Technology Today

2010 Issue 2

Counting Photons:
Advances in Passive Short Wave Infrared Imaging

Short Wave Infrared (SWIR) — the spectrum from nominally 1 to 3 microns in wavelength — has gone largely unexploited due to a lack of suit­able detectors and limited understanding of the image phenomenology in this band. Due to its shorter wavelength, SWIR offers the advantage of higher resolution and smaller optical systems than mid-wave and long-wave (LW) infrared systems, making it attractive for tactical applications. To en­able the exploitation of the SWIR band, Raytheon has been leading in the develop­ment of new detectors and cameras, as well as studying the imaging phenomenology.

Raytheon has made exciting advances in single-photon short-wave infrared detec­tors and imagers that open the door to new applications and operational advantages to the United States. New systems will lever­age the key strengths that SWIR sensors provide, including:

  • True low-light imaging capability — turns night into day
  • Penetration of haze better than visible cameras
  • Sharper images than conventional LW thermal imagers
  • Observation of covert lasers and beacons
  • Uncooled technology for size, weight and power advantages over cryo-cooled systems
  • Spectral phenomenologies that enable camouflage exposure, human-flesh de­tection, and the ability to see through glass
  • High resolution iris and 3D facial image capture for standoff biometrics

Helicopter

Raytheon continues to be the industry leader in delivering technologies to oper­ate in new optical wavelength ranges such as SWIR; achieving even higher sensitivities (down to single photon detection); and improving the size, weight and power of infrared imagers. Raytheon’s collabora­tion with the Defense Advanced Research Projects Agency (DARPA) and U.S. Army Night Vision and Electronic Sensors (NVESD) researchers has advanced the state of the art of SWIR imaging. Research has focused on developing SWIR imagers that exploit both urban light sources and natural nightglow. Significant advances have been made with higher sensitivities, smaller pix­els, larger imaging array sizes, and higher dynamic range. These advances will deliver new capabilities to the warfighter and are attracting interest for application to iris and 3D facial image capture for standoff biometrics.

Figure 1

SWIR Passive Imagers With Near Single Photon Sensitivity

Working with DARPA and NVESD research­ers during the last decade, Raytheon has played a key role in developing imaging technology in the SWIR with near single-photon uncooled imaging capability (see Figure 1). This challenges near-infrared image intensifiers in night-vision applications.

Sky nightglow comes from natural chemical reactions with oxygen–hydrogen molecules in the Earth’s mesosphere (50 to 80 km al­titudes) resulting in the release of energy in the form of photons in the SWIR band. This always-present light source is invisible to the human eye, yet provides 13 times the light available from visible light on a moonless night and comes from nearly the entire sky. Exploiting nightglow with advanced sensors can provide superior range for target identi­fication compared to conventional sensors. SWIR cameras can also see all common laser wavelengths (e.g., laser designators) in use today. Adoption of SWIR cameras and sensors into fielded systems will en­able more systems to transition to eye-safe lasers. Active and range-gated SWIR sensors that take advantage of materials reflectivity and contrast provide additional application opportunities. Under DARPA’s sponsorship of the Multi-spectral Adaptive Networked Tactical Imaging System (MANTIS) and Photon Counting Arrays (PCAR) programs, Raytheon has developed multiple gen­erations of SWIR cameras with dramatic improvements in sensitivity, resolution, and dynamic range — making Raytheon the current market leader in large-format and high-sensitivity uncooled SWIR focal planes.

Technology Leadership

Combining ultra-low dark-current indium gallium arsenide detectors and ultra-sensitive readout integrated indium gallium arsenide (InGaAs) circuits, Raytheon has maintained its uncooled (no cryogenic cooler) -SWIR technology leadership since 2004, when the company demonstrated the first megapixel (1280x1024) SWIR camera. In tests by NVESD and others, Raytheon SWIR outperforms third-generation image intensifiers and other competition with re­spect to sensitivity and resolution, providing superior night vision capability.

More recently under DARPA’s PCAR pro­gram, Raytheon invented and demonstrated an advanced SWIR sensor design. A remark­able feature of this sensor is the inclusion of novel high dynamic range circuitry in small 15×15 μm pixel. The result is a large-format, high-density 1280×1024 SWIR focal plane array. Dynamic range exceeding 16 bits (80,000:1) with less than five electrons read noise has been demonstrated with detector operation at or near ambient temperatures. High dynamic range performance is shown in Figure 2 in three representations obtained within one frame time and showing the resulting instanteous dynamic range image. With post processing, these 16-bit total dynamic ranges can be directly fed into an automated target recognition system, or individually contrast-adjusted and mapped into a typical 8-bit display.

These sensors’ extremely low noise and high dynamic range allow recognition of low-contrast targets, without saturation from bright sources within the same frame of information. This enables operation in both urban and rural environments with imaging under low levels of ambient illumination, while simultaneously seeing around bright sources that would otherwise saturate con­ventional high-sensitivity sensors.

Figure 2

Figure 3

Raytheon’s family of uncooled tactical SWIR focal planes and SWIR sensor technologies that were developed and matured through five years of DARPA programs is now ready for transition to production and integration into a wide range of applications, includ­ing: rifle sights, handheld targeting units, marine/ship-board surveillance cameras, port security systems, unmanned aerial vehicles, manned air vehicles, and long-range observation and targeting platforms. Raytheon’s innovative technology developed with support from DARPA has resulted in performance superior to conventional night vision systems. This has resulted in a com­pact, small-pixel, large-format SWIR camera which, within one scene, can operate over six orders of magnitude illumination, cover­ing conditions ranging from extremely low light to full daylight.

David Acton

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