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Cypress Image Sensors Glossary
1/f noise
Temporal noise with a dominantly low frequency content. In this context the origin
is typically in MOS or MOSFET interface states. A.k.a. flicker noise.
absorption depth
Light, as all electro-magnetic radiation, is absorbed (and detected!) in silicon over
a characteristic depth. Blue light is absorbed in about 0.5 to 1 um; green light in a
few um; red light in a layer over 5 to 10 um thick. Infrared light is absorbed in
deeper layers. Ultraviolet light is typically absorbed (and not detected!) in the top
oxide layers.
active pixel
Pixel, containing an active (= power dissipating) element. In most cases the active
element is a MOSFET in an amplifying or buffering configuration. The presence of
only one MOSFET acting as a switch only does not make a pixel 'active'.
ADC
Analogue to Digital Converter: An electronic circuit that converts an analogue
voltage, as comes out of an imager core, into a digital signal.
alchemy or black
magic
The part of our science that nobody really understands, but that seems to work
well until someone changes a tiny detail in some arcane procedure.
anthropomorphic
Properties of image sensors that are inspired by, or similar to, properties of the
human (- eye). Examples: logarithmic response, RGB sensitivity, and log-polar
geometry.
APS
Active Pixel Sensor: Image sensor using active pixels.
ARC
Anti Reflective Coating: Dielectric layers on top of the receptor with specific
thickness and refractive index, so that only a minimal fraction of the light impinging
on the receptor is reflected.
area sensor
Imager with a two dimensional array, or matrix, of pixels.
autosaturation time
The average time for a pixel to saturate due to dark current alone, i.e. nonilluminated. The absolute upper limit for useful integration time.
backside
illumination
An imager can be illuminated from the backside, if the silicon can be uniformly
thinned to a few micrometers. If successful, there is no obstruction of the light by
metalization and dielectric layers, and the Fill Factor and UV sensitivity can be
very high.
biCMOS
Bipolar + CMOS: CMOS technology extended with bipolar transistors.
binning
Mode of operation that is available only in some types of CCD (FTCCD). The
charge of multiple pixels is accumulated and read in a single operation. This type
of binning has no noise penalty. Binning is sometimes emulated in passive pixels,
active pixels or in the digital domain. But here the noise advantage is less
prominent.
Bayer pattern
A popular type of Red-Green-Blue CFA, invented by Bayer (Kodak).
BLIP
Background Limited Integrated Performance: Camera operation in a condition
where the noise is dominated by the inherent shot noise in the background
illumination level.
bulk
Substrate. In MOSFETs, the symbol for bulk or substrate is B.
bulk effect
Normally the source-drain current through a MOSFET is regulated by the gate
potential. Nevertheless, the bulk potential has a generally unwanted parasitic
effect too.
buried channel
Similar to a MOSFET inversion layer, but by the proper game of implantations the
charge is separated from the interface.
The better "buried channel" CCDs store and transfer their charge in a buried
channel under the electrodes.
CCD
Charge Coupled Device: Originally the name of a structure invented in 1970 of
adjacent MOS gates ("electrodes") that allows both confinement (storage) and
transport of free charge. Later the name of the modified MOS technology in which
the device is made. Even later the name of image sensors based on the CCD
principle. And finally the generic name of a solid state image sensor.
CDS
Correlated Double Sampling: A method to read the differential magnitude of a
charge packet, thereby canceling a source of FPN, as well as the kTC noise that is
caused by the reset of the charge packet.
CFA
Color Filter Array: The mosaic of colour filters overlaid on the pixel array.
CID
Charge Injection Device: CCD-based technique and image sensor using this
technique. Detection based on local charge transfers only and non-destructive
read out.
CIE diagram
Two-dimensional representation of color space, representing all physical colors,
i.e. more than what can be generated with the color triangle.
CIS
CMOS Image Sensor: go straight to jail, skip three turns, do not collect $200.
CMD
Charge Modulation Device: Image sensor technology based on the detection and
charge sensing in buried channel MOSFETs.
CMOS
Complementary MOS: The mainstream mass manufacturing Silicon technology.
Characterized by the fact that both nMOSFETs and pMOSFETs ("complementary"
pairs) can be manufactured on the same wafers.
COB
Chip On Board: Mounting the image sensor chip directly on the PCB, without
package. Cheap but a bit fragile, unless protected well.
co-integration
Or the question: Which part of the system (digital or analogue) intelligence is put
on the same chip as the analogue imager core?
color
reconstruction
The raw image from an imager with CFA has only one color component per pixel.
Trivial or more clever reconstruction algorithms must restore the missing color
components for every pixel.
column amplifier
Charge amplifier or voltage amplifier that is located at the edge of columns op
pixels. Serves to buffer or sense the signal of one, selected, pixel in the column.
Typically, the signals of the column amplifiers are multiplexed to one imager core
output.
cosmetic defect
As a spot or a mark in the face. Small defects, localized non-uniformities,
unwanted patterns, blemishes, dust particles, etc. that degrade the image quality.
CRT
Cathode Ray Tube: The classic display device, e.g. TV or computer monitor.
DAC
Digital to Analogue Converter: as ADC, but read the explanation back to front.
dark current
The parasite of photocurrent. In the absence of light, a non-zero current will flow in
the photodiode, spoiling the image and adding its own shot noise.
detector
"Detecting" apparatus. Most people don't tell the difference with "sensor". Sounds
more "active" than "sensor" - or does it sound more expensive? The nuclear
physics community considers a detector as the bigger apparatus containing many
smaller "sensors".
dielectric layers
Insulating, transparent layers on top of the IC. Typically SiO2, SiN and organic
materials.
diode
Electronic device with two terminals, that conducts the current in only one
direction. Typically it consists of a p-n junction, the juxtaposition of an n-type
semiconductor and a p-type semiconductor, typically on the same substrate.
double slope
integration
Method to extend the dynamic range of a normally linear-transfer imager, by
combining the images taken with a long and a short integration time into one
image. The resulting electro-optical transfer curve is bi-linear.
DS
Double Sampling: A method to read the magnitude of a charge packet, canceling a
main source of FPN.
DSP
Digital Signal Processor: software-programmed or hardwired logic device,
specialized in performing many calculations at high speed on a signal. The raw
image coming from an imager can be/needs to be post-processed in the digital
domain.
dummy pixels
A pixel array features some rows/columns of real but unused pixels at its
perimeter, thus ensuring that the "useful" pixels are not affected by peripheral
circuitry.
dynamic range
ratio of highest non-saturating optical flux on the smallest detectable flux for a
sensor. Equates to the signal to noise ratio if and when the sensor features a
linear transfer curve.
electron
Elementary particle with negative unit charge. Or was it a wave?
electronic shutter
A method to operate an electronic image sensor in a way that the effective
integration or exposure time (shutter time) is variable. The result is the same as
that of the mechanical shutter found on the classical photo camera.
EM
Electro-Magnetic radiation: Visible light is just one small part of the huge EM
spectrum.
EMI
Electro-Magnetic Interference: System noise.
epi
Epitaxial wafers. Most CMOS image sensors need processing on highly doped
wafers with on top a few microns thick "epitaxial layer" of lowly doped silicon.
FEA
Field Emission Array: Micro-machined silicon device that combines the light
detecting mechanism of a vidicon tube and the circuit possibilities of an integrated
circuit.
FF CCD
Full Frame CCD: A FT CCD lacking storage area. Needs an external shutter.
Fill Factor
FF: ratio [%] of the light sensitive area of a pixel on its total area. Should be quoted
in conjunction with spectral response.
FillFactory
Flemish commercial outfit. Much like a brewery, but focusing on custom-designed
and standard CMOS Image Sensors. Fill factors in and around the company are
reputed to be rather high.
floating diffusion
Popular circuit technique for the conversion of (photo) charge to a voltage. Used in
most CCDs and in most APSs.
flux
Light power per unit area [W/m2].
f-number
Ratio between the opening (diameter) of the lens and its focal length. A high fnumber means a large attenuation of the light power.
FPN
Fixed Pattern Noise, or spatial noise. Is the unwanted static (DC) variation of the
response of all pixels in the image. Expressed as an offset in volts or in ADC bits.
frame
The electronic form of one image.
frame rate
The frequency at which consecutive images are taken by a camera.
frame time
The time it takes for an imager to capture and output an image. Equals
1/frame_rate.
FTCCD
Frame Transfer CCD.
gamma
Response correction in cameras originally introduced to counteract the non-linear
response of CRTs. The typical gamma correction takes the signal luminance to the
power of 0.45
gate
Control terminal of a MOSFET. In a CCD the gates are called "electrodes".
ground or GND
The zero reference voltage in most electronic circuits. In our science it is typically
defined as equal to the substrate voltage, sometimes simplistically called substrate
potential.
hole
In a semiconductor the local absence of a bound electron, acting to all intents and
purposes as a free charged particle with positive unit charge, and thus opposite of
a negatively charged free electron. A hole is absolutely not the same as a positron,
the anti-electron, which also has a positive unit charge, but which is a real physical
(but rare, and thus expensive) particle.
hybrid
Imager consisting of a CMOS readout array chip and a separate detector chip.
IBIS
Cypress's family of integrating APS sensors with on chip FPN correction, intended
for photography and related applications. "IBIS" stems from I-BIS, I2, i.e
Integrating Imager. Eye-Bis is also a neat explanation.
ILCCD
InterLine CCD.
image processing
Digital domain operations in hardware or software intended to convert the raw
image from the imager to a "nicer" image. Typical image processing operators are:
spatial and temporal filtering, missing pixel correction, offset and gain correction,
gamma correction, color (CFA) reconstruction, white balance, saturation and hue
correction.
imager
Combination of the words image and sensor.
imager core
The essential part of an image sensor chip. In fact nothing more than the pixel
array and the immediate peripheral circuits required to operate the pixel array.
integration time
Exposure time or electronic shutter time. The time span during which light is being
converted to charge and accumulated in a pixel.
intelligent image
sensor
Smart sensor. The question is: what is "intelligent"? The first image sensor that
can give an answer to that question is really intelligent.
interface
In this science the border between Silicon and whatever is on top. The favorite
meeting place for embarrassing things like dark current generation centers and 1/f
noise centers.
interlacing
In the CCIR (PAL) and EIA (NTSC) video formats, the image information is divided
in odd and even (sub) frames. Alternating frames contain only the odd or even
lines. Imagers for these formats should read their lines in the same sequence.
intrinsic or i-type
Undoped Silicon has (in theory) a low concentration of both holes and electrons.
To distinguish it from n- or p- type, it is indicated by "i".
inversion layer
Under the influence of an electric field an n-type material can contain a majority of
holes, and thus "invert" effectively to p-type, or vice versa. The inversion layer
under a MOSFET gate is the conducting path of the MOSFET.
IR cut-off filter
Silicon photodiodes are sensitive for light with wavelengths between 350 and 1100
nm. The human eye sees light with wavelengths between 350 and 750 nm (the
"visible spectrum"). An IR cut-off filter is a piece of special glass, or another
material, put in front of the image sensor and absorbing or reflecting the light
outside the visible spectrum. Quite necessary for color sensitive imagers.
ISO (ASA)
Empirical measure of the light sensitivity or speed of photographic film.
Transferring this to an electronic image sensor spec is a real shot in the dark.
kTC
Bolzman's constant x absolute temperature x capacitance. The thermodynamic
uncertainty on the amount of charge left on a capacitor after disconnecting it from
a DC voltage source. kTC noise is also called reset noise.
LCD
Liquid Crystal Display: One type of flat panel display, based on the light polarizing
capabilities of so called liquid crystals in an electric field.
linear response
a light power (P) to voltage (V) conversion that can be approximated by a linear
relation as: V = something x P. Transfer curve of our IBIS range.
linear sensor
Image sensor with an (almost) one-dimensional array of pixels, i.e. one or a few
rows of pixels. Don't confuse it with a linear response image sensor!
logarithmic
response
A light power P to signal voltage V conversion that can be approximated by a
logarithmic relation as: V = something x log(P). Transfer mode used in our highdynamic range FUGAs.
log-polar geometry
Arrangement of pixels, organized in concentric circles, with a constant number of
pixels per circle. The pixel size is smallest in the center and grows towards the
edges. This mimics the human retina.
lux (lx)
Human eye equivalent (i.e. subjective) of the objective light flux measured in
W/m2. As the spectral sensitivities of the eye and Silicon are not matched, there is
no general conversion factor between lx and W/m2. As a rule of thumb, consider
1W/m2 = 70lx (visible white light) to 180lx (visible + NIR).
microlenses
Technique to increase the fill factor of pixels by giving each pixel a tiny individual
lens that converges all light on the even-tinier photodiode below it.
MOS
Metal Oxide Semiconductor: In ancient times MOSFETs were made as a stack of
aluminium (metal) on SiO2 (oxide) on Silicon (Semiconductor). Today the
aluminium is in most cases replaced with Polysilicon, but the name persisted.
Otherwise we would now be making POSFETs, and CPOS Image Sensors. Or
perhaps even SOSFETs ...
MOSFET
MOS Field Effect Transistor: transistor type based on the MOS layer structure.
The current between source (S) and drain (D) passes through the inversion layer
that is controlled by the voltage at the gate (G).
MTF
Modulation Transfer Function: Ratio between the amplitudes of a sinusoidal
pattern in the optical image and the resulting electronic image. A measure for a
sensor's inherent sharpness of focus.
NIR
Near InfraRed: EM radiation between 750 and 1100 nm, invisible for the eye, but
still visible for Silicon.
nMOSFET
n-type MOSFET, a MOSFET in which the conduction between n-type source and
drain happens by electrons.
noise
A word with many meanings. In our science it is the unwanted fluctuation of the
signal of one pixel over time. To differentiate from (static) FPN it is sometimes
called "temporal noise".
non-destructive
readout
Mode of operation of CCDs and other image sensors, where the size of a charge
packet is read, without resetting it. This information can thus be read several
times, so as to improve the S/N of the reading operation
n-type
Under zero electric field and equilibrium an n-type semiconductor contains a
majority of electrons and few holes. n-type Silicon is usually obtained by doping
with donor ions.
Nyquist limit
The Nyquist limit equals to the spatial frequency 1/(2 x pixel pitch) of a pixel array.
Image information beyond this limit is "aliased" to lower spatial frequencies. In
practice this aliasing appears as Moiré effects and color artifacts.
optical cross talk
Light impinging on a pixel can influence the signal of neighboring pixels through
various mechanisms: diffraction of light, diffusion of photo charge towards the
neighbor, or plain electrostatic cross talk. Together these make up optical
crosstalk.
passive pixel
Pixel not containing an active element, so essentially a photodiode with access
switches.
photo diode
All silicon diodes or p-n junctions are sensitive to light. A diode that explicitly is
designed for detecting light is a photo diode.
photo gate
Light receptor made as a MOS structure. Can be understood as a p-n photo diode,
where one of the parts is the inversion layer under the MOS.
photodiode array
Image sensor consisting of an array of passive pixels, where the passive pixels
contain only a photodiode and a multiplexing switch.
photoelectric effect
The basic mechanism of light detection in any solid state imager. A quantum of
energy from the impinging EM radiation (a photon) excites a bound electron to an
unbound state.
photon
The light quantum. The amount of energy exchanged in a single interaction
between EM radiation and a charged particle.
pin diode
p-n diode with a relatively important lowly doped (intrinsic) layer "i" sandwiched
between the p-type and n-type regions.
pinned diode
Photo diode which is covered by an additional junction on the top. The goal is to
pin the interface potential to a nearly fixed value, and to shunt the dark current
centers at the interface.
pixel
From picture + element. Originally the atom of a photograph, of an electronic
image, of an electronic display. In our profession the atom of an image sensor.
Minimally contains a receptor and elementary means to feed the information to the
outside world.
plasma display
Type of flat panel display, based on the emission of light by the interactions of free
electrons in an ionized gas (plasma).
pMOSFET
p-type MOSFET, a MOSFET in which the conduction in the inversion layer
between p-type source and drain happens by holes.
preview mode
Mode of operation of a large, slow-scan, imager: It is read at high frame rate but at
reduced resolution.
potential
Local property of a position in space. Energy that must be given to or taken from a
hypothetical free electron to be stable at that certain position. Is a relative
measure, thus it must be referred to some "reference". In absence of any
reference the reference is the potential in vacuum at an infinite distance. Ever
tried? For most earthlings potential is quite the same as "voltage".
PRNU
Photo Response Non-Uniformity: non-uniformity in the slope of the photo-electric
transfer curve between pixels. Also see FPN for offset non-uniformity.
PSD
Position Sensitive Device: A kind of solid state optical sensor that outputs analog
signals that are measures of the position of a light spot on the sensor.
p-type
Under zero electric field and equilibrium a p-type semiconductor contains a
majority of holes and few electrons. p-type Silicon is usually obtained by doping
with acceptor ions.
push broom
A linear imager scanning a two-dimensional scene. Often used for earth imaging.
quantum efficiency
QE. Ratio [%] between the number of generated electrons and the number of
impinging photons. Closely related to spectral response.
random addressing
A mode of operation of an image sensor, where pixels are addressed and read
individually and randomly, like a RAM or ROM.
receptor
The generic name of an elementary radiation sensitive device. Photodiodes, film
grains, eye cones and rods are examples of light receptors.
rel DE pixel
R*: Magnitude of the responsivity of a pixel referred to the threshold of nominal
light detectivity. Is a measure for the capabilities of a specifications writer.
reset
In the imager context, reset means clearing the charge contents of a pixel to the
initial dark value.
RGB
Red Green Blue: In most color cameras light is separated in 3 components labeled
red, green and blue. This matches the 3-color sensitivity of the human eye and the
3 phosphor emission of color TV displays.
Alternative, complementary, colors are sometimes used: Ye (yellow), Cy (cyan)
and Ma (magenta). But the choice is not even limited to these 6 colors.
rolling blade
shutter
A.k.a. curtain shutter. Moving slit mechanical shutter as used in SLR photo
cameras. Also used for the type of electronic shutter which does not expose all of
the image's pixels in the same period of time. Contrary of synchronous shutter.
RTS
Random Telegraph Signal: The noise of a single trap or interface state. Is of the
same origin as MOSFET 1/f noise
S/N or SNR
Signal to noise ratio: Ratio between the saturation signal (in volts) and the noise
level (in volts RMS).
saturation
The light level (the input quantity) or voltage (the output quantity) at which the pixel
has received so much light that its differential response drops to zero.
sensitivity
The property of a good sensor. A word with too many meanings to list here. The
difference with detectivity and responsivity is not clear.
sensor
"sensing" apparatus. In our world a circuit that in some way is able to sense the
amount of incoming electromagnetic radiation. Most people don't see the
difference with "detector".
sequencer
Digital circuit that generates the pulse trains that are necessary for the proper
operation of an analogue imager core.
shot noise
The fundamental statistical uncertainty on the amount of photoelectrons that are
generated by light falling on a photoreceptor.
SLR
Single Lens Reflex: denomination for standard reflex 35mm photo camera.
smart sensor
Imager combined with logic, on one chip. The chip should do something more than
just outputting the image contents.
spectral response
SR: Response of a photoreceptor over a spectral band. The ratio between the
photocurrent and the light energy. Expressed in [A/W]. Depends on the
wavelength of the light.
stare time
The total time impinging light has impact on the resulting frame. In the case of a
rolling blade shutter, stare time = exposure time + frame time. With a synchronous,
snap-shot, shutter, stare time = exposure time.
storage gate
MOSFET gate or CCD electrode that stores charge in the inversion layer or buried
channel underneath.
subsampling
Mode of operation of an image sensor. Pixels are read sequentially, but not
contiguously. I.e. some pixels are skipped, to obtain an image with lower
resolution, and perhaps at a higher frame rate.
substrate
The bulk of the integrated circuit. The "useful" circuits normally lie only on or just
below the surface.
synchronous
shutter
A type of electronic shutter, where all pixels in the imager are sensitivity to light
during exactly the same time span. Differs from the rolling curtain shutter, where
the instant of light sensitivity depends on the pixel's position in the image.
TDI
Time Delay Integration: Mode of operation of an area CCD to obtain very high
sensitivity.
transfer gate
MOSFET gate or CCD electrode that acts as a switch preventing or allowing the
transfer of (photo-) charge.
trappist
Benedictine monk, following the rule of the abbey of La Trappe. Later the beer
brewed by these monks. Later the reward for finding a fatal error in the design of a
CMOS image sensor. Later the name of such an error.
twin well
In a twin well CMOS process, there is of course the "real" well, but also an implant
of opposite type to increase the doping level of the substrate. All recent CMOS
processes are twin well.
VDD
Supply voltage of the polarity associated with "D", the drain of
nMOSFETs. The general supply voltage of an integrated circuit. Typically
5 or 3.3 volts referred to GND.
voltage
Potential, but corrected for the material's work function and diffusion potentials. Is
measured with a voltmeter. Reference is typically ground.
well
In Complementary MOS (CMOS), nMOSFETs and pMOSFETs cannot be placed
in the same substrate. One of them must reside in an area of the opposite type as
the substrate. In a p-type wafer the nMOSFETs are in the p-type substrate, and
the pMOSFETs are in the n-type (n-)well.
well pixel
Cypress's patented method to obtain a very high fill factor active pixel. The full
pixel area, except for the metallized areas, is light sensitive.
windowing
A mode of operation of an image sensor, where only the pixels within a window of
interest are read out.