Editing Charge-coupled device (section)

==Architecture==
[[Image:ArgusCCD.jpg|thumb|right|CCD from a 2.1-[[megapixel]] [[Argus (camera company)|Argus]] digital camera]][[Image:CCD line sensor.JPG|thumb|right|One-dimensional CCD image sensor from a [[fax machine]]]]The CCD image sensors can be implemented in several different architectures. The most common are full-frame, frame-transfer, and interline. The distinguishing characteristic of each of these architectures is their approach to the problem of shuttering.

In a full-frame device, all of the image area is active, and there is no electronic shutter. A mechanical shutter must be added to this type of sensor or the image smears as the device is clocked or read out.

With a frame-transfer CCD, half of the silicon area is covered by an opaque mask (typically aluminum). The image can be quickly transferred from the image area to the opaque area or storage region with acceptable smear of a few percent. That image can then be read out slowly from the storage region while a new image is integrating or exposing in the active area. Frame-transfer devices typically do not require a mechanical shutter and were a common architecture for early solid-state broadcast cameras. The downside to the frame-transfer architecture is that it requires twice the silicon real estate of an equivalent full-frame device; hence, it costs roughly twice as much.

The interline architecture extends this concept one step further and masks every other column of the image sensor for storage. In this device, only one pixel shift has to occur to transfer from image area to storage area; thus, shutter times can be less than a microsecond and smear is essentially eliminated. The advantage is not free, however, as the imaging area is now covered by opaque strips dropping the [[fill factor (image sensor)|fill factor]] to approximately 50 percent and the effective [[quantum efficiency]] by an equivalent amount. Modern designs have addressed this deleterious characteristic by adding microlenses on the surface of the device to direct light away from the opaque regions and on the active area. Microlenses can bring the fill factor back up to 90 percent or more depending on pixel size and the overall system's optical design.

The choice of architecture comes down to one of utility. If the application cannot tolerate an expensive, failure-prone, power-intensive mechanical shutter, an interline device is the right choice. Consumer snap-shot cameras have used interline devices. On the other hand, for those applications that require the best possible light collection and issues of money, power and time are less important, the full-frame device is the right choice. Astronomers tend to prefer full-frame devices. The frame-transfer falls in between and was a common choice before the fill-factor issue of interline devices was addressed. Today, frame-transfer is usually chosen when an interline architecture is not available, such as in a back-illuminated device.

CCDs containing grids of [[pixel]]s are used in [[digital camera]]s, [[image scanner|optical scanners]], and video cameras as light-sensing devices. They commonly respond to 70 percent of the [[Ray (optics)#incident ray|incident]] light (meaning a quantum efficiency of about 70 percent) making them far more efficient than [[photographic film]], which captures only about 2 percent of the incident light.

Most common types of CCDs are sensitive to near-infrared light, which allows [[infrared photography]], [[night-vision]] devices, and zero [[lux]] (or near zero lux) video-recording/photography. For normal silicon-based detectors, the sensitivity is limited to 1.1&nbsp;μm. One other consequence of their sensitivity to infrared is that infrared from [[remote control]]s often appears on CCD-based digital cameras or camcorders if they do not have infrared blockers.

Cooling reduces the array's [[dark current (physics)|dark current]], improving the sensitivity of the CCD to low light intensities, even for ultraviolet and visible wavelengths. Professional observatories often cool their detectors with [[liquid nitrogen]] to reduce the dark current, and therefore the [[thermal noise]], to negligible levels.

===Frame transfer CCD===
[[File:IECCD55-20.jpg|thumb|top|A frame transfer CCD sensor]]

The frame transfer CCD imager was the first imaging structure proposed for CCD Imaging by Michael Tompsett at Bell Laboratories. A '''frame transfer CCD''' is a specialized CCD, often used in [[astronomy]] and some [[professional video camera]]s, designed for high exposure efficiency and correctness.

The normal functioning of a CCD, astronomical or otherwise, can be divided into two phases: exposure and readout. During the first phase, the CCD passively collects incoming [[photon]]s, storing [[electron]]s in its cells. After the exposure time is passed, the cells are read out one line at a time. During the readout phase, cells are shifted down the entire area of the CCD. While they are shifted, they continue to collect light. Thus, if the shifting is not fast enough, errors can result from light that falls on a cell holding charge during the transfer. These errors are referred to as [[rolling shutter effect]], making fast moving objects appear distorted. In addition, the CCD cannot be used to collect light while it is being read out. A faster shifting requires a faster readout, and a faster readout can introduce errors in the cell charge measurement, leading to a higher noise level.

[[File:Propellor with rolling-shutter artifact.jpg|thumb|A [[de Havilland Canada Dash 8]] Q-400 six-blade propeller, with severe rolling shutter distortion from a [[Pixel 3]] camera]]

A frame transfer CCD solves both problems: it has a shielded, not light sensitive, area containing as many cells as the area exposed to light. Typically, this area is covered by a reflective material such as aluminium. When the exposure time is up, the cells are transferred very rapidly to the hidden area. Here, safe from any incoming light, cells can be read out at any speed one deems necessary to correctly measure the cells' charge. At the same time, the exposed part of the CCD is collecting light again, so no delay occurs between successive exposures.

The disadvantage of such a CCD is the higher cost: the cell area is basically doubled, and more complex control electronics are needed.

===Intensified charge-coupled device===
{{Main|Image intensifier}}
An intensified charge-coupled device (ICCD) is a CCD that is optically connected to an image intensifier that is mounted in front of the CCD.

An image intensifier includes three functional elements: a [[photocathode]], a [[micro-channel plate]] (MCP) and a [[phosphor]] screen. These three elements are mounted one close behind the other in the mentioned sequence. The photons which are coming from the light source fall onto the photocathode, thereby generating photoelectrons. The photoelectrons are accelerated towards the MCP by an electrical control voltage, applied between photocathode and MCP. The electrons are multiplied inside of the MCP and thereafter accelerated towards the phosphor screen. The phosphor screen finally converts the multiplied electrons back to photons which are guided to the CCD by a fiber optic or a lens.

An image intensifier inherently includes a [[shutter (photography)|shutter]] functionality: If the control voltage between the photocathode and the MCP is reversed, the emitted photoelectrons are not accelerated towards the MCP but return to the photocathode. Thus, no electrons are multiplied and emitted by the MCP, no electrons are going to the phosphor screen and no light is emitted from the image intensifier. In this case no light falls onto the CCD, which means that the shutter is closed. The process of reversing the control voltage at the photocathode is called ''gating'' and therefore ICCDs are also called gateable CCD cameras.

Besides the extremely high sensitivity of ICCD cameras, which enable single photon detection, the gateability is one of the major advantages of the ICCD over the [[#Electron-multiplying CCD|EMCCD]] cameras. The highest performing ICCD cameras enable shutter times as short as 200 [[picosecond]]s.

ICCD cameras are in general somewhat higher in price than EMCCD cameras because they need the expensive image intensifier. On the other hand, EMCCD cameras need a cooling system to cool the EMCCD chip down to temperatures around {{convert|170|K|°C|lk=on}}. This cooling system adds additional costs to the EMCCD camera and often yields heavy condensation problems in the application.

ICCDs are used in [[night vision devices]] and in various scientific applications.

=== Electron-multiplying CCD ===
[[Image:EMCCD2 color en.svg|thumb|Electrons are transferred serially through the gain stages making up the multiplication register of an [[#Electron-multiplying CCD|EMCCD]]. The high voltages used in these serial transfers induce the creation of additional charge carriers through impact ionisation.]]
[[Image:Output vs input electrons.png|thumb|in an [[#Electron-multiplying CCD|EMCCD]] there is a dispersion (variation) in the number of electrons output by the multiplication register for a given (fixed) number of input electrons (shown in the legend on the right). The probability distribution for the number of output electrons is plotted [[logarithm]]ically on the vertical axis for a simulation of a multiplication register. Also shown are results from the [[empiricism|empirical]] fit equation shown on this page.]]

An electron-multiplying CCD (EMCCD, also known as an L3Vision CCD, a product commercialized by e2v Ltd., GB, L3CCD or Impactron CCD, a now-discontinued product offered in the past by Texas Instruments) is a charge-coupled device in which a gain register is placed between the shift register and the output amplifier. The gain register is split up into a large number of stages. In each stage, the electrons are multiplied by [[impact ionization]] in a similar way to an [[avalanche diode]]. The gain probability at every stage of the register is small (''P'' < 2%), but as the number of elements is large (N > 500), the overall gain can be very high (<math>g = (1 + P)^N</math>), with single input electrons giving many thousands of output electrons. Reading a signal from a CCD gives a noise background, typically a few electrons. In an EMCCD, this noise is superimposed on many thousands of electrons rather than a single electron; the devices' primary advantage is thus their negligible readout noise. The use of [[avalanche breakdown]] for amplification of photo charges had already been described in the {{US patent|3761744}} in 1973 by George E. Smith/Bell Telephone Laboratories.

EMCCDs show a similar sensitivity to [[#Intensified charge-coupled device|intensified CCDs]] (ICCDs). However, as with ICCDs, the gain that is applied in the gain register is stochastic and the ''exact'' gain that has been applied to a pixel's charge is impossible to know. At high gains (> 30), this uncertainty has the same effect on the [[signal-to-noise ratio]] (SNR) as halving the [[quantum efficiency]] (QE) with respect to operation with a gain of unity. This effect is referred to as the Excess Noise Factor (ENF).  However, at very low light levels (where the quantum efficiency is most important), it can be assumed that a pixel either contains an electron—or not. This removes the noise associated with the stochastic multiplication at the risk of counting multiple electrons in the same pixel as a single electron. To avoid multiple counts in one pixel due to coincident photons in this mode of operation, high frame rates are essential. The dispersion in the gain is shown in the graph on the right. For multiplication registers with many elements and large gains it is well modelled by the equation:

<math display="block">P\left (n \right ) = \frac{\left
 (n-m+1\right )^{m-1}}{\left (m-1 \right )!\left
 (g-1+\frac{1}{m}\right )^{m}}\exp \left ( -
 \frac{n-m+1}{g-1+\frac{1}{m}}\right ) \quad \text{ if } n \ge m </math>
where ''P'' is the probability of getting ''n'' output electrons given ''m'' input electrons and a total mean multiplication register gain of ''g''.  For very large numbers of input electrons, this complex distribution function converges towards a Gaussian.

Because of the lower costs and better resolution, EMCCDs are capable of replacing ICCDs in many applications. ICCDs still have the advantage that they can be gated very fast and thus are useful in applications like [[range gate|range-gated imaging]]. EMCCD cameras indispensably need a cooling system—using either [[thermoelectric cooling]] or liquid nitrogen—to cool the chip down to temperatures in the range of {{convert|-65|to|-95|C}}. This cooling system adds additional costs to the EMCCD imaging system and may yield condensation problems in the application. However, high-end EMCCD cameras are equipped with a permanent hermetic vacuum system confining the chip to avoid condensation issues.

The low-light capabilities of EMCCDs find use in astronomy and biomedical research, among other fields.  In particular, their low noise at high readout speeds makes them very useful for a variety of astronomical applications involving low light sources and transient events such as [[lucky imaging]] of faint stars, high speed [[photon counting]] photometry, [[Fabry-Pérot|Fabry-Pérot spectroscopy]] and high-resolution spectroscopy. More recently, these types of CCDs have broken into the field of biomedical research in low-light applications including [[small animal imaging]], [[single-molecule|single-molecule imaging]], [[Raman spectroscopy]], [[super resolution microscopy]] as well as a wide variety of modern [[fluorescence microscopy]] techniques thanks to greater SNR in low-light conditions in comparison with traditional CCDs and ICCDs.

In terms of noise, commercial EMCCD cameras typically have clock-induced charge (CIC) and dark current (dependent on the extent of cooling) that together lead to an effective readout noise ranging from 0.01 to 1 electrons per pixel read. However, recent improvements in EMCCD technology have led to a new generation of cameras capable of producing significantly less CIC, higher charge transfer efficiency and an EM gain 5 times higher than what was previously available. These advances in low-light detection lead to an effective total background noise of 0.001 electrons per pixel read, a noise floor unmatched by any other low-light imaging device.<ref>{{cite journal
 | last1 = Daigle | first1 = Olivier
 | last2 = Djazovski | first2 = Oleg
 | last3 = Laurin | first3 = Denis
 | last4 = Doyon | first4 = René
 | last5 = Artigau | first5 = Étienne
 | title = Characterization results of EMCCDs for extreme low light imaging
 | date = July 2012
 | url = http://www.auniontech.com/ueditor/file/20170921/1505975389613590.pdf
|website=auniontech.com}}</ref>