ASTM E2642-08

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Designation: E 2642 – 08
Standard Terminology for
Scientific Charge-Coupled Device (CCD) Detectors
This standard is issued under the fixed designation E 2642; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope anti-blooming structure, n—a structure built into the pixel to
prevent signal charge above full-well capacity from bloom-
1.1 This terminology brings together and clarifies the basic
ing into adjacent pixels.
terms and definitions used with scientific grade cooled charge-
coupled device (CCD) detectors, thus allowing end users and
DISCUSSION—Anti-blooming structures bleed off any excess charge
vendors to use common documented terminology when evalu- before they can overflow the pixel and thereby stop blooming. These
structures can reduce the effective quantum efficiency and introduce
ating or discussing these instruments. CCD detectors are
nonlinearity into the sensor.
sensitive to light in the region from 200 to 1100 nm and the
terminology outlined in the document is based on the detection
antireflective (AR) coating, n—a coating applied to either the
technology developed around CCDs for this range of the
front surface of the CCD or the vacuum window surfaces, to
spectrum.
minimize the amount of reflected energy (or electromagnetic
radiation) so as to maximize the amount of transmitted
2. Referenced Documents
energy.
2.1 ASTM Standards:
back-illuminated CCD (BI CCD), n—a type of CCD that has
E 131 Terminology Relating to Molecular Spectroscopy
been uniformly reduced in thickness on the side away from
the gate structure (see Fig. 1b) and positioned such that the
3. Significance and Use
photons are detected on that side.
3.1 This terminology was drafted to exclude any commer-
DISCUSSION—A BI CCD leads to an improvement in sensitivity to
cial relevance to any one vendor by using only general terms
incoming photons from the soft X-ray to the near-infrared (NIR)
that are acknowledged by all vendors and should be revised as
regions of the spectrum with the highest response in the visible region.
charge-coupled device (CCD) technology matures. This termi-
However, compared to a front-illuminated CCD, it suffers from higher
nologyusesstandardexplanations,symbols,andabbreviations.
dark currents and interference fringe formation (etaloning) usually in
the NIR region. Also called back-thinned CCD.
4. Terminology
binning, n—the process of combining charge from adjacent
4.1 Definitions:
pixels in a CCD prior to read out.
advanced inverted mode operation (AIMO), n—a commer-
DISCUSSION—There are two main types of binning: (1) vertical
cial tradename given to a method of reducing the rate of
binning and (2) horizontal binning (see Fig. 2). Summing charge on the
generation of dark current. Also known as multi-pinned
CCD and doing a single readout results in better noise performance
phase operation.
than reading out several pixels and then summing them in the computer
analog-to-digital (A/D) converter, n—an electronic circuitry memory. This is because each act of reading out contributes to noise
(see noise).
in a CCD detector that converts an analog signal into digital
values, which are specified in terms of bits that can be
charge-coupled device (CCD), n—a silicon-based semicon-
manipulated by the computer.
ductor chip consisting of a two-dimensional matrix of photo
sensors or pixels (see Fig. 3).
This terminology is under the jurisdiction of ASTM Committee E13 on
DISCUSSION—The matrix is usually referred to as the image area.
Molecular Spectroscopy and Separation Science and is the direct responsibility of
Electronic charge is accumulated on the image area and transferred out
Subcommittee E13.08 on Raman Spectroscopy.
by the application of electrical potentials to shielded electrodes. The
Current edition approved Nov. 1, 2008. Published December 2008.
2 size of pixels in the sensor is typically 26 µm 3 26 µm; however,
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
sensorscanbemanufacturedinavarietyofdifferentpixelsizesranging
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
from 6 µm 36µmto50µm 3 50 µm. Although mathematically
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. incorrect, the dimension unit of a square pixel is typically given in
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E2642–08
FIG. 1 Cross Sections of Front-Illuminated (a) and Back-Illuminated (b) CCDs
FIG. 2 Example of a 2 3 2 Vertical and Horizontal Binning Methodology
square microns (for example, a pixel of dimension 26 µm 3 26 µm is
oftheCCDtotransferchargefromthepointofgenerationto
specified as 26 3 26 µm ).
the device output.
charge transfer, n—the process by which a CCD moves
DISCUSSION—It is defined as the fraction of the charge initially stored
electrons or charge from one pixel to the next.
in a CCD element that is transferred to an adjacent element by a single
charge transfer efficiency (CTE), n—measure of the ability
E2642–08
FIG. 3 Typical 1024 3 256 (26 3 26 µm pixel) Element CCD Sensor Used for Spectroscopy
clock cycle. The value for CTE is not constant but varies with signal
uniformity is lower. But the sensor can be built to require
size, temperature, and clock frequency.
less off-chip circuitry for basic operation (see Fig. 4).
correlated double sampling, n—a readout sampling tech-
column, n—a line of pixels in the CCD’s image area that is
nique used to achieve higher precision in CCD readout.
perpendicular to the horizontal register.
complementary metal oxide semiconductor (CMOS),
DISCUSSION—The sampling circuit is set to a predetermined reference
n—technology widely used to manufacture electronic de- level and then the actual pixel voltage is sampled in order to find the
difference between the two. The resulting correlation minimizes read
vicesandimagesensorssimilartoCCDs.InaCMOSsensor,
noise, especially in ultra-low-noise CCD detectors.
each pixel has its own charge-to-voltage conversion circuit,
and the sensor often also includes amplifiers, noise-
cosmic event, n—a spurious signal caused by a cosmic ray or
correction, and digitization circuits. Due to the additional
particle hitting the CCD sensor. It is typically observed to
components associated with each pixel, the sensitivity to
light is lower than with a CCD, the signal is noisier, and the
FIG. 4 Typical Architectures of CCD and CMOS Sensors
E2642–08
resultinahighintensitysignalcomingfromasinglepixelor has a two-way readout register, that is, the shift register and
small group of pixels. the gain register, each with its own output amplifier. When
dark current, n—a current that occurs naturally through the the charge is read out through the shift register, the detector
thermally generated electrons in the semiconductor material works like a standard CCD detector, and when the charge is
of the CCD. It is intrinsic to semiconductors and is indepen- read out through the gain register, it undergoes charge
dent of incident photons. amplification as a result of a different electrode structure
embedded underneath the pixels of this register (see Fig. 6).
DISCUSSION—Dark current is dependant on the CCD’s temperature. It
is expressed in electrons/pixel/unit time.
DISCUSSION—Passing charge through the gain register allows the
signal to be amplified before readout noise is added at the readout
dark noise, n—the shot noise associated with the dark current
amplifier, thus improving the signal-to-noise ratios making the camera
for the given exposure time, and is approximately equal to
highly sensitive in the low-light regime.
the square root of the dark current times the exposure time
etaloning, n—a phenomenon by which constructive and de-
used. It is usually expressed in terms of number of electrons.
structive interference fringes are produced in a back-
deep depletion CCD, n—a CCD that has been designed with
illuminated CCD caused by internal reflections between the
a thicker active area to provide enhanced sensitivity in the
two parallel surfaces of the CCD. Typically BI CCDs
NIR and hard X-ray regimes.
experience etaloning effects when subjected to NIR signals
DISCUSSION—Both front-illuminated and back-illuminated CCDs can
(see Fig. 5).
be manufactured with a deep depletion process to enhance the NIR
response; however, such devices cannot be operated in AIMO and are DISCUSSION—This effect causes the device to become transparent to
also more susceptible to cosmic rays. A back-illuminated deep deple- incoming photons in the NIR region.
tion CCD will have reduced etaloning effects that are typically
exposure time, n—the length of time for which a CCD
observed in back-illuminated devices exposed to NIR signals (see Fig.
accumulated charge.
5).
frame, n—one full image that is read out of a CCD.
dynamic range, n—the ratio of the full well saturation charge
frame-transfer CCD, n—a type of CCD whose active image
to the system noise level. It represents the ratio of the
area is divided into two sections, that is, image area and the
brightest and darkest signals a detector can measure in a
storage area. The image area is the light sensitive area of the
single measurement.
CCD and the storage area is masked to make it insensitive to
DISCUSSION—A true 16-bit detector will have a dynamic range of light (see Fig. 7).
65 535:1.
DISCUSSION—During operation the charge accumulated in the image
section is rapidly transferred to the storage section at the end of the
electron-multiplying CCD (EMCCD), n—type of CCD that
FIG. 5 Cross-Sections of Back-Illuminated (a) and Back-Illuminated Deep Depletion (b) Devices
E2642–08
FIG. 6 Typical Sketch of Full-Frame EMCCD Sensor
FIG. 7 Typical Sketch of a Frame-Transfer CCD
exposure time. The storage area is then readout as the image section
eliminate image smear (see Fig. 3).
accumulates charge for the next exposure.This type of CCD reduces or
full well capacity, n—the maximum number of photoelectrons
eliminates the need for a shutter, depending on the speed of the transfer
that can be collected on a single pixel in the image area or in
from image to storage.
the horizontal register of a CCD. It is typically specified in
front-illuminated CCD (FI CCD), n—a type of CCD in terms of number of electrons.
which the photons are detected through the gate structure gate structure, n—a polysilicon arrangement of electrodes
located in front of the silicon material of the semiconductor that create pixels and move charge.
(see Fig. 1a). horizontal binning, n—the process that allows charge from a
row of pixels to be combined on the CCD chip prior to
DISCUSSION—ThistypeofCCDhasmoderatequantumefficiency(see
readout (See Fig. 2). Horizontal binning is commonly used
Fig. 8) over the spectral range it covers and it is also free from any
in spectroscopy to increase the signal level of a data point,
etaloning effects that occur in the back-illuminated CCD when sub-
when less horizontal (or wavelength) resolution is not of
jected to NIR signals. These devices are relatively less expensive to
manufacture than the back-illuminated type.
concern.
horizontal register, n—a row of light insensitive pixels that is
full-frame CCD, n—a type of CCD that uses the entire silicon
located below the CCD’s image acquisition area into which
active area for photon detection. A shutter is required to
E2642–08
NOTE 1—Image used courtesy of E2V Technologies, 106 Waterhouse Lane, Chelmsford, Essex CM1 2QU, England, http://www.e2v.com.
FIG. 8 Typical QE Curves for FI and BI CCD Sensors
Alarge potential difference is applied across the ends of the multichan-
charge from the pixel columns is clocked and subsequently
nel plate to amplify the signal.There are two main types of intensifiers:
passed on to the output node to be read out. Also called the
GenIIandGenIII.Themaindifferencebetweenthemisinthematerial
serial register or readout register.
used in the photocathode. The Gen III models are a more advanced
indium tin oxide (ITO), n—a transparent conductive material
design and they provide higher quantum efficiencies than the Gen II
used in some CCD designs to provide an increase in
models.
quantum efficiency (QE) in the blue-green region of the
interline transfer CCD, n—a type of CCD designed with
spectrum.
columns of pixels alternated with masked storage registers
intensified CCD (ICCD), n—a type of CCD camera that has
so as to increase the rate of acquisition.The storage registers
an intensifier block attached in front of it. An ICCD is used
occupy a portion of the pixel area reducing the fill factor of
to amplify the incoming signal without varying the image
the diodes under the pixels, and hence, such a CCD
size so as to provide single-photon sensitivity and it can be
architecture has typically lower quantum efficiencies that
electronically gated down to nanosecond ranges (see Fig. 9).
other types of CCDs (see Fig. 10).
DISCUSSION—Intensifiers were initially designed for the military for
linear array CCD, n—a type of CCD that is comprised of a
night-vision ability and are now being widely used in applications that
single row of pixels that are used as the active area for
need nanosecond gate widths or single-photon sensitivity or both. The
capturing incident photons.
intensifier consists of a photocathode, multichannel plate and phosphor.
FIG. 9 Schematic of a Typical Intensifier Fiber Optically Coupled to a CCD Sensor
E2642–08
FIG. 10 Typical Sketch of an Interline CCD Sensor
light signal itself, and dark noise (shot noise from the dark signal). See
multi-pinned phase (MPP), n—mode of operation in CCDs
read noise, shot noise, and dark noise for further details.
that reduces dark charge.
open electrode CCD (OE CCD), n—type of front-illuminated
DISCUSSION—Also known as advanced inverted mode operation
(AIMO). CCD in which the electrodes are patterned such that a
portion of every pixel on the sensor remains open to direct
noise, n—unwantedrandomvariationsofoutputsignalthatare
illumination from incident photons (see Fig. 11). This
addedtotherealsignalandarenotsubtractable.Noisearises
minimizes absorption of charge between layers and leads to
from the statistical variations of both thermal
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