Instrumentation Yair Talmi
Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, TN 37830
Evaluation of the potential spectroscopic applicabilities of imaging devices is hard to comprehend, since most literature concerning their performance is written by the manufacturer and can be considered nonobjective, a t best. Even if we assume their complete innocence, these performance reports are tailored toward scenic rather than spectroscopic applications. Acknowledging these factors and being well aware of time and patience limitations of the reader, we hope that the following discussion and the preceding REPORT will serve as a limited-scope critical review intended primarily to draw the attention of the analytical chemist to the large variety of instrumental capabilities now becoming available. For more detailed reviews, we recommend the following sources (1-4). The definitions and acronyms that are repeatedly used throughout these reports and thus need clarification are given in the Glossary a t the end of this article. Photoelectric devices are similar to photographic cameras in the sense that a device consists of a means for collecting and focusing electromagnetic radiation and converting it by means of a sensing surface into a form that can be stored and/or observed. T h e TV cameras and the image intensifier (direct visual reading) merely replace the photographic plate in the focal plane with a photoelectronic transducing surface.
screen where the enhanced optical image is reconstructed. A light gain [(photon)output]/[(photon) input] is achieved since the photoelectrons produced a t the photocathode are accelerated to the phosphor. Through the use of optical-fiber faceplates, a few such intensifier tubes can be interfaced in tandem to form a cascade image intensifier with gains of up to IO6. The limit on gain is set primarily by the definition, geometry, and contrast losses characteristic of such devices. The photoelectrons can be focused by electrostatic or magnetic fields, with the former being cheaper and simpler to operate but the latter providing much better resolution. Another definite disadvantage of electrostatic focusing: it normally requires the use of an optical-fiber faceplate in the entrance window to comply with the electrostatic field contours. This limits the lower spectral response to about 370 nm.
A very useful device is the UV proximity-focused converter, from Galileo Electro-optic Corp. (Figure 1).Basically, the device is comprised of a MgF2 window, a Cs/Te UV solar-blind photocathode (as the photon-to-electron transducer) and a phosphor screen, and an optical-fiber faceplate which serve as the light output window. The device is very compact and serves as a limited gain (-20) UV-toVIS image converter (115-300 nm) with a resolution of 5 lp/mm a t 20% M T F and low dark current. Photoelectrons are accelerated by an electrical field over a very narrow gap, and thus a focusing section is not needed. Microchannel Plates (MCP) Image Intensifier. Until recently (1971), the MCP image intensifier, developed by the Army Night Vision Laboratories, was classified confidential. The MCP device (6),now available from Varian and Galileo, is basically a highly compact, disc-shaped
PHOTOCATHODE
1
ALUMINUM LAYER
-FIBER-OPTIC FACEPLATE (OUTPUT)
I m a g e Intensifier
Electron Image Intensifiers, These are devices which are capable of converting an input photon flux into an enhanced output flux and/or converting one type of energy to another, i.e., UV or X-ray to visible, in which case they are referred to as image conuerters. Through the use of a photocathode, a photon image is first converted into a corresponding electron image which is then focused by an electron-optics stage onto a phosphor
uv
1
Figure 1. Cross section of proximity-focused UV image converter (Galileo)
HIGH VOLTAGE
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heating and cooling
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AMPLIFIED ELECTRON OUTPUT EL
co
GLASS HOLLOW CHANNEL MULTIPLIER ARRAY
INCIDENT RADIATION (INPUT)
-
MICROCHANNEL
-
AMPLIFIED ELECTRON OUTPUT
__c
OUTPUT CONTACT INCONEL FILM
-
8).
VM GAIN CONTROL
(-1-2 in. diam. X 0.02-0.07 in. thickness), continuous dynode electron multiplier imager. I t consists of millions of microscopic (10-40 wm diam.) hollow-glass (fibers) conducting channels (fibers with a resistive coating on their inner surface), each acting as an individual electron multiplier with absolute position (geometrical) registration between the input and the output faceplates of the device. These channels are arranged in a highly patterned array and electrically connected, in parallel, by metal electrodes placed on either side of the disc (Figure 2). The MCP is very compact and thus amenable to cooling and interfacing. It can provide electron gains (variable) of iO”-lO4 over an active diameter of more than 30 mm with an electron equivalent input noise of iO-16-iO-15 A/cm2. When operated in tandem, gains of 106-10’ can be obtained with low background noise (1.5 counts/seccm2). The MCP is ideally suitable for high-speed spectroscopic studies since the electron transit time is less than 1 nsec. It is capable of detecting a large variety of radiation, Le., positive ions (0.5-50 keV), electrons (0.5-100 keV), soft X-rays (0.2-6.8 nm), and UV (30-150 nm), and can provide resolution of 30 lp/mm at high light levels and 10 lp/mm a t low light levels. At its limit the resolution is determined by the center-to-center spacing of the channels (p) and is equal to 1/2p. At the present time, there are still a few disadvantages involved in its operation, e.g., gain degradation over its lifetime (a few thousand hours) and its exponential pulse height distribution, causing a scintillation effect, especially a t low light levels. Its necessity to operate in vacuum restricts the flexi700A
capability in the ID which separates it from all other image devices. The design of the shape and size of the aperture makes it possible to enhance sensitivity a t the expense of resolution and vice versa, but once implemented, its design is permanent. Some of the most appealing characteristics of this detector are: very wide dynamic range, photon-counting capability, extremely rapid scan (up to 1 scan/nsec), high linear resolution, real-time output, and random access capability. Several schemes have been proposed to overcome the 1/N scan loss. Multiaperture Image Dissector. Increasing the number of apertures to M and using an equal number of separate P M T tubes or channeltron multipliers for detection will improve the SNR of the ID by a factor of M / N (7,
bility of its interfacing to TV camera tubes. MCP image intensifiers with gain of 1000 to 50,000 and designed to operate a t different spectral ranges are currently available from RCA, IT&T, and Galileo. Rapid Electron Scanners
Image Dissector (ID). The image dissector is basically a photomultiplier tube (PMT) in which a field of view is electronically scanned across a small aperture (Figure 3). Its photometric accuracy is that of a PMT, assuming that the photocathode is scanned on a microscopic scale comparable to the resolution of interest. The sensitivity of the system is determined by the fraction: area of defining aperture/ area of scanned spectrum. Thus, there is a 1/N scan loss (N, number of resolution elements to be scanned across the spectrum) or lack of integration
Smoothing Dissector. If the generated electrical image pulses are time stretched long enough compared to the duration of the readout sampling, they will be counted, and thus a partial integration capability is achieved. The interfacing of the ID to an image intensifier with a phosphor screen as the output readout provides such pulse stretching, since the finite decay time of the phosphor is utilized as a temporary storage register which is then read out by the dissector. The overall gain accomplished depends on the phosphor used and the image intensifier gain (9).Unfortunately, the very essential capability of the phosphor to store signals, especially a t high illumination levels, makes it impossible to simultaneously monitor very weak and very intense spectral features, thus reducing the effective dynamic range. Also, when the system is operated in the photon-counting mode, a substantial reduction in the SNR and the dynamic range can occur because of multiple count of the same event.
Figure 3. Schematic representation of image dissector
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
Signal-Generating Image Tubes Electron Beam Imagers. In this category are included imaging devices that use a focused electron beam to accomplish the readout of a latent image on a photoconductor or a dielectric storage layer. T o detect an optical image, the tubes are required first to transduce it into an analogous electrical charge pattern, then to integrate and store the charge image, and finally to read it out. The readout is accomplished by raster scanning the potential pattern on the target with a focused electron beam, which is also responsible for the restoration of the target to a uniform equilibrium potential (equal to the electron gun cathode potential), thus preparing it for the next image storage. Intensification (gain) of the image signal can he accomplished hy the following schemes: amplification of light input, i.e., image intensifier; amplification of charge modulation through characteristics of the integration-storage mode, e.g., gain in target as in EBT devices; multiplication of photoelectron prior to storage as in the microchannel plate; amplification through operation with
the electron beam as in the image orthicon tube. Basically, signal-generating tubes (Figure 4) can he separated into two groups. The first, image section tubes, includes image orthicon and isocon, SEC, SIT, and ECS. In these tubes the photon image is first transduced by a photocathode into a charge (or potential) image, which is then intensified and stored on the target. The second group includes the vidicon, Plumhicon, and silicon vidicon tubes. Here. a photoconductor target is used both as the photon-to-charge image transducer and as the charge image storage element. The first group of tubes offers versatility in the selection of sensors and charge storage capahilities, as well as high sensitivity, whereas the second group offers desirable simplicity and stability. Self-scanned Solid-state Imaging Systems. These are devices consisting of arrays of discrete photosensitive elements which are scanned by means of electronic circuits that are integrated on the same semiconductor wafer. At present, these devices are limited to "silicon technology."
/ELECTRON OPTICS lOEFLECTlON AND FOCUS COILS)
1%
d
SCANNING SECTION
VIDEO SIGNAL OUTPUl
10)
Electron Beam Imagers. Image Orthicon and Image Isoeon. Both devices utilize an image section to intensify the photoelectron signal and a thin insulating film as the storage element. When the readout electron beam strikes the target, a small fraction of i t is absorbed, another fraction is elastically scattered, and most electrons are simply reflected hack toward the electron gun. The return electron beam carries signal information (subtractive modulation), either in the reflected (orthicon) or the scattered (isocon) fractions [Figure 4(c)] which is detected by an electron multiplier to produce the raw video signal. The image isocon tube is far superior to the image orthicon. It provides higher sensitivity, Le., the minimum light input level detected is one tenth that of the orthicon, a t least three times higher SNR, and a ten times wider dynamic range. Direct Beam Readout Vidicon. Common to all vidicon T V tnhes is the use of a photoconductive target as both the sensor and the storage element and the mode of extraction of the video signal, which involves the following steps: restoration of the target equilibrium potential (by electron beam); discharge of the target capacitance by photoexcited electron-hole pairs to produce the charge (potential) image; and scanning of the target by the readout electron beam, resulting in a capacitively coupled video signal to the front-side transparent signal cathode. The various vidicon tubes differ mainly in their target composition and structure an,d thus in the nature of the charge image that is prodnchd. The most common vidicons use antimony sulfide as the photoconductive transducer. They are low priced and have a controlled gain hut suffer from excessive lag and nonlinear transfer characteristics (y = 0.6-0.7). Both the Plumhicon, with its nondisCrete PhO target acting as the p-i-n diode, and the silicon vidicon (SV), with its dense array of discrete p-n junction photodiodes, show improved sensitivity, linear transfer, and reduced lag; the SV tube also provides an extension of the spectral range to the near IR. The Plumhicon, although much more limited in its spectral coverage (max. 650 nm) has a very low dark current and can be integrated for significantly longer periods than the SV. In addition, the nondiscrete na-
Figure 4. Schematic representation of electron beam sensors (a) Vidicon (b) SEC and SIT (c)Image Onhicon and ismon
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7. JUNE 1975
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1
0
carbon in water: total organic carbon volatile organic carbon total carbon The Dohrmann DC-50 organic analyzer makes all of these measurements accurately and rapidly. Based on proven, E P A approved* methods, it avoids interferences and undesirable pyrolysis reactions t h a t historically have resulted in significant errors. D I R E C T R E A D O U T : Four-digit presentation shows carbon content directly in mg/liter or ppm. N o recorder needed! D I R E C T M E A S U R E M E N T : A single sample injection gives either Organic Carbon or Total Carbon content
directly, not by difference. I N D E P E N D E N T M E A S U R E M E N T : Volatile Organics are determined separately from Total Organics to aid in source identification. R E L I A B L E M E A S U R E M E N T S : Determines important, lightweight volatiles such as low molecular weight alcohols and ketones, normally lost by acidification and sparging. F A S T : 5 minutes per determination ACCURATE: Repeatability of f 1 mg/liter or f 270 0 W I D E R A N G E : 1 t o 2,000 mg/liter ( p p m ) without dilution $7,875, including start-up assistance .PRICE: and operator training
* ENVIRONMENTAL PROTECTION AGENCY, WATER PROGRAMS, Guidelines Establishing Test Procedures for Analysis of Pollutants. FEDERAL REGISTER Vol. 38, No. 199, Part II, Oct. 16, 1973.
for brochures, reprints, or dates and locations of seminars, contact:
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automatic determination of pore size, volume distribution and surface area? SORPTOMATIC Determination of complete adsorpti o n-deso rp t io n isotherms, su rf ace area and pore distribution. The readout of the pressure-volume values can be provided, according to the different models, on pneumatic recorder, strip chart recordkr and digital printer. Computer compatible.
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1 to 2000 m2/g Detectable pore radius: 15 to 300
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An instrument particu!arly versatile for quality control analysis, process control and many other applications requiring higly accurate surface area determinations in a short time. The surface area value can be directly expressed either on a digital counter or on a printer. Measurable surface area:
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POROSIMETERS Determination of pore size and volume distribution by mercury penetration. The pressure-penetration values are plotted both on a pressure recorder or directly printed in numerical form. The instruments are equipped with the hysteresis cycle for the evaluation of 10
55 35
15 5 0.3 0 20
0 1-5 I TT Westinghouse EMR
materials and photocathodes are used. These figures are based on relative response rather than % QE. A This extended spectral range IS obtained with PbO-PbS rather than PbO target. * Lag is defined as signal remaining on target after one readout of a 200-nA signal at standard TV rates. 1 Lag IS insi nificant here since functions of expasure, storage readout,,and erasures are totalfy separated. These tubes were specifically designed for 8 A S A . Lag is defined as signal remaining on target after third readout.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
705A
self-scanned array devices are similar in the mechanisms, limitations, and capabilities of the sensing elements hut differ most significantly in the arrangement for signal amplification and scanning. Light sensing is obtained by photoemission into a reverse-biased depletion layer operated in the storage mode. Two methods of self-scanning are used: digital multiplexing, (switching) as with photodiode and phototransistor arrays, and analog transfer by movement or injection of charge packets as in CCD devices and CID arrays. Amplification can he achieved hy using either one amplifier per sensor, one amplifier for many photosensors, or a single amplifier for the entire array. The fundamental performance criteria for solidstate arrays which set limits on their dynamic range and SNR are analogous to those of electron tube imagers. These criteria include: geometric resolution, determined hy the number of pixels and their size and spacing (density); quantum efficiency and its wavelength dependence, determined hy reflection and absorption losses in layers overlying the silicon and the ahsorption depth and by the electrode consistency and structure of the devices; and finally the output signal, determined by scan rates, integration periods, element size, quantum efficiency, and light exposure. Photodiode Arrays. Linear arrays with densities of up to 1000 photodiodeslin. are now available. T o perform the essential functions of the array, a sensor, an amplifier, and address and control circuitry are associated with each individual diode. A reverse-biased p-n junction diode serves as the photosensitive element, and addressing (readout) is done by a shift register. Integration i s accomplished during periods between reset pulses (which recharge the diodes) with a consequent efficiency proportional to the ratio of the integration period to the total periods between consecutive reset pulses. The sensitivity of the device is limited by l/f noise in the various components and by the systematic pattern noise derived from diode-todiode variations in current DC leakage from sensitivity gain and from charge, coupled by the reset switch. Phototransistors. Phototransistor arrays are not commercially available, although such devices have been constructed and studied for use in earth observation scanners. The phototransistor array hasically comprises an LSA (large-scale array) chip consisting of 195 phototransistors and their corresponding preamplifiers, and a SCE (signal conditioning electronics) chip consisting of five amplifier channels physically located on a common LSA substructure assembly. The five 706 A
Figure 6. Operation of CCD in storage and transfer modes (a) CCD storage mods
(bl CCD transfer mode
uurpur signals rrum the SCE require additimill signal conditiuning, multiplcxinq, and digital datu proreising. The reverse-biased wllecfor junction 01' earh photorran:'.litor ~ ~ i ~ as ta ' i phnrusensor. fne emitter junction prw vides the w i t ( hing, 2nd ilii individual praarnplif:~~ transistor provides the gain. Phototransisrim nrrays provide gain and arc more scnsirive than photodiode arrays; unforrimutely. pattern noiw is also more srvere Iwcause of variation9 in both tne responsivifv nnd dark current of rhe ind:viduiil pixel.; ('harw ( 'nupled Ikutc(,n iCCDI. CCU's are nearly ideal semic~mducror anolog xhiii r s g i i t t m which perfurm the iniiquc Task oi nianipularinx inlormarion ( 1 11. Linear c(.'ndevicrs u t i liied as imagers h i r a l l y cumprise an itrrav (l?X-1728) oi rlo.wlv spaced mprsl-insiilati~r-;eini~,,nditcti,r\ M I S ) rapacitors. Earh MIS pixel cunsisti of a mndurtive electrde and a rhix uxide layer insulator on u,p o i a semicmdiictor p-type silicon. When a positivc volrage is applied 1%) the conduct i v e Qlectrtrdein t h e MIS. ii inonequiIibrium (thermal, charge depletim region ,porential .'well''l is formed undwnearh ~n rhe jilicon (Figure G I .
ANALYTICAL CHEMISTRY, VOL 47, NO. 7, JUNE 1975
These potential wells serve as temporary individual storage elements for the photo-generated electrons (signal) as well as for the thermally generated electrons (dark current). The amount of charge accumulated is a linear function of the incident illumination intensity and the integration period. Typically, each well can store up to 1@-106 electrons. T o avoid ineffective coupling of charge across the interelectrode gaps, a "silicon gate" technology was developed. In this approach, the gaps between the electrodes are replaced by undoped polysilicon film, thus forming a highly resistive dielectric region which helps to define the potential in the interelectrode space and also to protect the active oxide from the environment (Figure 7). Readout is accomplished by a three-phase (or two-phase) voltage arrangement sequentially applied to the electrodes to ensure the unidirectional transfer of the accumulated packets of charge across the substrate to be serially sampled hy the readout amplifier (Figure 6). Two contrasting approaches to the operation of area imagers have aroused considerable controversy as to
METALLIC (AI) GATES
’OLY Si - GATES
Safety and Precision for
Si 02
SUBSTRATE ( p -TYPE SILICON)
OPTICAL INPUT
NONILLUMINATED
A
CCD REGISTERS-
PHOTOSENSOR ELEMENT
PHOTOSENSITIVE (IMAGING) ARRAY
The MLA PrePHOTOSE NSl TI VE ARRAY
’ORARY STORAGE ARRAY READOUT REGISTER tDOUT REGISTERFRAME TRANSFER READOUT
INTERLINE TRANSFER READOUT
.I. And safety!
Figure 8. CCD area array modes of operation
their relative merits: the frame transfer (vertical transfer), with separated photosensors and storage registers, and the interline transfer system, with alternate rows of photosensor elements and CCD shift registers. In the frame transfer mode, all charges collected during the integration time are shifted into a shielded storage section each I/& sec by simultaneously clocking all the vertical registers in unison. The signal is then read out by sequentially shifting in parallel one horizontal line (storage) after the other into the output CCD register where i t is read out as a linear CCD. In the interline transfer process, sensing sites integrate for a lj& sec as “A” sites are shifted sideways a t the end of the first integration and “B” sites are shifted in the opposite direction during the following integration time (Figure 8). With standard CCD’s, only the light that falls in the gaps between the
System, that is. Major hospital laboratories have standardized o n the MLA Precision Pipetting System after competitive evaluation for p r e c i s i o n and accuracy. The MLA Pipette displays outstanding repeatability
opaque electrodes (typically, 30-50% of the total illuminated area) is monitored. Such light losses can be eliminated if transparent (doped) polysilicon electrodes are used. Unfortunately, this design will not improve the UV response of the device. Alternatively, the light image can be monitored a t the back surface of the CCD. The silicon substrate has to be substantially thinned (20-25 Mm) so that the depletion region formed by the polysilicon gate potentials will reach the depth a t which the photo-generated charge image is formed (back surface) (Figure 7 ) .Essentially, all incident light is then effective in producing a signal charge. Another imaging problem, blooming, can be eliminated in linear CCD arrays (as well as photodiode arrays) by the implementation of sink diodes, always reverse-biased, along the sensor to drain any excessive overspilled charge from oversaturated
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
707A
wells. For two-dimensional arrays it seems more reasonahle to bury the sink diode under the photogate to avoid a reduction in the number of pixels, resolution, and format. The readout mode of CCD involves the transfer of charge packets across the target, and the transfer efficiency will determine the quality of the reconstructed output image. The transfer characteristics of a device are defined by the product of the transfer inefficiency, Ei (fraction of charge loss in each transfer step) and the number of transfer steps, N, required. Typically, a 1000 element linear CCD shows a total transfer inefficiency lower than 0.3, sufficiently low to produce an acceptable image fidelity. A different variation of CCD is produced by Bell Northern Research: a linear 256-photodiode array which is internally scanned by two charge coupled analog shift registers for readout. The device might he more amenable to silicon thinning and thus to UV and electron detection. Charge Injection Device (CID). This imaging device, recently developed by General Electric (151, utilizes an x-y addressed array of charge storage capacitors which store photongenerated charge.in MOS inversion regions. Each sensing site comprises a pair of charge storage capacitors of which one is controlled hy an x-row drive line, and the other by a y-column drive. The charge generated during the integration is collected in the potential wells formed by both storage capacitors: readout of a selected pixel is achieved when the voltage on both capacitors (electrodes) is removed. The x-y readout is accomplished by first removing the voltage from only one electrode, thus transferring the charge to the remaining potential well, now located under the second electrode only. Next, the voltage of the second capacitor is removed, and the charge is injected into the substrate (Figure 9). The resultant raw video signal contains also the parasitic capacitive coupling of the drive voltage to the substrate. This drive voltage interference is canceled through the use of an integrating readout technique. Charge injected from a sensing element must he diffused entirely to avoid lag. Injection to a regular suhstrate (hulk) is complete within about I fisec. For fast readout, an epitaxial structure is used in which the epitaxial junction is reverse-biased to form a charge collector under the array accomplishing a complete injection in less than 0.5 fisec. T h e epitaxial structure also provides better resolution hy reducing the charge cross-talk hetween sites, hut it is less sensitive (about half) than the hulk structure. 708A
vower COSV, avoiaance or cnarge transfer losses, and random access readout. The performance of various SSID’s is given in Table 11.
‘‘Application of SECTV to Neutron Diffraction,”Ref. No. 8. (13) Y.Talrni,Anol. Chern.,47(7),658A
References
Goldherg, C. M. Puckette, J. J. Tiemann, “Two Classes of Charge Transfer Devices for Signal Processing,”International Conference on the Technology and Application of Charge Coupled Devices, Edinburgh, Scotland, Sept. 25-27,1975. (15) G. J. Michon and H. K. Burke, Charge Injection Imaging,” IEEE Intern. Solid-state Circuits Conf. Digests, p 138,1973.
(1) G. R. Carruthers, Appl. Space Sci., 14, 239 1101711
~
.~
~~~~~~
~~
Vancouver, BC, Canada, May 1973. ’ (5) R. Bracewell, “The Fourier Transform and Its Application,” MeGraw-Hill, New Ymk., -NV lgfi.5 . ., . .- . ( 6 ) 6. ~ J. Ruggieri, IEEE Trans. Nuel. Sei., NS-19 (3) (1972). (7) S. W. Duckett, Ref. No. 3, p 119. (8) “SpectroscopicApplications of Image Dissectors,” E. H. Eberhardt, 18th Annual Conference on Analvtical Chemistrv in Nuclear Technology, “Symposium onApplicability of Multielement Detectors (9) E. H. Eb”&harht and R. J. Hertel, Appl. Opt., 10,1972 (1971). (10) W. J. Dreyer, A. Kuppermann, H. G. Boettger, C. E. Giffin,D. D. Norris, S. L.
* ANALYTICAL CHEMISTRY, VOL. 47. NO.
7, JUNE 1975
(12) J. B. David& ,ln.7E\
I’” I O , .
(14) R. D. Baertch, W. E. Engeler, H. S.
Glossary Resolution: Resolution will be given in lplmm at a given MTF value. Combined with tonal transfer, resolution defines how well the imager preserves the image details MTF: Modulation transfer function or sine-wave response of a system, normalized to unity, as function of spatial frequency, e.g., Ip/mm. It describes the change in amplitude of a sine-wave pattern transmitted through an imaging component and given as ( A - B ) / ( A+ B ) where A is the input amplitude, and B is the decrease observed in the output amplitude. The overall MTF of a device is the product of the individual MTF values of all components comprising the device, e.g., lens, intensifier stage, target, electron optics, optical fibers, etc. MTF describes the response of the detector to small details within an image. MTF values depend on the input light intensity
Table II. Characteristics of Solid-state Image Devices. Silicon photodiode arrays
Charge coupled devices (CCD)
Silicon hotodiodes with C 8 D readout
Format, no. of pixels (elements)
Linear arrays: 128-1872! elements. Area arrays: 32 X 32 and 50 X
Pixel size, pm
50 25.4 X 25.4 to 25.4 x 432. 13 X 17,30.5 X 30.5 (Fair- 18 ctc x 10 width 25.4 center-to-center child), 25-125 length (ctc). 15 ctc for 1872 (RCA). 30 ctc
Sensitivity,* photons/cm?
1.8 x 107 to 3 x
Charge injection devices (CID)
Linear arrays: 256-1728. Linear arrays: 128, 256, Area arrays: 100 X 100 and 188 X 244 and 512 Area arrays: 100 X 100 and 320 X 512
31.5 X 61L
array 108
Linear: 1.3 X IO8 to 4 X
100 x 100: 7 x 107 Linear: 4 X 1010 to 6 X 1010. Area: 1.3 X 1Olo 200-400 1000 5-10 mVh, -300-1000
2.2 x 108
108
4 x 10"
5 x 1011
108.
1010 to
Saturation exposure, photons/cmz Dynamic range Noise,c electrons/pixel Dark noise,d electron/pixel
2.4 X
Integration time, (sat. time by dark current), sec Clock rate, MHz
At 25°C: 1. At -40°C: 500 At 25°C: 1-2. At -40°C:
Resolution,e % MTF Manufacturer
1000-20,000Q 1000-2000 >loo0
5 X 1011
30-90 Min consists with inte- 0.05-10 gration time. Max: 10 40-60 58 Reticon
Ava ilabitity cost
200-500 500 1.0 mVk 5000" 1.0 mVh (dark noise not 3000-4000 available) A t 25°C: 0.5-1
At 25°C: 1-2. At 40°C:
0.01-2
240 0.05-4
66
85
Bell Northern Research General Electric Fairchild, RCA, Bell Northern Research* Now Now Now $65001 Linear: $275-1500. Area: Price available from company upon re100 x 100: $965 (Fairquest child). 320 X 512:$3800
Off the shelf $750 for 256 X 1 array. Other sizes, price is proportional to number of pixels. 1872 pixels N 3800 The reader is advised to use the data iven in this table very cautiously because: the data are based on information gathered only from the manufacturers; it is not easy to corn are t t e s e devices because the characteristics demanded by a particular application are important; and the various manufacturers disagree on $e definjtion of a few parameters (in particular, sensitivity, noise, a n d dark noise). Sensjtivity is,defined.as the ex osure (at 550 n m ) level at w h i c h the signal level equals the peak-to-peak random noise level. C Noise IS measured within the video period Of eacR pixel period a n d at a specified (by manufacturer) test frequency. Main noise source Is input capacitance n,oise. This,value refers to the shot d a r k noise rather than to the d a r k current e Resolution is given a s % MTF at the Nyquist,limit spatial resolution (see definition Of aliasing) and for visible I i ht. It,will be worse for IR ill;mination. I Soon to be announced. 0 Dynamic range will depend on amplifier used. This capability is supposedy achieved because of the larger storage capabi!ity available. h This value refers to the average d a r k current rather than to dark noise. T h i s company offers a 100 x 100 pixel area CCD. Price includes camera. k Value expressed in electrons(pixe1 was not available. This and all other parameters below are given for the 188 X 244 CID imager. m A new amplifier, now being designed, I S expected to lower this value by a factor of three.
Lag: The fraction of charge retained on the target after a single readout and thiis appears as interference (memory effect in succeeding frames)
Stellar Magnitude: Astronomical scale of brightness of stars and planets. One magnitude corresponds to a light ratio of (100)1'5 = 2.512 P M T : Photomultiplier tube
Dynamic Range: The range of radiance values, in a single detector frame, over which the detector is sensitive to changes in radiance. The limits of this range represent the lowest and highest intensity features that can be monitored in one frame
Searching (Random Access): The capability to scan locally only those regions of the image t h a t contain relevant information, rather than the entire image Blooming: Cross talk between adjacent channels because of spreading of charge to neighboring pixels. T h e result is loss of resolution and fidelity
S N R : Signal-to-noise ratio
Accumulative D,ynamic Range: The range of radiance that can be used by the system under separate illumination and integration time conditions for high and low radiances. This parameter is always significantly larger than the dynamic range but does not reflect the ability of the system t o measure high and low radiances simultaneously
Aliasing: When the spatial frequency of the image is greater than that of the imager (density of pixels), a Moire pattern is produced which reduces the monitoring accuracy. According to the Nyquist theorem (51, the sampling frequency should be a t least twice that of the image's spatial frequency to eliminate aliasing
Iplmm: Line pairs/mm. A white line and an adjacent black line are designated a line pair Tonal Transfer: T h e contrast characteristic of the image, corresponding to gamma in photographic systems
Integration Time: Time during which the storage target accumulates electrical charge generated hy image detections. Commercial T V systems use an integration time of sec Storage Time: The time during which the storage target can preserve the signal charge image without deterioration in fidelity Readout Rate: Rate at which the charge image stored on the target is read out. I t is determined by the electronic bandwidth available for transmission and by the efficiency of the readout mechanism
Pixel: Picture element. Resolution (sens. ing) element of the T V target matrix
RQE: Responsive quantum efficiency, Le., yield of photoelectrons per photon for a particular wavelength. This is a figure of merit for photocathodes
EBS: Electron bombardment silicon
EBT: Electron bombardment target
SIT: Silicon intensified target
SEC: Secondary electron conduction (tar-
Pattern Noise: Coherent nonrandom noise which originates from pixel-to-pixel variations in spatial response and dark current
get)
Nonflatness: Pixel-to-pixelvariation in the dark current, and wavelength response. This can he caused by numerous localized variations in transmission, absorption, and quantum efficiency of various image components, e.g., optical-fiber faceplate, photocathode and phos hor screen. These variations can also be &e to optical and electron-optical distortions. Nonflatness can be computer corrected by comparison with a prestored flat field exposure (from a uniform illumination of the imager)
CCD: Charge coupled device
RBV: Return-beam vidicon
CZD: Charge injection device Polysilicon: A multicrystalline form of silicon used in silicon gate MOS (metal-oxide silicon) technology. I t is electrically conductive and optically transparent Oak Ridge National Laboratory is operated by the Union Carbide Corp. for the U S . Energy Research and Development Administration.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
709A
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