Patent Description:
Optical systems for use with flight vehicles such as guided missiles or belly-mounted sensor pods on aircraft typically include an optical window (e.g., a dome) that protects the sensitive optical and electrical components. The optical window is transparent in a desired spectral band (e.g., the MWIR band from <NUM>-<NUM> microns) to pass emissions from a target in a scene through the optical window to the entrance pupil of focusing optics, which in turn route the incident radiation along an optical path and focus the radiation onto a detector. The detector may, for example, be a quad-cell detector for non-imaging applications such as spot tracking. The detector may, for example, be a focal plane array (FPA) for various imaging applications. The FPA generally includes an array of pixels, each pixel including a photo-detector that generates a signal responsive to the intensity of the incident. These signals are collected and combined to form a digital image of the object. The focusing optics may be fixed or gimbaled to increase the field-of-regard (FOR). Typically, the entrance pupil is symmetric about the central axis of symmetry of the optical window. Alternately, the entrance pupil may be offset such that the FOR does not cross the tip of the optical window to reduce distortion (See <CIT>).

Ideally, the only emissions sensed by the detector are those from the scene and particularly a specified target. However, in guided missiles or sensor pods there can be many different sources of parasitic radiation or "noise" that reduces the SNR of the target and the ability of the guidance unit to track the target. One such source is the self-emission of the optical window that may occur due to aero-thermal heating as the missile or pod travels through the atmosphere. The amount of aero-thermal heating depends on the flight speeds, aerodynamic design of the window that induces heating and the thermal design of the window that removes heat to cool the window. The window self-emissions can raise the general background noise or can induce a gradient in the detected signal (due to non-uniform heating of the window). In many instances, the self-emissions due to aero-thermal heating are insignificant. In others, it is desirable to try to mitigate the effects of window self-emissions.

One approach is to spectrally filter the incident radiation. Generally speaking, the temperature of the aero-thermally heated optical window is much higher than the temperature of the target. As such, the emissions of the target and the optical window will have different spectral characteristics. For example, the relative intensity of the window emissions will be stronger at the longer wavelengths in the MWIR band. Low-pass filtering the incident radiation can improve the contrast of the target radiation (signal) to the window self-emitted radiation (noise). See, for example, <CIT>.

Non-uniform aero-thermal heating can induce a gradient in the window self-emissions, hence the total detected incident radiation. This gradient is a form of "fixed-pattern noise". One way to remove this fixed pattern noise is by using Scene based Non-Uniformity Correction (NUC), in which the scene is intentionally blurred and the resulting image is recorded and then subtracted from non-blurred images of the scene. The blurring is usually done by moving an optical element, such as a lens, prism, or diffuser, into the beam, though some scene-based NUCing methods are completely software-based. The advantage of scene based NUC (as opposed to other NUCing methods, such as the use of a shutter in front of the detector) is that is can correct for contributions to fixed-pattern noise from every optical element in the system, including windows and domes. The disadvantage is that the method of blurring must be carefully designed so that the target is not inadvertently subtracted from the final image.

Polarimeters can be used to analyze the polarization components of light to, for example, extract shape information from an object. Some polarimeters use two or more linear polarizers (polarized pixels) that filter at least half of the incoming light and direct the remaining light to a focal plane. As a result, the brightness of the image at the focal plane is substantially reduced (e.g., by about half).

Polarimetry requires at least three measurements to analyze the polarization components of light; at least two different polarization components and possibly an unpolarized component. Typically, the pixelated filter array, and FPA, are divided into groups of four pixels (e.g., a <NUM>×<NUM> sub-array of pixels). The standard commercially available pixelated filter array is a <NUM>×<NUM> array of linear polarizers having angular values of Θ<NUM> = <NUM>°, Θ<NUM> = <NUM>°, Θ<NUM> = <NUM>° and Θ<NUM> = <NUM>°, respectively, which are optimum assuming perfect alignment between the pixelated filter array and the FPA. <CIT>entitled "Movable Pixelated Filter Array" describes a technique for using the data reduction matrix to account for misalignment. The electronics may compute an Angle of Linear Polarization (AoLP) image and a Degree of Linear Polarization (DoLP) image from the four linearly polarized pixel values in each grouping to extract shape information. The electronics may also compute an average of the four detector pixels in each grouping to form a reduced resolution intensity image.

<CIT> relates to a birefringent prism that is disposed in front of the entrance aperture of a Cassegrain-type telescope which constitutes the optical focussing assembly in a tracking system for a guided missile or the like. The prism refracts first radiation having a first polarization in a first direction, and refracts second radiation having a second polarization which is orthogonal to the first polarization in a second direction which is deviated from the first direction by a predetermined angle.

<CIT> relates to a multi-mode detector and detection method that utilize shared optical components to detect multiple different incoming wavelengths of energy.

<CIT> relates to an optical imaging system and method including a movable pixelated filter array, a shutter mechanism to which the pixelated filter array is attached, and a controller configured to implement a data reduction algorithm.

<CIT> relates to a vehicle including electro-optic imaging that has a vehicle body having an outer surface including a front portion and a side portion, wherein the side portion includes a plurality of portholes. A propulsion source is within the vehicle body for moving the vehicle.

<CIT> relates to an offset aperture two-axis gimbaled optical system that comprises a two-axis gimbal and an optics assembly that is mounted on the inner gimbal and offset radially from the rotation axis of the outer gimbal.

The following is a summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description and the defining claims that are presented later.

A polarizer is positioned in the optical path between the optical window and a detector (e.g., a FPA or quad-cell). The polarizer comprises at least one filter pixel that imparts a linear polarization of a certain angular value to filter the incident radiation as a function of the polarization of the incident radiation. At least one filter pixel is aligned to the P-polarization in the plane of incidence. If the polarizer is a sheet polarizer (effectively one large filter pixel), the sheet polarizer is aligned to the P-polarization in the plane of incidence. If the polarizer is a microgrid polarizer, at least one filter pixel and preferably multiple filter pixels in each sub-array are aligned to the P-polarization in the plane of incidence. This improves the SNR of the detected target radiation. In the case of the microgrid polarizer, the AoLP image removes the effects of non-uniform aero-thermal heating. The sheet polarizer may be used with imaging or non-imaging detectors whereas the microgrid polarizer is only applicable for imaging detectors such as a FPA.

In an embodiment, the flight vehicle is propelled toward the target at supersonic speeds in excess of Mach <NUM> and, in some applications, hypersonic speeds in excess of Mach <NUM>.

In different embodiments, a sheet polarizer passes P-polarized radiation thereby modulating the intensity of incident radiation as a function of its P-polarization and increasing a contrast ratio of target radiation to self-emitted radiation. In a fixed optical system, the sheet polarizer can be positioned at any point along the optical path between the optical window and the detector. In a gimbaled optical system, the sheet polarizer can be positioned off-gimbal or at any position on the inner gimbal. If off gimbal, the gimbal is rotated to align the plane of incidence to the sheet polarizer to read out the detector signals. Alternately, the sheet polarizer could be a dynamic microgrid array electronically controlled to present a single linear polarization that tracks the rotation of the gimbal. If on-gimbal, the sheet polarizer is mounted in alignment to the P-polarization in the plane of incidence and remains aligned as the gimbal rotates.

In different embodiments, a microgrid polarizer array having a plurality of polarized pixelated filter sub-arrays is positioned at the FPA or an intermediate image (conjugate plane) of the focal plane. Each sub-array comprises three or more filter pixels Q of which at least two filter pixels impart a linear polarization of a certain and different angular value. The microgrid polarizer is mounted such that at least one of the filter pixels (preferably multiple) in each sub-array is aligned to the P-polarization in the plane of incidence.

These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which:.

The present invention provides an optical system for use with flight vehicles subject to extreme aero-thermal heating in which window self-emission reduces the SNR of sensed target emissions complicating the task of target tracking. Supersonic (> Mach <NUM>) and more so hypersonic (> Mach <NUM>) weapons may produce such extreme aero-thermal heating of the window. Spectral filtering techniques may be effective to increase the SNR, however, in some systems, such techniques by themselves are not sufficient. The present invention provides a technique based on exploiting the polarization of the target and self-emitted radiation to lower the background window emission or to remove a gradient due to non-uniform heating.

Referring now to <FIG>, in an embodiment a hypersonic weapon <NUM> is subject to extreme aero-thermal heating of its optical window <NUM> when travelling at hypersonic speeds through the atmosphere. The weapon comprises an airframe <NUM> and a propulsion system <NUM> such as a liquid or solid rocket motor configured to propel the weapon toward the target in excess of Mach <NUM>. The temperature of the optical window is typically much higher than the temperature of a target. Consequently, the level of emissions <NUM> from optical window <NUM> is much higher than the level of emissions <NUM> from the target across the entire MWIR band from <NUM>-<NUM> microns. The integrated radiance <NUM> of the optical window over the MWIR band is several times the integrated radiance <NUM> of the target.

Target trackers require a certain minimum SNR to reliably track a target. For example, according to <NPL>) a minimum SNR of <NUM> dB (=~<NUM>) is required to track a target. Some trackers require a higher SNR, some less. In the illustrated example, the SNR would not surpass the minimum required for tracking. Since the emissions levels of the optical window dominate the target emissions across the MWIR, conventional spectral filtering would be ineffective.

Referring now to <FIG>, light <NUM> from a distant target <NUM> passes through both an outer surface <NUM> and an inner surface <NUM> of a tilted optical window <NUM>, representative of an off-axis portion of the hypersonic weapon's optical window. The aero-thermal heated window self-emits light in all directions. A portion of window light <NUM> is emitted in the same direction as the incident target light <NUM> and passes through only inner surface <NUM>.

A plane of incidence <NUM> is defined as the plane that contains the incident ray (light <NUM>) and a normal <NUM> to the surface of the window. If the outer and inner surfaces have the same symmetry (as they often do), the planes of incidence of the two surfaces are coincident.

The Fresnel equations describe the behavior of light when moving between media of differing refractive indices such as between the optical window and air. According to the Fresnel equations, the transverse electric field of light that is p-polarized oscillates in the plane of incidence <NUM>. The electric field of s-polarized light oscillates perpendicular to the plane of incidence. According to the equations, a majority of the incident energy is p-polarized.

The degree to which the light is p-polarized is most strongly affected by the angle-of-incidence (AOI) <NUM> between the target light <NUM> and the surface normal <NUM>, and to a lesser effect the refractive index difference between window material and air. The greater the AOI, the greater the p-polarization of the light.

As depicted in <FIG>, for a given AOI, the p-polarization <NUM> of the target light is greater than the p-polarization <NUM> of the window light. The p-polarization difference <NUM> between the two curves increases (to a point) with an increasing AOI. This difference is a result of the target light <NUM> passing through both the inner and outer surfaces, and thus twice transitioning between the window media and air, and the window light <NUM> passing through only the inner surface, and thus transitioning between the window media and air only once.

A typical target gives off a thermal signature in the MWIR band that is a combination of unpolarized and polarized light. The tilted window <NUM> induces a p-polarization bias to the unpolarized component of thermal signal <NUM> that is stronger than the polarized component of the signal <NUM>. Note this is only true if the entrance pupil of the optical system is positioned to look through an off-axis segment of the window. If the optical system is a more typical on-axis system, the light will not have a predominant p-polarization state because the rotational symmetry of the optical window will cancel the induced polarization.

The present invention combines an off-axis optical system, fixed or gimbaled, whose entrance pupil looks through an off-axis segment of the optical window with a polarizer that is positioned in the optical path between the optical window and a detector. The polarizer comprises at least one filter pixel that imparts a linear polarization of a certain angular value to filter the incident radiation as a function of the polarization of the incident radiation. At least one filter pixel is aligned to the P-polarization in the plane of incidence. The at least one filter pixel modulates the intensity of incident radiation as a function of its P-polarization. Because the target light is more p-polarized than the window light, this increases a contrast ratio of target light (radiation) to window light (radiation), which increases the SNR at the detector.

If the polarizer is a sheet polarizer (effectively one large filter pixel), the sheet polarizer is aligned to the P-polarization in the plane of incidence. The sheet polarizer modulates the intensity of incident radiation, both target radiation and self-emitted radiation, as a function of its P-polarization. This increases the contrast ratio of target radiation to self-emitted radiation, hence the SNR of the radiance image.

If the polarizer is a microgrid polarizer, at least one filter pixel and preferably multiple filter pixels in each sub-array are aligned to the P-polarization in the plane of incidence. For example, for a standard a 2x2 sub-array of linear polarizers having angular values of Θ<NUM> = <NUM>°, Θ<NUM> = <NUM>°, Θ<NUM> = <NUM>° and Θ<NUM> = <NUM>°, the angular value of one of the pixels is aligned to the plane of incidence. Another 2x2 sub-array may have angular values of, for example, Θ<NUM> = <NUM>°, Θ<NUM> = X°, Θ<NUM> = Y° and Θ<NUM> = <NUM>° where <NUM>° is aligned to the plane of incidence providing <NUM> pixels with higher SNR. The angular values X and Y are suitably selected to minimize the condition number (CN) of the data reduction matrix (DRM) subject to the constraint that <NUM> of the <NUM> pixels are aligned to the plane of incidence. There is a direct tradeoff between the number of pixels that have the same angular value and are aligned to the plane of incidence to improve SNR and the number of pixels having different angular values to improve polarization diversity. The size of the sub-array and the angular values may be optimized assuming perfect alignment between the microgrid polarizer and the FPA or, as described in co-pending patent application <CIT> to account for misalignment.

Generally speaking, the outputs Px of an FPA pixel corresponding to a polarizer at angle x in each sub-array can be processed to produce a radiance image (albeit at a reduced resolution) by averaging the outputs Px in each sub-array, to produce an angle of linear polarization (AoLP) image e.g., for a <NUM>×<NUM> sub-array AoLP = <NUM>*atan((P0-P90)/(P45-P135)) and to produce a degree of linear polarization (DoLP) image e.g., for a <NUM>×<NUM> sub-array DoLP=sqrt((P0-P90)^<NUM>+(P45-P135)^<NUM>)/(P0+P90) assuming a standard <NUM>×<NUM> sub-array. The optical window and polarization filtering can create a p-polarization bias in the AoLP and DoLP images. This bias can be mitigated or removed by calibrating the data reduction matrix for the microgrid polarizer.

In the case of non-uniform aero-thermal heating of the optical window, which produces a gradient in the incident radiance, it is important to note that the AoLP image will remove any gradient that has the same value for each <NUM>×<NUM> sub-array in addition to the back ground window self-emissions, whereas the radiance image and DoLP image will not. In certain extreme cases, the AoLP image will be the only data product used to track a target.

Combinations of the sheet polarizer, the micgrogrid polarizer and a transmissive element may be implemented using a mechanical shutter or a dynamic microgrid array. The sheet polarizer may be used with imaging or non-imaging detectors whereas the microgrid polarizer is only applicable with imaging detectors.

Referring now to <FIG>, an embodiment of an optical system <NUM> comprises an optical window <NUM> having interior and exterior surfaces <NUM> and <NUM>, respectively, with a curvature with respect to and symmetric about a central axis <NUM>. The optical window <NUM> may have a hemispherical or conformal shape. Typically, hypersonic weapons have a conformal shape with a relatively high AOI for aerodynamic considerations. The optical window may be formed from Germanium or a Nanocomposite Optical Ceramic (NCOC) material that is substantially transmissive in the MWIR.

Fixed focusing optics <NUM> have an entrance pupil <NUM> that is offset from central axis <NUM> to look through an off-axis segment <NUM> of optical window <NUM> such that a field-of-view (FOV) <NUM> does not cross central axis <NUM> behind optical window <NUM>. Focusing optics <NUM> is suitably configured to provide optical correction for distortion produced by passing through the optical window, route the incident radiation along an optical path (optic axis) <NUM> and focus the incident radiation at a focal plane. Focusing optics <NUM> may include corrective optic elements, focusing elements and turning mirrors, for example. Focusing optics <NUM> capture incident radiation <NUM> from a distant target <NUM> and self-emitted radiation from the off-axis segment <NUM> of the window.

A detector <NUM> at or near the focal plane generates a signal(s) responsive to the intensity of incident radiation in a spectral band (e.g., the MWIR band). The detector may, for example, be a quad-cell detector for non-imaging applications such as spot tracking. The quad-cell generates four signals responsive to the intensity of the incident radiation on each cell that are processed to generate an angle to the target. The detector may, for example, be a focal plane array (FPA) for various imaging applications. The FPA generally includes an array of pixels, each pixel including a photo-detector that generates a signal responsive to the intensity of the incident radiation. Detectors are not responsive to the polarization of the incident radiation. The detector may include a read out IC (ROIC) to readout the signals.

A polarizer <NUM> is positioned in optical path <NUM> between the off-axis segment <NUM> of optical window <NUM> and detector <NUM>. Polarizer <NUM> e.g., a sheet polarizer or microgrid polarizer comprises at least one filter pixel that imparts a linear polarization of a certain angular value to filter the incident radiation <NUM> as a function of its polarization. The sheet polarizer can be positioned anywhere along the optical path whereas a microgrid polarizer must be positioned at the FPA or an intermediate image of the focal plane. A mechanism <NUM> fixes the alignment of the at least one filter pixel to the p-polarization in the plane of incidence such that the polarizer prefers the target radiation to the self-emitted window radiation. This increases the contrast of the target radiation to the self-emitted window radiation, hence the SNR at the detector. Mechanism <NUM> may, for example, a mounting bracket and bolts to secure the polarizer <NUM>.

A processor(s) <NUM> processes the signal to generate data produces. For a sheet polarizer, the processor generates a radiance image. For a microgrid polarizer, the processor may generate a radiance image, an AoLP image and a DoLP image. Electronics <NUM> use the data products to track the target and provide command signals to guide the hypersonic weapon towards the target. Electronics <NUM> may be configured to use only the AoLP image.

Referring now to <FIG> and <FIG>, an embodiment of an optical system <NUM> comprises an optical window <NUM> having interior and exterior surfaces with a curvature with respect to and symmetric about a central axis <NUM>. Focusing optics <NUM> are mounted on a two-axis (roll/nod) gimbal <NUM> that rotates the optics' entrance pupil <NUM> to look through different off-axis segments <NUM> of the optical window to sweep a FOV <NUM> over a larger field-of-regard. The optics and entrance pupil are offset from the central axis <NUM> and rotated through allowed angles in roll and nod such that the FOV <NUM> does not cross central axis <NUM> behind the optical window.

In this embodiment, two-axis gimbal <NUM> comprises a roll gimbal <NUM> that is driven to rotate about a roll axis coincident with central axis <NUM> and a nod gimbal <NUM> mounted on the roll gimbal and driven to rotate about a nod axis <NUM> that is perpendicular or skew to central axis <NUM>. Each gimbal includes the rotating gimbal and a drive motor to affect rotation. Focusing optics <NUM> includes optical focusing or corrector elements <NUM>, <NUM>, <NUM> and <NUM> and turning mirrors <NUM> and <NUM> mounted on-gimbal to collect incident radiation within the entrance pupil, correct, focus and route the incident radiation along an optical path to a focal plane.

A polarizer is positioned in optical path between the off-axis segment <NUM> of optical window and a detector <NUM> (e.g., quad-cell or FPA) positioned at or near the focal plane. In a first embodiment, a polarizer <NUM> may be positioned off-gimbal. If polarizer <NUM> is a sheet polarizer it can be positioned at any arbitrary position along the optical path. If polarizer <NUM> is a microgrid polarizer it must be positioned at the FPA or an intermediate image of the focal plane. If off-gimbal, the polarizer is only aligned to the plane of incidence (as defined by surface normal <NUM> to the optical window) at a specified roll/nod orientation (e.g., <NUM>°, <NUM>°) and an orientation <NUM>° out of phase (e.g., <NUM>°, <NUM>°). To read out the detector, the gimbal is rotated to one of these two positions. The "mechanism" for aligning the polarizer is thus implemented by the nod gimbal and rotation of the nod gimbal. As will be discussed further, if the sheet polarizer is implemented with a dynamic microgrid array, that array may be reconfigurable to rotate with the gimbal to maintain alignment to the plane of incidence.

In a second embodiment, a polarizer <NUM> is positioned on the nod gimbal <NUM> ("on-gimbal"). If polarizer <NUM> is a sheet polarizer it can be positioned anywhere in the optical path on the nod gimbal and optically aligned to the plane of incidence, and remains in the plane of incidence as the roll and nod gimbals rotate about their respective axes. As shown in <FIG>, the lines of the sheet polarizer indicate the direction of the pass axis. If polarizer <NUM> is a microgrid polarizer it must be positioned at an intermediate image <NUM> of the focal plane on the nod gimbal and optically aligned to the plane of incidence, and remains in the plane of incidence as the roll and nod gimbals rotate about their respective axis.

A processor(s) <NUM> processes the signal to generate data produces. For a sheet polarizer, the processor generates a radiance image. For a microgrid polarizer, the processor may generate a radiance image, an AoLP image and a DoLP image. Electronics <NUM> use the data products to track the target and provide command signals to guide the hypersonic weapon towards the target. Electronics <NUM> may be configured to use only the AoLP image. If polarizer <NUM> is mounted off-gimbal, electronics <NUM> generate gimbal signals for the drive motors to rotate the gimbals to align the plane of incidence to the polarizer to read out the detector.

Referring now to <FIG> and <FIG>, a sheet polarizer <NUM> is positioned in the optical path behind an off-axis segment <NUM> of the optical window and aligned to the plane of incidence as defined by a surface normal <NUM> to the off-axis segment. Light <NUM> from a target is p-polarized by passing through both the inner and outer surfaces of off-axis segment <NUM>. Light <NUM> that is self-emitted by off-axis segment <NUM> is p-polarized by pass through only the inner surface. The percentage of p-polarization <NUM> of target light <NUM> is thus greater than the percentage of p-polarization <NUM> of window light <NUM>. The degree of p-polarization, and thus the difference in p-polarization increases as the angle of incidence <NUM> gets larger. The difference is suitably at leat <NUM>%.

Since sheet polarizer <NUM> is aligned to the plane of incidence it prefers p-polarized light. As a result, a greater portion of target light <NUM> is passed through the polarizer than window light <NUM>, thus increasing the contrast of target light to window light and SNR <NUM> at the detector. As shown, the SNR <NUM> at the detector also increases with increasing AOI.

As shown in <FIG>, left and right sided images <NUM> and <NUM>, respectively, are of untiltered radiation (no polarizer) and p-polarization filtered radiation (sheet) polarizer. Without the polarizer the background appears grey whereas with the polarizer the background appears black, which accounts for the improvement in SNR for the target in the scene. The improvement in SNR may allow the electronics to track the target notwithstanding the extreme aero-thermal heating of the optical window.

Referring now to <FIG>, a microgrid polarizer <NUM> is mounted on or off-gimbal in the optical path at a FPA <NUM> or an intermediate image of the focal plane. The gimbal rotates an entrance pupil <NUM> of the optics to look through different off-axis segments <NUM> and <NUM> of optical window <NUM> at a target <NUM>. Segment <NUM> at the tip of the optical window is heated to a higher temperature than segment <NUM>. The difference in temperature produces a gradient <NUM> in the radiance image <NUM> of the target.

Microgrid polarizer <NUM> comprises an array of polarized pixelated filter sub-arrays <NUM>. Each sub-array <NUM> comprises three or more filter pixels Q of which at least two filter pixels impart a linear polarization of a certain and different angular value. In this embodiment, each sub-array is a 2x2 grouping of filter pixels that impart linear polarizations of <NUM>°, <NUM>°, <NUM>° and <NUM>°, respectively, with one linear polarization aligned to the P-polarization in the plane of incidence. The FPA pixels are grouped into 2x2 super pixels <NUM> corresponding to each sub-array.

The outputs of a FPA super pixel in each sub-array can be processed to produce data products. Assuming that any gradient is constant over a <NUM>×<NUM> pixel grouping, each pixel output within a super pixel includes a Px contribution at angle x for target radiation and a constant contribution W for window radiation. A radiance image (albeit at a reduced resolution) is computed by averaging the outputs Px in each sub-array as RAD = <NUM>*((P0+W) + (P45+W) + (P90+W) + (P135+W). As such, the radiance image includes the window radiance component. An angle of linear polarization (AoLP) image <NUM> is computed as AoLP = <NUM>*atan(((P0+W)-(P90+W))/((P45+W)-(P135+W))) = <NUM>*atan((P0-P-<NUM>)/(P45-P135)). As such the radiance component of the window self-emissions, and thus any gradient component is canceled in the AoLP image. A degree of linear polarization (DoLP) image is calculated as DoLP = sqrt((P0-P90)^<NUM>+(P45-P135)^<NUM>)/(P0+P90+2W). The DoLP image includes the window radiance component. Thus, in cases of extreme aero-thermal heating, the AoLP image may be the only useful data product. Each of the data products can, to some extent, be improved by aligning one or more of the polarization pixels to the plane of incidence to improve the contrast of target radiance to window radiance.

Referring now to <FIG>, in an embodiment a sub-array <NUM> of a microgrid polarizer includes a <NUM>×<NUM> grouping of filter pixels Q of which at least two filter pixels impart a linear polarization of a certain and different angular value, and a third pixel either is unpolarized or imparts a linear polarization of a different angular value. The fourth pixels is not required to create the AoLP and DoLP data products but can be used to either increase the polarization diversity, thus improving the AoLP and DoLP data products or to increase SNR. As shown, two of the pixels impart a linear polarization of <NUM>° and the other two pixels impart linear polarizations of X° and Y°. The microgrid polarizer is aligned such that the <NUM>° linear polarization lies in the plane of incidence to pass p-polarized light. Note, the assignment of <NUM>° for alignment to the plane of incidence is arbitrary, any value of linear polarization could be so aligned. The values of X and Y are selected to provide diversity of polarization. In alternate embodiments, the sub-array may be larger than 2x2, in which case more than <NUM> pixels may have the same angular value that is aligned to the plane of incidence. The tradeoff is between SNR and polarization diversity.

Referring now to <FIG>, in an embodiment the polarizer may be implemented as a dynamic microgrid array <NUM> in which the individual pixels can be selectively configured to impart a linear polarization with different angular values or to be unpolarized. The array could be configured as a sheet polarizer <NUM> by setting all of the angular values to the same value. The array could be configured as a sub-array <NUM> of a microgrid polarizer by setting the angular values to different values. The array could be configured as a transmissive element <NUM> by setting all the pixels to be unpolarized "U". If the polarizer is mounted offgimbal, the angular values could be updated to effectively rotate the polarizer in sync with the rotation of the entrance pupil to maintain alignment of the polarizer to the plane of incidence. An embodiment of a dynamic microgrid array is disclosed in <CIT>.

In certain applications, the dynamic microgrid array could be reconfigured between a sheet polarizer, a microgrid polarizer and the transmissive element as a function of mission conditions. For example, if the unfiltered radiance provides sufficient SNR to track the target, the dynamic microgrid array may be configured as a transmissive element. If the SNR falls below the minimum threshold, the dynamic microgrid array may be reconfigured as the sheet polarizer or the microgrid polarizer.

Referring now to <FIG>, in an embodiment a fixed transmissive element <NUM>, a fixed sheet polarizer <NUM> and a fixed microgrid polarizer <NUM> are mounted on a carrier <NUM> in a fixed optical system or on or off-gimbal in a gimbaled optical system. A shutter mechanism <NUM> selectively actuates carrier <NUM> to move one of the transmissive element <NUM>, sheet polarizer <NUM> and microgrid polarizer <NUM> into an optical path <NUM> to an FPA <NUM>.

Claim 1:
An optical system for use with a flight vehicle subject to aero-thermal heating, said optical system comprising:
an optical window (<NUM>, <NUM>, <NUM>) having interior (<NUM>) and exterior (<NUM>) surfaces with a curvature with respect to a central axis, said optical window configured such that incident radiation from a target passes through both said interior and exterior surfaces each of which induces a P-polarization to the incident target radiation within a plane of incidence (<NUM>), said optical window configured to self-emit radiation that passes only through said interior surface which induces a P-polarization to the self-emitted radiation within the plane of incidence, said self-emitted radiation overlapping the spectrum of the target radiation;
focusing optics (<NUM>) comprising an entrance pupil (<NUM>) offset from the central axis to look through an off-axis segment (<NUM>, <NUM>, <NUM>, <NUM>) of the optical window (<NUM>), said focusing optics configured to route incident radiation along an optical path and focus the incident radiation at a focal plane;
a detector (<NUM>, <NUM>) at or near the focal plane configured to sense incident radiation;
a polarizer (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>,<NUM>,<NUM>) positioned in the optical path between the optical window and the detector, said polarizer comprising at least one filter pixel that imparts a linear polarization of a certain angular value to filter the incident radiation as a function of the polarization of the incident radiation; and
a mechanism configured to align said at least one filter pixel to the P-polarization within the plane of incidence, said mechanism comprising:
an outer gimbal (<NUM>) configured to rotate about a first rotation axis; and
an inner gimbal (<NUM>) mounted on the outer gimbal, said inner gimbal configured to rotate the entrance pupil about a second rotation axis perpendicular or skew to the first rotation axis.