Staring array

<h2 id="comparison-to-scanning-array">Comparison to scanning array</h2>
<p class="note">Further information: <a href="/facts/Push_broom_scanner/jgGG9C8w">Push broom scanner</a> and <a href="/facts/Whisk_broom_scanner/zqIzoUo0">Whisk broom scanner</a></p>
<p>Staring arrays are distinct from <a href="/facts/Line-scan_camera/sHkJxgEX">scanning array</a> and <a href="/facts/Time_delay_and_integration/RDgqtHmg">TDI</a> imagers in that they image the desired field of view without scanning. Scanning arrays are constructed from linear arrays (or very narrow 2-D arrays) that are rastered across the desired field of view using a rotating or oscillating mirror to construct a 2-D image over time. A TDI imager operates in similar fashion to a scanning array except that it images perpendicularly to the motion of the camera. A staring array is analogous to the film in a typical camera; it directly captures a 2-D image projected by the lens at the image plane. A scanning array is analogous to piecing together a 2D image with photos taken through a narrow slit. A TDI imager is analogous to looking through a vertical slit out the side window of a moving car, and building a long, continuous image as the car passes the landscape.
</p><p>Scanning arrays were developed and used because of historical difficulties in fabricating 2-D arrays of sufficient size and quality for direct 2-D imaging. Modern FPAs are available with up  to 2048 x 2048 pixels, and larger sizes are in development by multiple manufacturers. 320 x 256 and 640 x 480 arrays are available and affordable even for non-military, non-scientific applications.
</p>
<h2 id="construction-and-materials">Construction and materials</h2>
<p>The difficulty in constructing high-quality, high-resolution FPAs derives from the materials used. Whereas visible imagers such as CCD and CMOS image sensors are fabricated from silicon, using mature and well-understood processes, IR sensors must be fabricated from other, more exotic materials because silicon is sensitive only in the visible and near-IR spectra. Infrared-sensitive materials commonly used in IR detector arrays include <a href="/facts/Mercury_cadmium_telluride/kzgN6aJn">mercury cadmium telluride</a> (HgCdTe, "MerCad", or "MerCadTel"), <a href="/facts/Indium_antimonide/76EPwVLC">indium antimonide</a> (InSb, pronounced "Inns-Bee"), <a href="/facts/Indium_gallium_arsenide/DAHNSFPr">indium gallium arsenide</a> (InGaAs, pronounced "Inn-Gas"), and <a href="/facts/Vanadium(V)_oxide/LPtvz4pq">vanadium(V) oxide</a> (VOx, pronounced "Vox"). A variety of lead salts can also be used, but are less common today. None of these materials can be grown into crystals anywhere near the size of modern silicon crystals, nor do the resulting wafers have nearly the uniformity of silicon. Furthermore, the materials used to construct arrays of IR-sensitive pixels cannot be used to construct the electronics needed to transport the resulting charge, voltage, or resistance of each pixel to the measurement circuitry. This set of functions is implemented on a chip called the <a href="/facts/Multiplexer/lVro0Sfz">multiplexer</a>, or <a href="/facts/Readout_integrated_circuits/4kfWtmcL">readout integrated circuits</a> (ROIC), and is typically fabricated in silicon using standard CMOS processes. The detector array is then hybridized or bonded to the ROIC, typically using indium bump-bonding, and the resulting assembly is called an FPA.
</p><p>Some materials (and the FPAs fabricated from them) operate only at <a href="/facts/Cryogenic/cw7dHAkp">cryogenic</a> temperatures, and others (such as resistive amorphous silicon (a-Si) and VOx <a href="/facts/Microbolometer/390o08xp">microbolometers</a>) can operate at uncooled temperatures. Some devices are only practical to operate cryogenically as otherwise the <a href="/facts/Johnson%E2%80%93Nyquist_noise/ur238TgX">thermal noise</a> would swamp the detected signal. Devices can be cooled evaporatively, typically by <a href="/facts/Liquid_nitrogen/IGWI1h00">liquid nitrogen</a> (LN2) or liquid helium, or by using a <a href="/facts/Thermo-electric_cooler/fzQLmYjh">thermo-electric cooler</a>.
</p><p>A peculiar aspect of nearly all IR FPAs is that the electrical responses of the pixels on a given device tend to be non-uniform. In a perfect device every pixel would output the same electrical signal when given the same number of photons of appropriate wavelength. In practice nearly all FPAs have both significant pixel-to-pixel offset and pixel-to-pixel <a href="/facts/Photo_response_non-uniformity/9IWYg2WR">photo response non-uniformity</a> (PRNU). When un-illuminated, each pixel has a different "zero-signal" level, and when illuminated the delta in signal is also different. This non-uniformity makes the resulting images impractical for use until they have been processed to normalize the photo-response. This correction process requires a set of known characterization data, collected from the particular device under controlled conditions. The data correction can be done in software, in a <a href="/facts/Digital_signal_processor/YuBXweos">DSP</a> or <a href="/facts/Field-programmable_gate_array/M8LSGcAU">FPGA</a> in the camera electronics, or even on the ROIC in the most modern of devices.
</p><p>The low volumes, rarer materials, and complex processes involved in fabricating and using IR FPAs makes them far more expensive than visible imagers of comparable size and resolution.
</p><p>Staring plane arrays are used in modern <a href="/facts/Air-to-air_missile/82Dccmul">air-to-air missiles</a> and <a href="/facts/Anti-tank_missile/I3C5D1CS">anti-tank missiles</a> such as the <a href="/facts/AIM-9X_Sidewinder/LPm7nQTT">AIM-9X Sidewinder</a>, <a href="/facts/AIM-132_ASRAAM/E0gL2z0k">ASRAAM</a><a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a>
</p><p><a href="/facts/Crosstalk/IT2LIwth">Cross talk</a> can inhibit the illumination of pixels.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a>
</p>
<h2 id="applications">Applications</h2>
<h3>3D LIDAR Imaging</h3>
<p>Focal plane arrays (FPAs) have been reported to be used for 3D <a href="/facts/Lidar/b9SPUmLM">LIDAR</a> imaging.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a><a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a><a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a>
</p>
<h4>Improvements</h4>
<p>In 2003, a 32 x 32 pixel <a href="/facts/Breadboard/fokXRhUn">breadboard</a> was reported with capabilities to repress cross talk between FPAs. Researchers at the <a href="/facts/United_States_Army_Research_Laboratory/CULgwaW6">U.S. Army Research Laboratory</a> used a <a href="/facts/Collimator/UAsMlkeh">collimator</a> to collect and direct the breadboard’s laser beam onto individual pixels. Since low levels of voltage were still observed in pixels that did not illuminate, indicating that illumination was prevented by <a href="/facts/Crosstalk/IT2LIwth">crosstalk</a>. This cross talk was attributed to <a href="/facts/Capacitive_coupling/7m9pjCPF">capacitive coupling</a> between the <a href="/facts/Microstrip/QkKwyAN7">microstrip lines</a> and between the FPA’s internal conductors. By replacing the receiver in the breadboard for one with a shorter focal length,  the focus of the collimator was reduced and the system’s threshold for signal recognition was increased. This facilitated a better image by cancelling cross talk.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a>
</p><p>Another method was to add a flat thinned substrate membrane (approximately 800 angstroms thick) to the FPA. This was reported to eliminate pixel-to-pixel cross talk in FPA imaging applications.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> In another an <a href="/facts/Avalanche_photodiode/x2eUFfgF">avalanche photodiode</a> FPA study, the etching of trenches in between neighboring pixels reduced cross talk.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a>
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Focal-plane_array_(radio_astronomy)/DxmdLM89">Focal-plane array (radio astronomy)</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Air-to-Air Weapons - Royal Air Force <a href="http://www.raf.mod.uk/equipment/airtoair.html" target="_blank">http://www.raf.mod.uk/equipment/airtoair.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Goldberg, A.; Stann, B.; Gupta, N. (July 2003). "Multispectral, Hyperspectral, and Three-Dimensional Imaging Research at the U.S. Army Research Laboratory" (PDF). Proceedings of the International Conference on International Fusion [6th]. 1: 499–506. <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>Goldberg, A.; Stann, B.; Gupta, N. (July 2003). "Multispectral, Hyperspectral, and Three-Dimensional Imaging Research at the U.S. Army Research Laboratory" (PDF). Proceedings of the International Conference on International Fusion [6th]. 1: 499–506. <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>Marino, Richard M.; Stephens, Timothy; Hatch, Robert E; McLaughlin, Joseph L.; Mooney, James G.; O'Brien, Michael E.; Rowe, Gregory S.; Adams, Joseph S.; Skelly, Luke (2003-08-21). "A compact 3D imaging laser radar system using Geiger-mode APD arrays: system and measurements". In Kamerman, Gary W (ed.). Laser Radar Technology and Applications VIII. Vol. 5086. pp. 1–15. doi:10.1117/12.501581. S2CID 110267445. <a href="https://www.spiedigitallibrary.org/conference-proceedings-of-spie/5086/0000/A-compact-3D-imaging-laser-radar-system-using-Geiger-mode/10.1117/12.501581.short?SSO=1" target="_blank">https://www.spiedigitallibrary.org/conference-proceedings-of-spie/5086/0000/A-compact-3D-imaging-laser-radar-system-using-Geiger-mode/10.1117/12.501581.short?SSO=1</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>Marino, Richard M.; Davis, William Rhett (2004). "Jigsaw : A Foliage-Penetrating 3 D Imaging Laser Radar System". S2CID 18046922. {{cite web}}: Missing or empty |url= (help) <a href="/wiki/S2CID_(identifier)" target="_blank">/wiki/S2CID_(identifier)</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>Goldberg, A.; Stann, B.; Gupta, N. (July 2003). "Multispectral, Hyperspectral, and Three-Dimensional Imaging Research at the U.S. Army Research Laboratory" (PDF). Proceedings of the International Conference on International Fusion [6th]. 1: 499–506. <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
<li id="fn:7"><p>D., Gunapala, S.; V., Bandara, S.; K., Liu, J.; J., Hill, C.; B., Rafol, S.; M., Mumolo, J.; T., Trinh, J.; Z., Tidrow, M.; D., LeVan, P. (May 2005). "1024 x 1024 pixel mid-wavelength and long-wavelength infrared QWIP focal plane arrays for imaging applications". {{cite journal}}: Cite journal requires |journal= (help)CS1 maint: multiple names: authors list (link) <a href="https://trs.jpl.nasa.gov/handle/2014/39300" target="_blank">https://trs.jpl.nasa.gov/handle/2014/39300</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li>
<li id="fn:8"><p>Itzler, Mark A.; Entwistle, Mark; Owens, Mark; Patel, Ketan; Jiang, Xudong; Slomkowski, Krystyna; Rangwala, Sabbir; Zalud, Peter F.; Senko, Tom (2010-08-19). Dereniak, Eustace L; Hartke, John P; Levan, Paul D; Sood, Ashok K; Longshore, Randolph E; Razeghi, Manijeh (eds.). "Design and performance of single photon APD focal plane arrays for 3-D LADAR imaging". Detectors and Imaging Devices: Infrared, Focal Plane, Single Photon. 7780. SPIE: 77801M. Bibcode:2010SPIE.7780E..1MI. doi:10.1117/12.864465. S2CID 120955542. <a href="https://spie.org/Publications/Proceedings/Paper/10.1117/12.864465" target="_blank">https://spie.org/Publications/Proceedings/Paper/10.1117/12.864465</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li>
</ol>

Staring array open-in-new

Staring array