Image sensor: Difference between revisions

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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 |title=CCD and CMOS sensors |url=https://www.canon-europe.com/pro/infobank/image-sensors-explained/ |url-status=live |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.

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[[File:Absorption-X3.svg|thumb|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 conference <!-- 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 |conference=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|url-access=subscription }}</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|url-access=subscription }}</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 |url=https://ieeexplore.ieee.org/document/1479440|url-access=subscription }}</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.