The Image Sensor Lexicon ©
compiled by Bart Dierickx, version 19Jul2010.
|1/f noise||Temporal noise with a dominantly low frequency content. In this context the origin 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.|
|1.75T and 1.5T pixels||Are 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.|
|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.
Þ Another 4T pixels is the snapshot shutter pixel as used by FillFactory. This also resembles the 3T pixel, but a 4th MOSFET acts as a track&hold switch. .
Þ Caeleste’s “silencing reset” pixel
|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.|
|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.|
|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.
|Bayer pattern||A popular type of Red-Green-Blue CFA, invented by Brice E. Bayer (Kodak).|
|binning (charge 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. Charge binning can also be done in shared pinned photodiode pixels.|
|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?|
|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.|
|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.|
|CFD [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?|
|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.|
|color||human subjective experience of pure or a mixture of monochromatic wavelengths.|
|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.|
|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.|
|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).|
|DS||Dark signal. The signal due to dark current, expressed in [V/s].|
|Shorthand for “demolition”. The “demo-effect”: when a previously operational setup fails in front of you key costumer.|
|demosaicing||See color reconstruction. Recover RGB color information from a color filter mosaic pattern.|
|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. 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.|
|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.|
|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||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.|
|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 VRMS. 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.|
|dynamic range||ratio of highest non-saturating optical flux on the smallest detectable flux for a sensor. Is equal to the signal to noise ratio (SNR) 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.|
|EMCCD||Electron-multiplying CCD. A CCD with a very low read noise, as signal electrons are gradually multiplied in an output CCD section with high electric fields between CCD stages.|
|EMI||Electro-Magnetic Interference. In many cases the dominant fraction of the generic term “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:|
|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.|
|FireWire||FireWire or IEEE-1394: industry standard high speed serial link|
|floating diffusion||Circuit technique for the conversion of (photo) charge to a voltage, on a junction (that is also called floating diffusion of FD). Used in most CCDs and in most APSs.|
|flux||Light power per unit area [W/m2]. Light intensity.|
|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.|
|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|
|Control terminal of a MOSFET. In a CCD the gates are called “electrodes”.|
|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. It replaces the old unit “Rad”; 1 Gy is equal to 100 Rad.|
|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.|
|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.
|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.|
|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 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.
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:
|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.|
|I-SoC||Image sensor System-on-a-Chip|
|Motion image artifact in rolling shutter (CMOS 3T and 4T) image sensors. When panning the sensor left or right, object will appear to “wobble” due to the non-synchronous registration of the pixels from row to row.|
|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.|
|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.|
|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.|
|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.|
|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.|
|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 3×3 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.|
|multiple slope operation||High dynamic range method, as double slope, with more than 2 slopes.|
|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 Silicided Polysilicon, but the name persisted. yet in deep-deep-submicron processes, metal gates or fully silicided (FUSI) gates 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.|
|NA||Numerical aperture. 2NA = 1/f#|
|NEP||Noise Equivalent Power|
|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.|
|nMOSFET||n-type MOSFET, a MOSFET in which the conduction between n-type source and drain happens by electrons.|
|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 [VRMS] or QN [electrons])|
|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, 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 * 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.|
|photodiode array||Image sensor consisting of an array of passive pixels, where the passive pixels contain only a photodiode and a multiplexing switch.|
|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.|
|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.|
|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.|
|photon||The light quantum. The unit of energy exchanged in a single interaction between EM radiation and a charged particle. The photon, interpreted as a particle, cannot be in contradiction with the Maxwell relations, which are essentially the wave interpretation of light.
Reminds me of a quote: “Mr. Dierickx, photons are bullshit. All this quantum and particle blabla. I only believe in things that I can see with my own eyes.”
|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.|
|Ratio [%] between the number of generated electrons and the number of impinging photons. Closely related to spectral response.|
|QFW, Qsat||“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.|
|QN, Qnoise||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
|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.|
photo response (PR),
|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 (the McWorther model).
(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.
|Signal to noise ratio: Ratio between the signal (in volts) and the noise level (in voltsRMS). 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. Examples: GdOS, CsI, NaI, polystyrene, …|
|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.|
|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.|
|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 uniformities 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.|
|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.
|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..|
|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/|
|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.|
|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, that is 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, typicall the range 10nm to 400nm.
In dermatology, the following subclassification applies:
In astronomy and meteorology following terms have some traditional meaning:
|variability||Unlike outsiders tend to assume, transistors and other elementary components are not perfectly identical or reproducible amongst eachother. This variability is seen in image sensors as FPN, PRNU and DSNU. The main origins of variability are
· Lithographical, both due to the imperfection of the optics as to the randomness in the molecular nature of the photoresists and etchants
· Layer thickness variations, due to the stochastic nature of the deposition of growth processes
· Concentration, dopant density, variations due to the stochastic nature of the implantation or diffusion processes.
Variability is seen at several levels
· Local, purely stochastic variability. Also called “mismatch” or “intra-die variability”
· Local, reproducible, but not predictable, variability due to effects as proximity, orientation, density, equipment calibration…
· Wider scale (inter-die, inter-wafer, inter-batch) variability, also called “global” or “process” variability.
|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 drain 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).|
|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).|