Patent Application
20150226613
Uncooled microbolometer arrays (also called focal plane arrays, or FPAs) are used in a variety of thermal (infrared) imaging applications. As the microbolometers have been made more sensitive to incoming electromagnetic radiation, they have also become more sensitive to effects of self-heating, which causes a change in the intensity output from the pixels of the array. Changes in intensity tend to be non-uniform across the many pixels of the array, causing different pixels receiving the same input radiation to produce different outputs, and contributing to noise in the image.
One approach to non-uniformity correction (NUC) in uncooled microbolometer arrays is to periodically shutter the FPA, or the lens that focuses incident electromagnetic radiation onto the FPA, for a few seconds to allow for a non-uniformity correction of the image to be calculated. Some conventional uncooled microbolometer designs use video reference pixels (VRP) on the sides of the rows of the FPA to compensate for non-uniformity by continuously normalizing the row to row non-uniformity. However, non-uniformities also occur in the columns and high energy image locations, requiring the image to be corrected using a shutter.
Another method of non-uniformity correction is known as scene-based NUC, requires the viewed scene to be changing, and may involve complex image processing.
Aspects and embodiments are directed to methods for producing non-uniformity correction (NUC) without using a shutter outside the focal plane array (FPA). As discussed in more detail below, according to certain embodiments, designated reference pixels within the image area are used for the NUC calculations. Aspects and embodiments may provide numerous benefits, including removal of the need for a mechanical shutter, and avoidance of the loss of imaging time associated with the use of a shutter, as well as lower overall cost of a complete thermal imaging camera.
According to one embodiment, an imaging device comprises a focal plane array comprising an array of pixels arranged in rows and columns, the array of pixels corresponding to an imaging area of the focal plane array, the plurality of pixels including a first plurality of imaging pixels and a second plurality of reference pixels, wherein the first plurality of imaging pixels are configured to receive incident electromagnetic radiation from a viewed scene and provide image signals; and wherein the second plurality of reference pixels are shielded from receiving the incident electromagnetic radiation and are configured to produce non-uniformity correction signals.
The imaging device may further comprise optics configured to focus the incident electromagnetic radiation onto the imaging area of the focal plane array. In one example, the optics includes a lens having an f/#=1. The imaging device may further comprise a substrate, and the focal plane array may be disposed over the substrate. In one example, the substrate is a ceramic substrate. The imaging device may further comprise a thermocooler positioned between the substrate and the focal plane array. The imaging device may further comprise a reflective coating disposed over the second plurality of reference pixels, the reflective coating shielding the second plurality of reference pixels from receiving the incident electromagnetic radiation. In one example, the reflective coating includes a layer of gold. In one example, the layer of gold has a thickness in a range of approximately 2-300 nanometers. In another example, the reflective coating includes a layer of aluminum. In one example, the second plurality of reference pixels are arranged in a regular pattern over the array of pixels. In one example, the second plurality of reference pixels comprises between approximately 0.39% and 50% of the array of pixels. In another example, the second plurality of reference pixels comprises approximately 12.5% of the array of pixels. In another example, the second plurality of reference pixels includes at least one reference pixel in each row and column of the array of pixels. In another example, the focal plane array is a microbolometer array, and the incident electromagnetic radiation is infrared radiation.
Still other aspects, embodiments, and advantages of these exemplary aspects and embodiments are discussed in detail below. Embodiments disclosed herein may be combined with other embodiments in any manner consistent with at least one of the principles disclosed herein, and references to “an embodiment,” “some embodiments,” “an alternate embodiment,” “various embodiments,” “one embodiment” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described may be included in at least one embodiment. The appearances of such terms herein are not necessarily all referring to the same embodiment.
Various aspects of at least one embodiment are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of the invention. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. In the figures:
As discussed above, conventional uncooled microbolometer designs have always used a shutter to generate a single point offset non-uniformity correction (NUC). As the focal plane array (FPA) drifts in temperature, low frequency image artifacts appear making the image less usable. Furthermore, use of a shutter is undesirable in many applications. For example, a mechanical shutter may be noisy, making it unsuitable for use in applications where stealth is important. Additionally, the shutter is typically closed for a least a second, sometimes several seconds, rendering the imaging system “blind” and incapable of imaging the scene during this time. This may be particularly undesirable in applications and circumstances where continuous imaging is necessary and where losing the imaging capability for a second or more at a time may significantly negatively impact system performance (e.g., in target-tracking applications). As the shutter is a moving mechanical part, it may also be a point of failure in the system.
Aspects and embodiments are directed to apparatus and methods for shutterless NUC in microbolometer arrays or other FPAs. As discussed in more detail below, according to one embodiment, certain pixels within the imaging region of the array are designated as video reference pixels and applied with a coating that blocks incident electromagnetic radiation from being received. Accordingly, any output from these reference pixels is produced only by noise internal to the imaging system (e.g., thermal noise from self-heating of the pixels), not from any incident electromagnetic radiation, and may therefore be used to generate NUC signals. Unlike shutter-based solutions, embodiments of the devices and methods disclosed herein do not block or stop image collection during the correction. Additionally, embodiments disclosed herein do not require the viewed scene to be changing, as is the case for scene-based NUC. Furthermore, as discussed in more detail below, NUC correction may be applied on every frame, or with any spacing in time.
It is to be appreciated that embodiments of the methods and apparatuses discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and apparatuses are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms.
Referring to
The FPA 106 receives the electromagnetic radiation 112 from the viewed scene, and produces an image therefrom. However, as discussed above, one or more pixels of the FPA may also receive undesired thermal radiation from elsewhere, leading to noise in the image. For example, as shown in
According to one embodiment, a method for NUC in the FPA 106 includes designating certain pixels within the imaging area of the FPA 106 as video reference pixels, and shielding these pixels from the incident electromagnetic radiation 112. Referring to
The arrangements of reference pixels 210 illustrated in FIGS. 2 and 3A-C are exemplary only, and not intended to be limiting. Those skilled in the art will recognize, given the benefit of this disclosure, that numerous arrangements or patterns of reference pixels may be implemented. According to one embodiment, the reference pixels 210 may be arranged in a regular or irregular pattern within the imaging area 202, provided that there is at least one reference pixel in each row and column of the array, as illustrated in
As discussed above, the reference pixels 210 are shielded from the incident electromagnetic radiation 112, such that they measure only self-heating and other thermal noise and may therefore be used to perform NUC. In one embodiment, this shielding is implemented by coating the reference pixels 210 with a reflective coating, such that the incident electromagnetic radiation 112 is reflected from the reference pixels 210, and not absorbed and measured. In one example, the reference pixels 210 are coated with a layer of gold. Gold may be selected because it is highly reflective, namely, approximately 98% reflective, in the infrared spectral band, and also adheres well to the semiconductor materials used to form the FPA 106. In one example, the layer of gold used to coat the reference pixels 210 has a thickness in a range of approximately 2-300 nanometers. In another example, the reference pixels 210 may be coated with a layer of aluminum. Aluminum is approximately 90% reflective in the infrared spectral band.
Thus, aspects and embodiments provide an imaging device, in particular a microbolometer array, with built-in, shutterless NUC. Reference pixels are distributed within the imaging area of a focal plane array, providing numerous sample points over the imaging plane that can be used for NUC. As discussed above the reference pixels are permanently shielded from receiving incident electromagnetic radiation via the imaging optics, for example, by using a highly reflective coating over the surface area of the reference pixels. In this manner, signals from the reference pixels may be used by image processing firmware and/or software associated with the focal plane array to provide NUC signals which can be used to adjust the image produced by the imaging pixels of the array to compensate for non-uniformities in the photo-response of the array, for example, due to self-heating or other thermal noise. Unlike conventional scene-based NUC, which typically only addresses high-frequency content, aspects and embodiments of the present invention may provide the ability to remove low-frequency noise structures from the images produced by the array. Furthermore, NUC may be performed continuously during the imaging operation of the array, without requiring image collection to be stopped during the correction, as is the case in conventional shutter-based systems.
Having described above several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.
