We report on a technique for reducing the image degradation introduced by viewing through deep turbulence. The approach uses a variable aperture that was designed to maintain the telescope’s theoretical resolving power. The technique combines the variable aperture sensor with post processing to form a turbulence restored image. Local wavefront tilt is corrected using local image registration. Lucky look processing performed in the frequency domain is used to combine the best aspects of each image in a sequence of frames to form the final image product. The approach was demonstrated on imagery of targets of opportunity on the Boston skyline observed through a 55-mile nearlyhorizontal path from Pack Monadnock in southern New Hampshire. Quantitative assessment of image quality is based on the MTF which is estimated from edges within the images. This is performed for imagery acquired with and without the variable aperture, and the effectiveness of the approach is evaluated by comparing the results. In most cases, the reduced aperture is found to improve performance significantly relative to the full aperture.
Multi-spectral sensor systems that record spatially and temporally registered image video have a variety of applications
depending on the spectral band employed and the number of colors available. The colors can be selected to highlight
physically meaningful portions of the image, and the resulting imagery can be used to decode relevant phenomenology.
For example, the images can be in spectral bands that identify materials that are intrinsic to the target while uncommon
in the backgound, providing an anomaly detection cue. These multi-spectral video sensor engines can also be employed
in conjunction with conventional fore-optics such as astronomical telescopes or microscopes to exploit useful
phenomenology at dissimilar scales. Here we explore the relevance of multi-spectral video in a space application. This
effort coupled a terrestrial multispectral video camera to an astronomical telescope. Data from a variety of objects in
Low Earth Orbit (LEO) were collected and analyzed both temporally, using light curves, and spectrally, using principal
component analysis (PCA). We find the spectral information is correlated with temporal information, and that the
spectral analysis adds the most value when the light curve period is long. The value of spectral-temporal signatures,
where the signature is the difference in either the harmonics or phase of the spectral light curves, is investigated with
A novel spectral imaging sensor based on dual direct vision prisms is described. The prisms project a spectral image onto
the focal plane array of an infrared camera. The prism set is rotated on the camera axis and the resulting spectral
information is extracted as an image cube (x, y, λ), using tomographic techniques. The sensor resolves more than 40
spectral bands (channels) at wavelengths between 1.2 μm and 2.5 μm wavelength. The sensor dispersion characteristic is
determined by the vector sum of the dispersions of the two prisms. The number of resolved channels, and the related
signal strength per channel, varies with the angle between the prism dispersion axes. This is a new capability for this
class of spectral imaging sensor. Reconstructed short-wave imagery and spectral data is presented for field and
laboratory scenes and for standard test sources.
Spectral imaging is the art of quantifying the spectral and spatial characteristics of a scene. The current state of the art in spectral imaging comprises a wide range of applications and sensor designs. At the extremes are spectrometers with high spectral sampling over a limited number of imaging pixels and those with little spectral sampling over a large number of pixels. The predominant technical issue concerns the acquisition of the three-dimensional spectral imagery (X,Y,l) using an inherently two-dimensional imaging array; consequently, some form of multiplexing must be implemented. This paper will discuss a new class of sensors, broadly referred to as Spectral Temporal Sensors (STS), which capture the position and spectra of uncued point sources anywhere in the optical field. These sensors have large numbers of pixels (>512x512) and colors (>50). They can be used to sense explosions, combustion, rocket plumes, LASERs, LEDs, LASER/LED excitations and the outputs of fiber optic cables. This paper will highlight recent developments on an STS that operates in a Pseudo-imaging (PI) mode, where the location of an uncued dynamic event and its spectral evolution in time are the data products. Here we focus on the sensor's ability to locate the event to within approximately 1/20th pixel, however we will also discuss its capabilities at fully characterizing event spectral temporal signature at rates greater than 100Hz over a large field of view (greater than 30°).
A novel integration technology for the fabrication of active, or passive, focal plane array imagers has been developed. The integration scheme is based on the transfer of epitaxial layers to a surrogate substrate without critical alignment. Once the epitaxial layer is successfully transferred to the surrogate substrate, photodetector isolation, passivation, and fabrication are completed. To demonstrate the potential of the process, 320 x 256 arrays of InGaAs mesas were successfully transferred onto commercially-available focal plane array readout integrated circuits. Pitch and pixel resolution are limited by the available standard photolithography. InGaAs mesas transferred to silicon wafers with a pitch as small as 10 microns have been demonstrated. The process was optimized for the fabrication of high-performance vertical Schottky photodiodes. Dark-currents below 5 nA were observed with 44 mm diameter photodiodes. Responsivities of 0.55 A/W were obtained with a 1 micron InGaAs absorber. The new integration process can be used to easily achieve photodiodes with bandwidths higher than 20 GHz, without the use of an air-bridge.
We describe a model for cooled thermal imaging sensors, based on silicon Schottky diode bolometer arrays. The sensing mechanism is the modulation of Schottky diode dark current with temperature. The proposed array is identical to Schottky diode arrays, which would be used for uncooled thermal imaging, except for a change of the sensing electrode metal. We separate the thermal and electrical response of the detector elements and discuss sensor limitations related to detector thermal isolation. At a 180 K operating temperature, we project NEDT's in the 3 to 20 mK range, depending upon system f/number. A 20 cm aperture sensor based on this technology should have a noise equivalent power below 10-11 watts.
Infrared imaging based on photoemission in metal- silicide/silicon Schottky barrier arrays is a mature technology that is currently employed in both military and commercial applications. Metal-silicide/silicon Schottky diodes can also be employed in uncooled bolometer arrays. The bolometer detection mechanism is thermionic emission in the Schottky barrier. Schottky bolometer array technology is expected to have both performance and production advantages, when compared with current uncooled sensor technology. In this paper, we compare the physical mechanisms involved in the two Schottky barrier based infrared sensors. We will also present a simplified model for the noise equivalent temperature of each technology.
This paper reports on the design, performance and signal processing of a visible/near infrared (VIS-NIR) chromotomographic hyperspectral imaging sensor. The sensor consists of a telescope, a direct vision prism, and a framing video camera. The direct vision prism is a two-prism set, arranged such that one wavelength passes undeviated, while the other wavelengths are dispersed along a line. The prism is mounted on a bearing so that it can be rotated on the optical axis of the telescope. As the prism is rotated, the projected image is multiplexed on elements of the focal plane array. Computational methods are used to reconstruct the scene at each wavelength; an approach similar to the limited-angle tomography techniques used in medicine. The sensor covers the visible through near infrared spectrum of silicon photodiodes. The sensor weighs less than 6 pounds has under 300 in3 volume and requires 20 watts. It produces image cubes, with 64 spectral bands, at rates up to 10 Hz. By operating in relatively fast framing mode, the sensor allows characterization of transient events. We will describe the sensor configuration and method of operation. We also present examples of sensor spectral image data.
The thermionic thermal detector (TTD) sense IR radiation by temperature modulation of thermionic emission current within a silicon Schottky diode. The thermionic emission current is the well known Richardson dark current. The TTD operates in the LWIR band. The physics of TTD operation is distinct from that of silicon Schottky barrier MWIR detectors, such as PtSi/Si which are based on internal photoemission. In fact, the TTD has high detection efficiency. The architecture of a TTD array is very similar to that of microbolometer arrays, expect the detector elements are thermally isolated Schottky diodes, operating under reverse bias. When the TTD array is illuminated by an IR image, the temperature of individual detector elements will vary with the local incident power of the image. Under small signal conditions, the dark current of individual detectors will vary as temperature, resulting in an electronic image of the IR scene. The reverse bias dark current of a Schottky diode varies exponentially with temperature. For the small temperature variations observed on the focal pane of an uncooled sensor, this variation is approximately linear. The rate of temperature variation is determined by the Schottky barrier potential and, to a lesser extent by the applied bias potential. The operating temperature range of the detector can be designed into the device by selecting a metal with the appropriate Schottky barrier height. Experimental Schottky barrier heights were determined using Richardson dark current activation energy analysis. Devices optimized for operating at room ambient temperature have a 6 percent K temperature coefficient. The use of Schottky diode thermionic emission for uncooled IR imaging offers several advantages relative to current technology. TTD manufacture is 100 percent silicon processing compatible. Schottky barrier based thermionic emissions array have the same uniformity characteristics as MWIR Schottky barrier photoemissive arrays. Operating TTDs in reverse bias provides a high impedance 'current source' to the multiplexer, resulting in negligible Johnson noise. This mode of operation also results in negligible detector 1/f-noise and drift. In addition, the TTD thermionic emission detection process has high efficiency, fully comparable with the best current thermal detectors.
The responsivity of large scale platinum silicide arrays, having small pixels, is low compared to the responsivity of large area test diodes fabricated on the same wafer. Often, the responsivity loss is described by assigning a lower Fowler emission coefficient to the detectors. We find the reduced responsivity to be the direct result of a reduction in the effective active area of the detector. This reduction in effective active area becomes more pronounced as the detector cell size is reduced. We provide a simple model for the area reduction in terms of modulation of detector Schottky potential by the underlying depletion region of the detector guard ring. We also suggest changes in the detector array unit cell design, which will maximize responsivity.