Animage sensor orimager is a device that detects and conveys information used to form an image. It does so by converting the variableattenuation of lightwaves (as theypass through orreflect off objects) intosignals, small bursts ofcurrent that convey the information. The waves can be light or otherelectromagnetic radiation. Image sensors are used inelectronic imaging devices of bothanalog anddigital types, which includedigital cameras,camera modules,camera phones,optical mouse devices,medical imaging equipment,night vision equipment such asthermal imaging devices,radar,sonar, and others. Astechnology changes, electronic anddigital imaging tends to replace chemical and analog imaging.
The two main types of electronic image sensors are thecharge-coupled device (CCD) and theactive-pixel sensor (CMOS sensor). Both CCD and CMOS sensors are based onmetal–oxide–semiconductor (MOS) technology, with CCDs based onMOS capacitors and CMOS sensors based onMOSFET (MOS field-effect transistor)amplifiers. Analog sensors for invisible radiation tend to involvevacuum tubes of various kinds, while digital sensors includeflat-panel detectors.
The two main types ofdigital image sensors are thecharge-coupled device (CCD) and theactive-pixel sensor (CMOS sensor),fabricated incomplementary MOS (CMOS) orN-type MOS (NMOS orLive MOS) technologies. Both CCD and CMOS sensors are based on theMOS technology,[1] withMOS capacitors being the building blocks of a CCD,[2] andMOSFET amplifiers being the building blocks of a CMOS sensor.[3][4]
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.[5] 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 aCCD image 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.[6]
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.[6] Some CMOS imaging sensors also useback-side illumination to increase the number of photons that hit the photodiode.[7] CMOS sensors can potentially be implemented with fewer components, use less power, and/or provide faster readout than CCD sensors.[8] 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 arrays and has been adapted to silicon-based detector technology.[9] 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.[10]
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 thesize increases. It is because in a given integration (exposure) time, more photons hit the pixel with larger area.
Exposure time of 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 immediately 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.[11]
There are several main types of color image sensors, differing by the type of color-separation mechanism:
Special sensors are used in various applications such as creation ofmulti-spectral images,video laryngoscopes,gamma cameras,Flat-panel detectors and othersensor arrays forx-rays,microbolometer arrays inthermography, and other highly sensitive arrays forastronomy.[17]
While in general, digital cameras use a flat sensor, Sony prototyped a curved sensor in 2014 to reduce/eliminatePetzval 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.[18]
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-state CCD image sensors.[19]
The basis for modern solid-state image sensors is MOS technology,[20][21] which originates from the invention of the MOSFET byMohamed M. Atalla andDawon Kahng atBell Labs in 1959.[22] Later research on MOS technology led to the development of solid-statesemiconductor image sensors, including thecharge-coupled device (CCD) and later theactive-pixel sensor (CMOS sensor).[20][21]
Thepassive-pixel sensor (PPS) was the precursor to the active-pixel sensor (APS).[4] A PPS consists of passive pixels which are read out withoutamplification, with each pixel consisting of a photodiode and aMOSFET switch.[23] 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.[3] This was the basis for the PPS.[4] These early photodiode arrays were complex and impractical, requiring selection transistors to be fabricated within each pixel, along withon-chipmultiplexer circuits. Thenoise of photodiode arrays was also a limitation to performance, as the photodiode readoutbus capacitance resulted in increased noise level.Correlated double sampling (CDS) could also not be used with a photodiode array without externalmemory.[3] However, in 1914 Deputy Consul General Carl R. Loop, reported to the state department in a Consular Report onArchibald M. Low's Televista system that "It is stated that the selenium in the transmitting screen may be replaced by anydiamagnetic material".[24]
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.[25]
Thecharge-coupled device (CCD) was invented byWillard S. Boyle andGeorge E. Smith at Bell Labs in 1969.[26] 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 tofabricate a 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.[20] The CCD is a semiconductor circuit that was later used in the firstdigital video cameras fortelevision broadcasting.[27]
Early CCD sensors suffered fromshutter lag. This was largely resolved with the invention of thepinned photodiode (PPD).[4] It was invented byNobukazu Teranishi, Hiromitsu Shiraki and Yasuo Ishihara atNEC in 1980.[4][28] It was aphotodetector structure with low lag, lownoise, highquantum efficiency and lowdark current.[4] In 1987, the PPD began to be incorporated into most CCD devices, becoming a fixture inconsumer electronicvideo cameras and thendigital still cameras. Since then, the PPD has been used in nearly all CCD sensors and then CMOS sensors.[4]
TheNMOSactive-pixel sensor (APS) was invented byOlympus in Japan during the mid-1980s. This was enabled by advances in MOSsemiconductor device fabrication, withMOSFET scaling reaching smallermicron and then sub-micron levels.[3][29] The first NMOS APS was fabricated by Tsutomu Nakamura's team at Olympus in 1985.[30] TheCMOS active-pixel sensor (CMOS sensor) was later improved by a group of scientists at theNASAJet Propulsion Laboratory in 1993.[4] By 2007, sales of CMOS sensors had surpassed CCD sensors.[31] By the 2010s, CMOS sensors largely displaced CCD sensors in all new applications.
The first commercialdigital camera, theCromemco Cyclops in 1975, used a 32×32 MOS image sensor. It was a modified MOS dynamicRAM (DRAM)memory chip.[32]
MOS image sensors are widely used inoptical mouse technology. The first optical mouse, invented byRichard F. Lyon atXerox in 1980, used a5 μmNMOSintegrated circuit sensor chip.[33][34] Since the first commercial optical mouse, theIntelliMouse introduced in 1999, most optical mouse devices use CMOS sensors.[35]
In February 2018, researchers atDartmouth 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 aphoton.[36]
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