1、Designation: E 2642 09Standard Terminology forScientific Charge-Coupled Device (CCD) Detectors1This standard is issued under the fixed designation E 2642; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A
2、 number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This terminology brings together and clarifies the basicterms and definitions used with scientific grade cooled charge-coupled device
3、(CCD) detectors, thus allowing end users andvendors to use common documented terminology when evalu-ating or discussing these instruments. CCD detectors aresensitive to light in the region from 200 to 1100 nm and theterminology outlined in the document is based on the detectiontechnology developed a
4、round CCDs for this range of thespectrum.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.2. Referenced Documents2.1 ASTM Standards:2E 131 Terminology Relating to Molecular Spectroscopy3. Significance and Use3.1 This terminol
5、ogy was drafted to exclude any commer-cial relevance to any one vendor by using only general termsthat are acknowledged by all vendors and should be revised ascharge-coupled device (CCD) technology matures. This termi-nology uses standard explanations, symbols, and abbreviations.4. Terminology4.1 De
6、finitions:advanced inverted mode operation (AIMO), na commer-cial tradename given to a method of reducing the rate ofgeneration of dark current. Also known as multi-pinnedphase operation.analog-to-digital (A/D) converter, nan electronic circuitryin a CCD detector that converts an analog signal into
7、digitalvalues, which are specified in terms of bits that can bemanipulated by the computer.anti-blooming structure, na structure built into the pixel toprevent signal charge above full-well capacity from bloom-ing into adjacent pixels.DISCUSSIONAnti-blooming structures bleed off any excess chargebef
8、ore they can overflow the pixel and thereby stop blooming. Thesestructures can reduce the effective quantum efficiency and introducenonlinearity into the sensor.antireflective (AR) coating, na coating applied to either thefront surface of the CCD or the vacuum window surfaces, tominimize the amount
9、of reflected energy (or electromagneticradiation) so as to maximize the amount of transmittedenergy.back-illuminated CCD (BI CCD), na type of CCD that hasbeen uniformly reduced in thickness on the side away fromthe gate structure (see Fig. 1b) and positioned such that thephotons are detected on that
10、 side.DISCUSSIONA BI CCD leads to an improvement in sensitivity toincoming photons from the soft X-ray to the near-infrared (NIR)regions of the spectrum with the highest response in the visible region.However, compared to a front-illuminated CCD, it suffers from higherdark currents and interference
11、fringe formation (etaloning) usually inthe NIR region. Also called back-thinned CCD.binning, nthe process of combining charge from adjacentpixels in a CCD prior to read out.DISCUSSIONThere are two main types of binning: (1) verticalbinning and (2) horizontal binning (see Fig. 2). Summing charge on t
12、heCCD and doing a single readout results in better noise performancethan reading out several pixels and then summing them in the computermemory. This is because each act of reading out contributes to noise(see noise).CCD bias, nthe minimum analog offset added to the signalbefore the A/D converter to
13、 ensure a positive digital outputeach time a signal is read out.DISCUSSIONThe CCD bias is set at the time of manufacture andremains set over the lifetime of the camera.charge, nmeasure of number of electrons that are containedin a pixel potential well.charge-coupled device (CCD), na silicon-based se
14、micon-ductor chip consisting of a two-dimensional matrix of photosensors or pixels (see Fig. 3).1This terminology is under the jurisdiction of ASTM Committee E13 onMolecular Spectroscopy and Separation Science and is the direct responsibility ofSubcommittee E13.08 on Raman Spectroscopy.Current editi
15、on approved April 15, 2009. Published May 2009. Originallyapproved in 2008. Last previous edition approved in 2008 as E 2642 08.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume informatio
16、n, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.DISCUSSIONThe matrix is usually referred to as the image area.Electronic charge is accumulated on the image area and t
17、ransferred outby the application of electrical potentials to shielded electrodes. Thesize of pixels in the sensor is typically 26 m 3 26 m; however,sensors can be manufactured in a variety of different pixel sizes rangingfrom 6 m 3 6mto50m3 50 m. Although mathematicallyincorrect, the dimension unit
18、of a square pixel is typically given insquare microns (for example, a pixel of dimension 26 m 3 26misspecified as 26 3 26 m2).charge transfer, nthe process by which a CCD moveselectrons or charge from one pixel to the next.charge transfer efficiency (CTE), nmeasure of the abilityof the CCD to transf
19、er charge from the point of generation tothe device output.FIG. 1 Cross Sections of Front-Illuminated (a) and Back-Illuminated (b) CCDsFIG. 2 Example of a 2 3 2 Vertical and Horizontal Binning MethodologyE2642092DISCUSSIONIt is defined as the fraction of the charge initially storedin a CCD element t
20、hat is transferred to an adjacent element by a singleclock cycle. The value for CTE is not constant but varies with signalsize, temperature, and clock frequency.column, na line of pixels in the CCDs image area that isperpendicular to the horizontal plementary metal oxide semiconductor (CMOS),ntechno
21、logy widely used to manufacture electronic de-vices and image sensors similar to CCDs. In a CMOS sensor,each pixel has its own charge-to-voltage conversion circuit,and the sensor often also includes amplifiers, noise-correction, and digitization circuits. Due to the additionalcomponents associated w
22、ith each pixel, the sensitivity tolight is lower than with a CCD, the signal is noisier, and theuniformity is lower. But the sensor can be built to requireless off-chip circuitry for basic operation (see Fig. 4).correlated double sampling, na readout sampling tech-nique used to achieve higher precis
23、ion in CCD readout.DISCUSSIONThe sampling circuit is set to a predetermined referencelevel and then the actual pixel voltage is sampled in order to find thedifference between the two. The resulting correlation minimizes readnoise, especially in ultra-low-noise CCD detectors.cosmic event, na spurious
24、 signal caused by a cosmic ray orparticle hitting the CCD sensor. It is typically observed toFIG. 3 Typical 1024 3 256 (26 3 26 m2pixel) Element CCD Sensor Used for SpectroscopyFIG. 4 Typical Architectures of CCD and CMOS SensorsE2642093result in a high intensity signal coming from a single pixel or
25、small group of pixels.dark current, na current that occurs naturally through thethermally generated electrons in the semiconductor materialof the CCD. It is intrinsic to semiconductors and is indepen-dent of incident photons.DISCUSSIONDark current is dependant on the CCDs temperature. Itis expressed
26、 in electrons/pixel/unit time.dark noise, nthe shot noise associated with the dark currentfor the given exposure time, and is approximately equal tothe square root of the dark current times the exposure timeused. It is usually expressed in terms of number of electrons.deep depletion CCD, na CCD that
27、 has been designed witha thicker active area to provide enhanced sensitivity in theNIR and hard X-ray regimes.DISCUSSIONBoth front-illuminated and back-illuminated CCDs canbe manufactured with a deep depletion process to enhance the NIRresponse; however, such devices cannot be operated in AIMO and a
28、realso more susceptible to cosmic rays. A back-illuminated deep deple-tion CCD will have reduced etaloning effects that are typicallyobserved in back-illuminated devices exposed to NIR signals (see Fig.5).dynamic range, nthe ratio of the full well saturation chargeto the system noise level. It repre
29、sents the ratio of thebrightest and darkest signals a detector can measure in asingle measurement.DISCUSSIONA true 16-bit detector will have a dynamic range of65 535:1.electron-multiplying CCD (EMCCD), ntype of CCD thathas a two-way readout register, that is, the shift register andthe gain register,
30、 each with its own output amplifier. Whenthe charge is read out through the shift register, the detectorworks like a standard CCD detector, and when the charge isread out through the gain register, it undergoes chargeamplification as a result of a different electrode structureembedded underneath the
31、 pixels of this register (see Fig. 6).DISCUSSIONPassing charge through the gain register allows thesignal to be amplified before readout noise is added at the readoutamplifier, thus improving the signal-to-noise ratios making the camerahighly sensitive in the low-light regime.etaloning, na phenomeno
32、n by which constructive and de-structive interference fringes are produced in a back-illuminated CCD caused by internal reflections between thetwo parallel surfaces of the CCD. Typically BI CCDsexperience etaloning effects when subjected to NIR signals(see Fig. 5).DISCUSSIONThis effect causes the de
33、vice to become transparent toincoming photons in the NIR region.exposure time, nthe length of time for which a CCDaccumulated charge.frame, none full image that is read out of a CCD.frame-transfer CCD, na type of CCD whose active imagearea is divided into two sections, that is, image area and thesto
34、rage area. The image area is the light sensitive area of theCCD and the storage area is masked to make it insensitive tolight (see Fig. 7).DISCUSSIONDuring operation the charge accumulated in the imagesection is rapidly transferred to the storage section at the end of theFIG. 5 Cross-Sections of Bac
35、k-Illuminated (a) and Back-Illuminated Deep Depletion (b) DevicesE2642094exposure time. The storage area is then readout as the image sectionaccumulates charge for the next exposure. This type of CCD reduces oreliminates the need for a shutter, depending on the speed of the transferfrom image to sto
36、rage.front-illuminated CCD (FI CCD), na type of CCD inwhich the photons are detected through the gate structurelocated in front of the silicon material of the semiconductor(see Fig. 1a).DISCUSSIONThis type of CCD has moderate quantum efficiency (seeFig. 8) over the spectral range it covers and it is
37、 also free from anyetaloning effects that occur in the back-illuminated CCD when sub-jected to NIR signals. These devices are relatively less expensive tomanufacture than the back-illuminated type.full-frame CCD, na type of CCD that uses the entire siliconactive area for photon detection. A shutter
38、is required toeliminate image smear (see Fig. 3).full well capacity, nthe maximum number of photoelectronsthat can be collected on a single pixel in the image area or inthe horizontal register of a CCD. It is typically specified interms of number of electrons.gate structure, na polysilicon arrangeme
39、nt of electrodesthat create pixels and move charge.horizontal binning, nthe process that allows charge from arow of pixels to be combined on the CCD chip prior toreadout (See Fig. 2). Horizontal binning is commonly usedin spectroscopy to increase the signal level of a data point,when less horizontal
40、 (or wavelength) resolution is not ofconcern.horizontal register, na row of light insensitive pixels that islocated below the CCDs image acquisition area into whichFIG. 6 Typical Sketch of Full-Frame EMCCD SensorFIG. 7 Typical Sketch of a Frame-Transfer CCDE2642095charge from the pixel columns is cl
41、ocked and subsequentlypassed on to the output node to be read out. Also called theserial register or readout register.indium tin oxide (ITO), na transparent conductive materialused in some CCD designs to provide an increase inquantum efficiency (QE) in the blue-green region of thespectrum.intensifie
42、d CCD (ICCD), na type of CCD camera that hasan intensifier block attached in front of it. An ICCD is usedto amplify the incoming signal without varying the imagesize so as to provide single-photon sensitivity and it can beelectronically gated down to nanosecond ranges (see Fig. 9).DISCUSSIONIntensif
43、iers were initially designed for the military fornight-vision ability and are now being widely used in applications thatneed nanosecond gate widths or single-photon sensitivity or both. Theintensifier consists of a photocathode, multichannel plate and phosphor.A large potential difference is applied
44、 across the ends of the multichan-nel plate to amplify the signal. There are two main types of intensifiers:Gen II and Gen III. The main difference between them is in the materialused in the photocathode. The Gen III models are a more advanceddesign and they provide higher quantum efficiencies than
45、the Gen IImodels.interline transfer CCD, na type of CCD designed withcolumns of pixels alternated with masked storage registersso as to increase the rate of acquisition. The storage registersoccupy a portion of the pixel area reducing the fill factor ofthe diodes under the pixels, and hence, such a
46、CCDarchitecture has typically lower quantum efficiencies thatother types of CCDs (see Fig. 10).linear array CCD, na type of CCD that is comprised of asingle row of pixels that are used as the active area forcapturing incident photons.NOTE 1Image used courtesy of E2V Technologies, 106 Waterhouse Lane
47、, Chelmsford, Essex CM1 2QU, England, http:/.FIG. 8 Typical QE Curves for FI and BI CCD SensorsFIG. 9 Schematic of a Typical Intensifier Fiber Optically Coupled to a CCD SensorE2642096multi-pinned phase (MPP), nmode of operation in CCDsthat reduces dark charge.DISCUSSIONAlso known as advanced invert
48、ed mode operation(AIMO).noise, nunwanted random variations of output signal that areadded to the real signal and are not subtractable. Noise arisesfrom the statistical variations of both thermal and photon-generated signal as well as from electron conduction throughresistive material, and variations
49、 in the readout electronics.DISCUSSIONThe total noise in a signal measured by a CCD detectoris referred to as “system noise” and is the equal to the square root of thesum of the squares of each of the individual noise components. Themajor noise components present in CCD devices are: read noise causedby the systems output amplifier and electronics, shot noise from thelight signal itself, and dark noise (shot noise from the dark signal). Seeread noise, shot noise, and dark noise for further
