Image Sensor Lexicon

The one and lonely

Image Sensor Lexicon ©

compiled by Bart Dierickx, version 12JAN2012.

1.75T and 1.5T pixels	Are “shared” pinned photodiode pixels (see 4T) organized in groups of typically 2 or 4, whereby the floating diffusion is common. E.g. a group of 4 pixels, having 4 transfer gates, and 1 reset, 1 source follower and 1 column switch, have 7/4= 1.75 transistors per pixel.
1/f noise	Temporal noise with a dominantly low frequency content. In this context its physical is typically in MOS or MOSFET interface states. Synonym of flicker noise.
1T pixel	Consisting of one photodiode and one access MOSFET, the simplest passive pixel.
2T pixel	Two 2T pixels have had some limited success in the past Þ Hitachi’s 2T pixel is a variant of the passive 1T pixel, with two switches in series, which allows a faster readout and a better FPN cancellation Þ Toshiba’s 2T pixel is a variant of the 3T active pixel, not needing a select switch. Row selection is done by the supply line to the source follower transistor.
3T pixel	Most classic and most straightforward active pixel, having three (3) transistors (T)(MOSFETs) and one photoreceptor. One MOSFET resets the photoreceptor; the second is a source follower sensing and buffering the photoreceptor node voltage; the third is the row selection switch.
4T pixel	There exits several types of four transistor pixels. Þ The best known 4T pixel is also known as the pinned diode pixel. Take a 3T pixel, and extend the photodiode/floating diffusion node with a transfer gate and a pinned photodiode. Þ Another 4T pixel is the charge-skimming (or direct-injection) pixel, which is also a 3T pixel, where a 4th MOSFET is placed between the photoreceptor and the voltage sense node to increase the charge to voltage ratio. This topology is almost identical to the pixels above, except that the photodiode charge cannot be completely evacuated. Often used in hybrid IR FPAs.
absorption depth	Light, as all electro-magnetic radiation, is absorbed (thus detected!) in Silicon over a characteristic 'absorption' 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 (thus 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'.
active reset	Methods to reset a pixel, that results in kTC noise reduction based in sensing and feeding back to the sense node voltage during the reset operation.
ADC	Analog to Digital Converter: An electronic circuit that converts an analog 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.
Anna Karenina-effect	The observation that one cannot make one real general purpose image sensor (or anything else) that is “good” for all specifications. One-size-fits-all does not work as so easily a spec will fail. In the novel by Tolstoy, the opening sentence claims that there is only one type of happy (ideal) family, and innumerable ways to be an unhappy family.
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.
Anti-blooming	A feature in both CCD and CMOS pixels that counteracts “blooming”.
APD	Avalanche Photo Diode. Photo diode constructed such that it can be operated in “Geiger mode”, i.e. it can be biased beyond its breakdown voltage, and breakdown will only happen triggered by the absorption of a photon. This breakdown can be used in photon counting. See also SPAD.
APS (1)	Active Pixel Sensor: Image sensor using active pixels.
APS (2)	Advanced Photo System: small format film, 30.2 x 16.7 mm.
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.
auto-saturation time	The average time for a pixel to saturate due to dark current alone, i.e. non-illuminated. The absolute upper limit for useful integration time.
Bayer pattern	A popular type of Red-Green-Blue CFA, invented by Brice E. Bayer (Kodak).
binning	charge binning A 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. Charge binning can also be done in shared pinned photodiode pixels.
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. Used in IR imaging.
bolometer	Radiation detector sensor/pixel based on the change of temperature due to radiation absorption. The change in temperature is sensed and readout through a strongly temperature dependent resistance.
BSI/BST	backside illumination and backside thinning. 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 metallization and dielectric layers, and the fill factor and UV sensitivity can be very high.
bulk	Substrate. With 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.
Buried diode	Photo diode which by an additional junction/implant is separated from the Si-SiO₂ interface. This implant pins the interface potential to a fixed value, likely the substrate potential. Its main effect is a reduced dark current, as it electrically separates the interface dark current generation centers from the collecting junction.
Caeleste	Caeleste CVBA, image sensor design house, specialized in high-end space, medical and industrial image sensors. For the origin of the name, see www.caeleste.com/name.html
CameraLink	a historic serial communication protocol and cable format designed for computer vision applications. Successors to CameraLink are GigE, 10GigE, HDBaseT™, emcore, CoaxPress, LAG, USB3, …
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, sometimes, 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 color filters overlaid on the pixel array.
C_FD [fF]	Floating diffusion capacitance.
CFT	Contrast Transfer Function: MTF multiplied by S/N. An attempt to define an image sensor quality factor that favors both sharpness and contrast.
charge skimming	Also named "direct injection", is a technique to sense charge with a high charge-to-voltage conversion ratio. The photodiode (or photo resistor) is at the source of a DC-biased MOSFET. The charge is integrated at the drains side of the MOSFET. Is used in the classic 4T pixel.
cheerleader effect	A classic misconception that nature systematically compensates bonuses by flaws. e.g. that beautiful girls must be dumb; that smart people must be nerds or socially inapt; that CMOS image sensors must be low end.
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 [x,y] representation of [X,Y,Z] color space. It can represent all physical colors (and even more!). I.e. any combination of monochromatic wavelengths, which is much more than what can be generated with a set of three primaries (typically R,G,B). All colors that can be generated by the linear combination of such primaries form a color triangle. CIE = Commission Internationale de l' Eclairage
CIS	CMOS Image Sensor
CMD	Charge Modulation Device: Image sensor technology based on the detection and charge sensing in isolated bulk (B) node of a (buried channel) MOSFET.
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 well protected.
co-integration	Answer to the question: shouldn’t we put a larger part of the system (digital or analog) on the same chip as the analog imager core?
color	human subjective experience of pure or a mixture of monochromatic wavelengths.
color reconstruction or demosaicing.	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 of 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.
CoM	“Conservation of Misery”. When you improve on one parameter (say, noise), you will inevitably sacrifice another parameter (say power, area, speed, design time, profit, salary…). It does not work the other way around: when you deliberately worsen say, noise, power, speed or your salary will not improve. Has some affinity with the Laws of Thermodynamics (conservation of entropy), the Law of Murphy and the Anna Karenina effect (see above).
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.
cosmic radiation	see X-rays
CRT	Cathode Ray Tube: The ancient display device, used in TV and computer monitors. Now largely replaced by LCD.
CTIA	Charge Trans-Impedance amplifier. Amplifier, not in the sense of voltage multiplier, but in the sense of translating the signal to a different domain, in casu, charge to voltage domain. A "charge-to-voltage" amplifier thus. The classic full-bred CTIA is an inverting amplifier with a capacitor in the feedback; but simply storing a charge on a (parasitic) capacitance, and reading the voltage with a source follower, as in the 3T pixel, is also a CTIA in principle.
cushion effect, cushioning	light flux projected through a lens is more attenuated at the sides and corners of the image. The response over the image area has the shape of a pillow / cushion. The effect is caused by the reduced angle of incidence away from the image center, and by the non-Lambertian absorption of the imager. Is modeled by cos² to cos⁴ laws. Extreme cushioning is called “vignetting”. Sometimes also pincushion- or barrel distortion is called cushioning.
D*	Specific detectivity. A figure of merit for the capability of a receptor or receptor technology, which is invariant for area and integration time.
DAC	Digital to Analog Converter: as ADC, but read the explanation back to front.
I_dark	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. Typically expressed in [pa/cm²], in [electrons/second] or in [V/s] (see dark signal).
demo-effect	from “demolition-effect”. An empirical law stating that nice setups often fail at or just before an important person coming to see it. See also CoM.
demosaicing	See color reconstruction
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". In hybrid FPAs the detector is the non-Silicon part.
dicing	Dividing a wafer into dice by scribing/breaking or sawing
die	die (plural: dice): rectangular piece of semiconductor material, thus more or less a synonym for “chip”.
dielectric layers	Insulating, transparent (?) layers on top of the IC. Typically SiO₂, SiN. Recently "low-k" dielectrics (polymers, black diamond, porous materials) are being introduced to lower the inter-metal capacitance for high speed and RF applications.
diode	Electronic device with two terminals, that conducts the current in only one direction. Originally a two-terminal vacuum tube, invented by Edison in 1880. Nowadays 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.
DQE	Detective quantum efficiency [%]. Quality factor used of X-ray detectors, combining the absorption, S/N ratio and MTF.
DS	Dark signal. The signal due to dark current, expressed in [V/s].
DS	Double Sampling or Uncorrelated Double Sampling: A method to read the magnitude of a charge packet, canceling a main source of FPN, but not the kTC noise.
dSNR dS/dN	Differential or incremental or “AC” S/N ratio. Similar to S/N but the signal is considered explicitly as the small signal fit to the non-linear S/P (signal/power, or response) curve. dSNR is lower than SNR when considering the typical non-linearities seen in real systems.
DSNU	Dark Signal Non Uniformity, expressed in [V/s]_RMS or V_RMS. The spatial noise due to the pixel-to-pixel non-uniformity of the dark current.
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.
DR	dynamic range. ratio of highest non-saturating optical flux on the smallest detectable flux for a sensor. Is equal to the SNR_max if and when the sensor features a linear transfer curve. For more refined definitions see here.
electron	Elementary particle with negative unit charge.
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.
EMC	Electro-Magnetic Compliance: the property to emit no (or little) radio wave EM radiation, or to be insensitive for spurious EM radiation or EMI.
EMI	Electro-Magnetic Interference: System noise.
epi wafers	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.
eye	in many cases still the reference in electronic vision. Superb image quality mostly thanks to the huge image correction engine behind it.
f number	(sometimes written as f# or f/#) Ratio between the opening (diameter) of the lens and its focal length. A high f-number means a large attenuation of the light power. A low f number is beneficial for low light imaging, but low f-numbers have also low depth of focus and are vulnerable to lens imperfections:
FD	floating diffusion. Circuit technique for the conversion of (photo) charge to a voltage, on a (not tied to a DC voltage, thus “floating”) junction (junctions were historically created by impurity diffusion). Used in most CCDs and APSs. Invention attributed to Walter Kosonocky, but disputed to have been foreseen by Boyle and Smith in the original CDD invention.
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.
FEE	Front-end electronics. The driving electronics that resides close to the FPA.
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 image sensor design house, active from 2000 till 2004 focusing on custom-designed and standard CMOS Image Sensors. Predecessor of Caeleste.
FireWire	FireWire or IEEE-1394: industry standard high speed serial link
flux	Light power per unit area [W/m²]. Light intensity.
FPA	Focal Plane Array. The image sensor as placed in the focal plane of a telescope.
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 and be ready for the next frame. Equals 1/frame_rate.
FT CCD	Frame Transfer CCD.
G	gate Control terminal of a MOSFET. In a CCD the gates are called "electrodes".
gamma	Response correction in cameras originally introduced to counteract the non-linear response of CRTs. The typical gamma correction takes the signal - or each of the R, G, B components - to the power of 0.45.
gamma rays	see X-rays
GND	Ground. 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.
gray [Gy]	SI unit of radiation dose, used in X-ray and particle imaging. A dose of 1Gy corresponds to a radiation energy of 1J deposited in 1kg of material (body tissue). For a relation between dose in Gy see here.
handshake	Motion blur due to the shaking of the hand of the photographer.
handshaking	Feature of many transmission protocols, for establishing the data channel or to acknowledge correct receipt.
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.
horror vacui	Fear of emptiness (Latin). The natural fear of an image sensor designer for empty silicon, and her/his insuppressible desire to fill this space with circuits, test structures, yield killers, logos, etc.
hue	A number indicating the spectral component of a color. The "H" in LHS. Scalar number indicating the spectral component continuously from read-yellow-green-cyan-blue-magenta-red.
hybrid	Imager consisting of a CMOS readout array chip and a separate detector chip.
IC	Integrated circuit. An electronic circuit manufactured on one piece of semiconductor. There may be one or more ICs on a single die or chip.
IL CCD	InterLine CCD.
image processing	Digital domain operations in hardware or software on images or image sequences. · "early" operators are intended to convert the raw image from the imager to a "nicer" image. · "later" or "smart" operators do something with the image contents. Typical "early" image processing operators are: spatial and temporal filtering, non-uniformity calibration, defect corrections, offset and gain correction, gamma correction, color reconstruction, demosaicing, white balance, saturation and hue correction, “matrixing”. Amongst "smart" operators are: image compression, code recognition, image interpretation, segmentation, correlation, feature recognition, automatic measurements, demodulation, time gating, distance ranging, ...
imager	Combination of the words image and sensor.
imager core	The essential part of an image sensor chip. Nothing more than the pixel array and the immediate peripheral circuits required to operate the pixel array.
t_int	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 real question is: what is "intelligent"? The first image sensor that can give an answer to that question is really intelligent.
intensity	radiative power per unit area; flux. Expressed in [W/m²] or [lx] (see luminance)
interface (1)	In this science (CMOS technology) 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.
interface (2)	At system level it usually is the hardware/software entity that links camera or camera chip to a system. Examples: video (PAL, NTSC), FiWi, USB, CameraLink, ...
interlacing	In the CCIR (PAL) and EIA (NTSC) video formats, the image information was 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. Btw interlacing is a completely useless feature. It is maintained because of the historical way video standards have grown.
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	Infra Red: EM radiation with wavelengths longer than the visible light. The following names are often used to denominate IR bands: NIR: near infra red, 700-1100 nm ("Silicon" NIR), or 700-2000 (spectroscopy NIR) SWIR: short wave infrared: As NIR, but without the Silicon part, say 1200..2500nm. MIR: mid infra red, 3.5-5 µm, the first atmospheric thermal window TIR: thermal infra red, 8-14 µm, the second atmospheric thermal window, including the peak emission of blackbodies at ambient temperature. FIR: far infrared, 14-1000 µm, only of scientific interest. Terahertz waves / ~imaging: EM radiation 1-10 THz or 30-300µm Sub-millimeter waves / ~imaging: 300GHz-1THz or 300µm-1mm Millimeter waves: 30GHz-300GHz, or 1-10mm
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	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. See ISO standard 12232.
junction	The place where a two materials join. In semiconductor diodes: the border between the p- and the n- material.
knife edge method	A method to obtain the LFS, hence the MTF, by illuminating a pixel over a sharp edge, as that of a knife.
kTC	Boltzmann’s constant * absolute temperature * capacitance. The thermodynamic uncertainty on the amount of charge left on a capacitor after disconnecting it from a DC voltage source. kTC noise is sometimes 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.
LHS	luminance - hue - saturation: an alternate way to represent colors in a 3 dimensional space. Is more suitable for certain image processing algorithms than the RGB space.
linear response	a light power (P) to voltage (V) conversion that can be approximated by a linear relation as: V = something * P.
linear sensor	Image sensor with a 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 * log(P).
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.
LSF	Line Spread Function: the Point Spread Function (PSF, see there) projected on one dimension. MTF is the Fourier transform of the LFS or PSF.
luminance	The human vision spectrally weighted "light intensity" or "black&white" component of a color image, or of a color video signal. The "L" in LHS, the "Y" in YUV. Carries sharpness, contrast, dynamic, dark/light/grayscale. Luminance is proportional to light power. The CIE notion "lightness" (L*) is a non-linear perceptual derivate of luminance. The notion "Luma" (Y') is a video-technique non-linear look-alike of luminance.
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 = 100lx (visible white light) to 250lx (visible + NIR).
matrixing	Early image processing operator that can be described as a 3x3 matrix operation on the R, G and B color channels. It implies white balancing and hue/saturation correction.
micro lenses	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 medieval times MOSFETs were made as a stack of Aluminum (metal) on SiO₂ (oxide) on Silicon (semiconductor). Today the gate metal is in most cases replaced with Polysilicon or Silicide Polysilicon, but the name persisted. (yet in deep-deep-submicron processes, metal gates or fully silicided gates might reappear again).
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).
Motion blur	Unsharpness in an image causes by the object, the scene, or the camera moving during the integration time.
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 pixel’s / sensor's inherent sharpness of focus.
multiple slope operation	as double slope, with more than 2 slopes.
NA	Numerical aperture. 2NA = 1/f#
NDR	Non-Destructive Readout. Mode of operation of CCDs and CMOS image sensors, where the size of a charge packet is read, without resetting it. This information can thus be read several times (oversampled), so as to improve the S/N of the reading operation
NEP	Noise Equivalent Power. In [W] or [W_RMS]
NIR	Near InfraRed: EM radiation between 750 and 1100 nm, invisible for the eye, but still visible for Silicon. For non-Silicon guys, NIR means 750-2000nm. See also SWIR.
nMOSFET	n-type MOSFET, a MOSFET in which the conduction between n-type source and drain happens by electrons.
noise N, V_noise Q_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 spatial/static noise it is sometimes called "temporal noise" (N [V_RMS] or Q_noise [electrons])
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. The most obvious passive pixel is the 1T pixel.
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.
photo receptor	or light receptor. The light sensitive device in a pixel (in this science). Typically a photodiode, a photo transistor or a photo resistor.
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 unit of energy exchanged in a single interaction between EM radiation and a charged particle.
photon counting	Detecting individual photons and counting these events.
PIN diode	p-n diode with a relatively important lowly doped (intrinsic) layer "i" sandwiched between the p-type and n-type regions. Due to its deep collecting volume suitable for IR and X-ray detection. Due to its thick depletion layer having a low capacitance.
pinned diode	Buried (photo) diode with the additional feature that it can be completely depleted, and thus emulate complete charge transfer as in a CCD.
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.
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?
preview mode	Mode of operation of a large, slow-scan, imager: It is read at high frame rate but at reduced resolution. “Video mode”.
PRND	Do not confound with PRNU. Seen on automatic gear boxes in US cars. Park-Reverse-Neutral-Drive.
PRNU	Photo Response Non-Uniformity non-uniformity in the slope of the photo-electric transfer curve between pixels. Also see spatial noise.
PSD	Position Sensitive Device: A solid state optical sensor that outputs analog signals that are measures of the position of a light spot on the sensor.
pseudo-random CFA	mosaic color filter array, where the colors are attributed to pixels in a (pseudo) random way. The purpose is to suppress color-moiré or color-aliasing. There is some anthropomorphic aspect in it too.
PSF	Point Spread Function. The “effective shape” of a photodiode. MTF is the Fourier transform of the LFS or PSF.
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 imager	A linear imager scanning a two-dimensional scene. Often used for earth imaging.
QE	quantum efficiency Ratio [%] between the number of generated electrons and the number of impinging photons. Closely related to spectral response.
Q_FW, Q_sat	“Full well” (or “saturation”) charge, a term persisting from the CCD era: the maximum charge a pixel can contain and readout. Used as a synonym is saturation charge, the charge at which the pixel saturates.
Q_noise	Noise charge, generally expressed in “noise electrons”
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 (emptying) the charge contents of a pixel to the initial dark value.
response, photo response (PR), responsivity	What an image sensor or pixel really does: to respond with a “signal” (typically a signal is a voltage [V] or a voltage slope [V/s]) as a response to light (“photo”) (typically expressed in W, W/m², lx…). The usual units of responsivity are thus V/W, (V/s)/(W/m2), (V/s)/lx, and alike. Click here for a view on the contributors to photo response.
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). The choice is not even limited to these 6 colors.
ROI	Region of interest. See Windowing.
ROIC	Read out IC: the (often CMOS) readout part of a hybrid image sensor.
rolling shutter	Also curtain shutter, blade shutter. Moving slit mechanical shutter as used in SLR photo cameras. Ubiquitous type of electronic shutter which does not expose all of the image's pixels in the same period of time. Contrary of synchronous shutter. Btw rolling shutter on its own is a completely useless feature. It exists because it is suitable for the classic 3T and 4T pixels.
RTS	Random Telegraph Signal. The noise of a single trap or interface state, seen in (1) stepwise two-level variation of MOSFET Vth. Is of the same origin as MOSFET 1/f noise, presumably interface charge traps modulate the conductivity of part of the nearby MOSFET channel. (2) stepwise variations of rate of a generation center. This is seen in pixels with a random on/off keying of the dark current. the origin seems to be a geometrically close pair of a generation center and a coulomb trap. The coulomb trap's state modulates the generation center's rate. Are often seen as the result of displacement damage by proton or other heavy particle radiation.
RWI	Read while integrate Pipelined synchronous shutter
S	Signal, the output of a pixel or image sensor, usually in volts [V]
S	Source. On of the terminals of a MOSFET.
SF	Source follower. Near unity gain buffer amplifier very frequently used in CMOS pixels and CCD, whereby the output is the MOSFET source, following the input on the MOSFET gate.
SNR S/N	Signal to noise ratio: Ratio between the signal (in volts) and the noise level (in volts_RMS). By default, it is understood that S and N are considered in the same operation point, but often, marketing oriented people use S at large signal and N at low signal.
saturation (1)	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.
saturation (2)	Property of a color, more or less synonym to spectral purity. unsaturated colors are pale or greyish. Back, white and grey have zero saturation. Maximal saturation occurs for pure spectral colors. The "S" in LHS.
scintillator	Material in which X- (also γ and UV-) photons convert into a visible light flash, which are subsequently detected in an underlying image sensor.
scribe lane/line	Area between chips/dice on a wafer, intended to be scribed or sawed. These ~100µm waste gaps are often used for in-process or process validation test structures.
sensitivity	The desired property of a good sensor. The semantic difference with detectivity and responsivity is not clear. Often sensitivity is intended to express the ratio between voltage signal (V, V/s) versus light flux (W/m², lx), resulting in units as [V/lx.s] or [V.m²/W.s].
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 sense a difference with "detector".
sequencer	Digital circuit that generates the pulse trains that are necessary for the proper operation of an analog imager core.
PSN	Photon shot noise. The fundamental statistical uncertainty on the amount of photoelectrons that are generated by light falling on a photoreceptor.
SLR	Single Lens Reflex (-camera)
smart pixel	Pixel with built-in intelligence, which can be analog or digital operators, event triggering, event counters, smart reset, systolic arrays, ADC-in-pixels, etc.
smart sensor	Imager combined with logic, on one chip. The chip should interpret, rather than just output, the image contents.
SNR_max	Maximum SNR, whereby the S is taken at or near saturation, and N in the dark.
SPAD	Single Photon Avalanche Diode. See APD. The same as an APD in “Geiger” mode.
spatial noise	the generic description of static (non-temporal) non uniformity or variability in the imager. The three most prominent components are FPN, PRNU and DSNU. See under these chapters.
SR	Spectral Response. Response of a photoreceptor as function of wavelength. In most cases the ratio between the photocurrent and the light power (or: photo current density and light flux). Expressed in [A/W].
stitching	lithographical composition of a large area image sensor by many exposures of smaller entities
storage gate	MOSFET gate or CCD electrode that stores charge in the inversion layer or buried channel underneath.
striations	(1) year ring -like modulation of dopant concentration in grown crystals and thus in wafers. Can sometimes become visible in an image sensor's image, as offset variations (FPN), as PRNU, or as dark current variations. (2) radial lithographical faults due to photoresist hindered by the wafer topography during spinning.
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.
SWIR	Short wavelenght infrared
synchronous shutter	Also global 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 shutter, where the instant of light sensitivity depends on the pixel's position in the image. further definitions: pipelined, full, or true synchronous shutter: where the next image can be integrated during the readout of the previous one. snapshot, triggered or sequential synchronous shutter: where the light integration and readout are separated (alternating) in time.
tape-out	In earlier days the (CAD-) design file was sent to the foundry or the mask shop on a magnetic tape. Today this goes via email or FTP.
TDI	Time Delay Integration: Mode of operation of an area CCD to obtain very high sensitivity when used as line scan device..
The black shoe under the table -test	Quick image sensor test done by hasty procurement officers at camera system companies to evaluate the low-light capabilities of a camera or sensor. In an office, with a comfortable 200 to 1000 lux illumination, the darkest spot is under the table, the darkest object readily available there is a black shoe. Does it image well?
The Disneyland set	Yes! there exists a publicly accessible calibrated scene for camera evaluation that does not look like a nearly bankrupt furry animal and grocery shop: Disneyland under a blue sky. Make a set of pictures of your kid in every attraction while your boss signs the expense note.
TOF	Time of Flight: a nickname for “distance ranging” or “3D” cameras based on the capability of each pixel to measure the distance from the [camera + laser source] to any point in the scene by the time the light needs to travel form source via object to the imager.
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. http://www.trappist.be/
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. A “triple well” is a twin well combination embedded in a lower doped “tub”.
UDS	Uncorrelated Double Sampling, as opposed to CDS. See DS.
UV	Ultra Violet, EM radiation with wavelenghts shorter than the visible light, typically the range 10nm to 400nm. In dermatology, the following subclassification applies: UVA: 315-400 nm UVB: 280-315 nm UVC: 100-280 nm In astronomy and meteorology following terms have some traditional meaning: XUV: 1-10 nm (X-ray UV) EUV: 10-100 nm (extreme UV) VUV: 100-200 nm (vacuum UV) DUV: less than 300 nm (deep UV)
VDD	Supply voltage of the polarity associated with "D", the drain of nMOSFETs. The general supply voltage of an integrated circuit. Typically 5, 3.3, 2.5, 1.8, 1.5, 1.0 and even lower volts referred to GND.
vidicon	Vacuum tube (CRT-) based image sensor.
voltage	Potential, but corrected for the material's work function and diffusion potentials, and is thus identical to the “Fermi potential”. Is measured with a voltmeter. Reference is typically called “ground” (GND).
VSS	Supply voltage of the polarity associated with "S", the source of nMOSFETs. Typically equal to GND.
Wavefront coding	Spatially/optically modulate the phase in the wavefront. In particular, the technique pioneered by CDM-optics to create TrueFocus, a technique that blurs the image with a constant amount irrespective the depth of field. After a subsequent digital sharpening step, the depth-of-field dependent unsharpness is absent. Thus creating an extended depth of focus/field in exchange of the S/N loss in de sharpening operation.
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.
windowing	A mode of operation of an image sensor, where only the pixels within a region of interest (ROI) are read out.
wonderbra image sensor	Sales demonstration of a mediocre quality image sensor, where the quality of the resulting images are "pushed up" by external means like image post-processing, defect hiding, etc.
X-rays	In general the electromagnetic radiation with a wavelength (roughly) shorter than 1 nm. More specifically the radiation emerging from deceleration of electrons (Bremsstrahlung), which is the key difference with gamma rays (originating from nuclear reactions) or cosmic radiations (originating from astronomical objects).
Y	Luma. As in YUV. See Luminance.
yield	The “Y” word, the key specification of any electronic (and other) component. The ratio of devices manufactured and devices in spec (or actually used).
Y_SNR10	Scene illumination in [lx] whereby SNR=10:1, as measured on the Y (luma) channel. Silent assumptions: after RGB demosaicing but smoothing and image processing not allowed; the scene is a large, homogenous, grey 18% reflectivity patch; the illuminant is a 6000K blackbody; the lens is a perfect f-number 2.8 lens.
ZPS	Zero point something. Term used by Caeleste for pixels or image sensors that have a read noise far below 1 electrons_RMS.