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Image sensor

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ACCDimage sensor on aflexible circuit board
An American Microsystems, Inc., (AMI) 1-kilobitDRAMchip (center chip with glass window) used as an image sensor by theCromemco Cyclops

Animage sensororimageris asensorthat detects and conveys information used to form animage.It does so by converting the variableattenuationof lightwaves(as theypass throughorreflect offobjects) intosignals,small bursts ofcurrentthat convey the information. The waves can be light or otherelectromagnetic radiation.Image sensors are used inelectronicimaging devices of bothanaloganddigitaltypes, which includedigital cameras,camera modules,camera phones,optical mousedevices,[1][2][3]medical imagingequipment,night visionequipment such asthermal imagingdevices,radar,sonar,and others. Astechnology changes,electronic anddigital imagingtends to replace chemical and analog imaging.

The two main types of electronic image sensors are thecharge-coupled device(CCD) and theactive-pixel sensor(CMOSsensor). Both CCD and CMOS sensors are based onmetal–oxide–semiconductor(MOS) technology, with CCDs based onMOS capacitorsand CMOS sensors based onMOSFET(MOS field-effect transistor)amplifiers.Analog sensors for invisible radiation tend to involvevacuum tubesof various kinds, while digital sensors includeflat-panel detectors.

CCD vs. CMOS sensors

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A micrograph of the corner of the photosensor array of awebcamdigital camera
Image sensor (upper left) on the motherboard of aNikon Coolpix L2 6 MP

The two main types ofdigital imagesensors are thecharge-coupled device(CCD) and theactive-pixel sensor(CMOS sensor),fabricatedincomplementary MOS(CMOS) orN-typeMOS (NMOSorLive MOS) technologies. Both CCD and CMOS sensors are based on theMOS technology,[4]withMOS capacitorsbeing the building blocks of a CCD,[5]andMOSFETamplifiers being the building blocks of a CMOS sensor.[6][7]

Cameras integrated in small consumer products generally use CMOS sensors, which are usually cheaper and have lower power consumption in battery powered devices than CCDs.[8]CCD sensors are used for high end broadcast quality video cameras, and CMOS sensors dominate in still photography and consumer goods where overall cost is a major concern. Both types of sensor accomplish the same task of capturing light and converting it into electrical signals.

Each cell of aCCDimage sensor is an analog device. When light strikes the chip it is held as a small electrical charge in eachphoto sensor.The charges in the line of pixels nearest to the (one or more) output amplifiers are amplified and output, then each line of pixels shifts its charges one line closer to the amplifiers, filling the empty line closest to the amplifiers. This process is then repeated until all the lines of pixels have had their charge amplified and output.[9]

A CMOS image sensor has an amplifier for each pixel compared to the few amplifiers of a CCD. This results in less area for the capture of photons than a CCD, but this problem has been overcome by using microlenses in front of each photodiode, which focus light into the photodiode that would have otherwise hit the amplifier and not been detected.[9]Some CMOS imaging sensors also useBack-side illuminationto increase the number of photons that hit the photodiode.[10]CMOS sensors can potentially be implemented with fewer components, use less power, and/or provide faster readout than CCD sensors.[11]They are also less vulnerable to static electricity discharges.

Another design, a hybrid CCD/CMOS architecture (sold under the name "sCMOS") consists of CMOS readout integrated circuits (ROICs) that are bump bonded to a CCD imaging substrate – a technology that was developed for infraredstaring arraysand has been adapted to silicon-based detector technology.[12]Another approach is to utilize the very fine dimensions available in modern CMOS technology to implement a CCD like structure entirely in CMOS technology: such structures can be achieved by separating individual poly-silicon gates by a very small gap; though still a product of research hybrid sensors can potentially harness the benefits of both CCD and CMOS imagers.[13]

Performance

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See also:EMVA1288

There are many parameters that can be used to evaluate the performance of an image sensor, includingdynamic range,signal-to-noise ratio,and low-light sensitivity. For sensors of comparable types, the signal-to-noise ratio and dynamic range improve as thesizeincreases. It is because in a given integration (exposure) time, more photons hit the pixel with larger area.

Exposure-time control

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Exposure timeof image sensors is generally controlled by either a conventional mechanicalshutter,as in film cameras, or by anelectronic shutter.Electronic shuttering can be "global," in which case the entire image sensor area's accumulation of photoelectrons starts and stops simultaneously, or "rolling" in which case the exposure interval of each row immediate precedes that row's readout, in a process that "rolls" across the image frame (typically from top to bottom in landscape format). Global electronic shuttering is less common, as it requires "storage" circuits to hold charge from the end of the exposure interval until the readout process gets there, typically a few milliseconds later.[14]

Color separation

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Bayer pattern on sensor
Foveon's scheme of vertical filtering for color sensing

There are several main types of color image sensors, differing by the type of color-separation mechanism:

  • Integral color sensors[15]use acolor filter arrayfabricated on top of a single monochrome CCD or CMOS image sensor. The most common color filter array pattern, theBayer pattern,uses a checkerboard arrangement of two green pixels for each red and blue pixel, although many other color filter patterns have been developed, including patterns using cyan, magenta, yellow, and white pixels.[16]Integral color sensors were initially manufactured by transferring colored dyes through photoresist windows onto a polymer receiving layer coated on top of a monochrome CCD sensor.[17]Since each pixel provides only a single color (such as green), the "missing" color values (such as red and blue) for the pixel are interpolated using neighboring pixels.[18]This processing is also referred to asdemosaicingor de-bayering.
  • Foveon X3 sensor,using an array of layered pixel sensors, separating light via the inherent wavelength-dependent absorption property of silicon, such that every location senses all three color channels. This method is similar to how color film for photography works.
  • 3CCD,using three discrete image sensors, with the color separation done by adichroic prism.The dichroic elements provide a sharper color separation, thus improving color quality. Because each sensor is equally sensitive within itspassband,and at full resolution, 3-CCD sensors produce better color quality and better low light performance. 3-CCD sensors produce a full4:4:4signal, which is preferred intelevision broadcasting,video editingandchroma keyvisual effects.

Specialty sensors

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Infrared view of theOrion Nebulataken byESO's HAWK-I, a cryogenic wide-field imager[19]

Special sensors are used in various applications such asthermography,creation ofmulti-spectral images,video laryngoscopes,gamma cameras,sensor arraysforx-rays,and other highly sensitive arrays forastronomy.[20]

While in general, digital cameras use a flat sensor, Sony prototyped a curved sensor in 2014 to reduce/eliminatePetzval field curvaturethat occurs with a flat sensor. Use of a curved sensor allows a shorter and smaller diameter of the lens with reduced elements and components with greater aperture and reduced light fall-off at the edge of the photo.[21]

History

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See also:Digital imaging

Early analog sensors for visible light werevideo camera tubes.They date back to the 1930s, and several types were developed up until the 1980s. By the early 1990s, they had been replaced by modernsolid-stateCCD image sensors.[22]

The basis for modern solid-state image sensors is MOS technology,[23][24]which originates from the invention of the MOSFET byMohamed M. AtallaandDawon KahngatBell Labsin 1959.[25]Later research on MOS technology led to the development of solid-statesemiconductorimage sensors, including thecharge-coupled device(CCD) and later theactive-pixel sensor(CMOSsensor).[23][24]

Thepassive-pixel sensor(PPS) was the precursor to the active-pixel sensor (APS).[7]A PPS consists of passive pixels which are read out withoutamplification,with each pixel consisting of a photodiode and aMOSFETswitch.[26]It is a type ofphotodiode array,with pixels containing ap-n junction,integratedcapacitor,and MOSFETs as selectiontransistors.A photodiode array was proposed by G. Weckler in 1968.[6]This was the basis for the PPS.[7]These early photodiode arrays were complex and impractical, requiring selection transistors to be fabricated within each pixel, along withon-chipmultiplexercircuits. Thenoiseof photodiode arrays was also a limitation to performance, as the photodiode readoutbuscapacitance resulted in increased noise level.Correlated double sampling(CDS) could also not be used with a photodiode array without externalmemory.[6]However, in 1914 Deputy Consul General Carl R. Loop, reported to the state department in a Consular Report onArchibald M. Low'sTelevista system that "It is stated that the selenium in the transmitting screen may be replaced by anydiamagnetic material".[27]

In June 2022, Samsung Electronics announced that it had created a 200 million pixel image sensor. The 200MP ISOCELL HP3 has 0.56 micrometer pixels with Samsung reporting that previous sensors had 0.64 micrometer pixels, a 12% decrease since 2019. The new sensor contains 200 million pixels in a 1-by-1.4-inch (25 by 36 mm) lens.[28]

Charge-coupled device

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Main article:Charge-coupled device

Thecharge-coupled device(CCD) was invented byWillard S. BoyleandGeorge E. Smithat Bell Labs in 1969.[29]While researching MOS technology, they realized that an electric charge was the analogy of the magnetic bubble and that it could be stored on a tinyMOS capacitor.As it was fairly straightforward tofabricatea series of MOS capacitors in a row, they connected a suitable voltage to them so that the charge could be stepped along from one to the next.[23]The CCD is a semiconductor circuit that was later used in the firstdigital video camerasfortelevision broadcasting.[30]

Early CCD sensors suffered fromshutter lag.This was largely resolved with the invention of thepinned photodiode(PPD).[7]It was invented byNobukazu Teranishi,Hiromitsu Shiraki and Yasuo Ishihara atNECin 1980.[7][31]It was aphotodetectorstructure with low lag, lownoise,highquantum efficiencyand lowdark current.[7]In 1987, the PPD began to be incorporated into most CCD devices, becoming a fixture inconsumer electronicvideo camerasand thendigital still cameras.Since then, the PPD has been used in nearly all CCD sensors and then CMOS sensors.[7]

Active-pixel sensor

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Main article:Active-pixel sensor

TheNMOSactive-pixel sensor(APS) was invented byOlympusin Japan during the mid-1980s. This was enabled by advances in MOSsemiconductor device fabrication,withMOSFET scalingreaching smallermicron and then sub-micronlevels.[6][32]The first NMOS APS was fabricated by Tsutomu Nakamura's team at Olympus in 1985.[33]TheCMOSactive-pixel sensor (CMOS sensor) was later improved by a group of scientists at theNASAJet Propulsion Laboratoryin 1993.[7]By 2007, sales of CMOS sensors had surpassed CCD sensors.[34]By the 2010s, CMOS sensors largely displaced CCD sensors in all new applications.

Other image sensors

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The first commercialdigital camera,theCromemco Cyclopsin 1975, used a 32×32 MOS image sensor. It was a modified MOS dynamicRAM(DRAM)memory chip.[35]

MOS image sensors are widely used inoptical mousetechnology. The first optical mouse, invented byRichard F. LyonatXeroxin 1980, used a5μmNMOSintegrated circuitsensor chip.[2][1]Since the first commercial optical mouse, theIntelliMouseintroduced in 1999, most optical mouse devices use CMOS sensors.[36]

In February 2018, researchers atDartmouth Collegeannounced a new image sensing technology that the researchers call QIS, for Quanta Image Sensor. Instead of pixels, QIS chips have what the researchers call "jots." Each jot can detect a single particle of light, called aphoton.[37]

See also

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References

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  1. ^abLyon, Richard F.(August 1981)."The Optical Mouse, and an Architectural Methodology for Smart Digital Sensors"(PDF).In H. T. Kung; Robert F. Sproull; Guy L. Steele (eds.).VLSI Systems and Computations.Computer Science Press. pp. 1–19.doi:10.1007/978-3-642-68402-9_1.ISBN978-3-642-68404-3.S2CID60722329.
  2. ^abLyon, Richard F.(2014)."The Optical Mouse: Early Biomimetic Embedded Vision".Advances in Embedded Computer Vision.Springer. pp. 3–22 (3).ISBN9783319093871.
  3. ^Brain, Marshall; Carmack, Carmen (24 April 2000)."How Computer Mice Work".HowStuffWorks.Retrieved9 October2019.
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  6. ^abcdFossum, Eric R.(12 July 1993). Blouke, Morley M. (ed.). "Active pixel sensors: are CCDs dinosaurs?".SPIE Proceedings Vol. 1900: Charge-Coupled Devices and Solid State Optical Sensors III.Charge-Coupled Devices and Solid State Optical Sensors III.1900.International Society for Optics and Photonics: 2–14.Bibcode:1993SPIE.1900....2F.CiteSeerX10.1.1.408.6558.doi:10.1117/12.148585.S2CID10556755.
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  9. ^ab"CCD and CMOS sensors".Canon Professional Network.Archivedfrom the original on 28 April 2018.Retrieved28 April2018.
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  11. ^Moynihan, Tom (29 December 2011)."CMOS Is Winning the Camera Sensor Battle, and Here's Why".Archivedfrom the original on 25 September 2015.Retrieved10 April2015.
  12. ^scmos.comArchived2012-06-03 at theWayback Machine,home page
  13. ^ieee.org - CCD in CMOSArchived2015-06-22 at theWayback MachinePadmakumar R. Rao et al., "CCD structures implemented in standard 0.18 μm CMOS technology"
  14. ^Nakamura, Junichi (2005).Image Sensors and Signal Processing for Digital Still Cameras.CRC Press. pp. 169–172.ISBN9781420026856.
  15. ^Dillon, Peter (Dec 1976)."Integral color filter arrays for solid state imagers".1976 International Electron Devices Meeting.Technical Digest International Electron Device Meeting (IEDM), Washington, DC, Dec 1976. pp. 400–403.doi:10.1109/IEDM.1976.189067.S2CID35103154– via IEEE.
  16. ^Parulski, Kenneth (August 1985)."Color Filters and Processing Alternatives for One-chip Cameras".IEEE Transactions on Electron Devices.32(8): 1381–1389.Bibcode:1985ITED...32.1381P.doi:10.1109/T-ED.1985.22133.S2CID9008653.
  17. ^Dillon, Peter (February 1978)."Fabrication and performance of color filter arrays for solid-state imagers".IEEE Transactions on Electron Devices.25(2): 97–101.Bibcode:1978ITED...25...97D.doi:10.1109/T-ED.1978.19045.
  18. ^Dillon, Peter (February 1978)."Color imaging system using a single CCD area array".IEEE Transactions on Electron Devices.25(2): 102–107.doi:10.1109/T-ED.1978.19046.
  19. ^"Deepest Ever Look into Orion".Archivedfrom the original on 13 July 2016.Retrieved13 July2016.
  20. ^Gitto, Simone (2020).Arduino with MATLAB in the thermography: From the sensor to the thermal camera (Arduino and Beyond).Independently published.ISBN979-8698999171.
  21. ^Dent, Steve (8 July 2014)."Sony's first 'curved sensor' photo may herald better images, cheaper lenses".Archivedfrom the original on July 11, 2014.RetrievedJuly 8,2014.
  22. ^Musburger, Robert B.; Ogden, Michael R. (2014).Single-Camera Video Production.CRC Press.p. 64.ISBN9781136778445.
  23. ^abcWilliams, J. B. (2017).The Electronics Revolution: Inventing the Future.Springer. pp. 245–8.ISBN9783319490885.
  24. ^abOhta, Jun (2017).Smart CMOS Image Sensors and Applications.CRC Press.p. 2.ISBN9781420019155.
  25. ^"1960: Metal Oxide Semiconductor (MOS) Transistor Demonstrated".The Silicon Engine.Computer History Museum.RetrievedAugust 31,2019.
  26. ^Kozlowski, L. J.; Luo, J.; Kleinhans, W. E.; Liu, T. (14 September 1998). Pain, Bedabrata; Lomheim, Terrence S. (eds.)."Comparison of passive and active pixel schemes for CMOS visible imagers".Infrared Readout Electronics IV.3360.International Society for Optics and Photonics: 101–110.Bibcode:1998SPIE.3360..101K.doi:10.1117/12.584474.S2CID123351913.
  27. ^Daily Consular and Trade Reports.Department of Commerce and Labor, Bureau of Manufactures. 1914.
  28. ^Web, Desk (2022-06-25)."Samsung Electronics releases a sensor with 200 million pixels".BOL News.Retrieved2022-06-25.
  29. ^Janesick, James R. (2001).Scientific charge-coupled devices.SPIE Press. pp. 3–4.ISBN978-0-8194-3698-6.
  30. ^Boyle, William S; Smith, George E. (1970). "Charge Coupled Semiconductor Devices".Bell Syst. Tech. J.49(4): 587–593.Bibcode:1970BSTJ...49..587B.doi:10.1002/j.1538-7305.1970.tb01790.x.
  31. ^U.S. Patent 4,484,210: Solid-state imaging device having a reduced image lag
  32. ^Fossum, Eric R.(2007)."Active Pixel Sensors"(PDF).Semantic Scholar.S2CID18831792.Archived fromthe original(PDF)on 9 March 2019.Retrieved8 October2019.
  33. ^Matsumoto, Kazuya; et al. (1985). "A new MOS phototransistor operating in a non-destructive readout mode".Japanese Journal of Applied Physics.24(5A): L323.Bibcode:1985JaJAP..24L.323M.doi:10.1143/JJAP.24.L323.S2CID108450116.
  34. ^"CMOS Image Sensor Sales Stay on Record-Breaking Pace".IC Insights.May 8, 2018.Retrieved6 October2019.
  35. ^Benchoff, Brian (17 April 2016)."Building the First Digital Camera".Hackaday.Retrieved30 April2016.the Cyclops was the first digital camera
  36. ^Brain, Marshall; Carmack, Carmen (24 April 2000)."How Computer Mice Work".HowStuffWorks.Retrieved9 October2019.
  37. ^"Super Sensitive Sensor Sees What You Can't".npr.org.Archivedfrom the original on 24 March 2018.Retrieved28 April2018.
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