Imaging detectors for the ultraviolet

Imaging detectors for the ultraviolet

C. 1. Coleman

A review is presented of the available types of efficient middle- and far-UV imaging detectors. These in-clude luminescent phosphors, semitransparent and opaque photocathodes, mesh-based photocathodes, mi-crochannel plates, and solid-state devices. Certain important aspects of their performance and calibrationare discussed, and mention is also made of some applications of these systems, particularly in UV as-tronomy.

I. Introduction

Many areas of scientific research can benefit from theapplication of the latest generation of sensitive UVimaging detectors. Astronomical observations atwavelengths shorter than -320 nm (the atmospherictransmission limit) in the astrophysically interestingmiddle- and far-UV1 have been made possible in recentyears using rocket- or satellite-borne instrumentation,and many of the developments reported in this paperwere devised in the context of UV astronomy. Anothernotable application is in the field of plasma diagnosticsin fusion reactor research,2 and further ones includespectroscopy and UV microscopy.

No up-to-date review of the subject exists. Twobooks which are, however, still of considerable interestare Koller's ULTRAVIOLET RADIATION 3 andGreen's THE MIDDLE ULTRAVIOLET: ITSSCIENCE AND TECHNOLOGY. 4 Samson'sTECHNIQUES OF VACUUM ULTRAVIOLETSPECTROSCOPY 5 also contains much relevant ma-terial. The present paper draws upon a wide selectionfrom the more recently published literature.

Several types of detector (e.g., radiometric, photo-ionization) are capable of measuring UV flux, butwithout preserving spatial (2-D image) information.This paper, however, neglects these essentially single-point detectors-important as they are, especially forabsolute calibration purposes-and concentrates on thevarious types of imaging devices which are available foruse in the UV, particularly at low flux levels.

The author is with University College London, Department ofPhysics & Astronomy, London WC1E 6BT, U.K.

Received 14 April 1981.0003-6935/81/213693-11$00.50/0.©3 1981 Optical Society of America.

For the near UV one may employ fairly standardphotographic techniques or one of the more familiartypes of image tube systems (reviewed in Ref. 6), per-haps with slight modifications. Further into the UVmore special techniques have to be used, largely becausephotons of higher energies are not transmitted bycommonly available window materials.

11. Detectors for the UV

A. Classical Methods

1. General

Photography, one of the oldest methods of imagerecording, is a useful technique for the near and middleUV regions down to 200 nm, where absorption bygelatin becomes appreciable. Special purpose Schu-mann-type emulsions have been designed for workingat very short wavelengths; these are characterized by alow gelatin content and are essentially single-grainlayers. Examples are Kodak types 101 and 104 emul-sions such as were used on Skylab for EUV spectrohe-liograms7 and stellar spectra8 down to 17 nm. Unfor-tunately, however, these materials have very restricteddynamic range, poor resolving power, and high granu-larity, and yield a rather low image SNR. References9 and 10 are particularly useful papers reporting de-tailed performance measurements; they also includeconsideration of the nontrivial practical difficultiesencountered in handling these emulsions and describethe solutions adopted. Schumann emulsions have avery broad spectral response, and they are often usedin combination with a thin-film metal filter 1 to rejectunwanted long wavelength light.

A method of improving photographic sensitivity atshort wavelengths is to coat a visible light sensitiveemulsion with a wavelength-converting layer of a lu-minescent phosphor.5 Indeed, this method may alsobe used to confer UV sensitivity on other types of visibleimage detector, and it is particularly suitable for image

1 November 1981 / Vol. 20, No. 21 / APPLIED OPTICS 3693

100

cC

C

Oict

80

60

40

20

0 l 1 ' I l I - l

350 400 450 500 550

Wavelength (nm)

Fig. 1. Spectral distribution of the fluorescent radiation from sodiumsalicylate and tetraphenyl butadiene (TPB).

tubes with fiber-optic input faceplates or for solid-statedevices.

2. Luminescent PhosphorsSodium salicylate has been used for some time as a

detector in the far UV. Its popularity has been due toits simplicity of preparation (dissolve in ethanol andspray onto a substrate warmed to 500C) and to its highfluorescence quantum yield, which is almost indepen-dent of the wavelength of the stimulating radiation.1 2

The fluorescent emission occurs in a band peaking at443 nm (Fig. 1) and thus matches well to the responsesof S-1l, bialkali, or S-20 photocathodes. At an opti-mum layer thickness of 24 mg cm 2 , the quantum yieldapproaches 100% for incident UV radiation between 60and 360 nm.13 The fluorescent emission is isotropic sothat when the phosphor is on the fiber-optic faceplateof an image tube only about a quarter of the light can beaccepted by the fiber optics (numerical aperture -1);the overall efficiency, assuming a photocathode quan-tum efficiency of 20% in the blue, is thus <0.05 photo-electrons/UV quantum.

It has been found that there are significant advan-tages in using tetraphenyl butadiene (TPB) instead ofsodium salicylate. The spectral emission curves of bothphosphors peak at almost the same wavelength (Fig. 1).TPB can be vacuum evaporated at 2500C without de-composition and forms layers which are more repro-ducible in thickness, more stable, better adherent toglass, and more resistant to abrasion than sodium sali-cylate. TPB layers are also more uniform and ofsmaller grain size; at an optimum coating weight of -1mg cm 2 , the squarewave contrast transfer of the layeris 90% at 10 lp mm- 1 and 81% at 20 lp mm-1 ,1 4 so thatresolution with the phosphor is little degraded belowthat of an uncoated fiber-optic faceplate. The sensi-tivity of TPB appears to be somewhat higher but rathermore wavelength dependent than that of sodium sali-cylate, but the resolution is virtually independent ofwavelength (in contradistinction to UV photocath-odes).

Both phosphors are transparent to the visible spec-trum, so that a phosphor/fiber-optic/photocathodedetector cannot be made solar blind (see below). The

use of a luminescent phosphor on a fiber-optic faceplateconveniently provides almost constant sensitivity andhigh resolution throughout the UV but at the expenseof reduced sensitivity compared with an inherentlyUV-sensitive photocathode.

A currently planned application for a luminescentphosphor is in the Wide Field and Planetary Camerabeing built as one of the scientific instruments for theNASA Space Telescope.15 The detector used here-acharge-coupled device (CCD)-is primarily sensitiveto longer wavelength visible light; a 160-nm thickcoating of the organic phosphor coronene16 convertslight of wavelengths shorter than 400 nm to photonswith a wavelength distribution peaking at 520 nm,which is moderately well matched to the CCD sensi-tivity. The resultant quantum efficiency of the deviceis around 9% in the 100-300-nm wavelength range.Liumogen,'17 a phosphor with somewhat longer wave-length emission, has been considered too, but its effi-ciency is significantly less than that of coronene. 1 8 Acomposite (two-stage) phosphor of sodium salicylateand ruby (A1203) emitting at 700 nm has also beenproposed19 to provide a superior spectral match toCCDs.

B. Photocathodes for the Near and Middle UV

1. Broadband Semitransparent PhotoemittersThe semitransparent photocathodes which are gen-

erally used in image tubes for visible wavelengths arealso efficient in the UV provided that the substrate(usually the tube entrance window) is transmissive tothe wavelengths of interest (Fig. 2). Ordinary glassesand even UV glass do not usually transmit even downto 300 nm, while conventional fiber-optic faceplates are

LLUL-

7C,

I- )(2

< C

(0 0

60040 0

20-0

10 0

6040

2.0

060.4

02

100 120 160 200 250 300 400 500 600 800 1000 1200

WAVELENGTH (nm)

Fig. 2. Spectral response characteristics (schematic) of typicalsemitransparent photocathodes. The substrate materials are shownin parentheses; their approximate short wavelength transmissionlimits are indicated at the top of the diagram. The broken lines showhow the responses of the common visible-light-sensitive photoemitters

are modified by a borosilicate glass window.

3694 APPLIED OPTICS / Vol. 20, No. 21 / 1 November 1981

1. , N.

Table 1. Short Wavelength Transmission Limits of Some Useful UVWindow Materials

10% TransmissionMaterial limita (nm)

LiF 105MgF 2 115CaF 2 122BaF 2 135A12 03 (sapphire) 142SiO2 (fused) 160

a Figures are somewhat approximate as they depend, for example,on the purity of the material and the thickness of the sample (typically2 mm).

not transmissive below -380 nm (although UV fiberoptics which transmit to -320 nm are becoming avail-able). Consequently, it is not feasible to use electro-statically focused intensifiers (which normally have aplano-concave fiber-optic input) for the UV (exceptingcertain specialized designs; see, for example, Refs. 20and 21). It may be noted here that the principal aimof this section is to describe the primary process of UVimage photon detection; the standard electron-opticaldesigns and electron recording or television readouttechniques are not discussed in any detail here, as theyare well documented elsewhere (e.g., Ref. 6).

The transmission characteristics of some of theavailable window materials are summarized in Table I.Single-crystal LiF transmits down to 105 nm-theshortest wavelength transmission of any known bulkmaterial. Unfortunately, LiF is not particularly stable;its transmission suffers on exposure to damp air, and,additionally, it is highly susceptible to radiation dam-age,22 which is of consequence for satellite-borne in-struments. MgF2, with a slightly longer wavelengthlimit of -115 nm, is a much more practical materialfrom the point of view of long-term stability. Otheruseful materials are CaF2 and A1203 (sapphire-thislatter is the only VUV material which can be easilysealed to the borosilicate glasses used in many imagetubes; the other single-crystal materials mentioned hereneed special sealing techniques). A point to note inconnection with many of these UV-transmissive ma-terials is that they can be birefringent; thus, MgF2 (te-tragonal rutile-type crystallographic structure) shouldnormally be utilized with its faces cut perpendicular tothe tetrad axis of symmetry. Information on the tem-perature dependence of the transmissions of LiF, MgF2 ,CaF2 , BaF 2 , and A12 03 is to be found in Ref. 5.

A visible-light-sensitive photocathode has slightlydifferent characteristics when used in the UV. First,if optimized for UV wavelengths, its thickness andconsequently its long-wavelength response will be lowerthan normal. Second, as the incident photon energiesare high, the photoelectrons can be emitted with ener-gies of a few electron volts; this results in lower imageresolution which is also appreciably wavelength-de-pendent.23 Third, again because of the high photonenergies, electron pair production can take place in thephotocathode,24 increasing the apparent responsive

quantum efficiency (to greater than 100% in the middleUV in the case of the caesiated GaAs photocathode 2 5 )but actually decreasing the SNR and detective quantumefficiency.26 27 Note that other III-V photocathodes(e.g., InGaAsP 2 8 ), as well as more conventional onessuch as the bialkali and multialkali, 2 9 can all have highquantum efficiencies (10-20% or greater) in the UV.

A good example of a system using an extended visibledetector is the Faint Object Camera (FOC),30 which isthe European contribution to the Space Telescope in-strument complement. The FOC is designed princi-pally for direct imaging of astronomical sources at highangular resolution (0.1 sec of arc) from 115 nm into thevisible. Because of the Space Telescope's large aperture(2.4 m) and through the use of the image photoncounting detectors, the faint magnitude limit of thesystem should be mv -~ 28 (for an SNR of three in a 3-hexposure). The optical system is described in Ref. 15.The Photon Detector Assembly for the FOC is closelyrelated to the University College London (UCL) ImagePhoton Counting System (IPCS),30 but with severalimportant modifications: (1) extension of the spectralcoverage into the UV by using a MgF 2 entrance windowfor the image intensifier; (2) replacement of the focussolenoid for the high resolution intensifier by a speciallydesigned permanent magnet assembly3l (to reducepower consumption and mass); and (3) ruggedizationand space qualification of components.

The image photon counting concept is based on thefact that, using an intensifier of sufficiently high gain,individual photoelectron events resulting from a singlephoton's detection at the input photocathode can beamplified to a level such that the output scintillation canbe comfortably detected above system noise by a con-tinuously scanning television camera tube. Aftersuitable video processing the detected photoevents arerecorded directly in a computer memory location cor-responding to the image pixel in which the photon oc-curred.

The primary detector for the FOC is a ruggedized andslightly modified EMI 9914 three-stage intensifier (withMgF2 faceplate, hot bialkali photocathode for responsefrom 115 to 600 nm, and gain -105); the output is lens-coupled to a Westinghouse EBS (electron bombardedsilicon, alias SIT) camera tube.

The advantages of using an IPCS at low light levels,discussed in Ref. 30 and references cited therein, in-clude: (a) there is no low-level threshold for very weakexposures, so that exceedingly faint images can be re-corded; (b) the exposure linearity and system stabilityallow accurate photometric calibration and backgroundsubtraction (which may be carried out in real time); (c)rapid time-varying effects can be accommodated, witha resolution of a few milliseconds.

2. Spectrally Selective ResponsesBy appropriate choice of window material (deter-

mining the short wavelength limit) and photocathodematerial (determining the long wavelength limit), it ispossible to design detectors with tailored spectrallyselective responses.


lo

l0-

10

CE

10

10'

10O

Photon energy eVI

10 8 5 1

20 150 200 250 300 400 500 600 700

Wavelength Inl

Fig. 3. Spectral response of a semitransparent Cs2Te photocathodeon a MgF2 substrate.3 2 Note the tail of sensitivity which is actually

still measurable out to 800 nm.

For the middle UV it is often desirable to use photo-cathodes with solar blind response, so-called becausethey are relatively insensitive to the solar radiationtransmitted by the earth's atmosphere ( > 320 nm).The two most common solar blind materials are Cs2Te(Fig. 3) and Rb2 Te; the former has a long wavelengthlimit of -300 nm and the latter -250 nm, although infact both have a low-sensitivity tail extending at a verylow level throughout the visible. Peak quantum ef-ficiencies for semitransparent versions are -20% (see,for example, Ref. 32; also Fig. 3). The curves exhibita dip in sensitivity in the wavelength region where thephoton energy is around twice the semiconductorbandgap energy and electron pair production starts tobecome important. (This occurs short of -180 nm forCs2Te.)

The dark current of solar blind and far-UV photo-cathodes is extremely low, and it is not necessary to coolthem below room temperature. A typical dark countrate for Cs2Te is 4 X 10-3 cm- 2 sec-1 at 200 C.3 2 ,33 Thewavelength dependence of image resolution caused byvarying photoelectron emission energies is described inRef. 23. Further information on Cs2Te is contained inRefs. 34 and 35; Refs. 36 and 37 review the propertiesof solar blind and far-UV photocathodes.

A UV-to-visible image converter (UVC) with semi-transparent Cs2Te photocathode forms the input stageof each of the far-UV spectrograph cameras onboard theInternational Ultraviolet Explorer (IUE) satellite whichhas been operating successfully in geosynchronous orbitsince January 1978. The system obtains high signal-to-noise UV spectra of astronomical objects at a reso-lution of -0.01 nm in the 115-320-nm range, down toa limiting visual magnitude of -17. The IUE spacecraft

and scientific instrument are fully described in Refs. 38and 39. The converted image from the UVC is inte-grated and recorded by means of a TV camera with anSEC storage target.40 The UVC itself is a proximity-focused high-field device (5 kV across a 1.3-mm gap)having a gain of around 60,blue photons out/detectedUV photon. The UVC was chosen in preference to aluminescent phosphor deposited directly on the fiber-optic faceplate of the SEC camera tube because a solarblind response was desirable for observing objects in theUV in the presence of strong visible radiation; thismethod also provides a useful increase in system gainbut at the expense of a slight loss of resolution. Oneimportant point which emerged during the calibrationof the IUE detectors was that there is an appreciabledependence of the Cs2Te quantum efficiency on thevoltage applied to the UVC (Fig. 4), as it is a veryhigh-field device; thus, the quantum efficiency curvesderived from the camera calibration program weredifferent from those originally obtained for the UVCsalone (using them as photodiodes at a low bias voltageof -100 V) before they were built into the cameras. Adetailed study of the field-enhancement of photo-emission from Cs2Te is reported in Ref. 32. Analogousbehavior is to be expected with other UV photocathodesused in high-field devices.

C. Photocathodes for the Far UV

1. General

Photoemissive materials specific to the far UV (X <200 nm) have the distinct advantage of being stable indry air. Metzger4 l presents measured quantum effi-ciency data on 20 alkali halide materials over the rangeof 58.4-100 nm. CsI (Figs. 2 and 5)42 has a high sensi-tivity and is the most commonly used photocathode inthis region; it has a long wavelength cutoff at -200 nm,while that for KBr is at -160 nm and for NaCl at -150nm. The alkali halides also exhibit a distinct tailing ofresponse, and, if better discrimination against longwavelength background is required, cuprous halideswhich have a much sharper cutoff may be used; how-ever, the latter have only about half of the quantumefficiency of CsI. For wavelengths shorter than 105 nm(the transmission limit of LiF), one can no longer usesemitransparent photocathodes, and alternatives suchas those described below must be sought.

2. Reflective (Opaque) PhotocathodesAny material used as a semitransparent photoemitter

may also be used in a reflection mode. For the extremeUV (X < 100 nm) many metals (Pt, Au, Ni, Be, Pd, W,Cu) show reasonably good sensitivity, with quantumefficiencies from 1% up to >10% for Ni and Pt. Othermaterials which may be used specifically in the EUV arealkali and alkaline earth halides, e.g., LiF (which isLyman-oa blind), BaF2, and MgF2.43 For broader band(and higher) sensitivity materials previously mentioned,such as CsI and Cs 2Te, may be used; a typical layerthickness, sufficient for absorption of UV photons of allenergies, is between 0.1 and 1 Am.


2

Photon energy eV)

6 5 4 3.5 3 2.6 2.3 2.0 1.8

Wavelength Inm)

II - E m , I I I I I I100 200 300 400 500 600 700 800

Wavelength Inmi

Fig. 4. (a) Quantum efficiency of a Cs2 Te photocathode at low field (7.7 X 104 V m- 1 ), and field-enhanced (at 3.85 X 106 V m-1). (b) Quantumefficiency enhancement factor (ratio of high-field and low-field values) as a function of wavelength.

Opaque photocathodes have several advantages oversemitransparent ones: (a) the quantum yield can beconsiderably higher (about a factor of 5 for CsI at 120nm) as shown in Fig. 5; (b) the short wavelength cutoffis not limited by the substrate material; (c) for spaceapplications, background electron emission from thephotocathode due to energetic charged-particle-inducedscintillation or phosphorescence in the window material(and, over the long term, deterioration of windowtransmission) are eliminated; (d) opaque EUV photo-cathodes deteriorate little on exposure to air (providedthat the relative humidity is maintained below 25%),and they can therefore be incorporated in a windowlessconfiguration for spaceborne experiments.

Despite these advantages, opaque photocathodes aredifficult to use in electronic imaging devices because ofthe awkward geometry whereby the electrons areemitted from the same surface as that on which the lightis incident. Only a few practical designs have emerged,notably the following:

(1) The electronographic cameras and related deviceswith internal reflecting Schmidt optics built by Car-ruthers4 4 at the U.S. Naval Research Laboratory.These have been used with great success on rocketflights and on the Apollo 16 lunar mission.45 The re-flective-optics principle is illustrated in Fig. 6. Lightpassing through a figured corrector plate (which correctsspherical aberration) enters the reflecting optical system


C4

tL

CC'

3 10 10 8

(a)

10-2

O3 _ 6

100

15

10 (b)

C E5C c

CDSC

1 55 1~

100 150

electron

200

Wavelength nm)

Fig. 5. Typical spectral responses of opaque CsI and KBr photo-cathodes (continuous lines) compared with semitransparent versions

on LiF (broken lines). Data from Refs. 41 and 57.

Incident photons

I P o t o .II electron Figured corrector

_ -- plate le.g. LiFJ

Output electron 2

image plane

M agnelic/ Focused optical imagefocusing assembly on curved opaque

photocathodePrimarymirror surface

Fig. 6. Schematic cross-sectional diagram of a far UV camera withopaque photocathode and integral reflective Schmidt optics. Thephotocathode is supported by a spider and is operated at

about -25 kV.

and is directed by the primary mirror to the curvedopaque photocathode; the emitted photoelectrons areaccelerated and focused by means of electric and mag-netic fields to form an image which may be recorded ina conventional manner. The reflective type of detectorhas been used particularly for electronography, 4 6 whichis a method of recording several keV electrons onemulsion; it is characterized by linear response, highresolution, high storage, and good signal-to-noiseproperties. (The detailed performance of emulsionsexposed to electrons is described in Ref. 47.) Readouttechniques other than electronographic recording may,of course, be used as with any other electron-opticaldevice; for example, microchannel plates have been usedto provide additional prerecording gain.48 Because ofthe corrector plate the Schmidt cameras are limited toworking above 105 nm; for shorter wavelengths, spe-cial-purpose all-reflecting cameras have been de-signed.49,50 Note that the reflectances of mirror coat-ings such as Al/MgF 2 used down to 105 nm are very poorat shorter wavelengths; below 100 nm, surface coatingsof Os, Ir, or SiC may be used.5 1

Fig. 7. Schematic diagram of the oblique image converter. Theelectric field E and the magnetic field B are mutually inclined at an

angle a.

(2) The "oblique image converter." 52,53 This is aderivative of the traditional uniform electric and mag-netic field electron-optical system,5 4 and has the electricand magnetic fields at an angle to each other instead ofbeing parallel (Fig. 7). This has the effect of displacingthe electron trajectories away from the incident lightpath when an opaque photocathode is used; the imagequality is good [except for a small additional aberrationsimilar in appearance to astigmatism,26'55 and againsubject to the resolution limitation due to the highelectron emission energies (4 eV for CsI at 101.7nm56)], and the photocathode and output electronimage surfaces have the advantage of being plane.

Both the Schmidt-type cameras and the obliqueimage converter suffer from somewhat restricted opticalaccess, i.e., they cannot be used in very fast opticalsystems. They are also rather cumbersome because ofthe requirement for magnetic focusing assemblies.

D. Mesh-Based PhotocathodesAs indicated above, reflective photocathodes can only

be used in special situations, and what is really neededis a more generally applicable substitute for the semi-transparent photocathode for use in the EUV. Car-ruthers57 has developed a type of photocathode whichcombines many of the advantages of both opaque andsemitransparent varieties. It involves the use of anelectroformed nickel mesh upon which a photoemissivesubstance is deposited [Fig. 8(a)]. Photoelectronsemitted when the surface is illuminated are drawnthrough the interstices of the mesh and travel in thesame direction as the incident light. Thus, the mesh-based photocathode can be used in an otherwise con-ventional image tube.

Two mesh pitches were tried, namely, 40 lines mm-l(45% open area) and 60 lines mm-' (36% open area).The meshes were coated with a thin vacuum-evaporatedlayer of aluminium followed by a 1-Mm thickness of CsI.The image resolution of such a photocathode is ob-viously limited by the spatial sampling (Nyquist crite-


100

50

10

5

-

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LU

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CO

0

Cs IC I

kk rI I .I S.

% \Cs Ikr\ 1,

50

Incident photons

lr l Front bias rinq/ ( -ve w r to mesh)

Photoemissive coating

1----- ---. Photoelectrons Nickel mesh

40 lines mm-1)

I E (Electron-opticafocusing system)

Incident photons

I I Front bias rinn4 4 I

Photoemissive coati

\\\\\\\\\ \ \\N \\\\\\\\I " I

Photoelectrons

+ 'Venetian blind' grid

ng

Fig. 8. (a) Mesh-based photocathode; (b) proposed venetian blindtype photocathode (after Ref. 48).

rion) to around 30 lp mm-I; however, by way of com-pensation it is easy to manufacture these photocathodesin large formats.

For the mesh photocathode to work satisfactorily itis required that an electric field exists in the intersticesof the mesh, which is of sufficient magnitude to draw thephotoelectrons through into the electron-optical fo-cusing region. If this is achieved, the spectral responsecurve should be similar to that of an opaque photo-cathode, and the absolute quantum yield should just bereduced by an amount representing the fractional openarea. In practice, it has been found that not all photo-electrons are collected, but a combination of a front biasring and a high field (400 V mm-') behind the meshensured that some 80% of photoelectrons were collected.At 200 V mm-' (still a relatively high field for mosttypes of image tube, although not for proximity-focuseddevices), about two-thirds of the electrons were col-lected.

At 121.6 nm (Lyman-a) the measured responsivequantum efficiency of one of these photocathodes was-35% (this was calibrated with reference to a photo-ionization chamber containing methyl iodide gas asprimary standard, its quantum efficiency being 87% at121.6 nm). The mesh-based photocathode should beespecially useful in the VUV below 105 nm, but it is also

claimed to have significant advantages even at longerwavelengths, having perhaps twice the quantum effi-ciency of semitransparent photocathodes (on LiF orMgF2) and about 40-50% of the efficiency of an opaquephotocathode, despite the open area losses. Regardingthe latter, Carruthers48 has proposed the use of a ve-netian blind structure [Fig. 8(b)] which would interceptall the incident light and avoid the open area losses;there are, however, technical problems in manufactur-ing such a structure with the fine pitch required.

The mesh base can also be used for other photo-cathode materials such as Cs2Te and Cs3Sb with similargains over semitransparent types, although, of course,these would need to be in a sealed system as they cannotsurvive exposure to air.

Tests performed in electronographic cameras showedgood image quality, with the photocathode mesh pat-tern (60 holes mm-') clearly resolved; in practice, slightdefocusing gives a more acceptable image for manypurposes because the mesh structure is no longer visibleand because the storage capacity of the recording me-dium (electronographic emulsion) is more efficientlyemployed.

E. Microchannel Plates

The microchannel plate (MCP) is an imaging electronmultiplier which is capable of extremely high gain. TheMCP is a hexagonal-close-packed fused array of fine-bore (typically 12 or 25 gim) slightly conducting glasstubes of -4 -mm length and having a secondary emissioncoefficient greater than unity. For an up to date reviewof the technology of MCP manufacture and some per-formance data, see Ref. 58.

MCPs are most commonly used in the so-calledGeneration II image intensifier tubes, with a focusedelectron image incident from a photocathode. How-ever, MCPs also possess inherent photosensitivity in thefar-UV (also to x rays and charged particles); the UVsensitivity can be much augmented by having an effi-cient photoemitter such as CsI deposited on the hon-eycomb front face. (This technique can be extendedto longer wavelengths, even into the visible, using anappropriate photocathode material provided measuresare taken to protect the photocathode against poi-soning.)

A photoelectron entering one of the channels andstriking the wall (this, and the absorption of radiation,is helped by having the microchannels tilted at a slightbias angle with respect to the normal to the plate face)produces secondaries which in turn are accelerated bythe internal electric field to strike the wall; the processcontinues until an avalanche of electrons emerges fromthe output of the channel as a result of a single electroninput. Universal characteristics59 relate the electrongain to the length-to-diameter (L/d) ratio and thevoltage applied across the MCP (see also Refs. 60 and61). Typically, the gain may be 104 for L/d 45 and anapplied voltage of 1 kV. Normally, the maximum gainis limited by the onset of positive ion feedback (the ionsoriginating from gas released by electron bombardment)which degrades the photocathode, produces after-pulses


100

a

.9 c

w 0] LL

10

10 1

-2103l-3

50 100 150 200 250

Wavelength (nm)

Fig. 9. Detection efficiency of an MCP with and without a CsIphotocathode deposited on the surface (data from Refs. 66 and 72).Note that the results longward of 121.6 nm (curves labeled W) wereobtained in a device with a MgF2 window; the true peak quantumefficiency (attainable in a windowless device) of the CsI coated MCP

at 121.6 nm is therefore -48%.66

and can also lead to permanently switched-on channelsif there are enough ions to support a self-sustainingdischarge. The problem can be minimized by rigorousdegassing by electron scrubbing during manufacture.Recently, several techniques have been introduced tosuppress ion feedback, thereby prolonging device life-time and permitting considerably higher gain. Theseinclude the following:

(a) The "chevron plate" 62 where two MCPs withtheir channels at a small bias angle are placed close to-gether so that there is no line-of-sight path for an ion.This concept has now been extended to the use of threeor more stacked plates. While very high gains are at-tainable, image resolution suffers and the output pulseamplitude distribution may be rather poor.

(b) Curving the channels to a J or C shape.63 TheL/d ratio can be increased to around eighty, permittingan electron gain of 107-108 without ion feedback. Theoutput pulse amplitude distribution is also well peaked(the channel outputs are saturated) and does not exhibita long tail of very high amplitude pulses; these proper-ties enable the curved-channel MCP to be efficientlyused in a pulse counting mode.

The MCP may be used in a phosphor output imagetube, when the image resolution will be limited princi-pally by the structured nature of the MCP to perhaps30 lp mm-1. An optically coupled CCD [Sec. II.F.2]forms an attractive method of reading out the intensi-fied image.64 Several electrical readout schemesyielding somewhat lower resolution have also been de-vised for recording the MCP output pulses: (1) arraysof discrete anodes with individual amplifiers,2' 65' 66 (2)a self-scanned anode array,6 7 (3) resistive anodes,6 8'6 9

(4) coincidence crossed-wire anodes,6 5' 6 6'7 0 and (5)wedge-and-strip anodes. 71

In the far UV the detection efficiency of an uncoatedMCP has been measured50 67' 72' 73 as around 13% from40 to 90 nm (with a peak value of up to 17% in the regionof 70 nm), thereafter dropping fairly smoothly to about1% at 130 nm. With a CsI photocathode deposited onthe MCP surface, the quantum efficiency at this latter

wavelength became about 15% and remained high upto -170 nm66 (Fig. 9). (These particular measurementswere carried out in a device with a MgF2 window;without the window, the peak quantum efficiency of-19% at 121.6 nm would increase to nearly 50% at thatwavelength. Also, after taking account of the windowtransmission, the two curves for uncoated MCPs wouldbecome continuous.) Note that as with the mesh-basedphotocathode, a field electrode is required in front ofthe MCP to ensure efficient collection of photoelec-trons.66 Two points should be noted concerning-thedetection efficiencies of MCPs: first, the efficienciesturn out to be very similar for different manufacturers'MCPs (this is to be expected as they all use very similartypes of high lead content glass); second, the detectionefficiency depends upon the angle of incidence of theradiation, achieving its maximum value within a fewdegrees of normal incidence.73' 74 The high primaryyields coupled with the high MCP gain make such de-tectors very attractive for the far UV. A device of thistype with resistive anode readout has recently been usedon a rocket flight to obtain Lyman-a (121.6-nm) imagesof Jupiter.7 5

F. Photoconductive Detectors

1. Television Pickup TubesModified versions of the widely used vidicon-type

television pickup tube can operate in the UV. A goodexample is one which uses a double-layer CdSe/As2S3photoconductive target and a silica faceplate76 ; thisresponds efficiently from the red (-700 nm) all the waydown into the UV, limiting at -200 nm. However, aswith any other vidicon, it can only be used at moderatelyhigh light levels because of dark current and amplifiernoise limitations. The technique is also not capable ofextension to significantly shorter wavelengths by sub-stitution of, for example, a MgF2 faceplate because ofphoton absorption by the SnO2 signal electrode (evenif this is made as thin as possible).

2. Solid State ImagersCharge-coupled7 7 or other solid-state array imaging

devices are attractive alternatives to vacuum tubes forseveral reasons. They are robust and compact, need nohigh voltages, are stable, and provide good geometricalfidelity. The charge-coupled device (CCD) is generallythe preferred solid-state sensor for low light-levelimaging because of its low noise characteristics; darkcurrent can be reduced to negligible levels, even for longintegration times, by cooling to sufficiently low tem-perature (35 electrons/pixel/h can be achieved at-150'C). Resolution is limited principally by thespatial sampling due to the discrete charge collectionsites. Present-day imaging CCDs designed for me-dium-resolution television applications may have typ-ically around 350 X 500 pixels, each 20-30 ,um in width;arrays of 800 X 800 pixels and even larger are underdevelopment in several laboratories.

Little work has been done on the UV response ofCCDs. As noted above, UV sensitivity may be con-


II I I

Uncoated e X Csl coated -MCP (W) MCP

(W)

l , ___ I I I I

ferred on CCDs by coating them with a thin layer of anorganic phosphor. Here, it is considered whether CCDscan be inherently sensitive in the UV. It is known7 8that silicon photodiodes respond well between 50 and200 nm. However, all the photon absorption in thiswavelength range takes place within a depth of 0.01-0.1um in the silicon; hence, the surface layers play a deci-

sive role in device performance. This means that frontilluminated CCDs (with overlying polysilicon elec-trodes) cannot be used in the UV, and even their bluesensitivity is very poor. An exception to this is in thoseCCDs (still largely experimental) which have trans-parent SnO2 (Ref. 79) or In2 03 (Ref. 80) electrodes in-stead; their response in the blue is high but still does not.extend far into the UV. Back illuminated devices arebeing developed to avoid the problem of absorption inthe surface electrode layer. Most back illuminateddevices do not respond much below 400 nm, but, whenthe silicon chip is thinned [so that charge generationtakes place near the depletion (charge collection) re-gion], good response can be obtained in the near UV. Aquantum efficiency of 50% at 350 nm has been reportedfor one device8l; below this, the sensitivity is apparentlylimited by the transmission of the glass substrate towhich the thinned CCD is bonded, so it is very likelythat the response can be extended further into the UVby using a different substrate material.

CCDs also respond to soft x rays,82 but the responsediminishes toward longer wavelengths, limiting at >5nm for some types; for others, response up to -75 nmhas been reported.81 83 It is possible, therefore, thatuseful response may be obtainable over much of the UVusing thinned back illuminated CCDs, but it seemslikely that there will be some intermediate range ofphoton energies to which CCDs may not respond effi-ciently. This is one area in which further investigationand development effort are required.

Ill. Calibration

Each detector has its own specific calibration re-quirements. By way of illustration the following aresome of the problems encountered in the preflight cal-ibration of the IUE detectors at University CollegeLondon and which are likely to require considerationwhen dealing with other UV systems:

(1) Nonlinearity of response as a function of exposurelevel. (In photon-counting devices, a related butslightly different phenomenon-nonlinearity in in-tensity due to count-rate saturation-must be takeninto account.)

(2) Sensitivity (quantum efficiency) dependent onwavelength.

(3) Quantum efficiency also dependent on electricfield strength in front of the photocathode.

(4) Nonuniformity of response and of background(global and pixel-to-pixel).

(5) Image resolution (and long-range halation) de-pendent upon signal level, wavelength, position in theimage, and field strength in front of the photo-cathode.

(6) Image distortion.

Fortunately, some of the variables are separable, andthe problems are not completely intractable! At UCL,a computer-controlled vacuum optical facility wasemployed to facilitate the acquisition of the very largenumber (several hundred) of calibration images re-quired to take account of as many as possible indepen-dent combinations of variable parameters: wavelength,exposure level, test pattern (e.g., uniform illumination,bars at various spatial frequencies), and camera gainsettings. For absolute calibration of quantum effi-ciency, a standard Cs2 Te photodiode cell originatingfrom Stanford Electronics Laboratories8 4 was used. Animportant feature of the detectors is the incorporationof a network of regularly spaced fiducial marks by ref-erence to which computer image processing operationscan be carried out. Image processing techniques, in-cluding the Jet Propulsion Laboratory's VICAR systemused inter alia for the IUE images, are described in Ref.85.

For an instrument designed to operate unattendedin space for several years, onboard facilities for periodiccalibration checking are required. On the IUE space-craft, small low-power mercury arcs8 6 provide flat-fieldillumination at 254 nm. An additional onboard cali-bration facility-to supply an accurate wavelengthscale-is provided by a space-qualified platinum-neonhollow-cathode lamp with MgF2 window; this yields aspectrum rich in sharp and intense lines between 115and 310 nm, and the wavelengths of several hundred ofthese emission lines have been precisely measured. 8 7

IV. Conclusion

This review has illustrated several different lines ofapproach to high-sensitivity UV image detection.Many of the methods described are, of course, appli-cable in other regions of the electromagnetic spectrum,from soft x rays through to the near IR. Some of thetechniques are becoming well established, while otherssuch as UV-sensitive CCDs are still very much in theirinfancy.

It is expected that further new detectors will emergeshortly. An example is the recently announced Ma-gellan project, which is a space astronomy experimentdesigned to work at very short wavelengths (primaryrange 80-136 nm, additionally capable of working in therange of 27-68 nm). For highest sensitivity (exceedingthat of the IUE or Space Telescope instruments in theoverlap region above 115 nm) a photon counting de-tector is required. The final choice of a suitable sensorfor this application is awaited but might well com-prise a high gain MCP with KBr or CsI photocathodecoating, using either direct electrical or optical CCDreadout.

The author wishes to thank A. Boksenberg for re-viewing the manuscript and contributing several valu-able suggestions.


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Patents Patter continued from page 3692

strictly from the solar disk, eliminating circumsolar radiation (radiation scat-tered by dust and the atmosphere). By eliminating the circumsolar inputs,more-precise measurements can be made of energy captured by the receiverapertures of highly concentrating solar thermal-energy converters.

Version (a) field limiter, 41 cm long, uses an achromatic objective lens (similarto those used in small refractor telescopes) to form an image of the sun at anaperture just ahead of the radiometer cavity. Version (b) is shorter, 18.8 cm,and hence more convenient. Its shorter focal-length achromatic objective formsa smaller image that is magnified by another lens and imaged onto an aper-ture.

The diameter of the image at the aperture (3.8 mm) is the same in both ver-sions. Aperture diameter determines the instrument acceptance angle, i.e.,the aperture is a field stop. An aperture the same diameter as the solar imagewould prevent circumsolar radiation from entering the radiometer cavity.

Concentrating or focusing solar-energy collectors form an image of the sunon the collecting element. The sun subtends an angle of 32 min of arc. Prac-tical solar collectors accept radiation from a somewhat larger angle to avoidhaving the intense solar image fall on anything but the collector element. Theangle required depends on collector aiming accuracy and image definition. Acollector performance evaluation requires accurate radiation measurementswith the same acceptance angle as the collector. Measurements of radiationfrom the solar disk alone are also needed for comparison. Either type of mea-surement can be made by placing an aperture of the proper diameter in the fieldlimiter.

Either version of the field limiter would require calibration against a standardradiometer because reflection and absorption losses in the lenses prevent ac-curate sensitivity calculations from geometry alone. A solar-tracking mountkeeps the sun image centered on the field-limiting aperture.

This work was done by C. Martin Berdahl of Caltech for NASA's Jet Pro-pulsion Laboratory. Refer to NPO-14781.

Multibeam collimator uses prism stackMany precisely divergent beams are created from a single laser beam by a

stack of prisms in a new optical instrument. Each beam points in a slightlydifferent direction with a precise angular relationship to the others. Themultiprism collimator can be used to measure angles in surveying land andaligning machine elements. Unlike many other devices for measuring angles,the multiprism collimator is nearly immune to vibration, changes in gravita-tional force, temperature variations, and mechanical distortion. It is accurate

STACK OF

COVENION.L5 PRISMS

COLIMTO

E-L>0-'. o ' w r -UGT

PINHOLE

UT S I5DEVME FRONTSIEW

Fig. 16. Fanlike stack of prisms separates one beam of light intomany, each having a precise directional relationship to the others.The light source, a He-Ne laser, was selected for its high intensity andthe ease with which it can be collimated. A mask with aligned parallel

slits is mounted at the entrance to the prism stack.

within 1 grad in its present form, and it would be even more accurate if it werebuilt in larger sizes.

As shown in Fig. 16, the beam from a helium/neon laser is expanded andcollimated by a conventional optical collimator. The beam is then directedonto a stack of prisms, so that each prism intercepts a different portion of thewave front. The angles of the front and rear prism faces and their refractiveindices are selected to divert the beam by a given amount different for eachprism. The angle of the emerging beam thus changes by increments from oneend of the prism array to the other. For example, the direction may vary from+70 to -7° from one side of the array to the other, with the prisms providing1° increments and the deviation of the center wedge being 0°.

The prisms are mounted at their positions of minimum deviation. Thismeans that the angle of incident light is at the minimum of the deviation-vs-incidence-angle curve. In this region, the directions of the emerging beamsare least sensitive to changes in incidence angle. The accuracy of the deviation,therefore, is relatively unaffected by dimensional distortions. Mechanicalmotion of the collimator causes all the beams to shift in unison but does notchange the angular separation between the beams.

The glass used for the prisms is selected for its low-temperature coefficientof refractive index. This property ensures that the instrument accuracy ishardly affected by temperature changes.

In a collimator that has been built, the prisms are 4.7 mm thick and 82.5 mmwide. A stack of 15 such prisms in the mounting fixture provides a clear ap-erture 70.7 mm high and slightly more than 70.7 mm wide.

Prism arrays can be designed to produce various increments and ranges ofdeviation, since the parameters are determined chiefly by the angles of theentrance and exit faces for a given index of refraction. The important con-straint is that the deviation of an individual prism should approximate theminimum deviation.

This work was done by Peter 0. Minott of Goddard Space Flight Center.Refer to GSC-12608.

Dual-laser schlieren systemA proposed schlieren system uses two lasers and two knife-edges to simul-

taneously view perpendicular refractive-index gradients in a test volume. Itis an improvement over conventional schlieren systems, which monitor thegradient along only one axis. Although the system was originally developedto monitor materials-processing experiments in space, it should find usewherever there is a need to study 2-D temperature, pressure, concentration,or other gradients related to the index of refraction.

In the modified system, shown schematically in Fig. 17, two laser beamsseparated in wavelength by at least 1000 A are combined at a beam splitter.The composite beam is expanded, collimated, and then passed through the testvolume. The emergent light is focused by a second lens and is passed througha transmission grating that separates the laser beams.

Each of the beams is focused on one of two perpendicular knife-edges. Inthe figure, one knife-edge is in the plane of the page, normal to the optical axis;the other is normal to the page and to the axis. The images on the screen arethus a side-by-side display of the perpendicular refractivity gradients at theobject point in the test volume.

This work was done by Robert B. Owen and William K. Witherow of MarshallSpace Flight Center. Refer to MFS-25315.

6,328-A 5,145-AIMAGE IMAGE

Fig. 17. Dual-laser schlieren system employs two lasers of differentwavelengths and two perpendicular knife edges. The simultaneousside-by-side images on the screen display mutually perpendicularrefractivity gradients in the test volume. One gradient is in the planeof the page, normal to the optical axis; the other is normal to the page

and to the axis.

continued on page 3706


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Imaging detectors for the ultraviolet