Instrumentation Yair Talmi Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, TN 37830
TV-Type Multichan 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, at 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 at 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. The 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 at 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 10fi. 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.
Image 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 converters. 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
Figure 1. Cross section of proximity-focused UV image converter (Galileo)
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 at 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
( ~ l - 2 in. diam. X 0.02-0.07 in. thick ness), continuous dynode electron multiplier imager. It consists of mil lions of microscopic (10-40 μηι diam.) hollow-glass (fibers) conducting chan nels (fibers with a resistive coating on their inner surface), each acting as an individual electron multiplier with ab solute position (geometrical) registra tion between the input and the output faceplates of the device. These chan nels 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 ame nable to cooling and interfacing. It can provide electron gains (variable) of 10 3 -10 4 over an active diameter of more than 30 mm with an electron equivalent input noise of 1 0 - I 6 - 1 0 - 1 5 A/cm 2 . When operated in tandem, gains of 10 6 -10 7 can be obtained with low background noise (1.5 counts/seccm 2 ). 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, i.e., 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 resolu tion of 30 lp/mm at high light levels and 10 lp/mm at low light levels. At its limit the resolution is determined by the center-to-center spacing of the channels (p) and is equal to l/2p. At the present time, there are still a few disadvantages involved in its op eration, e.g., gain degradation over its lifetime (a few thousand hours) and its exponential pulse height distribution, causing a scintillation effect, especial ly at low light levels. Its necessity to operate in vacuum restricts the flexi
bility of its interfacing to TV camera tubes. MCP image intensifies with gain of 1000 to 50,000 and designed to operate at 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 l/N scan loss {N, number of reso lution elements to be scanned across the spectrum) or lack of integration
capability in the ID which separates it from all other image devices. The de sign of the shape and size of the aper ture makes it possible to enhance sen sitivity at 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 pro posed to overcome the 1/N scan loss. Multiaperture Image Dissector. Increasing the number of apertures to M and using an equal number of sepa rate P M T tubes or channeltron multi pliers for detection will improve the SNR of the ID by a factor of M IN (7, 8). Smoothing Dissector. If the gener ated electrical image pulses are time stretched long enough compared to the duration of the readout sampling, they will be counted, and thus a par tial 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 in tensifier gain (9). Unfortunately, the very essential capability of the phos phor to store signals, especially at high illumination levels, makes it impossi ble to simultaneously monitor very weak and very intense spectral fea tures, thus reducing the effective dy namic 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
700 A · 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. To 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 be accomplished by 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 be 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, Plumbicon, 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 capabilities, 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 Beam Imagers. Image Orthicon and Image Isocon. 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 it is absorbed, another fraction is elastically scattered, and most electrons are simply reflected back 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, i.e., the minimum light input level detected is one tenth that of the orthicon, at least three times higher SNR, and a ten times wider dynamic range. Direct Beam Readout Vidicon. Common to all vidicon TV tubes 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 and thus in the nature of the charge image that is produced. The most common vidicons use antimony sulfide as the photoconductive transducer. They are low priced and have a controlled gain but suffer from excessive lag and nonlinear transfer characteristics (7 = 0.6-0.7). Both the Plumbicon, with its nondiscrete PbO 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 Plumbicon, 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) V i d i c o n (b) SEC a n d SIT (c) I m a g e o r t h i c o n a n d i s o c o n
ANALYTICAL
CHEMISTRY,
VOL.
47,
NO.
7, JUNE
1975
·
701 A
Figure 5. Schematic representation of electrostatic storage camera
ture of the PbO target efficiently eliminates aliasing (Moiré effects) which is a problem with discrete silicon targets. Common to all silicon imagers, including the SV, is the development of interference patterns between the front and back surfaces of the target when narrow bandwidth light is incident and the aperture of the light beam is low. A very attractive feature of the vidicons is the commercial availability of a large variety of tubes designed for specific needs, e.g., low-light level detection, IR response, long integration time, high resolution, etc. Return-Beam Vidicon (RBV). These tubes contain the internal elements of conventional vidicons but utilize the return-beam readout mode [Figure 4 (c)] of the image orthicon. They were specifically designed for high-resolution earth observation studies (NASA) using antimony trisulfide oxysulfide (ASOS) as the photo-
transducer target. Silicon RBV tubes have also been constructed and should become excellent photometric detectors, since they combine the attractive features of SV tubes: excellent quantum efficiency, wide dynamic range, and UV response (limited only by faceplate transmission), with the excellent resolution and greatly improved SNR provided by the returnbeam readout mechanism. On the other hand, RBV tubes exhibit much more lag than SV tubes and are not commercially available. Dielectric Cameras. Various dielectric tape cameras, e.g., photoconductive and grating targets, have been developed for recording, storing, readout, transmitting (data), and reconstructing images for applications which require repetitive visual images and large capacity storage for long storage periods. Of particular interest is the electrostatic camera system
(ECS) now available from CBS Laboratories (Figure 5). The unique feature of the ECS is a rotating multipleframe storage drum which permits a total separation between the exposure, storage, readout, and erasure procedures. This feature makes the device unusually amenable to specific tailored designs and practically eliminates lag problems. The dielectric, large-frame storage element provides high resolution and unusual integration and storage capabilities. Readout is accomplished when the exposed storage target is scanned by the highenergy reading beam. Secondary electrons are thus produced at the target, with velocity proportional to the target potential pattern (charge image) and thus to the input light image. These electrons are deflected and returned into a velocity selector (monitor) which generates the video signal. This readout procedure does not erase the charge image which can be read again. ECS is the most promising universally applicable spectroscopic multichannel detector yet designed. Unfortunately, at the present time, its "custom-made" price tag ($10,000500,000) is highly prohibitive. Electronic Bombarded Target (EBT)—High-Sensitivity Tubes. High sensitivity is required when image tubes are interfaced to optically slow high-resolution spectrometers or when monitoring photon-limited spectroscopic sources. The EBT tubes SIT and SEC are basically vidicons with
Table I. Characteristics of Electron Beam Image Tubes Sb^Ss-vidicon
Format: diam, mm Sensitivity, photons/cm-" Max signal to rms, SNR (sat.) Dynamic range Resolution: Ip/mm MTF = 20% MTF = 50% No. of resolving picture elements,' pixels Spectral response, nm' % QE at 300 nm 400 nm 600 nm 800 nm 1000 η m Image retention (lag), %' Max integration time, sec Price estimate for standard tube, thousands of dollars Manufacturer
Plumbicon (PbO)
Si-vidicon
ASOS-RBV
Si-RBV
25,50 8.3 X 10»
16,25 2.8 X 10'"
25,46 4 X 10»
16,25 8.3 X 109
8, 16, 25 8.3 X 10"1
200
320 100 Linear 14 9 4 X 101
480 500-1000 Linear 32 18 2X106
30 80 Nonlinear 75 45 13 X 10°
100 Linear 60 40 10«
380-800'· 380-«)0
350-1100
450-850
350-1100
62 15 0 0 9 10 0.5-1.0
68 78 82 15 14 >1.0 0.3-0.5
RCA Phillips Labs
RCA GE Westinghouse Hammamatsu
250 Nonlinear 45 24 4 X 105 300-800
13 24 17 15
30 0.9 0.1 RCA GE Hammamatsu
30» 80 5 0
>50 >1.0 Commercially unavailable* RCA
50» 60 45 2 Not available >1.0 Commercially unavailable'' RCA
" S e n s i t i v i t y is d e f i n e d for SNR = 1 a t 550-nrn i l l u m i n a t i o n for '/so-sec i n t e g r a t i o n t i m e . b d is t h e a p e r t u r e ; E, t h e f a c e p l a t e i l l u m i n a t i o n in f o o l c a n d l e ; a n d Λί, t h e d w e l l t i m e in s e c o n d s . ' D y n a m i c r a n g e r e fee r s t o t h e w o r k i n g r a n g e a n d is d e f i n e d by t h e t r a n s f e r c h a r a c t e r i s t i c s of t h e t u b e . '• B a s e d on a p r i v a t e c o m m u n i c a t i o n w i t h C B S L a b o r a t o r i e s . N u m b e r of p i x e l s w a s d e t e r m i n e d a s s u m i n g a s q u a r e f o r m a t of t h e t a r g e t a n d r e s o l u t i o n a t 50% M T F . •' T h e s p e c t r a l r a n g e g i v e n for t h e s t a n d a r d t u b e s . D i f f e r e n t r a n g e s c a n b e a c h i e v e d w i t h t h e s e t u b e s if d i f f e r e n t w i n d o w
704 A
·
ANALYTICAL CHEMISTRY,
V O L . 4 7 , N O . 7, J U N E
1975
an image-intensification stage, which can produce fully exposed images under integrated flux (light) 100-1000 times lower than the vidicons. The gain mechanism of EBT tubes is based on the large number of hole-electron pairs produced in the target by each high-energy (7-8 keV) bombarding photoelectron. The high gain achieved is, however, inadequate for single pho ton-counting (event) readout, and intensified-SIT (I-SIT) or I-SEC tubes with an additional intensification stage are required. The use of differ ent window materials for interfacing to image converters can extend the ap plicability of EBT tubes to vacuum UV and X-ray spectrometry as well as to electron, ions (10, 11), neutrons (12), and γ-spectrometry. Secondary Electron Conduction (SEC) Tube. The target in SEC is made out of very low density (~0.02 g/cm 3 ) KC1. A positive charge image pattern is produced when secondary electrons, generated by the high-ener gy bombarding photoelectrons (gain si 100), move to the signal plate through the vacuum interstices of the target structure under the influence of the electric field within the target (secondary electron conduction). The high resistivity of the KC1 target and the resultant very low dark current (less than 0.008 electron/pixel-sec) permit the integration and storage of electron images for very long periods without degradation of image fidelity, although resolution is somewhat poor
SEC SIT (EBS)
Electrostatic
Magnetic
er than that obtained by conventional vidicons owing to lateral diffusion of charges. Yet, the large nondiscrete for mat of the SEC target can provide up to 106 pixels at 45% MTF, and the tubes show low lag. Gain is controlled by photocathode voltage adjustment, and electronic shuttering is available. The transfer characteristics show line arity over a limited dynamic range (13). Silicon Intensified Target (SIT). SIT is basically an intensified SV tube whose target operates in the electron, rather than the photon-detection mode. The controlled gain range (200-400) and the inherent wide dy namic range of the silicon target pro duce an accumulated linear dynamic range of at least 10 4 :1. At low light lev els with a maximal gain of 1500 (QE of photocathode 20%), the SIT can im prove SNR by a factor of 300 over the ordinary SV. Above 850 nm and below 370 nm, the SV is superior to the SIT (13). As generally is the case, resolu tion decreases with signal intensity, but owing to lateral charge spread, a few channels will always be required for each spectral element, thus reduc ing the effective multichannel capabil ity of the device. The lag is similar to that of the SV with 50% image reten tion after 0.15 sec. As with all silicon detectors, integration time can be largely extended by cooling. Other EBT devices utilize silicon solid-state sensor arrays as the target (13).
The performance of the various TV-type imagers is given in Table I. Solid-State Imaging Devices (SSID)—Silicon Devices. Silicon self-scanned solid-state image arrays are intrinsically small and require lit tle power for their operation. Their most attractive features include: solidstate (silicon) reliability, i.e., very sta ble geometric, radiometric, and elec trical characteristics; excellent re sponse of silicon in the vacuum UV to near IR range; digital scanning which provides geometric accuracy; high ver satility in addressing and in some cases (CID) random access capability; compactness and flatness which pro vide unusual interfacing flexibility and simplified cooling; and finally on the basis of experience with integrated circuits, a predicted substantial reduc tion in future cost. Most disadvan tages of SSID's, including a small for mat (up to xh X % in.), a small number of pixels, an inavailability of intensi fied (EBT)-SSID, and a limited UV response, should all be eliminated in the near future. More serious short comings, inherent in state-of-the-art SSID technology, are the high dark current and thus the limited integra tion time and the coherent systematic pattern noise responsible for the nonflatness of the sensors. At least four types of photosensitive elements are available with silicon de vices: p-n junction photodiodes, pho totransistors, photoconductors, and photovoltaic cells. The various silicon
Image orthicon
Image isocon
Electrostatic "camera
Image dissector
16, 25, 40 5.5 X 107
25,50 8.3 X 10'
40 2.5 X 10»
40 5.8 X 10'
50 1.0 X 10»
32 500-1000 Linear 15 10 6 X 104
90 ~ 5 0 0 O n l y partial linearity 14 30 8 20 2-4 X 108
45 100 Linear 12 6 6X10*
100 1000 Linear 14 10 2.0 Χ 105
1000" Linear 50 30 6 X 106
19,43 7.2 X 10», high resolution 6.0 X 10">, low resolution 1.22 d V I S ? 10,000 Linear 55 35 2-4 X 10«
100-900
300-800
300-775
300-800
350-850
3-5
4» 100 16 0 0 10 >1.0 1-2
18» 100 54 1.8 0 10' >1.0 2-3
RCA GE
RCA GE
300-800 360-850 15 9 0.3 0 15 >1.0 1.2-1.5 RCA GE
12 5 0.8 0 6 >5hr 1-2
Westinghouse
60" 60 40 1.0 0 i Hundreds of hr >10 CBS Labs
15 5 0.3 0 20 0 1-5 ITT Westinghouse EMR
materials and photocathbdes are used. « These figures are based on relative response rather than % QE. ''This extended spectral range is ob tained 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. > Lag is insignificant here since functions of exposure, storage readout, and erasures are totally separated. k These tubes were specifi cally designed for NASA. ' Lag is defined as signal remaining on target after third readout.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975 • 705 A
self-scanned array devices are similar in the mechanisms, limitations, and capabilities of the sensing elements b u t differ most significantly in the arrangement for signal amplification a n d 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 m o v e m e n t or injection of charge packets as in CCD devices a n d CID arrays. Amplification can be achieved by using either one amplifier per sensor, one amplifier for m a n y photosensors, or a single amplifier for the entire array. T h e fundamental performance criteria for solidstate arrays which set limits on their dynamic range and S N R are analogous to those of electron t u b e imagers. These criteria include: geometric resolution, determined by the number of pixels and their size a n d spacing (density); q u a n t u m efficiency and its wavelength dependence, determined by reflection and absorption losses in layers overlying the silicon and t h e absorption d e p t h and by the electrode consistency and structure of the devices; and finally the o u t p u t signal, determined by scan rates, integration periods, element size, q u a n t u m efficiency, and light exposure. Photodiode Arrays. Linear arrays with densities of u p to 1000 photodiodes/in. are now available. T o perform the essential functions of t h e array, a sensor, a n amplifier, a n d address a n d 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 is accomplished during periods between reset pulses (which recharge the diodes) with a consequent efficiency proportional to t h e ratio of the integration period to the total periods between consecutive reset pulses. T h e sensitivity of the device is limited by 1/f noise in the various components and by the systematic p a t t e r n 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. T h e phototransistor array basically comprises an LSA (large-scale array) chip consisting of 195 phototransistors a n d their corresponding preamplifiers, and a S C E (signal conditioning electronics) chip consisting of five amplifier channels physically located on a common LSA substructure assembly. T h e five 706 A ·
Figure 6. Operation of CCD in storage and transfer modes (a) CCD storage mode (b) CCD transfer mode
o u t p u t signals from t h e SCE require additional signal conditioning, multiplexing, and digital d a t a processing. T h e reverse-biased collector junction of each phototransistor serves as a photosensor, the emitter junction provides the switching, and an individual preamplifier transistor provides t h e gain. Phototransistor arrays provide gain and are more sensitive t h a n p h o todiode arrays; unfortunately, p a t t e r n noise is also more severe because of variations in both the responsivity and dark current of the individual pixels. Charge Coupled Devices (CCD). CCD's are nearly ideal semiconductor analog shift registers which perform t h e unique task of manipulating information (14). Linear CCD devices utilized as imagers basically comprise an array (128-1728) of closely spaced metal-insulator-semiconductor (MIS) capacitors. Each M I S pixel consists of a conductive electrode and a t h i n oxide layer insulator on top of a semiconductor p-type silicon. W h e n a positive voltage is applied to t h e conductive electrode in t h e M I S , a nonequilibrium (thermal) charge depletion region (potential "well") is formed und e r n e a t h in t h e silicon (Figure 6).
ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 7, JUNE
1975
These potential wells serve as t e m p o rary individual storage elements for the photo-generated electrons (signal) as well as for t h e thermally generated electrons (dark current). T h e a m o u n t of charge accumulated is a linear function of t h e incident illumination intensity and the integration period. Typically, each well can store u p to 10 5 -10 6 electrons. T o avoid ineffective coupling of charge across the interelectrode gaps, a "silicon g a t e " technology was developed. In this a p proach, the gaps between the electrodes are replaced by u n d o p e d polysilicon film, t h u s 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). R e a d o u t is accomplished by a three-phase (or two-phase) voltage arrangement sequentially applied to the electrodes to ensure t h e unidirectional transfer of the accumulated packets of charge across t h e s u b s t r a t e to be serially sampled by t h e readout amplifier (Figure 6). Two contrasting approaches to t h e operation of area imagers have aroused considerable controversy as to
for Safety and Precision You can't beat the system
The M LA Precision Pipetting System, that is. Major hospital laboratories have s t a n d a r d i z e d on the MLA Precision Pipetting System after competitive evalu a t i o n for p r e c i s i o n and accuracy. The MLA Pipette displays outstanding repeatability. 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 y6o 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 it is read out as a linear CCD. In the interline transfer process, sensing sites integrate for a %o sec as "A" sites are shifted sideways at 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
. . . A n d safety'.
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 at 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 at 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
Your hand need never touch the disp o s a b l e t i p . I m p o r t a n t for q u a l i t y control before use because you don't contaminate the clean tip. Important for your own safety after use because you minimize your exposure to residual pathogenic material. S h o u l d n ' t you s t a n d a r d i z e on the MLA System? MLA Pipettes are a v a i l a b l e f r o m selected Laboratory Supply Houses. The P i p e t t i n g D e c i s i o n . . . i s w o r t h your time. Use the Reader Service Number to send for our Pipette Information Pack.
Medical Laboratory Automation, Inc. 520 Nuber Ave., Mt. Vernon, N.Y. 10550 914/664-0366 CIRCLE 159 ON READER SERVICE CARD
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975 . 707 A
wells. For two-dimensional arrays it seems more reasonable t o bury the sink diode under t h e photogate to avoid a reduction in the n u m b e r of pixels, resolution, and format. T h e readout mode of CCD involves the transfer of charge packets across the target, and t h e transfer efficiency will determine t h e quality of the recon structed o u t p u t image. T h e transfer characteristics of a device are defined by the product of t h e transfer ineffi ciency, Ei (fraction of charge loss in each transfer step) and t h e number of transfer steps, N , required. Typically, a 1000 element linear CCD shows a total transfer inefficiency lower t h a n 0.3, sufficiently low to produce an ac ceptable image fidelity. A different variation of CCD is pro duced by Bell N o r t h e r n Research: a linear 256-photodiode array which is internally scanned by two charge cou pled analog shift registers for readout. T h e device might be more amenable to silicon t h i n n i n g and t h u s to UV a n d electron detection. Charge Injection Device (CID). T h i s imaging device, recently devel oped by General Electric (-75), utilizes an x-y addressed array of charge stor age capacitors which store photongenerated c h a r g e i n M O S inversion re gions. E a c h sensing site comprises a pair of charge storage capacitors of which one is controlled by an x-row drive line, a n d t h e other by a y-column drive. T h e charge generated during the integration is collected in t h e po tential wells formed by b o t h storage capacitors: readout of a selected pixel is achieved when t h e voltage on both capacitors (electrodes) is removed. T h e x-y r e a d o u t is accomplished by first removing t h e voltage from only one electrode, t h u s transferring the charge to t h e remaining potential well, now located under the second elec trode only. Next, t h e voltage of the second capacitor is removed, a n d the charge is injected into t h e substrate (Figure 9). T h e resultant raw video signal contains also the parasitic capa citive coupling of t h e drive voltage t o the substrate. This drive voltage inter ference is canceled t h r o u g h t h e use of an integrating readout technique. Charge injected from a sensing ele m e n t m u s t be diffused entirely to avoid lag. Injection to a regular sub s t r a t e (bulk) is complete within about 7 Msec. For fast readout, an epitaxial structure is used in which the epitaxial junction is reverse-biased to form a charge collector u n d e r t h e array ac complishing a complete injection in less t h a n 0.5 μββα T h e epitaxial struc ture also provides better resolution by reducing t h e charge cross-talk be tween sites, b u t it is less sensitive (about half) t h a n t h e bulk structure.
'-mmi READOUT CURRENT Figure 9. CID mode of operation (a) Storage mode (b) x-y readout mode (c) Charge injection
T h e potential advantages of t h e CID over the CCD are simple fabrication (lower cost), avoidance of charge transfer losses, and r a n d o m access readout. T h e performance of various SSID's is given in T a b l e II. References (1) G. R. Carruthers, Appl. Space Sci., 14, 332 (1971). (2) L. M. Biberman and S. Nudelman, "Photoelectronic Imaging Devices," Vols I & II, Plenum, New York, NY, 1971. (3) "Advanced Scanner and Imaging Sys tems for Earth Observations," NASA, SP-335 Report, 1972. (4) "Astronomical Observations with Tele vision-Type Sensors," Proceedings of a symposium held at the University of British Columbia, J. W. Glaspey, G. A. H. Walker, and C. M. Anderson, Eds., Vancouver, BC, Canada, May 1973. (5) R. Bracewell, "The Fourier Transform and Its Application," McGraw-Hill, New York, NY, 1965. (6) D. J. Ruggieri, IEEE Trans. Nucl. ScL, NS-19 (3) (1972). (7) S. W. Duckett, Ref. No. 3, ρ 119. (8) "Spectroscopic Applications of Image Dissectors," Ε. Η. Eberhardt, 18th An nual Conference on Analytical Chemistry in Nuclear Technology, "Symposium on Applicability of Multielement Detectors to Spectroscopy—Present Status," Yair Talmi and John Lowrance, Coordinators, Gatlinburg, TN, October 1974. (9) E. H. Eberhardt 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.
708 A · ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
Grotch, and L. P. Theard, Clin. Chem., 20, 998 (1974). (11) H. G. Boettger, Ref. No. 8. (12) J. B. Davidson, "Application of SECTV to Neutron Diffraction," Ref. No. 8. (13) Y. Talmi, Anal. Chem., 47 (7), 658 A (1975). (14) R. D. Baertch, W. E. Engeler, H. S. Goldberg, C. M. Puckette, J. J. Tiemann, "Two Classes of Charge Transfer Devices for Signal Processing," International Conference on the Technology and Ap plication of Charge Coupled Devices, Ed inburgh, Scotland, Sept. 25-27, 1975. (15) G. J. Michon and Η. Κ. Burke, "Charge Injection Imaging," IEEE In tern. Solid-State Circuits Conf. Digests, ρ 138, 1973. Glossary Resolution: Resolution will be given in lp/mm 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., lp/mm. It describes the change in am plitude of a sine-wave pattern transmitted through an imaging component and given as (A — B)/(A + B) where A is the input amplitude, and Β 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, tar get, 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'1 Silicon photodiode arrays
Format, no. of pixels (elements) Pixel size, μτη
Sensitivity,6 photons/cm2 Saturation exposure, photons/cm2 Dynamic range Noise, electrons/pixel Dark noise/ electron/pixel Integration time, (sat. time by dark current), sec Clock rate, MHz Resolution,' % NITF Manufacturer Availability Cost
Charge coupled devices (CCD)
Charge injection devices (CID)
Silicon photodiodes with CCD readout
Linear arrays: 128-1872' Linear arrays: 256-1728. Linear arrays: 128, 256, Area arrays: 100 X 100 elements. Area ara n d 188 X 244 Area arrays: 1 0 0 x 1 0 0 and 512 rays: 32 X 32 and 50 X a n d 320 X 512 50 31.5 X 6 1 ' 25.4 X 25.4 to 25.4 X 432. 1 3 X 1 7 , 30.5X30.5 (Fair- 18 etc X 10 width 25.4 center-to-center c h i l d ) , 25-125 length (etc). 15 etc for 1872 (RCA). 30 etc array 7 8 s Linear: 1.3 Χ 108 to 4 X 2.2 X 10 1.8 X 10 to 3 X 10 108 2.4 X 10"> to 5 X 10" 1000-20,000» 1000-2000 >1000
10». 100 X 100: 7 X 107 5 X 10» 4 X 10" Linear: 4 X 10'° t o 6 X 101». Area: 1.3 X 10'° 500 200-400 200-500 5000™ 1000 1.0 mV* 1.0 mVA (dark noise not 3000-4000 5-10 mV», ~300-1000
available) At25°C:l. At-40°C:500 At 25°C: 1-2. A t - 4 0 ° C : At25°C: 0.5-1 Min consists with inte gration time. Max: 10 58 Reticon Off the shelf $750 for 256 X 1 array. Other sizes, price is proportional to num ber of pixels. 1872 pixels ~ 3800
30-90 0.05-10
A t 25°C: 1-2.
At 40°C:
240 0.01-2
0.05-4
40-60 Fairchild, RCA, Bell Northern R e s e a r c h
66 Bell Northern Research
General Electric
Now
Now Price available from company upon re quest
Linear: $275-1500. Area: 100 X 100: $965 (Fairc h i l d ) , 320 X 512: $3800 (RCA)»
85 Now $6500'
" The reader is advised to use the data given in this table very cautiously because: the data are based on information gathered only from the manufacturers; it is not easy to compare these devices because the characteristics demanded by a particular application are important; and the various manufacturers disagree on the definition of a few parameters (in particular, sensitivity, noise,c and dark noise). ° Sensitivity is defined as the exposure (at 550 nm) level at which the signal level equals the peak-to-peak random noise level. Noise is measured within the video period of each pixel period and at a specified (by manufacturer) test frequency. Main noise source is input capacitance noise. * This value refers to the shot dark noise rather than to the dark current. ' Resolution is given as % MTF at the Nyquist limit spatial resolution (see definition of alias ing) and for visible light. It will be worse for IR illumination. / Soon to be announced. » Dynamic range will depend on amplifier used This capability is supposedly achieved because of the larger storage capability available. ' T h iks value refers to the average dark current rather than to dark noise. * This company offers a 100 X 100 pixel area CCD. ' Price includesm camera. Value expressed in electrons/pixel was not available. ' This and all other parameters below are given for the 188 X 244 CID imager. A new amplifier, now being designed, is expected to lower this value by a factor of three.
Ip/mm: L i n e p a i r s / m m . A w h i t e line a n d an a d j a c e n t b l a c k line are d e s i g n a t e d a line pair Tonal Transfer: T h e c o n t r a s t c h a r a c t e r i s tic of t h e i m a g e , c o r r e s p o n d i n g to g a m m a in p h o t o g r a p h i c s y s t e m s Dynamic Range: T h e r a n g e of r a d i a n c e v a l u e s , in a single d e t e c t o r frame, over w h i c h t h e d e t e c t o r is sensitive t o c h a n g e s in r a d i a n c e . T h e limits of t h i s r a n g e r e p r e s e n t t h e lowest a n d h i g h e s t i n t e n s i t y feat u r e s t h a t can b e m o n i t o r e d in one f r a m e Accumulative Dynamic Range: T h e r a n g e of r a d i a n c e t h a t can be used b y t h e s y s t e m under separate illumination and integration t i m e c o n d i t i o n s for h i g h a n d low r a d i a n c e s . T h i s p a r a m e t e r is always signific a n t l y larger t h a n t h e d y n a m i c r a n g e b u t d o e s n o t reflect t h e ability of t h e s y s t e m t o m e a s u r e high a n d low r a d i a n c e s simultaneously Integration Time: T i m e d u r i n g w h i c h t h e s t o r a g e t a r g e t a c c u m u l a t e s electrical c h a r g e g e n e r a t e d b y image d e t e c t i o n s . C o m m e r c i a l T V s y s t e m s use an i n t e g r a t i o n t i m e of '/JO sec Storage Time: T h e t i m e d u r i n g w h i c h t h e s t o r a g e t a r g e t can p r e s e r v e t h e signal c h a r g e image w i t h o u t d e t e r i o r a t i o n in fidelity Readout Rate: R a t e a t w h i c h t h e c h a r g e i m a g e s t o r e d on t h e t a r g e t is r e a d o u t . I t is d e t e r m i n e d by t h e electronic b a n d w i d t h available for t r a n s m i s s i o n a n d b y t h e efficiency of t h e r e a d o u t m e c h a n i s m
Lag: T h e fraction of c h a r g e r e t a i n e d on t h e t a r g e t after a single r e a d o u t a n d t h u s a p p e a r s as i n t e r f e r e n c e ( m e m o r y effect in s u c c e e d i n g frames)
Stellar Magnitude: A s t r o n o m i c a l scale of b r i g h t n e s s of s t a r s a n d p l a n e t s . O n e m a g n i t u d e c o r r e s p o n d s t o a light ratio of ( 1 0 0 ) 1 / 5 = 2.512
Searching (Random Access): T h e capabilit y t o scan locally only t h o s e regions of t h e image t h a t c o n t a i n r e l e v a n t i n f o r m a t i o n , r a t h e r t h a n t h e e n t i r e image
PMT:
Blooming: Cross talk b e t w e e n a d j a c e n t c h a n n e l s b e c a u s e of s p r e a d i n g of c h a r g e t o n e i g h b o r i n g pixels. T h e r e s u l t is loss of reso l u t i o n a n d fidelity
SNR:
Aliasing: W h e n t h e s p a t i a l f r e q u e n c y of t h e image is g r e a t e r t h a n t h a t of t h e i m a g e r ( d e n s i t y of pixels), a M o i r é p a t t e r n is p r o d u c e d w h i c h r e d u c e s t h e m o n i t o r i n g accuracy. A c c o r d i n g t o t h e N y q u i s t t h e o r e m (5), t h e s a m p l i n g f r e q u e n c y s h o u l d b e a t least twice t h a t of t h e i m a g e ' s s p a t i a l freq u e n c y t o e l i m i n a t e aliasing Pattern Noise: C o h e r e n t n o n r a n d o m noise w h i c h o r i g i n a t e s from pixel-to-pixel variat i o n s in s p a t i a l r e s p o n s e a n d d a r k c u r r e n t Nonflatness: P i x e l - t o - p i x e l v a r i a t i o n in t h e dark current, and wavelength response. T h i s can be c a u s e d by n u m e r o u s localized v a r i a t i o n s in t r a n s m i s s i o n , a b s o r p t i o n , a n d q u a n t u m efficiency of various i m a g e comp o n e n t s , e.g., optical-fiber f a c e p l a t e , p h o t o c a t h o d e a n d p h o s p h o r screen. T h e s e v a r i a t i o n s can also b e d u e t o o p t i c a l a n d elect r o n - o p t i c a l d i s t o r t i o n s . N o n f l a t n e s s can b e c o m p u t e r corrected by comparison with a p r e s t o r e d flat field e x p o s u r e (from a u n i form i l l u m i n a t i o n of t h e imager)
Photomultiplier tube
Pixel: P i c t u r e e l e m e n t . R e s o l u t i o n (sensing) e l e m e n t of t h e T V t a r g e t m a t r i x Signal-to-noise r a t i o
RQE: R e s p o n s i v e q u a n t u m efficiency, i.e., yield of p h o t o e l e c t r o n s per p h o t o n for a p a r t i c u l a r w a v e l e n g t h . T h i s is a figure of m e r i t for p h o t o c a t h o d e s EBS: EBT: SIT:
E l e c t r o n b o m b a r d m e n t silicon Electron bombardment target Silicon intensified t a r g e t
SEC: S e c o n d a r y e l e c t r o n c o n d u c t i o n (target) RBV:
R e t u r n - b e a m vidicon
CCD: C h a r g e c o u p l e d device CID: C h a r g e injection device Polysilicon: A m u l t i c r y s t a l l i n e form of silicon u s e d in silicon g a t e M O S ( m e t a l - o x i d e silicon) t e c h n o l o g y . I t is electrically cond u c t i v e a n d optically t r a n s p a r e n t 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 · 709 A
Table II, Characteristics of Solid-State Image Devices"
Format, no. of pixels (elements)
Pixel size, μπι
Sensitivity,' photons/cm ' Saturation exposure, photons/cm 2 Dynamic range Noise," electrons/pixel Dark n o i s e / electron/pixel Integration time, (sat. time by dark c u r r e n t ) , sec Clock rate, M H z Resolution/ % MTF Manufacturer Availability Cost
Silicon photodiode arrays
Charge coupled devices (CCD)
Linear arrays: 128-1872/ e l e m e n t s . Area arrays: 32 X 32 a n d 50 X 50 25.4 X 25.4 to 25.4 X 432. 25.4 center-to-center (etc). 15 etc for 1872 array 1.8 X 107 t o 3 X 108
Linear arrays: 256-1728. Area arrays: 100 X 100 a n d 320 X 512
Linear arrays: a n d 512
1 3 X 1 7 , 3 0 . 5 X 3 0 . 5 (Fairc h i l d ) , 25-125 l e n g t h (RCA). 30 etc
18 etc X 10 w i d t h
Charge injection devices (CID)
Silicon photodiodes with CCD readout 128, 256,
Linear: 1.3 X 108 t o 4 X 2.2 X 108 108. 100 X 100: 7 X 107 Linear: 4 Χ 1010 t o 6 X 4 X 10" 2.4 Χ 1010 to 5 X 10» 10 l °. Area: 1.3 X 1010 200-500 1000-20,000" 200-400 1000 1.0 m V 1000-2000 5-10 m V \ ~300-1000 1.0 mV» ( d a r k noise not >100Q available) A t 2 5 ° C : l . A t - 4 0 ° C : 5 0 0 A t 25°C: 1-2. A t —40°C: A t 2 5 ° C : 0.5-1 30-90 0.01-2 M i n consists w i t h inte 0.05-10 gration t i m e . Max: 10 40-60 66 58 Bell N o r t h e r n Research Reticon Fairchild, RCA, Bell Northern Research' Now Off t h e shelf Now Linear: $275-1500. Area: Price available f r o m $750 for 256 X 1 array. 100 X 100: $965 (Fairc o m p a n y u p o n re Other sizes, price is c h i l d ) , 320 X 512: $3800 quest p r o p o r t i o n a l to n u m (RCA)» ber of pixels. 1872 pixels ~ 3800
Area arrays: 100 X 100 a n d 188 X 244
31.5 X 6 1 '
108 5 X 10» 500 5000™ 3000-4000 A t 25°C: 1-2. 240 0.05-4
At 40°C:
85 General Electric Now $6500'
" The reader is advised to use the data given in this table very cautiously because: the data are based on information gathered only from the manufacturers; it is not easy to compare these devices because the characteristics demanded by a particular application are important; and the various manufacturers disagree on the definition of a few parameters (in particular, sensitivity, noise,c and dark noise). ' Sensitivity is defined as the exposure (at 550 nm) level at which the signal level equals the peak-to-peak random noise level. Noise is measured within the video period of each pixel period and at a specified (by manufacturer) test frequency. Main noise source is input capacitance noise. d This value refers to the shot dark noise rather than to the dark current. e Resolution is given as % MTF at the Nyquist limit spatial resolution (see definition of aliasing) and for visible light. It will be worse for IR illumination. / Soon to be announced. " Dynamic range will depend on amplifier used. This capability is supposedly achieved because of the larger storage capability available. ''This value refers to the average dark current rather than to dark noise. * This company offers a 100 X 100 pixel area CCD. > Price includes camera. * Value expressed in electrons/pixel was not available. ' This and all other parameters below are given for the 188 X 244 CID imager. "» A new amplifier, now being designed, is expected to lower this value by a factor of three.
Lag: The fraction of charge retained on the target after a single readout and thus 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) I,/5 = 2.512 PMT: 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 that contain relevant information, rather than the entire image Blooming: Cross talk between adjacent channels because of spreading of charge to neighboring pixels. The result is loss of resolution and fidelity
SNR: Signal-to-noise ratio
Accumulative Dynamic 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 to 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 Moiré pattern is produced which reduces the monitoring accuracy. According to the Nyquist theorem (5), the sampling frequency should be at least twice that of the image's spatial frequency to eliminate aliasing
Ip/mm: Line pairs/mm. A white line and an adjacent black line are designated a line pair Tonal Transfer: The contrast characteristic of the image, corresponding to gamma in photographic systems
Integration Time: Time during which the storage target accumulates electrical charge generated by image detections. Commercial TV 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. It is determined by the electronic bandwidth available for transmission and by the efficiency of the readout mechanism
Pattern Noise: Coherent nonrandom noise which originates from pixel-to-pixel variations in spatial response and dark current Nonflatness: Pixel-to-pixel variation in the dark current, and wavelength response. This can be caused by numerous localized variations in transmission, absorption, and quantum efficiency of various image components, e.g., optical-fiber faceplate, photocathode and phosphor screen. These variations can also be due 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)
Pixel: Picture element. Resolution (sensing) element of the TV target matrix
RQE: Responsive quantum efficiency, i.e., 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 (target) RBV: Return-beam vidicon CCD: Charge coupled device CID: Charge injection device Polysilicon: A multicrystalline form of silicon used in silicon gate MOS (metal-oxide silicon) technology. It 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 · 709 A
