low light level tv techniques

References

1. E. Baumann, J. Brit. Instrum. Radio Eng. 12, 69 (1952).2. W. E. Good, Proc. Nat. Electron. Conf. 24, 771 (1968).3. M. Auphan, L'Onde Elec. 36, 1040 (1956).4. R. Whiddington, Proc. Roy. Soc. (London), Ser. A 86, 360

(1912); 89, 559 (1914); also K. Gentner, Ann. Phys. 5.Folge 31, 407 (1938).

5. S. Timoshenko and G. MacCullough, Elements of Strength ofMaterials (Van Nostrand, Princeton, N.J., 1955).

6. Ref. 5, pp. 122, 171, 172.

7. K. G. Sodeberg and A. K. Graham, 34th Annual Proceedings,American Electroplaters' Society (American Electroplaters'Society, Jenkintown, Pa., 1947), pp. 74-95; W. M. Phillipsand F. L. Clifton, pp. 97-110.

Low Light Level TV Techniques

J. Gildea

As the science of low light level sensing becomes better understood, the demand for systems with this ca-

pability has increased considerably in recent years. Low light level television systems are part of these lowlight sensing devices in which interest has grown. Development of low light level TV systems has, inturn, stimulated technical advances in new tube types with improved performance, development of elec-tronic techniques which enhance the over-all performance, and design techniques which make the systemmore versatile and adaptable. A general look at some of these developments and techniques gives in-sight into the versatility and adaptability of low light level TV.

Introduction

The capability of obtaining reliable information underlow light level conditions has been actively pursued formany years. Until recently, however, progress hasbeen slow and field success limited, primarily due toLow Light Level (L3 ) weak equipment performance andto the lack of a real need for L3 sensing systems. Witha sharpened interest created by military ventures,developments in new image tubes, optics, and elec-tronics have triggered a revolution. Increased use ofthe devices created breakthroughs and stressed equip-ment performance, versatility, and adaptability.

Technical developments not only provided a betterunderstanding of low light level requirements but alsohelped to simplify system configuration and enhanceperformance. These developments included: imagetubes with improved characteristics, photoemitterswith a broadened spectral response and higher sensi-tivity, electronic techniques for automatic control,electronic zoom and shuttering functions, and wide-spread use of semiconductor technology. As the per-formance of many military systems continues to im-prove, their complexity will lessen, the quantity in usewill multiply, the cost of the basic sensors will plummet,and applications to solving more of man's everydayproblems will become more evident.

The author is with the Radio Corporation of America, Bur-lington, Mass. 01803.

Received 20 April 1970.

Two categories of L3 sensing systems have promotedrecent interest: remote viewing and direct viewing.Optics and electronic functions are usually similar ineach, but their image tube complex categorizes their use.The direct view consists of a light-collecting lens,several stages of light intensifiers, and an eyepiece.The remote type, where detectors are electronicallyscanned, produces a video signal which can be remotelydisplayed. It basically consists of a light-collectinglens, light amplifier stage, readout image tube, andelectronics. This type encompasses LTV systems.This paper stresses the characteristics of these systemsthat have been most instrumental in establishingL3TV as a solution to L3 sensing.

Low Light Level TV Characteristics

Except for increased sensitivities, L3TV cameras aresimilar to those presently existing in TV studios or indaylight surveillance systems. Basically, this type oftelevision utilizes the ambient illumination of starlight,moonlight, or sky glow light falling on objects in orderto create a useful visual image to be displayed. Illu-minance levels under these conditions on the earth'ssurface are indicated in Fig. 1. The camera's useful-ness lies in its ability to intensify the reflected lightfrom an object at these levels so that what the eye alonecannot see, it can see with the aid of the TV system.In effect, it extends the range of visualization and signif-icantly sharpens the clarity of the user's night vision.However, scenes observed through the device will nothave the same clarity of image definition at any given

2230 APPLIED OPTICS / Vol. 9, No. 10 / October 1970

range as they would in daylight. This is due primarilyto the loss of resolution and contrast that are a funda-mental physical consequence of the lower ambientlight level, and secondarily to losses in resolution andcontrast inherent in the electrooptical system. Inpractice, color is also lost. However, the degradationresulting is not so great as to prevent useful data,particularly with image tubes whose spectral responseoverlaps that of the eye.

In comparison with the unaided eye, L3TV representsa step function increase in the ability to visually detectand recognize objects and forms at night. It is not anall-weather seeing device. It cannot see through fog,haze, smoke, rain, or snow any better than the unaidedeye or an optical instrument. It does, however, im-prove night resolving capability when not limited byatmospheric conditions.

Principally the L3TV features that give it outstandingadvantages over unaided vision or aided vision withconventional optical devices (binoculars, etc.) as shownin Fig. 2, are as follows:

(1) The objective lens can have a light collectingaperture area much greater than the pupil of the eye.

(2) The sensitive photocathode area of the detectingmedia can be made much greater than its counterpart,the retina of the eye.

(3) The quantum yield, or efficiency, of the firstdetector photocathodes can be substantially higherthan that of the retina.

(4) The observer's eye can remain light-adapted(photopic vision) in viewing the bright picture on thedisplay screen. The resolution of the eye thus remainshigh and no waiting period for dark adaptation (scotopicvision) is needed.

(5) The remote view systems give more field flexibil-ity and adaptability.

The effectiveness of the L3 system in seeing at lowambient light levels is therefore a function of its com-ponents and features which make it perform to man'sadvantage. Effective systems are characterized by:(1) high sensitivity image tubes that may be coupled forimproved sensitivity or may have different spectralcharacteristics for various applications, (2) automaticlight control over ranges of scene illumination fromdaylight to night, (3) electronic shuttering for lightattenuation or implementing pulsed illumination sys-tems, (4) electronic zoom in lieu of complex opticalzoom, (5) low noise video channels, (6) wide aper-ture, long focal length optics, (7) environmental pack-

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aging design. These developments have contributedsignificantly to the success of L3TV systems. Severalare basic to any TV system but most have strengthenedL3TV system capability, making it a multiapplicationaldevice. A closer look at some of these developmentswill clarify L3TV science.

Image Tube Criteria

The image tubes are the heart of the system. Newtypes which have overcome problems encountered inearlier developed tubes and the development of fiberoptics have given great impetus to the low light level(L3) TV field. Tube types include vidicons and orthi-cons and recent new types such as isocons, SEC target,and silicon target tubes, and image intensifiers (seeTable I). Except for the intensifier, all the tubes func-tion similarly in that they have a target which accumu-lates a charge until the image signal is read out by ascanning electron beam. In LTV applications thescan tubes are usually not sensitive enough to be usedalone in view of all other aspects of the system. There-fore, all are used with intensification stages simplycoupled to the scan tube through the medium of fiberoptics, which gives a highly efficient coupling. Hereto-fore these intensifier stages were included as part of thebasic tube, which made it complex and reduced yield,or were added through optical couplings, which weak-ened the intensification by optical coupling loss.Therefore, the fiber optic-coupled technique has re-duced complexity and has assisted in bringing forthnew tube types and in improving L3TV performance.

The significant goal of L3TV is to achieve as highresolving capability as is possible in the presence of lowlight levels. This depends upon how much detail theeye, as the integrating medium, can resolve in thepresence of the fluctuating nature of light. Utilizingspecific system parameters which include the firstphotodetector sensitivity, contrast of the target on itsbackground, integration time of the human eye, and afactor for the signal-to-noise ratio required to detectan isolated round target, one may determine a curveof limited resolving capability as a function of light

October 1970 / Vol. 9, No. 10 / APPLIED OPTICS 2231

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Table I. LTV Image Tubes

Type Signal generating process Main features

ISOCON Photocathode generated electrons bombard glass target with High aperture response.500-V acceleration potential. No blooming.

Scan beam produced nonspecularly reflected electrons at target High signal-to-noise ratio.form a return beam which is amplified by an electron multi- Excellent high light over-plier. load operation.

SEC Photocathode generated electrons bombard AlO, Al, KCI tar- Low lag. No blooming.get with 8-kV acceleration potential, producing high gainthrough generation of secondary electron conduction.

SIT Photocathode generated electrons, with 8-12 kV acceleration High sensitivity, good lightpotential bombard a silicon solid state target producing high overload operation, smallgain through generation of electron hole pair charge carriers. size and rugged.

INTENSIFIER Photocathode generated electrons with 1-kV acceleration Makes available a variety ofpotential bombard phosphor screen. photocathode responses and

provides for increase gain toimage tube.

level. For a system utilizing a first photocathodesensitivity of 400 ,A/lm and assuming a 100% contrastscene, the limited resolving capability in televisionlines per picture height of a 24-mm image is as shown inFig. 3. Combinations of image tubes ISOCON, SEC,SIT coupled to an intensifier with such a 400 A/lmphotocathode are also shown indicating their capabilityin achieving the desired goal. Each curve representslimiting resolution at a specific light level, as observedby the eye on a display. The curves therefore have aconstant signal-to-noise ratio at the respective linenumbers. The variation from combination to combi-nation is a function of the tube sensitivity, systemnoise, signal current, and over-all amplitude responsecharacteristic of the image tubes, video channel, anddisplay. Each plays a role in making an effectivesystem.

As shown, the I-SIT combination closely approxi-mates the desired low light level requirement. Theupper section or highlight limiting resolution is a func-tion of the system noise and the image tube amplitude

Fig. 3. Rlesolution characteristics of L3 TV image tubes.

response. Thus the lower the noise and higher theresponse, the better the performance. The I-SEC andI-ISOCON both lack gain, but could also be made toapproximate the requirement by the addition of anotherstage of intensification. However, their highlightresolving capability would decrease due to the increasein system noise and the loss in amplitude response.

The curves represent the measured performance ofthree typical L3TV systems for a 100% contrast scene.They may be made to indicate performance undervarious other scene contrast as well as image motion.Both cause a reduction in the signal developed at aspecific line number, thus causing a shift toward higherlight level to maintain equivalent resolving capability.Each combination is affected in much the same waysuch that curves for lower contrast and dynamic condi-tions would have the same relative position to oneanother as for the static 100%o contrast curve of Fig. 3.

As mentioned, the video channel plays an importantrole in low light level systems. Due to the video band-width required to see high light resolution, the noisegenerated in developing a wide bandwidth becomessignificant. The video channel must have sufficientbandwidth not to limit daylight performance, but lowenough noise so as not limit the low light level perfor-mance. The system resolution (Fig. 3) is high at thehigher light levels and the video channel needs a widebandwidth to pass this resolution. As the light leveldecreases, the resolution also decreases since the ampli-fier noise level begins to affect the signal-to-noise ratio.Therefore we can see the need for wide bandwidth,low noise video channels. However, as the resolutionfalls off at the low light levels, the wide bandwidth isnot required and an advantage can be gained in lowlight level TV systems by reducing the bandwidth.This is accomplished by the automatic light control inthese systems, yielding an improvement of better than3 to 1 signal-to-noise ratio at the lower light levels withsystems which are, in effect, video channel noise-limited.


In addition to the primary performance characteris-tics just discussed, image tubes are looked at for othercharacteristics such as blooming or spreading out ofsmall high light images in the scene, damage susceptibil-ity due to target rupture or burn, environmental capa-bility, and physical size. The tradeoff that one canmake depends upon the application. Instantaneoushigh light can be sustained by vidicon, isocon, andsilicon target tubes. Orthicons and SEC target tubesare subject to catastrophic failure due to either targetrupture or burn. Under environmental conditions,particularly vibration, tubes with targets having aclosely spaced mesh are susceptible to generation ofmicrophonic signal modulation. SEC tubes, orthicons,and isocons have this problem. Therefore for presentday low light level TV applications where sudden highlight flashes occur in the field of view, and where ex-treme vibration or shock occurs, it is obvious then thattubes with poor performance under these conditionscannot be used. The sensitivity and resolution per-formance therefore do not give the entire picture forlow light level TV applications. It is best to select animage tube for a particular application based on itscapability for meeting the application requirements.As shown, performance is different and other character-istics make them more or less desirable. For futureL3TV systems data indicate that the silicon target tubewith its high sensitivity, ruggedization, nondamagecapability, and its small size will perform best in themajority of applications.

Spectral Characteristics

Although the basic scan tube is the heart of thesesystems, the intensifier stages fiber optically coupled toit offer more than just an increase in gain. They offerthe following: capability for utilizing different photo-emitters for changing the spectral region of interest;use of different phosphor screens for providing betterspectral matches to the scan tube and reduction ofphosphor lag; a variety of sizes for matching the imagetube photoemitter size; capability of zoom; and capa-bility for implementing range-gated systems throughelectronic shuttering. All these advances have in-creased the use of the L3 TV system.

The advantage in having a selection of photoemittersis twofold. First, it permits different applications and,second, it permits upgrading the system performanceas improved sensitivity becomes available. Typicalphotoemitters indicate the old standby S-20 and -1and the present day extended red S-20 or S-25.

As nighttime radiation is decidedly toward the red,the S-25 response with high quantum efficiency in thered is very popular for nighttime reconnaissance. Eventhough the S-1 has good red response, it is usuallycharacterized by high clark current background withincreased temperatures and therefore is most alwayscooled. As L3TV systems often have the first photo-cathode floating at -25 kV to -30 kV potential, con-siderable complexity is added to cool an -1 surface.Therefore one can see the widespread use of the S-25for the first photoemitter surface.

Use of the S-25 on the second photocathode, generallythe readout tube, is not so important as on the first sur-face, since this surface is looking into an intensifier phos-phor screen with a P-20 or P-i spectral response. It isimportant that the two surfaces spectrally match eachother since this determines the additional electron gainprovided to the system by the intensifier addition. Theelectron gain is determined by integrating the product ofthe phosphor spectral output and the photocathode spec-tral sensitivity over the common spectral region, thusgiving a ratio between photocathode current and powerincident on the phosphor. Therefore the more energycollected, the better the gain. For the typical P-20-S-20sandwich, with a photocathode sensitivity to whitelight (2870 K) of 200 uA/lm, an electron gain of fortyhas been achieved. It is usable with typical electrongains from published characteristics of scan tubes todetermine approximate performance of the coupled com-binations. Curves estimated in this way comparefavorably with those of Fig. 3. The spectral responseof the image tubes thus affects the over-all L3TV per-formance, and the previous discussion briefly indicatesits significance to this science.

Circuit Features

Intensifier stages coupled to readout tubes come in avariety of sizes. As some readout tubes have 18-mm,25-mm, and 40-mm photocathode sizes, the need for amatching size of phosphor screen is evident. Sinceintensifier photocathode sizes need not be the same, thesizes vary from 80 mm to 18 mm. Tubes are made asone-to-one versions, i.e., 18-18, or demagnified versions,80-25, 60-18, 40-18, etc. All types are desirable. Somedemagnified versions offer more gain than one-to-oneversions by the square of the ratio between screen andphotocathode size. Some have zoom capability andothers gating capability or both. Some one-to-oneversions are diodes or have gating grids. Althoughall these items are important, two significant functionsof the intensifier enhance the L3TV system performanceand applicability: electronic shuttering and electroniczoom.

Electronic Shuttering

The use of electronic shuttering in low light level TVcameras allows the system to function as a light atten-uator, and as a background discriminator when usedwith pulsed illuminators. In both cases, complexcircuitry operates with gated intensifiers to shut off theflow of electrons from the intensifier photocathode tothe phosphor screen for discrete time intervals.

Gated intensifier stages are not unusual devices. Inmost gated systems, triode intensifier stages are used,but tetrode and pentode units have also found limiteduse. No matter which tube is used, the focus gridwhich sits at approximately +200 V to +400 V withrespect to the photocathode is pulsed in the shutteringmode at least 1500 V negative with respect to thephotocathode to prevent electrons from reaching theanode or phosphor screen. The effectiveness of thiscutoff is determined by a so-called cutoff ratio, the


Fig. 4. Range-gated electronic shutter.

ratio of illuminance out of the phosphor screen beforeand after cutoff. Tubes with less than 100,000:1 areusually not good shutter tubes when the photocathodeillumination is sufficient to generate 0.1 MA/cm2 photo-cathode current. In addition to this good cutoff ratio,the tube must be capable of maintaining good focusquality with on time as low as 50 nsec and with repeti-tion rates as high as 15 kHz. Of all tube types thatoffer gating, the RCA C33014 triode exhibits the bestover-all characteristics and performance for use in gatedsystems. Such tubes are available with either S-25or S-1 photocathode response, thus making them evenmore adaptable.

Sharing equal billing with tube capability is thecircuitry required to accomplish the shuttering func-tion. The primary requirements for the circuit neededto produce electronic shuttering of these image intensi-fiers operating in television systems are that it shouldbe capable of switching the tube grid in nanosecondsat high repetition rates and float at the -30 kV poten-tial of the intensifier photocathode.

A typical electronic shutter meeting the abovecharacteristics is shown in Fig. 4. This circuit pro-vides a minimum pulse width of 2 Msec, with a 2200-Vamplitude and 500-nsec rise time. All pulsing is insynchronism with the TV scanning frequencies toeliminate interference and the pulse top is flat within5 V to maintain image focus during the on time.

This circuit operates with pulsed illuminators or asa light attenuator. In applications where a shuttercircuit is required to operate as a light control device,the ratio of tube on time to the 1/60 sec readout timeof the TV scan develops an attenuation characteristic.Approximately three orders of magnitude of light canbe handled in this way in lieu of other methods.

The most common use of electronic shuttering tech-niques is with pulsed illuminators such as GaAs. Inthis mode, the illuminator emits a wavefront of lightand the turn-on time of the intensifier is delayed until

energy is returned from a particular range. Tube ontime depends on the depth of field desired. The riseand fall time of the pulse determines the sharpness ofthe imagery, and any wider pulse than the depth offield determines the contrast since a wider pulse thannecessary collects unwanted background lighting.Approximately 400 TVL/PH can be resolved at arange of 1000 m for a 15-W pulsed GaAs illu-minator and 40-mm I-SEC camera with a 15° fieldof view operating in this mode.

Electronic Zoom

Another feature which the additional intensifierstage can supply to the low light level TV system iselectronic zoom. Tubes exhibiting this characteristicare of the demagnified type, and zoom is accomplishedby varying the intensifier grid voltages to focus electronson the screen from concentric areas on the photo-cathode. Although the zoom tube performance playsan important role, the more significant part of im-plementing zoom is the development of the high voltagepower supply.

The voltages required by the zoom image tubes areparticularly critical if these tubes are used in cameraswhere their gain is varied to attenuate light. Eachvoltage must remain stable and proportional to thephotocathode potential within 0.1% at any zoom posi-tion in order to maintain focus and size as the gain ischanged. It is common to build these supplies in adc-to-dc converter configuration to get synchronismwith the TV scan rates. Each output has 50-MA loadcapability to obtain stability and fast time constantwhen zoom is activated. Such supplies are packagedin sections similar in configuration and size to theelectronic shutters in Fig. 4. They represent state ofthe art in miniature zoom high voltage supply design.

Control Features

A TV system whose characteristics permit highresolution during periods of high scene illumination andhigh sensitivity at low scene illumination is moreversatile if it can operate over a wide range of sceneillumination, thus accomplishing both tasks within asingle system. Controlling a light range of 104 to 10-s

ft L is usually required. Two means of attenuation arefrequently used in L3TV systems requiring both day-light and nighttime operation with the same camera.These means are neutral density filter screens withapproximately a 10b attenuation and an iris diaphragmin conjunction with so-called bull's eye filters which alsoprovides attenuation of 106.

Functionally, the bull's eye is made of neutral densitystep rings which operate with the lens iris to attenuatethe incoming light. The attenuation characteristicis usually not too linear due to the discrete steps in therings, although some units with gradual changes in thedensity rings provide a more linear attenuation.Nevertheless, it is useful in providing the necessaryattenuation. A second method of obtaining a widedynamic range of light control is to use an attenuatorbelt. A belt is made in such a way that its two sides


Fig. 5. I-SEC L3TV with range-gated electronic shuttering.

have a graduated density characteristic. As the twosides with the density variation in the opposite direc-tion are passed by each other, uniform attenuation isobtained throughout the field of view.

Daylight operation of LTV systems cannot beaccomplished with neutral density filtering only.Therefore, additional attenuation is provided byelectronic shuttering or adjusting the gain of the intensi-fier stages by varying the acceleration potential acrossthe tubes. The two types of attenuation, optical andelectronic, are sequenced in operation such that, oninitial system turn-on, the iris or attenuator starts inits most dense positions while application of the acceler-ation potential is delayed until the tube heaters warmup. When the potential is applied, it is at a slowly in-creasing rate so that at high light levels the tube ac-celeration potential has a chance to control and not puta sudden surge of light on the target. As the level ofproper video is established, a level of high voltage isreached where tube performance is optimum. As thelight level increases or decreases from optimum and theacceleration potential is driven to preset limits, theiris or attenuator is actuated so as to drive the voltageback to the optimum operating point. Thus, tubesare always utilized in their best performance range, anddaylight-night operation is achieved.

Systems

Examples of L3 TV systems utilizing the technologyand features previously discussed are shown in Figs.5 and 6. These are representative of cameras usingfiber optically coupled intensifiers and readout tubessuch as vidicons, isocons, secondary electron, and silicontarget devices. The systems include such features aspushbutton electronic zoom, electronic shuttering, day-night automatic light control, state of the art in lowlight level lenses, techniques for cooling photocathodesfloating at high potential, and state of the art in elec-tronic circuitry, and packaging for hostile environ-ments. Although systems as complex and expensiveas these will probably not find widespread commercialuse, more economical approaches can be developed.

As described, LTV systems operational today re-sulted from a steady stream of technical developmentsthroughout the years. There are many incidentalswhich must be considered in their design, many tech-niques which can enhance their performance, and manycombinations of tubes and electronics which can bestfit the application. Low light level TV is therefore ascience whose evolution brings forth new impetus forsolutions of the environmental problem of seeing underlow ambient conditions.

Fig. 6. I2 vidicon L'TV with cooled S-1 and range-gated elec-tronic shuttering.


low light level tv techniques

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