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{{Short description|Device that converts images into electronic signals}}

An '''image sensor''' or '''imager''' is a [[sensor]]device that detects and conveys information used to form an [[image]]. It does so by converting the variable [[attenuation]] of light [[wave]]s (as they [[refraction|pass through]] or [[reflection (physics)|reflect off]] objects) into [[signal (electrical engineering)|signals]], small bursts of [[electric current|current]] that convey the information. The waves can be light or other [[electromagnetic radiation]]. Image sensors are used in [[electronics|electronic]] imaging devices of both [[analogue electronics|analog]] and [[digital electronics|digital]] types, which include [[digital camera]]s, [[camera module]]s, [[camera phones]], [[optical mouse]] devices,<ref name="VLSI Systems and Computations">{{cite book | chapter = The Optical Mouse, and an Architectural Methodology for Smart Digital Sensors | title = VLSI Systems and Computations | pages = 1–19 | last1=Lyon | first1=Richard F. | author1-link=Richard F. Lyon |editor1=H. T. Kung |editor2=Robert F. Sproull |editor3=Guy L. Steele | publisher=Computer Science Press |date=August 1981 | doi=10.1007/978-3-642-68402-9_1 | chapter-url=http://bitsavers.trailing-edge.com/pdf/xerox/parc/techReports/VLSI-81-1_The_Optical_Mouse.pdf| isbn = 978-3-642-68404-3 | s2cid = 60722329 }}</ref><ref name="Springer">{{cite book |last1=Lyon |first1=Richard F. |author1-link=Richard F. Lyon |chapter=The Optical Mouse: Early Biomimetic Embedded Vision |title=Advances in Embedded Computer Vision |date=2014 |publisher=Springer |isbn=9783319093871 |pages=3–22 (3) |chapter-url=https://books.google.com/books?id=p_GbBQAAQBAJ&pg=PA3}}</ref><ref>{{cite web |last1=Brain |first1=Marshall |last2=Carmack |first2=Carmen |title=How Computer Mice Work |url=https://computer.howstuffworks.com/mouse4.htm |website=[[HowStuffWorks]] |access-date=9 October 2019 |date=24 April 2000}}</ref> [[medical imaging]] equipment, [[night vision]] equipment such as [[thermography|thermal imaging]] devices, [[radar]], [[sonar]], and others. As [[technological change|technology changes]], electronic and [[digital imaging]] tends to replace chemical and analog imaging.

[[File:Ccd-sensor.jpg|A [[Charge-coupled device|CCD]] image sensor on a [[Flexible electronics|flexible circuit board]]|thumb|230px]]

[[File:Cromemco_Cyclops_Camera_Board_1.jpg|An American Microsystems, Inc., (AMI) 1-kilobit [[Dynamic random-access memory|DRAM]] chip (center chip with glass window) used as an image sensor by the [[Cromemco Cyclops]]|thumb|230px]]

An '''image sensor''' or '''imager''' is a [[sensor]] that detects and conveys information used to form an [[image]]. It does so by converting the variable [[attenuation]] of light [[wave]]s (as they [[refraction|pass through]] or [[reflection (physics)|reflect off]] objects) into [[signal (electrical engineering)|signals]], small bursts of [[electric current|current]] that convey the information. The waves can be light or other [[electromagnetic radiation]]. Image sensors are used in [[electronics|electronic]] imaging devices of both [[analogue electronics|analog]] and [[digital electronics|digital]] types, which include [[digital camera]]s, [[camera module]]s, [[camera phones]], [[optical mouse]] devices,<ref name="VLSI Systems and Computations">{{cite book | chapter = The Optical Mouse, and an Architectural Methodology for Smart Digital Sensors | title = VLSI Systems and Computations | pages = 1–19 | last1=Lyon | first1=Richard F. | author1-link=Richard F. Lyon |editor1=H. T. Kung |editor2=Robert F. Sproull |editor3=Guy L. Steele | publisher=Computer Science Press |date=August 1981 | doi=10.1007/978-3-642-68402-9_1 | chapter-url=http://bitsavers.trailing-edge.com/pdf/xerox/parc/techReports/VLSI-81-1_The_Optical_Mouse.pdf| isbn = 978-3-642-68404-3 | s2cid = 60722329 }}</ref><ref name="Springer">{{cite book |last1=Lyon |first1=Richard F. |author1-link=Richard F. Lyon |chapter=The Optical Mouse: Early Biomimetic Embedded Vision |title=Advances in Embedded Computer Vision |date=2014 |publisher=Springer |isbn=9783319093871 |pages=3–22 (3) |chapter-url=https://books.google.com/books?id=p_GbBQAAQBAJ&pg=PA3}}</ref><ref>{{cite web |last1=Brain |first1=Marshall |last2=Carmack |first2=Carmen |title=How Computer Mice Work |url=https://computer.howstuffworks.com/mouse4.htm |website=[[HowStuffWorks]] |access-date=9 October 2019 |date=24 April 2000}}</ref> [[medical imaging]] equipment, [[night vision]] equipment such as [[thermography|thermal imaging]] devices, [[radar]], [[sonar]], and others. As [[technological change|technology changes]], electronic and [[digital imaging]] tends to replace chemical and analog imaging.

The two main types of electronic image sensors are the [[charge-coupled device]] (CCD) and the [[active-pixel sensor]] ([[CMOS]] sensor). Both CCD and CMOS sensors are based on [[metal–oxide–semiconductor]] (MOS) technology, with CCDs based on [[MOS capacitor]]s and CMOS sensors based on [[MOSFET]] (MOS field-effect transistor) [[amplifiers]]. Analog sensors for invisible radiation tend to involve [[vacuum tube]]s of various kinds, while digital sensors include [[flat-panel detector]]s.

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[[File:Image sensor and motherbord nikon coolpix l2.JPG|thumb|Image sensor (upper left) on the motherboard of a {{nowrap|Nikon Coolpix L2 6 MP}}]]

The two main types of [[digital image]] sensors are the [[charge-coupled device]] (CCD) and the [[active-pixel sensor]] (CMOS sensor), [[semiconductor device fabrication|fabricated]] in [[complementary MOS]] (CMOS) or [[N-type semiconductor|N-type]] MOS ([[NMOS logic|NMOS]] or [[Live MOS]]) technologies. Both CCD and CMOS sensors are based on the [[MOS technology]],<ref>{{cite book |last1=Cressler |first1=John D. |title=Silicon Earth: Introduction to Microelectronics and Nanotechnology, Second Edition |date=2017 |publisher=[[CRC Press]] |isbn=978-1-351-83020-1 |chapter=Let There Be Light: The Bright World of Photonics |page=29 |chapter-url=https://books.google.com/books?id=i-5HDwAAQBAJ&pg=SA12-PA29}}</ref> with [[MOS capacitor]]s being the building blocks of a CCD,<ref>{{cite book |last1=Sze |first1=Simon Min |author1-link=Simon Sze |last2=Lee |first2=Ming-Kwei |chapter=MOS Capacitor and MOSFET |title=Semiconductor Devices: Physics and Technology : International Student Version |date=May 2012 |publisher=[[John Wiley & Sons]] |isbn=9780470537947 |chapter-url=https://www.oreilly.com/library/view/semiconductor-devices-physics/9780470537947/13_chap05.html |access-date=6 October 2019}}</ref> and [[MOSFET]] amplifiers being the building blocks of a CMOS sensor.<ref name="fossum93">{{cite journalbook |last1=Fossum |first1=Eric R. |author1-link=Eric Fossum |titlechapter=Active pixel sensors: areAre CCDsCCDS dinosaurs? |journalauthor1-link=SPIEEric Proceedings Vol. 1900: Charge-Coupled Devices and Solid State Optical Sensors IIIFossum |seriestitle=Charge-Coupled Devices and Solid State Optical Sensors III |volume=1900 |date=12 July 1993 |doi=10.1117/12.148585 |bibcode=1993SPIE.1900....2F |publisher=International Society for Optics and Photonics |pages=2–14 |editor1-last=Blouke |editor1-first=Morley M.|citeseerx=10.1.1.408.6558 |s2cid=10556755 }}</ref><ref name="Fossum2014">{{cite journal |last1=Fossum |first1=Eric R. |author1-link=Eric Fossum |last2=Hondongwa |first2=D. B. |title=A Review of the Pinned Photodiode for CCD and CMOS Image Sensors |journal=IEEE Journal of the Electron Devices Society |date=2014 |volume=2 |issue=3 |pages=33–43 |doi=10.1109/JEDS.2014.2306412 |bibcode=2014IJEDS...2...33F |doi-access=free }}</ref>

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.<ref>{{cite web |url=http://www.techhive.com/article/246931/cmos_is_winning_the_camera_sensor_battle_and_heres_why.html |title=CMOS Is Winning the Camera Sensor Battle, and Here's Why |website=techhive.com |date=2011-12-29 |access-date=2017-04-27 |url-status=live |archive-url=https://web.archive.org/web/20170501024004/http://www.techhive.com/article/246931/cmos_is_winning_the_camera_sensor_battle_and_heres_why.html |archive-date=2017-05-01 }}</ref> 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 a [[Charge-coupled device|CCD]] image sensor is an analog device. When light strikes the chip it is held as a small electrical charge in each [[photo 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.<ref name="auto">{{cite web|url=http://cpn.canon-europe.com/content/education/infobank/capturing_the_image/ccd_and_cmos_sensors.do |title=CCD and CMOS sensors|website=Canon Professional Network|access-date=28 April 2018|url-status=live|archive-url=https://web.archive.org/web/20180428122601/http://cpnwww.canon-europe.com/content/educationpro/infobank/capturing_the_imageimage-sensors-explained/ccd_and_cmos_sensors.do |archive-url= |archive-date= |access-date=28 April 2018 |website=Canon Professional Network}}</ref>

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.<ref name="auto"/> Some CMOS imaging sensors also use [[back-illuminated sensor|Back-side illumination]] to increase the number of photons that hit the photodiode.<ref>{{cite web |url=http://www.techradar.com/news/photography-video-capture/cameras/what-is-a-backlit-cmos-sensor-1086234 |title=What is a backlit CMOS sensor? |website=techradar.com |date=2012-07-02 |access-date=2017-04-27 |url-status=live |archive-url=https://web.archive.org/web/20170506184555/http://www.techradar.com/news/photography-video-capture/cameras/what-is-a-backlit-cmos-sensor-1086234 |archive-date=2017-05-06 }}</ref> CMOS sensors can potentially be implemented with fewer components, use less power, and/or provide faster readout than CCD sensors.<ref>{{cite web|last1=Moynihan|first1=Tom|title=CMOS Is Winning the Camera Sensor Battle, and Here's Why|work=TechHive |date=29 December 2011|url=http://www.techhive.com/article/246931/cmos_is_winning_the_camera_sensor_battle_and_heres_why.html?page=0|access-date=10 April 2015|url-status=live|archive-url=https://web.archive.org/web/20150925220239/http://www.techhive.com/article/246931/cmos_is_winning_the_camera_sensor_battle_and_heres_why.html?page=0|archive-date=25 September 2015}}</ref> 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 infrared [[staring array]]s and has been adapted to silicon-based detector technology.<ref name="test3">[http://www.scmos.com/ scmos.com] {{webarchive|url=https://web.archive.org/web/20120603082322/http://www.scmos.com/ |date=2012-06-03 }}, home page</ref> 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.<ref name="test4">[http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4489895 ieee.org - CCD in CMOS] {{webarchive|url=https://web.archive.org/web/20150622073305/http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4489895 |date=2015-06-22 }} Padmakumar R. Rao et al., "CCD structures implemented in standard 0.18 µmμm CMOS technology"</ref>

==Performance==

{{see also|EMVA1288}}

<!--[[File:Removed EOS 350D IR-blocking filter.jpg|thumb|An infrared-blocking filter removed from a [[Canon EOS 350D]] [[Digital single-lens reflex camera|DSLR]]]]

anti-infrared filter has little to to with a sensor -->

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

==Exposure-time control==

[[Exposure time]] of image sensors is generally controlled by either a conventional mechanical [[shutter (photography)|shutter]], as in film cameras, or by an [[electronic 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.<ref>{{cite book |last1=Nakamura |first1=Junichi |title=Image Sensors and Signal Processing for Digital Still Cameras |date=2005 |publisher=CRC Press |isbn=9781420026856 |pages=169–172 |url=https://books.google.com/books?id=UY6QzgzgieYC&pg=PA170}}</ref>

[[Exposure time]] of image sensors is generally controlled by either a conventional mechanical [[shutter (photography)|shutter]], as in film cameras, or by an [[electronic 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.<ref>{{cite book |last1=Nakamura |first1=Junichi |title=Image Sensors and Signal Processing for Digital Still Cameras |date=2005 |publisher=CRC Press |isbn=9781420026856 |pages=169–172 |url=https://books.google.com/books?id=UY6QzgzgieYC&pg=PA170}}</ref>

==Color separation==

[[ImageFile:Bayer pattern on sensor profile.svg|thumb|right|200px|Bayer pattern on sensor]]

[[File:Absorption-X3.svg|right|thumb|200px|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'''<ref>{{Cite bookconference <!-- Citation bot no -->|last=Dillon |first=Peter |title=1976 International Electron Devices Meeting |chapter=Integral color filter arrays for solid state imagers |date=Dec 1976 |chapter-url=https://ieeexplore.ieee.org/document/1478779 |journalconference=Technical Digest International Electron Device Meeting (IEDM), Washington, DC, Dec 1976 |pages=400–403 |doi=10.1109/IEDM.1976.189067 |s2cid=35103154 |via=IEEE}}</ref> use a [[color filter array]] fabricated on top of a single monochrome CCD or CMOS image sensor. The most common color filter array pattern, the [[Bayer 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.<ref>{{Cite journal |last=Parulski |first=Kenneth |date=August 1985 |title=Color Filters and Processing Alternatives for One-chip Cameras |journal=IEEE Transactions on Electron Devices |volume=32 |issue=8 |pages=1381–1389 |doi=10.1109/T-ED.1985.22133 |bibcode=1985ITED...32.1381P |s2cid=9008653 |url=https://ieeexplore.ieee.org/document/1484879}}</ref> 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.<ref>{{Cite journal |last=Dillon |first=Peter |date=February 1978 |title=Fabrication and performance of color filter arrays for solid-state imagers |journal=IEEE Transactions on Electron Devices |volume=25 |issue=2 |pages=97–101 |doi=10.1109/T-ED.1978.19045 |bibcode=1978ITED...25...97D |url=https://ieeexplore.ieee.org/document/1479439}}</ref> 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.<ref>{{Cite journal |last=Dillon |first=Peter |date=February 1978 |title=Color imaging system using a single CCD area array |journal=IEEE Transactions on Electron Devices |volume=25 |issue=2 |pages=102–107 |doi=10.1109/T-ED.1978.19046 |urlbibcode=https://ieeexplore1978ITED..ieee.org/document/147944025..102D }}</ref> This processing is also referred to as [[demosaicing]] or 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.

* '''[[Three-CCD camera|3CCD]]''', using three discrete image sensors, with the color separation done by a [[dichroic prism]]. The dichroic elements provide a sharper color separation, thus improving color quality. Because each sensor is equally sensitive within its [[passband]], and at full resolution, 3-CCD sensors produce better color quality and better low light performance. 3-CCD sensors produce a full [[Chroma subsampling|4:4:4]] signal, which is preferred in [[television broadcasting]], [[video editing]] and [[chroma key]] visual effects.

==Specialty sensors==

[[File:A deep infrared view of the Orion Nebula from HAWK-I - Eso1625a.jpg|right|thumb|Infrared view of the [[Orion Nebula]] taken by [[ESO]]'s HAWK-I, a cryogenic wide-field imager<ref>{{cite web|title=Deepest Ever Look into Orion|url=http://www.eso.org/public/news/eso1625/|access-date=13 July 2016|url-status=live|archive-url=https://web.archive.org/web/20160713170150/http://www.eso.org/public/news/eso1625/|archive-date=13 July 2016}}</ref>]]

Special sensors are used in various applications such as [[thermography]], creation of [[multi-spectral image]]s, [[Laryngoscopy|video laryngoscopes]], [[gamma camera]]s, [[Flat-panel detector]]s and other [[sensor array]]s for [[x-ray]]s, [[microbolometer]] arrays in [[thermography]], and other highly sensitive arrays for [[astronomy]].<ref>{{Cite book |last=Gitto |first=Simone |title=Arduino with MATLAB in the thermography: From the sensor to the thermal camera (Arduino and Beyond) |publisher=Independently published |year=2020 |isbn=979-8698999171 |language=en}}</ref>

While in general, digital cameras use a flat sensor, Sony prototyped a curved sensor in 2014 to reduce/eliminate [[Petzval field curvature]] that 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.<ref>{{cite web |url=https://www.engadget.com/2014/07/08/sony-shows-off-first-picture-taken-with-curved-sensor/ |title=Sony's first 'curved sensor' photo may herald better images, cheaper lenses |first=Steve |last=Dent |date=8 July 2014 |access-date=July 8, 2014 |url-status=live |archive-url=https://web.archive.org/web/20140711224002/http://www.engadget.com/2014/07/08/sony-shows-off-first-picture-taken-with-curved-sensor/ |archive-date=July 11, 2014 }}</ref>

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The first commercial [[digital camera]], the [[Cromemco Cyclops]] in 1975, used a 32×32 MOS image sensor. It was a modified MOS dynamic [[Random-access memory|RAM]] ([[Dynamic random-access memory|DRAM]]) [[memory chip]].<ref name="hackaday">{{cite web |last1=Benchoff|first1=Brian|title=Building the First Digital Camera|url=http://hackaday.com/2016/04/17/building-the-first-digital-camera/|website=[[Hackaday]]|access-date=30 April 2016|date=17 April 2016|quote=the Cyclops was the first digital camera}}</ref>

MOS image sensors are widely used in [[optical mouse]] technology. The first optical mouse, invented by [[Richard F. Lyon]] at [[Xerox]] in 1980, used a [[6 µmμm process|5{{nbsp}}µmμm]] [[NMOS logic|NMOS]] [[integrated circuit]] sensor chip.<ref name="Springer">{{cite book |last1=Lyon |first1=Richard F. |author1-link=Richard F. Lyon |title=Advances in Embedded Computer Vision |date=2014 |publisher=Springer |isbn=9783319093871 |pages=3–22 (3) |chapter=The Optical Mouse: Early Biomimetic Embedded Vision |chapter-url=https://books.google.com/books?id=p_GbBQAAQBAJ&pg=PA3}}</ref><ref name="VLSI Systems and Computations">{{cite book |last1=Lyon |first1=Richard F. |author1-link=Richard F. Lyon |title=VLSI Systems and Computations |date=August 1981 |publisher=Computer Science Press |isbn=978-3-642-68404-3 |editor1=H. T. Kung |pages=1–19 |chapter=The Optical Mouse, and an Architectural Methodology for Smart Digital Sensors |doi=10.1007/978-3-642-68402-9_1 |editor2=Robert F. Sproull |editor3=Guy L. Steele |chapter-url=http://bitsavers.trailing-edge.com/pdf/xerox/parc/techReports/VLSI-81-1_The_Optical_Mouse.pdf |s2cid=60722329}}</ref> Since the first commercial optical mouse, the [[IntelliMouse]] introduced in 1999, most optical mouse devices use CMOS sensors.<ref>{{cite web |last1=Brain |first1=Marshall |last2=Carmack |first2=Carmen |title=How Computer Mice Work |url=https://computer.howstuffworks.com/mouse4.htm |website=[[HowStuffWorks]] |access-date=9 October 2019 |language=en |date=24 April 2000}}</ref>

In February 2018, researchers at [[Dartmouth College]] announced 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 a [[photon]].<ref>{{cite news|url=https://www.npr.org/sections/alltechconsidered/2018/02/13/585149644/super-sensitive-sensor-sees-what-you-cant|title=Super Sensitive Sensor Sees What You Can't|website=npr.org|access-date=28 April 2018|url-status=live|archive-url=https://web.archive.org/web/20180324010947/https://www.npr.org/sections/alltechconsidered/2018/02/13/585149644/super-sensitive-sensor-sees-what-you-cant|archive-date=24 March 2018}}</ref>