assembled in an array comprising a focal plane array (FPA) for imaging the intensity of incident infrared (IR) electromagnetic radiation. Detector material in the micro-bolometer is subject to temperature changes. As the material is illuminated by the focused image, it becomes warmer or colder depending on the temperature of the object being imaged in contrast to the ambient temperature of the detector material. The temperature change, in turn, causes the electrical resistance of the micro-bolometer to change. The resistance change is measured for each pixel and collectively processed into images, which can be interpreted as temperature values based on prior calibration of the response of the device.
Non-uniformity correction (NUC) of video is performed by scaling and offsetting the detected signal for each pixel based on gain and offset terms calculated from video imagery of a uniform target scene. The gain and offset terms calculated are dependent upon the intensity of the uniform scene being imaged; for an IR imaging system, the intensity of the scene is related to the temperature of the scene as a radiating black body. The effectiveness of NUC gain and offset terms is to correct the spatial non-uniformity of a video image, known as Fixed Pattern Noise (FPN). The FPN is a measure of the spatial variation of the response of the pixels across the FPA as referenced for a uniform level of irradiance.
A non-uniformity correction (NUC) is typically performed to decrease the non-uniformity (i.e., FPN) of the IR FPA, which is an inherent characteristic due to different response rates among pixels having the same IR radiance and the relative offset of initial detection levels. Applying NUC gain and offset terms increases sensitivity of the detector and increases the spatial and thermal resolution of the imaging system. Additionally, as a result of thermal equilibration over time with the surrounding environment, the thermal state of the detector material changes accordingly, resulting in a degradation of quality of previously applied NUC terms. This leads to an increase in FPN and a loss in sensitivity and resolution.
The variation in the rate of change in response from pixel to pixel over temperature is corrected by applying the appropriate multiplier terms to each pixel that results in a uniform response, referred to as gain compensation. Offset terms are applied to bring all pixels to the same response level for a given uniform irradiance over the FPA. As the thermal environment of the FPA and/or scene change new offset and gain terms need to be calculated, i.e., a “re-NUC”. The effectiveness of applying NUC to IR video is dependent on the portion of the dynamic range of the FPA thermal response in which the scene temperatures for calibration were acquired. Video from uniform image scenes at different black body temperatures bound the range of temperatures of blackbodies expected to be imaged. The sensitivity of the detector to resolve temperature differences between objects is the noise equivalent change in temperature (NEAT) defined as the level of temperature difference (signal) that can be detected for a signal-to-noise ratio (SNR) of one. Reducing the FPN of the imaged video improves the sensitivity of the imaging system, as measured by NEAT.
The FPN can also occur when a response is inconsistent within the calibrated range due to changes in position of various components in communication with the detector, such as optics, mechanics and electronics, or because of a physical change in the structure of the detecting material within the array. Such changes can occur during launch, as projectiles can experience launch shock events up to 10 KG deforming the structure of the detector elements thus changing the response. Therefore, FPN can affect the sensitivity metrics of the IR FPA by many means.
Non-uniformity can be corrected in two degrees of fidelity. The first degree is a one-point, or single-point NUC which involves normalizing the level of response of the array by offsetting each pixel by an appropriate amount. These offset terms are calculated based on illuminating the FPA with a scene of uniform intensity. The level of intensity of the scene is dependent on the temperature of the blackbody source projecting the IR radiation, and must be within the dynamic range of the detector. The dynamic range being the range between the lowest temperature blackbody detectable and the highest temperature blackbody filling the capacity of FPA to detect the object. The second degree of fidelity to compensating Non-uniformity of a FPA is by modifying the different response rates over temperature of individual pixels by applying calculated gain terms. Gain correction terms are calculated based on the response images of two uniform scenes of illumination of known blackbody temperatures. From a two-point NUC, both gain and offset terms are derivable from the same set of calibration data. Both means of determining gain and offset NUC terms are routinely derived in a laboratory or factory setting.
Previous attempts to provide NUC in flight include a one-point correction method in which a mechanical shutter or “spade” is placed in front of an IR FPA. However, accuracy is quite limited with a one-point NUC as only one source of known intensity is being imaged which may or may not be within the thermal dynamic range of the scene to be imaged.
Another attempt to provide NUC in flight includes rapid motion of the projectile through a field of regard that produces a blurred (or approximately uniform) image of the scene.
In contrast, an in flight two-point NUC sequentially requires imaging two uniform sources of known intensity. One known two-point NUC method involves use of a variable thermal source which requires a secondary mirror as a thermal flood source, such that two different thermal target of known temperatures need to be mechanically positioned for imaging by the FPA using secondary mirror.
Other two-point correction systems are mechanically-based systems and rely on reference-based correction using calibrated images on startup.
Other in flight one and two-point NUC methods involve various software algorithms based on data from one or a series of scene images that may involve averaging schemes, or software based filter modification of the image(s) to simulated a uniform scene.
Essentially, many of the differences between the responses and signal levels of an IR FPA are removed through calibration during production of a sensor by measuring the signal at two or more known temperatures and correcting the response (gain) and signal level (offset) for all the detector elements.
Other known two-point methods are considered to be “scene-based” in that they are capable of continually recalibrating the sensor for parameter drifts. A scene based NUC refers to uniformity terms calculated from the intensity of video images bounding the temperature of expected targeted scenes. However, such methods rely on software to provide one or more algorithms to calculate the NUC. These methods are complex and, once the algorithm is entered, the system is unable to provide a rapid response rate to any real-time issues that may arise. (See, for example, John G. Harris, et al., Nonuniformity Correction of Infrared Image Sequences Using the Constant-Statistics Constraint, IEEE Transactions on Image Processing, Vol. 8, No. 8, August 1999). Other software methods are designed to perform NUC in a variety of ways, such as by applying a filter.
In contrast, the novel embodiments described herein provide an in situ, real-time, scene-based two-point correction system which is not mechanically-based, and, although utilizing pre-flight calibration data on the relative difference of intensity levels of transmission states, does not rely on conventional pre-loaded signal processing algorithms. As shown in
Equivalent temperature can be determined as follows:
in which “Responsivity” is the responsivity of the detector 208, which relates the temperature to signal levels. (This responsivity pertains to the response signals of a known difference in blackbody temperatures and is calibrated prior to the use of the projectile). “Transmission” is the level of attenuation that the signal is reduced by, with the term in brackets representing the uniformity/scattering processes that take place. Scene uniformity, expressed in the last term, is where energy of the target scene 206 in the Field of View (FOV) is summed from and normalized to the number of pixels in the array so that each pixel sees the same energy, i.e., a substantially uniform image.
In one embodiment, ADNUC is accomplished through a polarization neutral transmission of IR radiation, such as long wave IR (LWIR) radiation with an IR modulator containing smart glass, such as a liquid crystal (LC) using a pair of cholesteric right and left handed LC molecules (CLC) as described herein. As a result, accuracy is improved over conventional methods, including methods using simulated pre-flight temperatures for target temperature outside the calibration temperature range (
As used herein the term “smart glass” refers to any type of material in which optical properties can be dynamically change electronically without any mechanical means. A smart glass can be thermochromic (optical properties altered by temperature), electrochromic (electroactive materials that present a reversible change in optical properties when electrochemically oxidized or reduced) or both. Smart glass may be formed into a film of any desired thickness, which is then either applied to another component or may be a separate element and placed in front of another component. The LC smart glass can be any suitable thickness, d, which can be calculated by,
In one embodiment, the thickness is between about 10 and 30 microns. However, if the smart glass is too thick, however, the quality of the temporal and/or spatial response of the device can be compromised.
In one embodiment, at least two different scenes of effective temperatures, T1 and T2, at two different locations within the blackbody are detected. In one embodiment, the operating temperatures, i.e., the “effective” temperature of the scene, range from about 0° C. to 50° C. In this embodiment, NUC is performed at effective temperatures of 20° C. and 30° C., such that calibration is provided over the entire range of 0° to 50° C. In one embodiment, the temperature difference is about 10° C.
In one embodiment, a system is provided which is capable of producing at least two diffuse images that can be associated with effective temperatures such that a NUC can be calculated. For a single scattering state, i.e., a “one-point” NUC, the offset terms for the Focal Plane Array (FPA) can be calculated. For two scattering states, i.e., a “two-point” NUC, both the pixel offset and pixel gain factors can be calculated. In one embodiment, the final state of the IR modulator can allow for “fully” transparent transmission (i.e., maximum transparency possible such that the response of LC alignment to the applied electric field is fully achieved) of the thermal target to the FPA providing high quality imagery for sensor functions. (See also
Operational steps of the IR modulator 306 positioned between the FPA 310 and the optical elements 312 are shown in
Various types of smart glass can be used in the IR modulators described herein. In one embodiment a mesomorphic material, such as a liquid crystal (LC) is used, which can change between a liquid and a crystal. LC molecular species have many phases of formation, including the nematic (i.e., threadlike) phase. As shown in
Upon application of an electric field (E), such as in
The occurrence of birefringence is a feature of liquid crystals in general, and is exploited in displays with polarizers attached to the substrates, forming character shapes commonly seen in liquid crystal displays. The difference between the indices of refraction of the two polarization states of the transmitted light is the measure of birefringence, Δn.
Various types of IR scattering materials can be used herein, including, but not limited to, any type of metal oxide which is thermochromic, electrochromic or both. In one embodiment, metal oxides, using metals such as vanadium, tungsten, nickel, and indium may be used. It is possible other materials or combinations of materials can be used, as long as they possess the desired features as described herein. In one embodiment, vanadium oxide is used. Vanadium oxide can provide a reflection to transmission state from about 10% to about 80% for switching speeds from about two (2) milliseconds (ms) up to about 0.5 ms. (See, for example, U.S. Pat. No. 4,283,113 to Eden).
In one embodiment, a cholestric liquid crystal (CLC) 700, as shown in
In one embodiment, the nematic host is a mixture containing cyano biphenyls and cyano terphenyls with lateral fluoro substitutions with the amount of rotation per unit length defined as the rotary power (P), with P=1/(helical twisting power)×(concentration of the chiral dopant).
A CLC has selective Bragg reflection because of its periodic helical structure when confined in a homogeneous cell. As such, and as shown in
The wavelength reflected is determined by the average refractive index, <n>, of the CLC and the cholesteric pitch length, P. The chirality of a CLC limits reflection to one circular polarization (left or right handed). In one embodiment, two cells having substantially the same pitch, but opposite handedness, can be stacked in order to achieve polarization independence. In one embodiment, as described in Example 1, the NUC is accomplished through polarization neutral transmission of long wave infrared (LWIR) radiation through a liquid crystal (LC) device using a pair of cholesteric right and left handed LC molecules (CLC).
The two-stacked CLC cell concept is shown in
The optical rotation rate per unit thickness of material, or rotary power, ρ, can be calculated according to the following formula:
In the testing performed, this was calculated to be −2.8 μrad/μ. Variation in the residual birefringence between the left and right handed CLCs is expected to produce a measureable rotary power, comparable to theory, relevant for the resulting image quality.
In one embodiment, suspended particle devices (SPDs) are used. A SPD incorporates the alignment of microscopic charged particles suspended in a fluid which align from a random state to an ordered state under an applied electric field (similar to
Embodiments of the invention will be further described by reference to the following examples, which are offered to further illustrate various embodiments of the present invention. It should be understood, however, that many variations and modifications may be made while remaining within the scope of the present invention.
The CLC mixture for the LWIR demonstration was a Merck Liquid Crystals (MLC 7247 and MLC 6248) comprising a nematic host and a 1.67 wt % chiral dopant(s). The two CLC mixtures were formulated, with one left-handed and the other right-handed. The pitch of the chiral mixtures was approximately six (6) microns. Stacking of two cells with the same pitch, but opposite handedness was used to achieve polarization independence as shown in
The prototype parts were fabricated using “in house” components and commercially available Ge substrates purchased from Mellos Griot. The final assembled component was an IR modulator, which was roughly the size of an American dime, although thicker, and substantially square in shape.
Fabrication also involved surface preparation of the Ge substrates for polyimide deposition. Specifically, about 0.5 grams of right handed and left handed CLC was deposited between the two pairs of prepared substrates. The gaps, constrained by glass shims, were approximately 22 μm and sealed by UV cured epoxy.
Thermal IR data was collected with a Holraum test set-up as shown in
Additional testing measuring the spectral transmission of the IR spatial modulator 1102 shown in
Spectral Transmission
Preliminary proof of concept testing was performed using a ZnSe substrate. Thereafter the Ge substrate described herein was used.
It is known that the birefringence, Δn, of the IR spatial modulator 1102 is 0.24 in the LWIR at room temperature (using MLC 7247 and 6248). The pitch, P, of the cholesteric mixture is approximately six (6) μm, with the band pass calculated to be 1.44 μm using the following equation: Δλ=Δn·P. With detailed information of the electro-optic behavior of the LC in combination with the substrate absorption characteristics, modeling of the transmission spectrum can be derived for comparison with measured results in
Switching Speed and Effective Temperature
Referring again to
The radiometric transmission performance of the IR spatial modulator 1102 was measured using the blackbody 1104 set to 37° C. in the Holraum test-setup as shown in
The effective temperature imaged was measured in both the “on” and “off” states. A 4° C. difference in transmitted temperature was measured with transition times between states of under one second. (See
LWIR Video Images
Using the FLIR camera 1108 operating at 60 Hz, transmission of the (Ge-based) IR modulator 1102 was recorded switching between “on” and “off” states while backlit with the blackbody 1104 set at 37° C.
Thus, the ability of the system to produce a diffuse scene that can be associated with an effective temperature has been demonstrated.
The image quality (i.e., Modulation Transfer Function (MTF))) in the LWIR transmitted through the device in operation will be measured.
The degree of scattering (or reflection) achieved in the “off” or intermediate (10 to 20 V) states will be measured.
Thermal behavior of the IR modulator over typical operational temperatures will also be determined, i.e. (LC response to applied voltage varies with temperature, and with eventual phase changes at critical temperatures such that no applied voltage can alter the state.
The degree of polarization neutrality, i.e., amount of residual birefringence for image quality in the transparent state will also be measured.
Embodiments may be implemented in one or a combination of hardware, firmware and software. In these embodiments, as shown in
In an alternative embodiment as shown in
In the various embodiments described herein the target is a blackbody viewed radiometrically and the IR associated with the blackbody has a temperature associated with it. By changing the amount of LWIR radiation that goes through an IR modulator, a different effective temperature is obtained, thus allowing for the NUC correction.
Additional benefits of this technology include the ability to shield un-cooled LWIR FPAs from thermal damage due to exposure of direct sunlight or intense radiation. The novel embodiments described herein also have application to short wave IR (SWIR) and mid wave IR (MWIR) bands, dynamic spectral filters throughout the IR spectrum, focal plane noise filtering by simulating a “chopped” IR signal using the dynamic shuttering capability, and possible applications to computational optics.
The Abstract is provided to comply with 37 C.F.R. Section 1.72(b) requiring an abstract that will allow the reader to ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to limit or interpret the scope or meaning of the claims. The following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate embodiment.
