Pyroelectric vidicon: a new multichannel spectrometric infrared (1.0-30-itm) detector Yair Talmi A pyroelectric (triglycine sulfate) vidicon tube has been used in conjunction with an optical multichannel an- alyzer (OMA) to obtain ir (1-30-gm) spectral information. The system can detect continuous as well as pulsed ir signals. Improvement of SNR through accumulation of data in memory has been demonstrated. Various parameters that affect the performance of the system include variation of sensitivity across the tar- get, thermal diffusion,discharge lag, thermal lag, and noise. The applicability of the system to ir absorption and laser (cw and pulsed emission) spectrometry has been demonstrated. Introduction TV-type multichannel detectors have been applied to numerous spectrometric measurements in the vac- uum uv to near ir region. 1 Such detectors provide several hundred independent optical channels in the linear mode and many thousands in the 2-D mode. The simultaneous detection of all dispersed radiation by such detectors improves SNR (multiplex advantage) by a factor of (N) 1 / 2 in the accumulation mode (continuous target readout) and by a factor of N in the integration mode (long exposure followed by a single readout), where N is the number of spectral or spatial resolution elements covered by the detector. Practically, an im- provement in SNR leads to a corresponding improve- ment in detectability or reduction in the sampling (observation) time. Another important feature of multichannel detectors is their photographiclike ca- pability to monitor simultaneously a complete spectra of ultrarapid pulse phenomena, i.e., at the nsec to psec pulse width range. Multichannel optical detectors, when interfaced on-line to a minicomputer, provide unprecedented flexibility in spectrometric data han- dling including spectral response normalization, various modes of integration, elimination of undesired spectral information by spectral stripping, etc. Until now the applicability of optical multichannel detectors was restricted to the vacuum uv to near ir spectral region. This paper introduces a new ir multi- channel detector, based on the use of a pyroelectric The author is with Princeton Applied Research Corporation, P.O. Box 2565, Princeton, New Jersey 08540. Received 16 June 1977. 0003-6935/78/0815-2489$0.50/0. C) 1978 Optical Society of America. vidicon image tube, that responds to radiation with wavelengths between 1.0-Am and 30-Am spectral range. Pyroelectric vidicons sense directly the difference in the thermal signal from the background; they do not require cooling and have a limiting resolution of about 100 TV lines (TVL) with a minimum resolvable tem- perature (MRT), a fraction of a C. The operation of the pyroelectric vidicon and its use as an ir radiation sensor depend on the pyroelectric effect. The pyroe- lectric effect occurs in materials which have a macro- scopic electric polarization even in the absence of an applied (external) electric field. This polarization re- sults from the alignment (poling) of internal microscopic electric dipoles in the material. The change in the po- larization due to a temperature change is called pyro- electricity and is proportional to the energy absorbed on the target for small signals. It is the intent of this paper to discuss the use of this detector in conjunction with the Princeton Applied Research Corporation (PARC) optical multichannel analyzer (OMA) ad evaluate its performance and po- tential applicability to ir spectrometric measure- ments. Instrumentation and Principles of Operation Detector Head The Thomson CSF TH-9840 pyroelectric vidicon (Pyricon) is offered with either a Ge or a KRS-5 face- plate window. The over-all design of the detector head assembly is similar to that of the silicon vidicon head (PARC's model 1205B), 56 with the following exceptions: the preamplifier bandwidth was reduced from 40 kHz to 16 kHz, the preamplifier gain was reduced by a factor of about 20, a third preamplifier stage was added in the 15 August 1978 / Vol. 17, No. 16 / APPLIED OPTICS 2489
Transcript
Pyroelectric vidicon: a new multichannel spectrometricinfrared (1.0-30-itm) detector
Yair Talmi
A pyroelectric (triglycine sulfate) vidicon tube has been used in conjunction with an optical multichannel an-alyzer (OMA) to obtain ir (1-30-gm) spectral information. The system can detect continuous as well aspulsed ir signals. Improvement of SNR through accumulation of data in memory has been demonstrated.Various parameters that affect the performance of the system include variation of sensitivity across the tar-get, thermal diffusion, discharge lag, thermal lag, and noise. The applicability of the system to ir absorptionand laser (cw and pulsed emission) spectrometry has been demonstrated.
Introduction
TV-type multichannel detectors have been appliedto numerous spectrometric measurements in the vac-uum uv to near ir region. 1 Such detectors provideseveral hundred independent optical channels in thelinear mode and many thousands in the 2-D mode. Thesimultaneous detection of all dispersed radiation bysuch detectors improves SNR (multiplex advantage) bya factor of (N)1 /2 in the accumulation mode (continuoustarget readout) and by a factor of N in the integrationmode (long exposure followed by a single readout),where N is the number of spectral or spatial resolutionelements covered by the detector. Practically, an im-provement in SNR leads to a corresponding improve-ment in detectability or reduction in the sampling(observation) time. Another important feature ofmultichannel detectors is their photographiclike ca-pability to monitor simultaneously a complete spectraof ultrarapid pulse phenomena, i.e., at the nsec to psecpulse width range. Multichannel optical detectors,when interfaced on-line to a minicomputer, provideunprecedented flexibility in spectrometric data han-dling including spectral response normalization, variousmodes of integration, elimination of undesired spectralinformation by spectral stripping, etc.
Until now the applicability of optical multichanneldetectors was restricted to the vacuum uv to near irspectral region. This paper introduces a new ir multi-channel detector, based on the use of a pyroelectric
The author is with Princeton Applied Research Corporation, P.O.Box 2565, Princeton, New Jersey 08540.
Received 16 June 1977.0003-6935/78/0815-2489$0.50/0.
C) 1978 Optical Society of America.
vidicon image tube, that responds to radiation withwavelengths between 1.0-Am and 30-Am spectralrange.
Pyroelectric vidicons sense directly the difference inthe thermal signal from the background; they do notrequire cooling and have a limiting resolution of about100 TV lines (TVL) with a minimum resolvable tem-perature (MRT), a fraction of a C. The operation ofthe pyroelectric vidicon and its use as an ir radiationsensor depend on the pyroelectric effect. The pyroe-lectric effect occurs in materials which have a macro-scopic electric polarization even in the absence of anapplied (external) electric field. This polarization re-sults from the alignment (poling) of internal microscopicelectric dipoles in the material. The change in the po-larization due to a temperature change is called pyro-electricity and is proportional to the energy absorbedon the target for small signals.
It is the intent of this paper to discuss the use of thisdetector in conjunction with the Princeton AppliedResearch Corporation (PARC) optical multichannelanalyzer (OMA) ad evaluate its performance and po-tential applicability to ir spectrometric measure-ments.
Instrumentation and Principles of Operation
Detector Head
The Thomson CSF TH-9840 pyroelectric vidicon(Pyricon) is offered with either a Ge or a KRS-5 face-plate window. The over-all design of the detector headassembly is similar to that of the silicon vidicon head(PARC's model 1205B),5 6 with the following exceptions:the preamplifier bandwidth was reduced from 40 kHzto 16 kHz, the preamplifier gain was reduced by a factorof about 20, a third preamplifier stage was added in the
Fig. 1. Schematic diagram of the pyroelectric vidicon tube and itsoperation.
Fig. 2. (A) Scanning pattern of the TGS target; overscanned anduniform regions. (B) On-target light-dark correction.
console to provide a TV monitor video output displaywith a 500-kHz bandwidth, and an additional inputcable was added to allow poling of the target wheneverrequired.
Infrared OMA Principles of Operation
The operation of the OMA ARC model 1205A hasbeen described in detail elsewhere 7 and, therefore, willbe discussed here only briefly, with particular emphasison the modifications that were necessary for its opera-tion with the Pyricon detector.
Basically, the OMA is a modular electrooptical signalprocessing system for the acquisition of optical andthermal data. The ir signals are detected and tempo-rarily stored on the triglycine sulfate (TGS) target of thePyricon, which functions in a fashion analogous to aphotographic emulsion. The target is coated on thefront with an ir transparent conductor which serves asthe signal electrode. A scanning electron beam reads(destructively) the stored signal by depositing a negativecharge on the positive areas (analogous to signal-sen-sitized areas in a photographic emulsion), thus resultingin a capacitively coupled signal to the front side signalelectrode (video signal) (Fig. 1). The complete readoutprocedure of the target including scanning, integration,digitization, and storing in memory is done in real time,every 32 msec. The target, 16 mm in diameter, mustbe fully interrogated by the scanning beam. (In fact thetarget is slightly overscanned, about 16.5 mm.) Al-though the entire horizontal scan of the target providesa detection of ir signal in 500-input channels (useful inapplications requiring maximal spectral coverage), onlythe central region [Fig. 2(a)] (approximately 300 chan-nels) is sufficiently uniform (within 20-30%) for mostreal time quantitative spectrometric measurements.Over that region, the scanning pattern of the target thusproduces the equivalent of 300 adjacent ir detectionchannels, each approximately 0.33 X 10 mm.
To improve further the channel-to-channel unifor-mity across the target, each channel is divided into anactive (signal sensing) half and an inactive (dark) half[Fig. 2(b)]. The signal readout during scan abc (dark)is electronically subtracted from that during scan cde(light). The current is integrated for one completechannel to convert current back to charge. This way,charge on the target is read as charge by the OMAelectronics. An analog-to-digital converter counts thecharge from each channel and stores it at the appro-priate address in a 500-word memory.
Principles of a Pyroelectric Target Detector
The TGS target has an initial temperature distribu-tion T, and its free surface is brought to a uniform ref-erence potential (Fig. 3). Energy AE, emitted by thehot object, is locally absorbed in the target and causesvariations of the initial temperature distribution AT.These localized temperature variations produce pro-portional variations in the electrical polarization of theTGS target AP (pyroelectricity), thus causing a corre-sponding variation in the surface distribution of chargesAb and potential AV.
An artificial positive current pedestal is continuouslybeing applied to the TGS target (see p. 9). This ped-estal current serves as a charge reservoir from which thetarget can borrow or to which it can add, in response toalternating signals. An increase of target temperaturecauses the release of positive charge, which has beenbound to the internal polarization field. [Charge can-not escape the target because it is an electrical insulatorand is removed from target only by the reading (scan-ning) electron beam.] Similarly, when the target tem-
Fig. 3. Principles of operation: pyroelectric target.
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It also implies, however, that the device is insensitiveto large steady thermal background fluxes. An im-portant consideration for ir spectrometric (and evenmore so, imaging) applications is the contrast ratio be-tween the photon flux generated by an incrementalchange in background temperature and the total flux.For instance, a 0.1-K change of the average 300-Kbackground (room temperature) corresponds to a con-trast ratio of about 1% at 2 Am and less than 0.1% at the8-14 spectral region. Channel-to-channel responsevariations (nonuniformity) in a photoconductive orphotovoltaic diode ir array, which exceed the contrastratio, seriously degrade the minimum resolvable tem-perature (or spectral signal) and result in a significantpattern noise. Because the Pyricon does not respondto background (unmodulated) signals, response uni-formity requirements are greatly reduced,
Another fundamental advantage of the Pyricon overphotoconductive and photovoltaic ir detectors is itscapability to operate at room temperature. The highbackground irradiance at 8-14 ,m (assuming blackbodybehavior), approximately 10-17 photon/sec cm2, sig-nificantly reduces the saturation time (further reducedby internal thermally generated charges) of photo-electric detectors. The Pyricon, however, is a thermalenergy detector that operates at room temperature, andits saturation (exposure) time is limited only by thethermal diffusion characteristics of the target ( 3 X 107m2/sec). In fact, cooling of the TGS target reduces itsresponsitivity due to reduction in its pyroelectric coef-ficient (see Fig. 4).
It is important, however, to recall that energy mustbe absorbed to be detected with a pyroelectric detector.Thus, the spectral response of the detector is deter-mined by the thickness, ir absorption characteristics,
TEMPERATURE C
Fig. 4. Pyroelectric coefficient (pyroelectricity)constant vs temperature for TGS.14
and dialectric100
perature decreases, e.g., blocking of the incident ra-diation, the internal polarization field increases, lockingup some pedestal charge. Thus the net result of targettemperature variation is either a positive or a negativevideo signal for AT > 0 and AT < 0, respectively, rela-tive to the constant pedestal current.
A constant signal produces no changes in the polar-ization field of the target, and the surface charge dis-tribution depends only on the pedestal current. Con-sequently, the target response becomes independent ofthe incident radiation level until it changes. The de-tector, therefore, senses only temperature variationsbetween sampling times and does not respond to asteady temperature signal.
This immediately implies that although the Pyriconcan readily detect rapidly changing ir phenomena, e.g.,laser pulses with widths in the msec to psec range, itcannot detect a steady cw signal unless it is modulated.
o
WOO -
00
2 3 5 t 0 20 30
WAVELENGTH, pm
Fig. 5. Typical spectra response of pyroelectric vidicons; )1) aKRS-5 window (above 13-,um data are from Thomson-CSF) and (2)a Ge window coated for optimum transmission in the 8-14-,um range.Experimental data from Thomson-CSF. The dotted circles representabsolute spectral response values (Table I), except for the first three
(below 2m) which are only semiquantitative (50%) values.
and heat capacity of the target in addition to thetransmission of the window faceplate. Yet pyroelectricdevices are thermal energy detectors, and, consequently,their spectral response characteristics (D*) are muchmore uniform (see Fig. 5).
Detector Operation in the Chopped Mode
To obtain a signal from continuous thermal emitters,their incident radiation on the target must be chopped.Alternatively, although impractical for spectrometricapplications, a relative displacement must be createdbetween the target and the emitter's image; two com-mon modes are orbiting and panning.
The chopping mode of operation produces a stableimage position and a constant field of view in contrastto orbiting or panning. However, alternate positive andnegative signals of equal magnitude will be producedwhen the shutter is open and closed, respectively. Inthe cathode potential stabilization mode (CPS) utilizedin this tube, the scanning electron beam is incapable ofreading out the surface charge information when thepotential of the target becomes more negative than thatof the cathode. Thus, because of the negative chargesproduced during the closed period of the shutter, thetube becomes inoperable. To eliminate this problem,an artificial polarization (a positive charge pedestal) isproduced which prevents the negative overcharging ofthe target. Mechanical chopping of ir signals (prior toentering the polychromator) will, therefore, cause theproduction of alternating polarity spectra by the de-
tector, superimposed on a positive pedestal. The pos-itive charge pedestal in the Pyricon is produced byionization of a noble gas (or hydrogen) present in thetube in the region between the mesh grid and the target(see Fig. 1).
Unfortunately, the alternating polarity display ishighly objectionable, since it makes interpretation ofspectral data more tedious and less accurate. A mereinversion of polarity of the video signal, during theclosed chopper period, will not overcome this problembecause the flicker of the high level pedestal signal willpractically mask any low intensity features.
To eliminate these signal display alternations as wellas the pedestal itself, a signal processing method, re-ferred to as spectrum difference processing (SDP), wasused. This method is very similar to the image differ-ence processing technique used for pyroelectric vidiconimaging applications.8 9
Basically, the SDP method requires chopping of theincident signal at the entrance slit of the polychromatorat half of the OMA scan rate (two 32-msec OMA framesduring each open and closed period of the chopper).Figure 6 illustrates the operation of the system in theSDP detection mode. The synchronization of the tar-get readout, the data acquisition, and the signal mod-ulation (chopping) processes are accomplished by al-lowing the console to control the velocity of the chopperand the scan rate and the chopper to control the phase.In the real time display mode, the last frame in eachopen and closed chopper period is stored, the former in
VARIABLE SPEED CHOPPER PAR M191
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OPTICAL MULTICHANNEL8 ACCUM (FRAME 4)
OPTICAL MULTICHANNEL ANALYZER PAR 1204A
Fig. 6. Over-all schematic diagram of the Pyricon OMA (PARC model 1204A and 1204P) system.
PYRICON SCANS 1- FIRST FRAME -- 1.-ISECOND FRAME 1 THIRD FRAME -*1.FOURTH FRAME-4-1(FRAMES) { I I I
0 32 64 96 128 s
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FOURTH FRAME FOURTH FRAME THIRD FRAME FOURTH FRAMEI I ~ ~ ~ ~ ~~~~~I II
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Fig. 7. Real time mode displayand timing diagram (choppedcontinuous signal). In the A-Baccumulation mode, only frames2-4 are displayed so that only one
spectrum is displayed.
FRAME NO. 1-3 and 2-41
A-B
memory A and the latter in memory B. The basictiming and display diagram for the real time mode ispresented in Fig. 7. In the A-B mode of operation, thedisplay, during a full chopper revolution, consists offrames 1-4, 2-4, 2-3, and 2-4, sequentially appearingevery 32 msec. In this mode, a complete elimination ofthe pedestal from the display is achieved, as shown inFig. 8. (The tube was intentionally misadjusted toproduce a nonuniform pedestal.)
In the accumulation mode, only the second frame ofeach chopper period (open and closed) is stored. Upto 5000 separate frames can be sequentially accumu-lated into each memory in order to enhance the SNR of
the detector. Following accumulation, the content ofmemory B can be subtracted from that of memory A soas to present a uniform display. The reason for dis-carding the first frame of each chopper period will bediscussed later.
Poling
The TGS target is pyroelectric in nature, i.e., it isspontaneously polarized in the absence of an externalfield. The alignment (poling) of the internal micro-scopic electric dipoles in the material (formation ofdomain volumes polarized in the same direction) is ac-complished during or after the manufacture of the tar-
get. To optimize the performance of the tube, espe-cially after long periods of nonuse, repoling is necessary.Heating the target above its phase-transition (Curie)temperature (490C), underscanning it (by the electronbeam), or accidentally misaligning grossly, the tube gridpotentials can bring about a depoling of the target thatnevertheless can be easily remedied by repoling it.Figure 9 schematically illustrates the automatic polingsequence used in the OMA.10
During this sequence, the target is scanned with ahigh energy electron beam and charges positively, dueto secondary electron emission on the target, until targetpotential equals that of the deceleration grid (mesh grid4). The resultant electric field established across thetarget is sufficient to repole it.
A
B
Fig. 8. Real time display of a single spectral line superimposed on the
current pedestal: (A) display of memory A, chopper in open position;(B) display of memory B, chopper in closed position; (C) spectrum
difference processing (A-B) display.
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Fig. 9. Automatic poling sequence.
Fig. 10. Uniformity of pedestal and sensitivity across the target. (A)
Pedestal display preamplifier (first stage) output). A negative irsignal is superimposed on the pedestal. (B) Pedestal display following
blanking of upper and lower target regions [Fig. 2(a)] and on-targetlight-dark correction [Fig. 2(b)]. (C) Variation of signal sensitivity
Fig. 11. Loss of resolution with time (frame-and channel number);mechanically modulated (chopped) continuousir signal.' (A),Channel75; At = 5 msec; width at half peak height:' first frame 5 channels,second frame 7 channels. (B) Channel- 150;,At 10 msec, width athalf peak height: first frame 6-7 channels, second frame 7-8 chan-nels. (C) Channel 350; At-= 23 msec, width at half peak height: first
frame 7 channels, second frame 8 channels.
Limitations on Pyricon Performance
Systematic Variations in Sensitivity Across Oie TargetThe TGS target is a light (power) integration device.
A radiant flux density, Wo (W/M2), incident on thetarget for a time interval At (At is the time elapsed be-tween two consecutive readouts), will cause a variationin its stored energy, AE= AtW 0 and thus a corre-sponding variation in its surface electric charge densityAq = At6o. The video signal current is i = kAt for a
Fig. 12. Loss of resolution with time because of thermal diffusion.Multiple exposure (Polaroid film) of five consecutive TEA CO 2 laser
pulses.
steady radiant flux density Wo(k = Abo/r). A is thescanned area, is the frame scan period (32.8 msec).During the first frame (in either the closed or open po-sition of the chopper), At varies proportionally from At= 0 to Dt = 32 msec for channels 0 and 500, respectively,thus causing a monotonic variation in sensitivity acrossthe target. During the second frame, however, At = 32msec for all channels (the light is incident on the targetfor at least 32 msec), and, therefore, the sensitivityshould be uniform across the entire target. In reality,the sensitivity is fairly uniform (20%) from channels100-400 [Fig. 10(c)] but gradually drops below andabove these values. This behavior corresponds to asimilar variation in the positive charge pedestal on thetarget [Fig. 10(a)] as described below.
Lateral Heat Spread in the TargetThermal diffusion causes a loss in sensitivity and
resolution (information) because the localized hot re-gions (where radiation has been absorbed) on the targetspread over to the neighboring cooler regions and,therefore, reduce the over-all contrast of the imager.This degradation in performance can be reduced byincreasing the chopping and target scan rates but, un-fortunately, only at the expense of reducing the targetreadout efficiency (as will be discussed below). Figure11 shows the loss of resolution with time, observed fora continuous (chopped) signal. Resolution is degradedfrom frame 1 to frame 2 and from channel 75 to channel350. When using the system for pulsed measurementswith a CO2 TEA laser, a more significant degradationof resolution was observed (see Fig. 12)."
Discharge Lag
As with all other electron beam imagers, the positivecharge pattern (electron image) on the pyroelectricvidicon target cannot be completely readout in a singlescan.'2 In fact, the measured readout efficiency witha single scan (first field) is only 50-60%.13 A rathersimplistic interpretation of this phenomenon is that the
Fig. 13. Thermal lag combined with discharge lag. Rapid shuttering
of a continuous ir signal through a spectrometer slit. (a) Signal am-
plitude vs time, a trace every 32 msec. Real time display of memory
A. (b) Signal amplitude vs time. Spectrum difference processing(A-B) real time display.
discharge time constant, roughly a product of the beamimpedance and the target capacitance, is longer than theframe scan period of 32 msec. The lag constant a 1 4 is
defined as
a~ CTB (Ip)
where Ip, = pedestal current,TB UIP) = beam temperature at Ip current,
C = target capacity,rf= field (scan) time,
q = electron charge, andK = Boltzmann constant.
If a < 1, lag is significant. If a > 1, most signal on targetis read out efficiently in one scan.
The high capacitance of the target (due to the highdielectric constant at the operating temperature) andthe beam temperature are inherent in the design of thePyricon. Therefore, more efficient signal readout re-quires a high pedestal current and a long if. The ped-estal current value is limited to about 70 nA, but,practically, above 35 nA the uniformity of the pedestalacross the target is degraded, and the SNR is reduced.
As will be shown later, the over-all noise of the systemis largely limited by the pedestal ion gas noise. Increaseof Tf will improve the readout but will also increasethermal diffusion on the target.
Improvement in future pyroelectric vidicons will mostprobably involve a reduction of the target's dielectricconstant (use of deuterated triglycine fluoroberyllatesas target material), reduction in beam temperature (useof a laminar flow electron gun), and reduction in ther-mal diffusivity (using a reticulated TGS target).
Thermal Lag
The thermal time constant of the pyroelectric vidiconis a few seconds. In imaging applications, this will re-sult in a negative after image (dark) that slowly decayswhen the radiating object is removed. The SDP (A-B)display technique, however, minimizes the apparentafter image (Fig. 13). The explanation for that im-provement is the following: the thermal time constant(decay time) is a few second, whereas a frame scan pe-riod is only 32 msec. Thus, a few tens of scans (read-outs) occur during the decay time of the image. TheA-B real time display mode involves the subtraction ofconsecutively read out signals because, following theremoval of the object, both A and B memories will beloaded only with the (negative) decay signal. Conse-quently, A-B display continuously represents the dif-ference between signals of close intensities, tframe scan/
tdecay time << 1, and, therefore, it remains practically flatduring the (thermal) recovery time of the target.
For studies requiring a wide spectral coverage witha high resolution, the polychromator has to be scannedcontinuously in order to monitor different spectralwindows (a spectral region simultaneously monitoredby the Pyricon). The elimination of thermal lag by theSDP technique allows a real time display even at highpolychromator scan rates. Unfortunately, the Pyriconis a piezoelectric detector, and a rapid (mechanical) scanwill produce intolerable microphonic interferences.
Spectral Response
Methods of Calibration
Calibration of the pyroelectric vidicon for spectralresponse was performed by first measuring the relativespectral response over the 2-13.7-Am wavelength rangeand then performing an absolute calibration using theinterference filter technique. A 10000C blackbody wasimaged on the entrance slit of a single grating mono-chromator (0.3-m Spex Czerny-Turner). Gratingsblazed at 4 gm and 10 gm were used along with appro-priate cutoff filters to reduce second order overlappingand stray light interferences. Both a precalibratedthermopile and a Pyricon with a KRS-5 window wereplaced at the focal plane of the spectrometer. A 1-mmwide exit slit was used with the thermopile. No exit slitwas used with the Pyricon. Instead, the sum total count(response) of channels 270-300 (-1.00-mm X 5.0-mmportion of the target) was recorded, thus simulating a1.00-mm X 5.00-mm exit slit. By performing themeasurement that way, the spectral characteristics of
a Light sensitive channel area is 33 X 10-4 cm (width) X 0.50 cm (length) = 1.65 X 10-3 Cm2
.
the energy incident on both detectors were similar forevery wavelength setting of the polychromator. Toimprove the SNR of the Pyricon measurements, 250OMA (32 msec each) frames were accumulated inmemory (SNR improvement of about 16). The abso-lute response measurements consisted of calibrating athermopile using an NBS standard of total irradiance(tungsten lamp). The calibrated thermopile was thenused to measure the discrete spectral irradiance of anumber of filter-blackbody combinations. The py-roelectric vidicon was then irradiated by the same fil-ter-blackbody combinations. The uncertainty in theflux incident on the vidicon was estimated to be 3%.Figure 5 shows the absolute response values along withthe normalized spectral response curve. The dashedportion of the curve and the Pyricon-Ge (window) curverepresent data obtained from the manufacturer(Thomson-CSF) of the tubes.
Below 2 m only semiquantitative measurementswere taken, although the tube was sensitive down to atleast 1.0 um. Table I gives the response of the pyroe-lectric vidicon in counts/frame (32 msec)/channel/Wcm-2 at the six wavelengths where absolute measure-ments were made. The average rms noise per framewas 0.9 counts under the measurement operating con-ditions: G1 =-24 V; G2 = 160 V; G3 = 200 V; G4 = 250V; frame scan = 6 V; line scan = 15 V; and pedestalcurrent = 23 nA. Therefore, the detectability (minimalenergy detected by each channel), 4-10 X 10-9 W/count,actually represents a SNR higher than 1. It should berecalled, however, that using a 2 0 0 -Am wide entranceslit, the average FWHM (full width at half-maximum)of a spectral line image (entrance slit with 200,um) is atleast 6-7 OMA channels (200-230, m).
Electron Noise
Several sources of random electrical noise are presentin the Pyricon OAM system. At a 16-kHz amplifierbandwidth (BW), the main noise sources are pedestalgas-ion noise [Np = (2eIpBW)1/2 , where e is the electroncharge, and Ip is the pedestal current] about 11.5 pA atIp = 25 nA, preamplifier noise about 8 pA, and the in-tegration and ADC noise about 15 pA. The gain of theintegrator and ADC (and correspondingly their noise)were adjusted so that the over-all noise (rms) of theOMA-Pyricon system was approximately 1 count. Atthis noise level the SNR multiplex (Fellgett) advantage,
i.e., the improvement in SNR proportional to the squareroot of the number of frames accumulated in memory,was maintained over the full accumulation range (5000)of the system. The effect of the pedestal current levelon SNR is shown in Table II. It is interesting that themaximal SNR has been achieved at a pedestal currentof 25 nA compared with 50-70 nA recommended by thetube manufacturer for imaging applications (whereamplifier BW is at least 1.5 MHz).
Spatial Noise
Spatial (fixed pattern) noise is an important consid-eration in the evaluation of a multichannel imagingdevice because it is nonrandom in character. Conse-quently, its effect cannot be reduced by prolonged signalintegrations. Spatial noise in the Pyricon originatesfrom nonuniformities in the TGS target (referred to astarget blemishes in the TV industry) as well as in thepedestal current across the target. Although thestate-of-the-art Pyricons have an acceptable targetuniformity (by TV industry standards), the uniformityof the pedestal is harder to control and maintain, andit becomes more so as the pedestal level is raised.
The SDP technique (previously discussed) producesa stationary (nonflicker) spectrum, reduces considerablythe after image (streaking) effect due to thermal lag,enhances the spatial resolution, and, most importantly,eliminates spatial noise. However, this is only a displaycompensation technique which does not correct forvariations of sensitivity (and noise) and resolutioncaused by these fixed-pattern noise features.
To minimize the spatial noise effects (prior to dis-play), the following signal processing steps are per-formed: only the central 10 X 10-mm (300 OMAchannels) portion of the target is used, although all 500
Fig. 14. Absorption spectra of thin films of (a) polycarbonate,(b) polytoluene, and (c) polystyrene. Operation conditions:
Light source: Glow bar (800'C),Slit: 200 gm wide, 4.5 mm long,
Grating: Blazed at 2 Am, 75 g/mm,SDP display (A-B): 400 accumulations in memory.
Application Demonstration
Infrared Absorption Spectrometry
Polymer films a few microns thick of polyethylene,polycarbonate, and polystyrene were placed in front ofthe entrance slit of a polychromator (JACO, EbertMount 0.3 m, PARC model 1208) with a 2 -,4m and a10-,um blazed gratings; the entrance slit was 250,Mm. Aglow bar (set approximately at 850"C) placed 4 cm awayfrom the entrance slit served as the blackbody radiationsource. Absorption spectra of all three polymers at the3-4-gtm (C-H stretching regions) spectral range wereobtained (see Fig. 14).
Figure 15 demonstrates the effect of accumulation (inmemory) on SNR (polystyrene absorption spectra).Figure 16 shows the blackbody transmission spectrum(Io), the polystyrene absorption (I), and absorbance (A= logIo/I) spectra. The absorbance spectrum was ob-tained using an HP-9825A desk calculator interfacedto the OMA. Absorbance or transmittance spectra can
z0
vzI2n:
channels (full target, 16-mm diam) can be displayed oncommand, and only the lower half of each channel (5mm long) is utilized for radiation detection. The upperhalf of the channel is used as a background detector andis electronically subtracted from the lower half. Figure10 shows the uniformity improvements achieved aftereach signal processing step. The uniformity in sensi-tivity across the target was determined by recording thezero-order (slit image) signal (gradually displaced fromchannel 50 to channel 450 by grating rotation) via amultiple exposure of a Polaroid film. The nonunifor-mity over the recommended 300 channels span (channel100-400) was less than 20% at a pedestal current levelof 30 nA.
Fig. 16. Blackbody emission (A), polystyrene absorption (B), andabsorbance (C) spectra. Operating conditions as in Fig. 14.First-order spectral coverage (window), 0.726 m for channels
50-450.
be more accurate than absorption spectra because theyautomatically provide a normalized sensitivity acrossthe detector. Thus they compensate for wavelengthdependent variations in the light emission character-istics of the blackbody, in the polychromator trans-mission efficiency (especially due to gratings blazing),and for detector sensitivity variations due to target andpedestal current nonuniformities. On the other hand,unless appropriate data smoothing techniques are ap-plied, absorbance spectra can result in a considerablereduction in SNR as demonstrated for polyethylene,Fig. 17.
and absorbance (loglo/I) spectra. Operating conditions as in Fig. 14.Spectral coverage as in Fig. 16.
As previously discussed, the typical resolution ob-tained with the Pyricon-OMA system is approximatelyseven channels (peak width at half height). Thus, noimprovement in resolution is achieved upon reducingthe slit width (zero-order image) below 200 gm (see Fig.18).
Infrared Laser Emission SpectroscopyFigure 19 shows the second-order separation of two
adjacent CO2 rotation lines [CO2 transitions P(24) andP(22), emitted from a cw CO2 laser] obtained with a0.3-m polychromator (see above) equipped with a 10-Am
Fig. 18. Effect of slit width on the spectral resolution of the PyriconOMA system: (A) Slit width 50 ,um (peak height 195 counts); (B) slitwidth 150 gm (peak height 890 counts); (C) slit width 2 0 0 ,m (peakheight 1080 counts), obtained at a lower display vertical expansion.
Fig. 19. Second-order separation of adjacent CO2 (laser) emissionrotation lines [CO2 transitions P(22) and P(24)].
blazed 50-g/mm grating. The radiant flux through theentrance slit was approximately 5 mW. The second-order simultaneous spectral coverage of the polychro-mator was set by adjustment of scan frame to 0.5 gim.The peak-to-peak separation between the two rotationlines was nineteen channels or 0.0190 ± 0.0005 gim.This value agrees well with the reported value of 0.0207gm. 15
The applicability of the system to ir pulse laserstudies, where mechanical signal modulation is unnec-essary, is demonstrated in Fig. 12. A single CO2 TEAlaser pulse [an R(18) transition line, 9.28 gm]11 with arise time of approximately 30 nsec was detected througha 160 gm wide slit (same polychromator as above).Lasing was triggered by the OMA display-cursor.
A theoretical analysis was performed 16 to determinethe loss of resolution and response of the target withtime, following the irradiation of the target by a singlepulse incident through the entrance slit of a polychro-mator.
If W is the total energy absorbed per unit length ofthe slit, the temperature variation of the target 0 as afunction of time, t and the distance y (laterally from thecenter of the slit) may be written as
W [. y y+B y-B0(t y) = 4CeB exp [- (erf 2 + erf D )]
4CeB T 2 (Dt)"/ 2 2(Dt)"1i
where 2B is the slit width,C is the specific heat per unit volume of the
TGS material,e is the target thickness,t is the thermal time constant of the target
(8 sec),D is the thermal diffusivity of the pyroelec-
tric material (about 2-3 X 10-7 m/sec), and
erf(x) = - f e-u 2 du is the error function.
Figure 20 shows the relative temperature1 +u 1u-'erf + u+erf-l'flmax 2 er W )1/2 +ef(,)1/2'
(which holds for t << r) vs the reduced distance u = yIBand as a function of the reduced time r = 4Dt/B 2.
Table III compares the theoretical calculations for 2B
Table ll. Effect of Time on (a Single Pulse) the Resolution and Responseof the Pyricon Detector
Fig. 20. Temperature response of a TGS target irradiated by a rapid pulse through a slit of variable width 2B (Table III). Reduced variableu y/b, r = 4Dt/B2.
= 160 gm with the experimental results obtained withthe Pyricon-OMA system.
It is interesting to note that the two D values obtainedwere not identical. At least partially, this discrepancywas caused by loss of resolution in the polychromator(in particular, due to vignetting at the edges of the16-mm wide focal plane) and to the finite size of theelectron scan beam and the finite bandwidth of thepreamplifier. The limited accuracy of the A\y mea-surement (approximately +16,m, 1/2 channel) and theirreproducibility of the laser pulses (approximately± 30%) are additional potential sources of error. Therelatively more pronounced loss of resolution (wider Ay)after 3 msec is in accord with the lower sensitivity ratio1.5 (experimental) obtained, compared with the theo-retically calculated ratio of 1.72 (indeed, 190 m X1.5/1.72 = 166 gm).
Nevertheless, these D values are in rather goodagreement with the reported values of 1.95 X 10-7 and2.8 X 10-7 m2 /sec in the c and a crystal axes (eigendi-rections), respectively.17 Thus, it seems that the lossof resolution and sensitivity (with time) of a single pulsepractically follows the limitations set by the thermaldiffusivity of the target.
We acknowledge C. A. Sabato and D. R. Mohr of the
Princeton Applied Research Corporation for the elec-tronic design and construction of the ir optical multi-channel analyzer.
References
1. "Advanced Scanner and Imaging Systems for Earth Observations," NASAReport SP-335 (December 1972).
2. M. J. Milano, H. L. Pardue, T. E. Cook, R. E. Santini, D. W. Margerum, andJ. M. T. Raycheba, Anal. Chem. 46, 374 (1974).
3. Y. Talmi, Anal. Chem. 47, 658A (1975).4. Ref. 3, p. 697A5. G. G. Olson, American Laboratory (February 1972).6. OMA Catalog, Princeton Applied Research Corporation, P.O. Box 2565,
Princeton, N.J. 08540.7. K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem. 46, 575
(1975).8. C. N. Helmick, Jr., SPIE Proc. 62,177 (1975); see also C. N. Helmick, Jr.,
and W. H. Woodworth, Ferroelectrics 11, 309 (1976).9. "Supplement to Application Note APV-6075," Thomson-CSF, Groupment
Tubes Electroniques (February 1976).10. "Pyricon-An Introduction to Modern Thermal Imaging Technology,"
Thomson-CSF, Application Note APV-6075 (November 1975).11. Experiments were carried out at the Department of Chemistry, Booklyn
College, SUNY, Brooklyn, N.Y. 11210, courtesy of A. Ron.12. A. L. Harmer, IEEE Trans. Electron Devices ED-23, 3120 (1976).13. P. Felix, Thomson-CSF; private communication (April 1977).14. T. Conklin and B. Singer, "High-Performance Pyroelectric Vidicon," In-
ternational Electron Devices Meeting, Washington, D.C. (1975).15. K. M. Baird, H. D. Riccius, and K. J. Siemsen, Opt. Commun. 6, 92
(1972).16. P. Felix, Thomson-CSF, Division Tubes Electroniques; private commu-
nication (June 1976).17. P. Felix, G. Moiroud, and S. Veron, "Recent Improvement of the Pyricon,"
Fourth European Symposium on Military Infrared, Malvern (May1974).
