Patent Grant
8306786
This disclosure relates to imagers that utilize compressive sensing and in particular to millimeter wave or higher frequency imaging.
Millimeter wave imaging sensor arrays have been used to detect energy present in an imaging focal plane. These prior art sensor arrays produce images by directly sensing the energy in the focal plane, and each detector in the sensor array corresponds to one pixel in the image. Thus, sensitivity for these prior art sensors is limited by the noise at each sensor and by the amount of energy collected by each sensor. An example of such an array structure is disclosed in U.S. Pat. No. 6,828,556 to Pobanz et al. Another example of such an imaging sensor array is disclosed in “A Wideband Radiometer Module for an Unamplified Direct Detection Scalable W-band Imaging Array”, by James H. Schaffner et al., Proc. SPIE Vol. 6948, 694807 (2008).
Compressive sampling, also known as compressive sensing and sparse sampling, is another prior art technique that has been used to sense energy in a scene. The key concept of compressive sampling is to exploit the structure and redundancy in an image. The advantage of compressive sampling is that a detector is not needed for each pixel to obtain a particular image resolution and thus this technique promises to be less expensive than requiring a detector in the sensor array corresponding to each pixel in the image.
Such a technique is described in “Single-Pixel Imaging via Compressive Sampling” by M. F. Duarte, et al., IEEE Signal Processing Magazine, Vol. 25, No. 2, pp. 83-91, 2008. The described device consists of a digital micromirror device, two lenses, a single photon detector, and an analog-to-digital (A/D) converter that computes pseudo-random linear measurements of the scene under view. The image is then recovered by processing the measurements with a digital computer. The technique utilizes spatial light modulators or mirrors to modulate the signals at the optical visible wavelengths.
The compressive sampling technique described by Duarte utilizes a single “pixel” to detect the combined signals from all of the modulators or mirrors. For the compressive sampling technique to work in typical situations with moderate to low signal-to-noise ratio (SNR), the single “pixel” must be able to detect a large number of electromagnetic modes so that the resulting signal power is larger than that for a single mode. EM modes are an orthogonal set of EM field patterns that describe the manner in which energy is radiated from or absorbed by an electrically active device, including antenna elements, antenna arrays, antenna-coupled detectors, and photodetectors. EM modes are further described in “Performance Limitations of Compressive Sensing for Millimeter Wave Imaging” by Jonathan Lynch, Roy Matic, and Joshua Baron, Proc. SPIE 7670, 7670D 2010, which is incorporated herein by reference as though set forth in full. Detecting a large number of electromagnetic modes is relatively easy to accomplish at visible wavelengths, because electrically large multi-mode detectors are common and convenient. However, millimeter wave detectors typically detect only a single mode, so employing the prior art single “pixel” technique of Duarte would result in a significant attenuation of the image and a poor signal to noise ratio, and thus poor image quality.
Research, such as described by Duarte, has focused on imaging at visible or infrared wavelengths and used moveable mirrors as the method to implement compressive sensing. At visible wavelengths, which are 1 micron or less, the movable mirrors used for single “pixel” compressive sampling generally have a size of about 50 wavelengths or about 50 microns, in order to achieve an electrically large multi-mode detector. Millimeter wavelengths are on the order of 3 millimeters, so a mirror size of 50 wavelengths would be 150 millimeters or about 5.9 inches in size, which makes this approach impractical for millimeter wave imaging.
What is needed is a device that provides detection of multiple independent modes to obtain a high signal to noise ratio and high sensed image quality for imaging sensors and in particular imaging sensors for millimeter wavelengths, while providing the benefits of compressive sensing. The embodiments of the present disclosure answer these and other needs.
In a first embodiment disclosed herein, a compressive sensor for sensing energy comprises a plurality of detectors for detecting a plurality of electromagnetic modes, wherein each detector has an output and detects at least one electromagnetic mode of the energy, a summer coupled to the plurality of detectors for forming a composite sum of the outputs of the plurality of detectors, and a plurality of switches, each respective switch coupled to a respective detector, wherein the respective switch may be switched to remove the respective detector from the composite sum.
In another embodiment disclosed herein, a compressive sensor for sensing energy comprises a plurality of detectors for detecting a plurality of electromagnetic modes, wherein the plurality of detectors are connected serially, wherein each detector detects at least one electromagnetic mode of the energy and has an output, and wherein the outputs of the serially connected detectors form a composite sum; and a plurality of switches, each respective switch coupled to a respective detector, wherein the respective switch may be switched to remove an output of the respective detector from the composite sum.
In yet another embodiment disclosed herein, a method of compressive sensing of energy comprises summing outputs of a plurality of detectors to form a plurality of composite sums each composite sum corresponding to a switch state, wherein each detector detects at least one electromagnetic mode of the energy, wherein each respective detector has a respective switch for removing the respective detector from a composite sum, and wherein a switch state comprises a pattern of the plurality detectors not removed from a composite sum; and processing M composite sum measurements made for M different switch states, wherein M is less than N and the sensed energy is from an image having N pixel resolution.
These and other features and advantages will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features, like numerals referring to like features throughout both the drawings and the description.
In the following description, numerous specific details are set forth to clearly describe various specific embodiments disclosed herein. One skilled in the art, however, will understand that the presently claimed invention may be practiced without all of the specific details discussed below. In other instances, well known features have not been described so as not to obscure the invention.
Referring to
For example, the detector 16 has a corresponding switch 17. The inputs to detector 16 and switch 17 are connected to the output of detector 14 and to switch 15. When switch 15 is open then the inputs to detector 16 and switch 17 are connected to the output of detector 14. When switch 15 is closed then the inputs to detector 16 and switch 17 are connected to the output of either detector 12 or switch 13, depending on whether switch 13 is opened or closed.
By closing some of the switches and opening other switches in the detector array 10, the detector array may be made to have multiple electromagnetic modes because different combinations of detectors with open switches in the detector array observe different radiation patterns. The outputs of the detectors in the detector array 10 that have their switches in an open position are summed in series providing a single composite output signal 52. As further described below, by collecting and processing composite output signals 52 with the switches in different combinations of open and closed, an image may be formed.
The detectors in the detector array 10 may have horizontal and vertical spacings that correspond to the wavelengths (λ) of interest. Optical resolution is constrained by a diffraction limit that is approximately λ/2. Thus, there is little benefit to have the detectors on a horizontal and vertical spacing that is less than λ/2. However, the horizontal and vertical spacing of the detectors in the detector array 10 may be more than λ/2 and can be up to a maximum of 10λ. Also, as shown in
As described above, the outputs of the detectors in the detector array 10 that have their switches in an open position are summed in series providing a single composite output signal 52, as shown in
The advantage of the using the detector array 10 versus a single pixel detector for compressive imaging is that there is an increase in the SNR of the generated image. As noted above, the resolution of an image is diffraction limited by the optics to be λ/2, which determines the N pixels of possible resolution for an image for a particular wavelength. In compressive imaging M measurements are made of an N pixel resolution image, where M<N. Nonlinear optimization techniques are then processed in processor 82 to form an N pixel image from the M measurements.
If a detector is a single mode detector, then there is an attenuation factor of 1/N at the detector. If instead, a multi-mode detector array, such as detector array 10, is used to make the measurements, the attenuation factor is Nm/N, where Nm is the number of discrete detectors summed in the detector array 10. Because there are Nm detectors in the detector array and because the outputs of the detectors that have their corresponding switches open are summed, there is an increase in the signal that is proportional to Nm, while the noise has an increase that is proportional to √{square root over (Nm)}, because the noise is uncorrelated. Therefore there is a net gain in the signal to noise ratio (SNR) of the generated image of √{square root over (Nm)} compared to the SNR of the generated image using a single mode detector.
From the foregoing, a method of compressive sensing of energy may include summing the outputs of the plurality of detectors to form a plurality of composite sums, wherein each composite sum corresponds to a switch state. Then the M composite sum measurements made for M different switch states are processed, where M is less than N and the sensed energy is from an image having N pixel resolution. The detectors may be connected serially and the number of detectors in the plurality of detectors may be less than or equal to N. The plurality of detectors may be arranged in an array with horizontal and vertical spacing of λ/2 to a maximum of 10λ between detectors. The plurality of detectors may also be arranged in an array with a random spacing between detectors. As discussed above with reference to
Having now described the invention in accordance with the requirements of the patent statutes, those skilled in this art will understand how to make changes and modifications to the present invention to meet their specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention as disclosed herein.
The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the Claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the step(s) of . . . .”
| Number | Name | Date | Kind |
|---|---|---|---|
| 6828556 | Pobanz et al. | Dec 2004 | B2 |
| 20090121145 | Rostaing et al. | May 2009 | A1 |
| 20100264316 | Delaney et al. | Oct 2010 | A1 |
