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An image sensor or imager is a that detects and conveys information used to make an . It does so by converting the variable of light (as they or reflect off objects) into signals, small bursts of that convey the information. The waves can be light or other electromagnetic radiation. Image sensors are used in imaging devices of both analog and digital types, which include , , , devices,

(1981). 9783642684043, Computer Science Press.
(2021). 9783319093871, Springer.
equipment, equipment such as devices, , , and others. As technology changes, electronic and 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 ( sensor). Both CCD and CMOS sensors are based on metal–oxide–semiconductor (MOS) technology, with CCDs based on and CMOS sensors based on (MOS field-effect transistor) . Analog sensors for invisible radiation tend to involve of various kinds, while digital sensors include flat-panel detectors. Image sensors with built-in processing units for are known as smart image sensors or intelligent image sensors.

CCD vs. CMOS sensors
The two main types of sensors are the charge-coupled device (CCD) and the active-pixel sensor (CMOS sensor), fabricated in complementary MOS (CMOS) or N-type MOS ( or ) technologies. Both CCD and CMOS sensors are based on ,
(2021). 9781351830201, .
with being the building blocks of a CCD,
(2012). 9780470537947, John Wiley & Sons.
and amplifiers being the building blocks of a CMOS sensor.

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. 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 CCD image sensor is an analog device. When light strikes the chip it is held as a small electrical charge in each . 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.

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. Some CMOS imaging sensors also use Back-side illumination to increase the number of photons that hit the photodiode. CMOS sensors can potentially be implemented with fewer components, use less power, and/or provide faster readout than CCD sensors. They are also less vulnerable to static electricity discharges.

Another design, a hybrid CCD/CMOS architecture (sold under the name "") consists of CMOS readout integrated circuits (ROICs) that are bump bonded to a CCD imaging substrate – a technology that was developed for infrared and has been adapted to silicon-based detector technology. , home page 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. - CCD in CMOS Padmakumar R. Rao et al., "CCD structures implemented in standard 0.18 µm CMOS technology"

There are many parameters that can be used to evaluate the performance of an image sensor, including , signal-to-noise ratio, and low-light sensitivity. For sensors of comparable types, the signal-to-noise ratio and dynamic range improve as the size increases.

Exposure-time control
of image sensors is generally controlled by either a conventional mechanical 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.
(2021). 9781420026856, CRC Press. .

Color separation
There are several main types of color image sensors, differing by the type of color-separation mechanism:
  • , low-cost and most common, using a color filter array that passes red, green, and blue light to selected pixel sensors. Each individual sensor element is made sensitive to red, green, or blue by means of a made of chemical dyes patterned over the elements. The most common filter matrix, the , uses two green pixels for each red and blue. This results in less resolution for red and blue colors. The missing color samples may interpolated using a algorithm, or ignored altogether by lossy compression. In order to improve color information, techniques like color co-site sampling use a mechanism to shift the color sensor in pixel steps.
  • 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.
  • , using three discrete image sensors, with the color separation done by a . The dichroic elements provide a sharper color separation, thus improving color quality. Because each sensor is equally sensitive within its , and at full resolution, 3-CCD sensors produce better color quality and better low light performance. 3-CCD sensors produce a full signal, which is preferred in television broadcasting, and visual effects.

Specialty sensors
Special sensors are used in various applications such as , creation of multi-spectral images, , , sensor arrays for , and other highly sensitive arrays for .

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.

Early analog sensors for visible light were video 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 modern solid-state CCD image sensors.
(2021). 9781136778445, . .

The basis for modern solid-state image sensors is MOS technology,

(2021). 9783319490885, Springer. .
(2021). 9781420019155, . .
which originates from the invention of the MOSFET by Mohamed M. Atalla and at in 1959. Later research on MOS technology led to the development of solid-state image sensors, including the charge-coupled device (CCD) and later the active-pixel sensor ( sensor).

The passive-pixel sensor (PPS) was the precursor to the active-pixel sensor (APS). A PPS consists of passive pixels which are read out without , with each pixel consisting of a photodiode and a switch. It is a type of , with pixels containing a , integrated , and MOSFETs as selection . A photodiode array was proposed by G. Weckler in 1968. This was the basis for the PPS. These early photodiode arrays were complex and impractical, requiring selection transistors to be fabricated within each pixel, along with on-chip circuits. The noise of photodiode arrays was also a limitation to performance, as the photodiode readout capacitance resulted in increased noise level. Correlated double sampling (CDS) could also not be used with a photodiode array without external .

Charge-coupled device
The charge-coupled device (CCD) was invented by Willard S. Boyle and George E. Smith at Bell Labs in 1969.
(2021). 9780819436986, SPIE Press. .
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 tiny . As it was fairly straightforward to fabricate 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. The CCD is a semiconductor circuit that was later used in the first digital video cameras for television broadcasting.

Early CCD sensors suffered from . This was largely resolved with the invention of the pinned photodiode (PPD). It was invented by Nobukazu Teranishi, Hiromitsu Shiraki and Yasuo Ishihara at in 1980. It was a structure with low lag, low noise, high quantum efficiency and low dark current. In 1987, the PPD began to be incorporated into most CCD devices, becoming a fixture in consumer electronic and then digital still cameras. Since then, the PPD has been used in nearly all CCD sensors and then CMOS sensors.

Active-pixel sensor
The active-pixel sensor (APS) was invented by Olympus in Japan during the mid-1980s. This was enabled by advances in MOS semiconductor device fabrication, with reaching smaller micron and then sub-micron levels. The first NMOS APS was fabricated by Tsutomu Nakamura's team at Olympus in 1985. The active-pixel sensor (CMOS sensor) was later developed by 's team at the Jet Propulsion Laboratory in 1993. By 2007, sales of CMOS sensors had surpassed CCD sensors. By the 2010s, CMOS sensors largely displaced CCD sensors in all new applications.

Other image sensors
The first commercial , the in 1975, used a 32×32 MOS image sensor. It was a modified MOS dynamic RAM (DRAM) .

MOS image sensors are widely used in technology. The first optical mouse, invented by Richard F. Lyon at in 1980, used a 5µm integrated circuit sensor chip.

(2021). 9783319093871, Springer.
(1981). 9783642684043, Computer Science Press.
Since the first commercial optical mouse, the introduced in 1999, most optical mouse devices use CMOS sensors.

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 .

See also
  • List of sensors used in digital cameras
  • Contact image sensor (CIS)
  • Electro-optical sensor
  • Video camera tube
  • Semiconductor detector
  • Fill factor
  • Full-frame digital SLR
  • Image sensor format, the sizes and shapes of common image sensors
  • Color filter array, mosaic of tiny color filters over color image sensors
  • , the scientific study of light-sensitive materials
  • History of television, the development of electronic imaging technology since the 1880s
  • List of large sensor interchangeable-lens video cameras
  • Oversampled binary image sensor
  • Push broom scanner
  • Whisk broom scanner

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