This disclosure relates to passive imaging systems, and more particularly to systems that perform wavelength conversion of passively collected millimeter wave input power levels to signal output intensities for imaging.
Millimeter Wave (MMW) is the radiation band with wavelengths from one millimeter to 10 millimeters. As radio waves are considered low band frequency, millimeter waves are designated as very high frequency. Regardless of the frequency of the radiation, just like light, millimeter waves can be found throughout the environment. For example, the human body is a natural emitter of such radiation and thus, imagers can be designed to passively (without emitting power) capture radiation emitted from the body including use in non-invasive scanning technologies (e.g., explosive device detection on humans). There are several advantages to passively image millimeter wave radiation from scenes. First, imaging through atmospheric obscurants like fog, smoke, dust, and even clothing is possible. Second, the person is not exposed to any radiation that is not already all around them so there is no concern that the imaging device is harmful, such as from x-ray machines, for example. Further, the length of the millimeter wave is large, so the inherent imaging resolution is low, reducing the impact on personal privacy (e.g., images are not so revealing). Third, there are fewer man-made sources in this domain to clutter the scene. Finally, imaging can be performed covertly as there are no emissions from a passive imager.
In earlier passive millimeter wave (PMMW) imager configurations, a lens is employed to passively collect the millimeter wave radiation from the scene and focus it onto a two-dimensional focal plane array (FPA) of monolithic microwave integrated circuit (MMIC)-based receivers. These receivers (e.g., radiometers) convert the radiation into a usable electrical signal. These signals are then electronically processed (amplified, digitized, multiplexed), and sent as a digital stream to a bank of processors where an image is constructed and displayed. The process of image conversion can be expensive both in economic complexity of integrated circuits to perform the conversion and in terms of processing expense to produce the image from the captured waveforms.
This disclosure relates to systems and methods for performing passive millimeter wave (PMMW) imaging. In one example, an imaging system is provided. The system includes a receiver to receive a millimeter wave (MMW) power level input from a portion of the scene and generates an analog output signal. A driver circuit receives the analog output signal and generates a drive output signal based on the amplitude of the analog output signal. A wavelength converter generates a light intensity output in response to the drive output signal, wherein the light intensity output is a replica of a portion of the scene associated with the MMW power level input at a different wavelength range than the MMW power level input wavelength range.
In another example, an imaging system includes an optical front end for passively capturing a millimeter wave image of a scene. The system includes an imaging assembly that also includes a plurality of focal plane array receivers that each receives a MMW power level input associated with the MMW image and each generates a respective analog output signal. The assembly includes a plurality of driver circuits that each receives a respective analog output signal and generates a respective drive output signal based on the amplitude of the corresponding analog output signal. The assembly also includes a plurality of wavelength converters that each generates a light intensity output in response to a respective drive output signal. The light intensity output is a replica of a portion of the scene associated with the MMW power level input at a different wavelength range than the MMW power level input wavelength range.
In yet another example, an imaging system includes an optical front end for passively capturing a millimeter wave (MMW) image of a scene. The system includes a focal plane array (FPA) consisting of a plurality of feed antennas that each samples a portion of the MMW image of the scene coming from the optical front end, and generates an antenna output signal. The FPA includes a plurality of low noise amplifiers that boost the antenna output signal to generate a MMW power level input associated with the MMW image. The FPA includes a plurality of detectors that each receives the MMW power level input associated with the MMW image and each generates a respective analog output signal. The FPA includes a plurality of driver circuits that each receives a respective analog output signal and generates a respective drive output signal based on the amplitude of the corresponding analog output signal. The FPA includes a plurality of light emitting diodes (LEDs) that each generates a light intensity output in response to a respective drive output signal. The light intensity output is a replica of a portion of the scene associated with the MMW power level input from one of the feed antennas, at a different wavelength range than the MMW power level input wavelength range. The imaging system also includes an image capture device that captures an image of the scene formed by the light intensity outputs of the LEDs.
This disclosure relates to passive millimeter wave imaging systems. Millimeter wave (MMW) radiation is passively captured from a scene in the form of an input power level associated with the radiation. For example, MMW radiation at 94 GHz can be received at a measureable power level from the human body. The imaging system can include a lens to focus a portion of the passively collected MMW radiation onto a feed antenna which collects the MMW power (e.g., collected from a body or other radiating object at a checkpoint). A low noise amplifier amplifies the received MMW power level input from the feed antenna to increase the available signal level in view of background noise. A detector (e.g., diode detector) receives the amplified MMW power level input from the low noise amplifier and generates an electrical signal output where a driver circuit generates a drive output signal in response to the electrical signal output from the detector. A wavelength converter such as a light emitting diode (LED), for example, receives the drive output signal from the driver circuit and generates a light intensity output that is proportional to the MMW power level input from the scene.
Although visible light imagers are typically employed, other non-visible wavelength conversions such as infrared light or ultraviolet light are also possible. After converting the input power level from the scene to a given wavelength via the wavelength converter, an image capture device (e.g., video camera, pixilated integrated circuit sensor) can be employed to generate an image of the scene from the signal intensity output of the wavelength converter. In some examples, the images can be viewed directly with the eye at the output of the wavelength converter (e.g., goggle-like or binocular-like optical imagers). In another example, a camera or other capture device can capture the respective images at the output of the wavelength converter. In yet another example, fiber optic couplings can be employed to couple the output from the wavelength converter to the image capture device which may be at a remote location. By converting captured MMW power levels directly to visible wavelengths, (or other non-visible wavelengths), complex phase and image reconstruction processing hardware can be eliminated while simplifying overall system design. In some examples, the systems can be conveniently fabricated on an integrated circuit such as a focal plane array (FPA).
By converting the MMW power level input to an analog signal, no phase information is produced at the output of the wavelength converter circuit 130 which greatly simplifies operation and expense of the image conversion system 100. For example, images can be directly viewed at the output of the wavelength converter circuit 130 as a pixilated array displaying the scene (e.g., viewing the backside of a FPA). Such viewing is a result of the direct image reconstruction of the scene at the output of the wavelength converter circuit 130 without the need for complex digital image reconstruction processing and phase-management hardware as in conventional PMMW imaging systems. The light intensity output from the wavelength converter circuit 130 can also be captured as an image via an image capture device 140 which could include the human eye, digital cameras, film cameras, pixilated sensors, and so forth. In another example, the light intensity output from the wavelength converter 130 could also be transported to remote locations via fiber optic cabling (e.g., each cable coupling one pixel's worth of information from a pixilated array output of the wavelength converter) where the image capture device 140 could be located remotely and the image captured by filming or recording the light intensity coming out of the ends of the fiber cables.
As will be illustrated and described below with respect to
The wavelength converter circuit 130 can also include a low noise amplifier (LNA) to boost the received MMW power level input from the scene above background noise levels. A feed antenna within the wavelength converter circuit 130 collects the MMW power level input from the scene and provides the MMW power level input from the scene to the low noise amplifier which is then received by the detector. As will be illustrated and described below, the feed antenna, the low noise amplifier, the detector, the driver circuit, and the wavelength converter in the wavelength converter circuit 130 can be configured as a single integrated circuit pixel to provide one pixel's worth of information from the scene. By configuring a plurality of integrated circuit pixels, the wavelength converter circuit 130 can be configured as a focal plane array (FPA) assembly, for example. In one example, the wavelength converter inside the wavelength converter circuit 130 can be a light emitting diode (LED). Other wavelengths than from the visible spectrum can also be supported. For example, the LED can be configured to emit infrared spectrum frequencies, visible spectrum frequencies, or ultra violet spectrum frequencies. When non-visible conversions are performed by the wavelength converter circuit 130, the image capture device 140 can be configured accordingly. For example, an infrared sensor would be utilized in the image capture device 140 for infrared conversions or an ultraviolet sensor for ultraviolet conversions from MMW wavelengths.
In conventional passive millimeter wave camera configurations, a lens passively collects the millimeter wave radiation from the scene and focuses it onto a two-dimensional focal plane array (FPA) of MMIC-based receivers. These receivers, also referred to as radiometers, convert the radiation into a usable electrical signal. These signals are amplified, demodulated, digitized, multiplexed, and sent as a digital stream to a bank of processors where an image is constructed and displayed. Such processing, depending on the front end optics, can include both complex amplitude and phase processing of the received MMW signals. The wavelength converter circuit 200 can simplify the image reconstruction process from the output of the MMIC receivers. Instead of the complex image reconstruction electronics described above, each receiver in an FPA configuration can drive a LED such that the backside of the FPA ends up having an array of LEDs (see
An image visible to the eye can be created by the wavelength converter circuit 200 (if the LEDs are emitting visible light) that corresponds to the MMW image collected by the FPA. In one example, an off-the-shelf video camera can be directed at the LED array to create a digital video stream that can be sent to a computer where image processing can be performed in real-time with software, and can easily be displayed. Thus, all the complex electronics of conventional PMMW imaging systems can be reduced to a straightforward analog LED driver and an off-the-shelf video camera (or other image capture device). In one example variation from using an image capture device, the output of each LED can be fed through separate fiber optic cables, and these fibers can be bundled and the light sent remotely to a video camera CCD array. The fibers should be arranged in a suitable order to correspond to their relative location at the back end of the FPA where the wavelength converters 250 would be mounted. This method would be useful if a video camera cannot be located near the FPA, where, for example, the size or volume of the camera is a consideration, and/or where electrical isolation is desired.
What have been described above are examples. It is, of course, not possible to describe every conceivable combination of components or methodologies, but one of ordinary skill in the art will recognize that many further combinations and permutations are possible. Accordingly, the disclosure is intended to embrace all such alterations, modifications, and variations that fall within the scope of this application, including the appended claims. As used herein, the term “includes” means includes but not limited to, the term “including” means including but not limited to. The term “based on” means based at least in part on. Additionally, where the disclosure or claims recite “a,” “an,” “a first,” or “another” element, or the equivalent thereof, it should be interpreted to include one or more than one such element, neither requiring nor excluding two or more such elements.