THE VITASCANLIVE FLYING-SPOT COLOR SCANNER

Jesse H. Haines and G. Richard TingleyAllen B. DuMont Laboratories, Inc.

Passaic, New Jersey

Introduction

The DuMont "Vitascan" Color TV system pro-duces live color television pictures of highquality without the use of expensive and compli-cated color television cameras or highly trainedand skilled camera and maintenance crews. It is,in fact, a modern all-electronic version of theearliest forms of mechanical live televisionpickup which employed the flying-spot scanningprinciples.

While the Vitascan cannot displace storage-type color TV cameras, it is expected that theunique advantages of the Vitascan will earn it amodest place in color TV programming. It is thepurpose of this paper to explain its virtues andshortcciings.

On an historical level, it is our hope toexplain how the present technical state of theart has allowed this revitalization of the oldflying-spot principle. The basic principles arediscussed and the evolution of a workable systemleading to the present commercially availableequipment is traced.

Historical Review

J. L. Baird, in England in 1932, vas thefirst to achieve the analysis and synthesis of asubject by television. He employed a 30-linemechanical scanning system. As late as 1937, theBBC in London programmed with a Baird mechanicallive pickup flying-spot scanner operating at 240lines.

In advancing from 30 to 240 lines, the mech-anical scanner system was doomed for two majorreasons. First, the extremely acute mechanicalproblems and, secondly, the lack of sensitivityof the then available photocells. Since multi-plier photocells were not available, the lowsignal level gave excessive noise when amplified,thus limiting the action to head and shouldercloseups.

The first non-mechanical live pickup, usinga cathode-ray tube, was demonstrated early in1947 during the first FCC color hearings inconnection with the RCA simultaneous color TVsystem.1 The 18" x 24" scanned area limited thedevice to head and shoulder pickups.2 Although

1. RCA Staff, "An Experimental Simultaneous ColorTV System," PROC IREE, Vol. 35, PP 861-870.Sept., 1947

2. RCA Review Publication - Television Vol. V(1947-1948): Simultaneous Color Television -The Flying-Spot 'Live" Pickup. PP 244-248.

931A multiplier phototubes were used, the lightcollection was severely restricted by the 5/16"x 15'16" pickup area.

The DuMont color scanner development program,which began in 1949, took advantage of the newlyavailable 2 multiplier phototubes. Although thetransmission of slides, film, and opaques wasachieved with considerable success, it was notuntil 1954 that components and techniques hadsufficiently improved to allow the contemplationof a satisfactory color scanner for limited livepickup work which would surmount some of the short-comings of earlier equipments. The real technicalbreakthrough came with the commercial availabilityof the first 5" multiplier phototubes, the DuMontType 6364.

At first, our efforts were directed to pro-viding a live-action color pickup of a 7-2 x 10foot scanned area solely ass a signal source forresearch purposes. However, notwithstanding itsinherent limitations, it became apparent that avery definite use could be made of the principlein normal color broadcasting operations. A com-mercial version of this device was dubbed the"Vitascan" and a prototype was first publiclydemonstrated at the NARTB Convention in May, 1955.This paper is not intended as a detailed descrip-tion of the present commercially available equip-ment, but rather as a history of its parentage.

Basic Principle of Operation

The light from a high-voltage, high-beamcurrent flying-spot scanning tube which emanatesfrom the unmodulated rectangular scanning patternis focused onto the subject or scene being tele-vised through a suitable lens system.

As this light spot traverses the scene in itsregular pattern it is reflected from the variousobjects, the amount and color of reflected lightat any given moment varying with the reflectionfactor and color characteristics of that part ofthe scene. The scanning light incident on thescene is therefore modified or modulated by thedifferent reflective characteristics of the scenefrom one part to another. These variations inintensity and color of the reflected light arepicked up by groups of multiplier photocells,suitably disposed around the scene, which linearlyconvert the light energy to varying currents forprocessing through the normal color video apparatusfor flying-spot scanners.

The phototubes are covered with suitablecolor filters which, in combination with the spec-tral emission characteristic of the scanning tubeand the response of the phototubes determine thespectral characteristics of the system.

11

The major differences between the flying-spot scanning operation for the transmission oflive pickup and that for transparencies are:

1. The scanned area is many thousand timeslarger for live pickup than for slide or filmoperation.

2. The scanning light is reflected fromsolid objects rather than transmitted through flatmaterial as in the case of a transparency.

3. Using a multiplicity of phototubes,lighting effects similar to other live-action TVsystems are provided.

4. Controlled illumination additional to thelight from the flying-spot must be provided in thestudio.

5. Special TV previewing techniques must beemployed.

In visualizing how the Vitascan live-actionscanner operates, it is helpful to regard thesystem as being the reverse of conventional studiotelevision operation. Clusters of phototubes re-place the studio lighting fixtures and the flying-spot tube is substituted for the image-orthiconcamera tube.

Normally, light from the studio fixturesilluminates the scene, and reflected light isgathered by an objective lens onto a light-sensitive electrode, where it is transformed bythe scanning process within the camera tube intoelectrical currents. In contrast, the scanningtube and its lens, placed where the camera wouldnormally be, produces the light and performs thescanning operation at the same time. Some ofthis light reflected from the scene is inter-cepted by the phototube cluvters and convertedinto electrical currents.

The analogous relationship described abovewas recognized by the early pioneers in televi-sion. One such printed reference by Lance andPercy in 1935, describes this relationship aptly:

"In this studio there has been a great deal ofof work done to produce an artistic effect bycorrect lighting of the artists; in fact, it wasfound that, since each photocell could be con-trolled separately to give the correct balancebetween top, side, and back lighting, in imitationof the lighting of a floodlight studio, thesimplest way of working out the lighting effectswas to regard the scanning projector as a camera,and the photocells as light sources, placing themand altering their gain from that point of view."

Studio Requirements

Since the "Vitascan" System operates byvirtue of a scanning light beam, no illuminationother than that from the controlled scanningsource can be allowed to reach the phototubes dur-ing the scanning period, since this extraneouslight will only produce noise.

The studio, therefore, must be sealed againstambient light. Eliminating all ambient light,however, would leave the studio in relative dark-ness since the light from the scanner is insuffi-cient to adequately illuminate the studio.

This acute problem was literally a stumblingblock in earlier versions of live scanner opera-tion, since lack of light seriously restrictedthe movement of performers about the studio , madeit impossible to read, and produced a staringexpression on the faces of the actors.

The difficulty of providing adequate lightin the studio without interfering with the Vita-scan operation has been resolved by utilizing the60 cycle vertical blanking period, during whichtime the scanning beam is extinguished.

During this blanking period, pulsed or strob-oscopic lights synchronized with the TV system areflashed on in the studio, providing adequate illu-mination for studio action without upsetting thescanning operation. During this same period thatthe lights are pulsed on, the phototubes or asso-ciated video amplifiers, or both, are keyed offto prevent overloading. In addition to beinglight-tight, noise is further reduced by employingreflective materials on the unscanned walls andceiling, thus increasing the light directed to thephototubes and additionally softening the shadows.

Equipment Complement

In _ts present form, the "Vitascan" is oper-ated as a one or two "camera' system. The"cameras" which, in actuality, are flying-spotscanners, may be fixed or mobile. One favorablearrangement, which was used at the initial publicdemonstration at the 1955 NARTB Convention, in-cludes the DuMont "Multiscanner' equipment toprovide a fixed scanner augmented by a mobilescanner packaged to resemble a standard image-orthicon camera.

Figure 1 shows the studio of 200 square feetat the NARTB Convention. The equipment complementincluded the following items:

a. Two scanning light sources, one mobileand one fixed unit - utilizing the DuMont Multi-scanner.

b. Seven clusters of DuMont 5" multiplierphototubes.

c. Six pulsed light fixtures.

d. Processing amplifiers.

The DuMont Multiscanner, as seen in Figure 2,is a flying-spot scanner system for slide, opaqueand motion picture film pickup. It utilizes a 7"high voltage cathode-ray tube operating at 40 kv.The tube, which is aluminized, employs a shortpersistence zinc oxide phosphor coated onto ahigh-quality, non-browning, neutral density faceplate. Use of this gray face plate, rather thana clear face plate, improves the small area con-

12

Fig. 1 - Vitascan studio.

trast by a factor of ten by the reduction of hala-tion flare.

Figure 3 shows how the Multiscanner iseasily adapted to live pickup work by the removalof the cabinet for 4" x 5" opaques and the addi-tion of the desired focal length lens to theflying-spot cabinet.

The mobile light source, seen in Figure 4,resembles a conventional TV camera and includesa lens turret to provide a choice of focal leng-ths for close-up, medium, or long-shot work. Thismobile scanner employs a 5" high-voltage cathode-ray tube operating at 35 kv in which specialprecautions have been taken to minimize the phos-phor grain size by careful control of the screen-ing process.

The phototube clusters consist of four DuMont5" diameter 10 stage multiplier phototubes, Type6364, shown in Figure 5. They are closely spacedin a compact housing as depicted in Figure 6.Each phototube is covered with an appropriatecolor filter in order to analyce the reflectedlight from the scene into three primary colorchan.Dels. One cell is allocated to the greenchannel and one to the blue channel. Two cellsare supplied for the red channel in order tocompensate for the lack of sensitivity at the redend of the spectrum. The phototubes identifiedwith each color channel are bridged onto a coaxline which is terminated at each end, thus allow-ing the addition of as many clusters as desired.

The pulsed or stroboscopic, lights are oftwo types. In one version, small commerciallyavailable Xenon flash lamps are used, five to areflective fixture. These lamps are triggeredto flash during the vertical scan blanking inter-val. In the second developmental version, minia-ture CRTS are employed, three to a reflectivefixture. These tubes are operated at 15 kv andare pulsed on during the vertical blanking inter-val.

Fig. 2 - DuMont Multiscanner.

Operation on the R, B, G signals developedfrom the combined outputs of the phototube clus-ters is accomplished in conventional video pro-cessing amplifiers, providing phosphor correction,gamma correction, and blanking insertion, asnormally used in flying-spot scanners.

We hope, in the previous paragraphs, to havewhetted your appetites with a brief general lookat the Vitascan System. We now plunge into anexpanded explanation of some of the more novelfeatures of the system.

Vitascan Photometry

In returning now to the basic principles ofthe Vitascan, it is helpful to give our defini-tion of a live-action flying-spot scanner. Alive scanner may be defined as one which tele-vises an object having depth, and which, conse-quently, must provide the appearance of "effectslighting". To show simply how light modelingeffects are produced, we refer to Figure 7 depic-ting a schematic top view of a simple monochromeVitascan. The scene being televised is a squarewhite card suspended some distance from a graybackground. When a single phototube pickup isplaced at the right of the scanner, it will beshown that the TV picture produced looks like onefrom a storage-type TV camera viewing, from theCRT position, the scene lit by a single lamp atthe phototube position. It is obvious from theslide that a shadow from the white card would bethrown to the left on the gray background.

Now, to see why the Vitascan signal alsoexhibits the same shadow, we consider what hap-pens at seven equal increments of time, Tlthrough T7. At the instant of Time Tl, supposethat the scan is just beginning a new line in themiddle of the picture. The tiny spot on the CRTis imaged by the objective lens onto the left-hand side of the gray background. The lightreflects in all directions from that particularspot on the gray background, as indicated in the

13

Fig. 3 - Multiscanner used as fixed Vitascan.

Fig. 4 - Mobile Vitascan "camera."

14

Fig. 5 - A 5" multiplier phototube,type 636h.

Fig. 6 - Phototube cluster.

figures. A small portion of the light is pickedup by the phototube and a signal corresponding togray is generated and thence displayed on thescreen of the TV monitor. As the spot moves fromTi to T2, the phototube continues to generate thegray signal level. But, between times T2 and T3,the light reflected from the gray background can-not reach the phototube, since the white squareblocks the path, and hence no signal is producedby the phototube. This results in a black shadowon the TV monitor. At T3 and continuing throughT5, the flying spot hits the white square and thephototube transmits a white signal and finally,from T5 to T7, a gray signal is again displayed.If now the whole scanning period is viewed, theresulting picture resembles that on the monitor.

Suppose now that the same phototube ismoved to a corresponding position to the left ofthe scanner. The shadow would then move to theright. In ract, if only a single phototube isemployed, the resultant picture will appear to beilluminated by a light at the phototube position.For this reason, we normally think of the photo-tubes as representing "lights".

By employing a number of phototubes3 withsuitable reflectors, any desired lighting effect

can be produced such as "key", "fill", and "back"lighting. The apparent intensity of these"lights" are controlled simply by the photomulti-plier gain. Thus, the video level and the light-ing effects are controlled by only one individual.This compares to the case with storage systemswhere the video operators and the lighting staffare distinct groups and thus considerable co-ordination must be exercised.

With the color Vitascan, the "lightingfixture" normally consists of a cluster of fourphototubes, one Blue, one Green, and Two Red.From the sketch, it is apparent that the physicalspacing of the 5" phototubes is apt to causecolored shadows if only one cluster is employed.Again, the picture is analogous to having red,blue, and green lamps illuminating the scenerather than a single white lamp. However, withthe number of phototube clusters normally employedto obtain the desired artistic effect, coloredshadows are not apparent. Added to the 'filllighting" produced by tne clusters themselves isthe considerable diffusion of light provided bythe various inter-reflections in the scene. Inaddition, reflectors around the clusters are agreat help in mixing tne red, blue, green light toavoid colored shadows. Another useful "fill-

V

4

..

Fig. 7 - Principle of Vitascan "lighting."t

light" technique, especially in small studios, isto cover the unscanned walls and ceiling withreflective material.

At this point, the operation of the fixed Vit-ascan has been briefly covered from an optical-photometric point of view. Depth-of-field consi-derations are straightforward and are treatedlater.

But now, what additional theoretical problemsare raised when the fixed Vitascan is replaced bya mobile Vitascan? By mobile Vitascan, we mean ascanner which can be moved around the studio andwhich employs a turret with a variety of objectivelens focal lengths. Actually, no special problemsarise although the situation may appear filledwith difficulties at first glance. The firstsituation likely to raise a mental block is wherethe scanner is dollied very close the subject. Itmight be supposed that the signal level would begreatly increased. This supposition is incorrect.For simplicity, refer again to the figure, butmentally remove the white square so that only thegray background is scanred. Instead of consider-ing the scanning, as before, on an instantaneousbasis, observe tLat a certain total amount oflight flux passes through the objective lens inthe same unit time and is spread over the scene.Of the total amount of light lux leaving the lens,a certain proportion is picked up by the phototube.Suppose now that a 6' x 8' area is scanned on

the gray background, producing the correspondinggray signal amplitude. If now the scanner dolliesin to scan a 3' x 4' area, the sigrnal level willremain exactly the same. This is because thephototube reins at the same distance and there-fore still collects the same proportion of thesame total light flux leaving the scanner. Look-ing at the situation in another way, the 3' x 4'area will be four times as bright as the 6' x 8'area, but since it has only on-fourth the areathe total light picked up will be constant.

Thus, with the mobile Vitascan, the sameflexibility of movement is allowed as with any TVcamera. The same argument holds when the lensturret is changed from narrow to wide lens. Inany given scene, as long as the effective f'numbers of the lenses are the same, the signallevel will not vary with focal length.

Another question that might be asked is"what happens when an actor gets very close toa phototube"? The answer is that he will appearvery bright, just as if he had moved close to alight in a studio employing storage-type TVcameras.

One of the important reasons for our develop-ment of the color Vitascan was to produce a live-action signal source totally devoid of colorregistration errors for Research Division testingof color TV equipment. Today, it is still the

16

uest signal source we have for this purpose.This absence of color misregistry is readily ap-preciated by reference again to the sKetch. Sup-pose we consider the white-to-gray transition atT5. Just prior to T5, the R., B., G signals areequal and produce white. When the spot moves tothe gray background, all the phototube outputsmust change together. There is no way for onecolor channel to change at a time different fromanother. Thus, color misregistry, such afamiliar problem with storage-type color TVcameras, is completely avoided.

Spectral Characteristics

When the Vitascan is used for monochrome TVpickup, the over-all spectral characteristic isdetermined by the product of the phosphor timesthe phototabe spectral responses. This resultantcurve is shown in Figure 8. Note that the peaksensitivity occurs around 500 mu and fallsgradually to zero at 400 and 700 mu. In order toproperly shape the Red, Blue, and Green channelsensitivities for color TV, appropriate colorfilters are placed before the phototubes.

Unavoidably, as in any color camera, only aminor part of the sensitivity remains after thefilter shaping. The resultant R, B, G sensiti-vities, drawn to the same scale, are shown in thefigure. The most serious loss is in the Redcuanel because of the poor Red response in boththe phosphor and the phototube. The R, B, Gsensitivities relative to monochrome, as deter-mined by the integral area under the curve or bydirect measurement, give Red 1%, Blue 17%, andGreen 50%.

At the outset of our work, the Red outputwas doubled to 2% by employing two Red phototubes

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for each Blue and Green. To obtain equal R, B,G signals for white balance, as is customary incolor TV, it is necessary to suitably adjust thephotomultiplier gains. The spectral character-istics for equal-signal white, with the corres-ponding gains indicated, are shown in Figure 9.

The extremely poor Red sensitivity, requir-ing increased Red photomultiplier gain, results inthe Red channel noise determining the over-allacceptability of the signal-to-noise ratio in thesimultaneous color signal.

Figure 10 shows the spectral transmissioncurves of the shaping filters used in the proto-type color Vitascan. To produce the desiredsloping response with the peak at 600 mu, it wasnecessary to fabricate a composite filter with asmall area of orange. This was because the&gamutof normally available red filters is characterizedby a sharp cutoff on the short wavelength side.Note also that the desired Green sensitivity shapeobtained with a yellow shaping filter gives aboutdouble the output compared to a green shapingfilter.

Improved Red and Blue shaping filters arenow being manufactured. The Red is approximatelythe same shape, but one piece rather than com-posite and about 20% greater in sensitivity. TheBlue has higher transmission but gives the sameoutput since the over-all sensitivity shape ismoved bodily about 10 mu dowh in wavelength, adesirable improvement.

As a practical matter, the Vitascan spectralsensitivities are a compromise between theoreti-cally exact color fidelity and signal-to-noiseratio. The fact that the Vitascan does exhibitsuch remarkably good color fidelity is due more to

700

Fig. 8 - Color Vitascan filter shaping.

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WAVELENGTH ( MILLIMICRONS)

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WAVELENGTH

Fig. 9 - Color Vitascan

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spectral characteristic.

400 500 600 700WAVELENGTH (MILUMICRONS)-*

Fig. 10 - Color Vitascan shaping filters.

the linearity of the transfer characteristic,rather than to our choice of spectral sensitivit-ies which are not theoretically perfect.

With the storage-type color TV cameras, nomatter how close to the ideal the spectral charac-teristics may be, color contamination is presentdue to the non-linear transfer characteristic ofeach camera tube.

From the standpoint of color fidelity, weprefer the Vitascan to a slide scanner as a color

signal source because we avoid contending with thecolorimetric failures of the film processes. In

addition, a great variety of subject matter isimmediately available for test and demonstration.

Optical Parameters

From the beginning of our work, signal-to-noise and depth-of-field considerations haveproven to be serious problems. Although theoreti-cally quite independent, the economics of equip-ment design makes them mutually antagonistic.

18

GAINS ADJUSTED AS SHOWNTO GIVE EQUAL OUTPUTS FOR WHITE

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Our first simple experiments utilized astandard DuMont Multiscann*r which employs a 7"CRT and 7" f/2.8 lens, and a small light-tightroom where a 7-2 x 10 foot raster was scanned onthe back wall. At first, two clusters of fourDuMont Type 6364 five-inch multiplier phototubeswere used. With this arrangement, the depth-of-field was only about two feet. In order toincrease the depth-of-field, the lens was stoppeddown, but this made the signal-to-noise ratioobjectionable.

To improve the depth-of-field, it isnecessary to use a smaller lens aperture, orsmaller scan size, or both. In either case,the amount of light incident on the scene isdecreased. Unless the same total amount of lightis collected by the phototubes, the signal-to-noise will suffer. In our present Vitascan atthe Research Division, which is in daily use asa high-quality signal source for color TV develop-ment, higher collection efficiency was accom-plished in three ways. To recapitulate, we firstadded simple reflectors to each phototube cluster.This improved the efficiency for exactly thesame reason as with any lighting fixture.Secondly, the walls were covered with aluminumsheeting. This increased the light level andsoftened the shadows. Thirdly, the total ofphototube clusters was increased. An importantreason for this was to obtain more interestingmodeling effects in the lighting as well asgreater light collection.

Although these approaches to greater lightcollection efficiency were entirely feasible inour small studio and allowed increased depth-of-field, it was apparent that a considerablygreater economic problem would exist in largerstudios employing mobile Vitascan cameras. Herethe cost of the phototube clusters can be .a sub-stantial matter. And so before proceeding withthe development of the mobile Vitascan camera, itwas necessary to consider the depth-of-fieldproblem purely from the standpoint of the cameradesign itself. This inquiry divided itself intotwo parts, first, a general analysis of depth-of-field as a function of CRT raster size and othervariables compared to competing color TV cameras.Secondly, the final optimizing of the cameradesign for signal-to-noise was determined bypractical problems of CRT operation.

It is well known that cameras with small filmhave greater depth-of-field than those with largefilm. For example, a 4" x 5." camera at f/8 hasthe same depth-of-field as an 8" x lo" camera atf/16.

The range of CRT sizes employed in flying-spot scanners today is from 4 to 7 inches nominaldiameter. The useful screen area available dic-tates scan sizes of 2.1" x 2.8" to 3.9" x 5.2".Present image orthicon color cameras have a scansize of about 1.05" x 1.40". It is apparent thenthat with any of the flying-spot raster sizesabove, the theoretical depth-of-field will beless than the image orthicon if the same lens f/numbers are used.

To be sure, the present Vitascan units oper-ate at a distinct theoretical disadvantage toimage orthicon cameras in regard to depth-of-field, but it is important to emphasize that, inreality, this difference is less than that indi-cated by the figures since the theoretical depth-of-field advantage is far from being attained inimage orthicon color cameras because of certaineffects which reduce the sharpness. Anyone whohas compared the softness of the average livecolor TV signal with that from monochrome imageorthicon origination is aware of this fact. Thislack of crispness is caused, first, by opticalaberrations introduced by the complicated lensrelay system and second, to misregistry of theRed, Blue, and Green camera scans.

The inherent Vitascan sharpness is defi-nitely superior to the image orthicon; whetherexpressed in aperture response or in limitingresolution. Stated another way, the Vitascanwill produce a crisper image of a test pattern.On the other hand, with the usual lens aperturesemployed, the depth-of-field will be less.

Figure 11 compares the theoretical depth-of-field at various apertures for the image orthiconcoldr camera and illustrative scan sizes forfixed and mobile Vitascan cameras. It must beclearly kept in mind that the apparent depth-of-field of Vitascan signal, because of its in-herently crisper response, will more nearly ap-proach the theoretical figures. The raster sizesand lenses were selected so that all the systemshave the same field angle of 30 x 40 degrees. Innormal color TV studio operation, the imageorthicon color camera is operated at f/5.6. Thetheoretical depth-of-field produced at variousdistances is shown along the bottom row. Inorder to match these figures, the Vitascan camerascan size and f/number must obviously equal theige orthicon camera. Unfortunately, underthese conditions, it is simply not possible toachieve the necessary CRT brightness because ofexisting phosphor light output limitations. Inaddition, phosphor grain size with the 1.05" x1.40"t scan is excessive. On the other handthese limitations are not serious with the 3.9"x 5.2" scan size of the fixed Vitascan but aglance at the table reveals that the depth-of-field seriously limits the TV programming appli-cations. A compromise clearly had to be madein the mobile Vitascan design. A 5" CRT allowsa 3.0" x 4.0" scan size or smaller. Our exper-iments indicate the smallest practical scan sizeto be 2.1" x 2.8", determined primarily by thecurrent saturation, grain size and life of thephosphor. Typical operating conditions are 35kv accelerating voltage at 250 ua beam current.Countering these disadvantages is the consider-ably improved depth-of-field revealed in thetable. Coupled with the inherent sharpness ofthe Vitascan, we feel that the depth-of-field ofthe 2.1" x 2.8" raster at f/j2.8 is adequate formany types of TV programming. Notice in thetable that at 10 foot focus, nearly a four footdepth-of-field is realized. This is a markedimprovement over the fixed Vitascan at f/2.8 andten foot focus where only a two foot depth-of-

119

FOCUS DISTANCE - FIELD AREA

SYSTEM RASTER APERTURE 5 0 15 20QLENGTH 30 x 4 0 52" x 611 7' 0 x 1O'6" 10'6" x 1 4'

ff'2.8 + 3" + 'I" +2'6" + 5'FIXED toi

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VITASCAN 5.2" ff 4.0 +4 + 1 8"1 + 4 4+8

-4" -1 -2.6 -46"

ff2.8 +5 + 22" +5'6" +11'

|M081 LE 2 OOmr -5" -1 6 -3' -5VITASCAN 2.8" f / 5.6 +12 +56 + 1 8' + 50'

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IMAGE 1.05" 9" -2 8" 5' -8OTH ICONX 5 OmmORTHICON I1.40" ff5.6 + 33" + 25' + co +

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Fig. 11- Depth-of-field tables (based on 600 line tv cutoffC).

field is obtained. Figure 12 was photographedfrom a 15" color monitor and shows a pictureproduced from the fixed Vitascan focused at 10feet. While the depth-of-field is perfectlyadequate for this type of subject, a 4 footdepth obviously gives more programming flexibi-lity. Incidentally, the slide from which thisfigure was reproduced is in color and the absenceof color shadows is readily apparent in the ori-ginal.

The complement of lenses available for thecommercial mobile Vitascan unit includes 100 mmf/2.8, 150 mm f/2.8, 210 mm f/2.8 and 300 mm f/3.5.This corresponds exactly with a 50 to 150 mmcomplement for the image orthicon camera.

Studio Strobe Lighting

As previously pointed out, we have developedtwo different methods of lighting during thevertical blanking interval. The first methodutilizes miniature Xenon flash lamps and thesecond method small, short-persistence cathode-ray tubes.

At the present time, the production versionof the Vitascan uses Xenon flash lamps, pendingthe quantity manufacture of the light sourcecathode-ray tubes. While there are points forand against both methods, we believe that thescheme utilizing cathod-ray tubes is ultimatelyto be preferred.

The Sync-Lite equipment, as it is called,operates on the same principle as the Xenon photo-graphic flashlamp. A trigger voltage of severalthousand volts is applied to the external elec-trode of a U-shaped Xenon Amglo Type U-35K shownin Figure 13. This voltage ionizes the gas anda DC potential of 350 volts from a charged con-

denser applied across the internal electrodesmaintains the ionization, producing a blue-whitelight output for approximately 100 microsecondsduration. Since the trigger voltage is syn-chronized with the vertical drive from a standardsync generator, the flash occurs during thevertical blanking interval.

To provide a suitable light level, fiveXenon lamps are used in each fixture, as shownin Figure 14. The fixtures are constructed ina housing similar to the Vitascan pickup clusters,the lamps being inserted in a chrome reflector,on the front of which is mounted a glass cover toconfine the noise caused by the high current surgethrough the lamps. Additional units required forthe operation of the Sync-Lite are the TriggerUnit and the Power Panel shown in Figures 15 and16, respectively.

In contrast to the Xenon flash lamp, thesecond method of providing strobe lighting uti-lizes miniature short persistence cathode-raytubes as light sources, DuMont Type K1388 asshown in Figure 17. Each tube consists of a lirdiameter glass envelope with a special triodegun at one end and a metal phosphor plate, in-clined at 450, welded to the other end. Thismethod of construction gives a three times im-provement in light output over conventional tubeconstruction by enabling the phosphor to beefficiently air-cooled. The special gun is de-signed to produce an unfocused flood of electronsover the phosphor surface, typical operatingfigures being 15 kv accelerating voltage at 200uamps average current.

The short persistence DuMont P15 phosphoris used in these tubes. This is a zinc oxide,zinc activated phosphor with a decay of 1 usecond to the 1 point, having high efficiency and

20

Fig. 12 - Vitascan reproduction (panchromatic copy from ori-ginal color positive transparency).

ez, Fig. 13 - Xenon flash lamp.

TRIGGERTRI¢ REBANDLEAD

SIDE VIEW

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Fig. 14 - Xenon sync light fixture,

21

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Fig. 15 - Sync light trigger unit.

Fig. 16 - Sync light power panel.

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Fig. 17 - CRT strobe light, type K1388.

22

i ~ i~' -, 'B

considerable resistance to burning under electronbombardment.

The tubes are operated by keying their gridson, with positive pulses of approximately 700 useconds duration, derived from the vertical driveof the sync generator. The vertical drive pulsesare passed through a variable delay and widthcontrol unit before amplification, to allow thelight to be centered accurately within the verti-cal blanking period.

An experimental light fixture utilizingthree of these tubes is shown in Figure 18. Itconsists of an aluminum reflector housing intowhich the cathode-ray tubes are inserted withtheir metal phosphor plates uppermost. This

Fig. 18 - Strobe light fixture.

enables efficient cooling to be obtained by asmall fan mounted above the tubes at the top ofthe reflector. In addition to the light fixtures,the associated units required for the operationof the CRT pulsed light system are the Delayand Pulse Width Control unit and the High Voltagesupply.

With either of the above methods, a highcurrent pulse is generated at the output of thephototubes during the strobe lighting period. Itis, of course, essential to prevent this pulsefrom appearing in the video signal leaving theprocessing amplifier. There are two convenientways to accomplish this: First, by keying thephototubes off so as to make them inoperativeduring the vertical blanking interval. This hasthe advantage of protecting each phototube fromdamage which might arise due to the high peakcurrent overheating the anode. The second methodclips and keys out the unwanted pulse in the

video processing amplifier. This has the ad-vantage of requiring less circuitry than operat-ing on the phototubes and is the method used inthe commercial units.

The main advantage of generating the pulsedlighting by means of Xenon flash lamps is that,apart from a low current trigger voltage of afew thousand volts, there is no really highvoltage associated with the system. On the otherhand, there are three main disadvantages. One isthe tendency for erratic ionization of the Xenongas giving rise to a slight flickering of thelight, although this is minimized by using fivelamps in each fixture. Secondly, the noisegenerated by the high current surges in the Xenonlamps. Thirdly, tbelife expectancy of the Xenonlamp is 500 hours.

On the other hand, the light-source cathode-ray tubes generate a perfectly stable lightoutput, are noiseless, and have much longer life.There are two disadvantages to this method.First, the P15 color is slightly greenish but iscorrectable to white by available color filters.Secondly, a moderately high voltage of 15,000volts is required of approximately 3 milliampscapacity.

Preview Facilities

We would now like to discuss briefly thevexing problem of providing preview facilitieswhere more than one scanner televises the samescene.

Suppose that two Vitascan cameras are scan-ning the same scene from dirferent positions.Since both light sources would be operating atthe same time, a mixed signal is generated bythe phototubes. On the picture monitor, thiswould appear as two superimposed images as seenfrom the respective camera positions. Theproblem, therefore, is to derive two separatesignals, without crosstalk, from the two scanninglight sources. Obviously, some multiplexingsystem must be employed.

There are several possible ways of providinga preview picture for the Vitascan, the morepromising of which have been investigated indetail. The suggested methods were Judged forsuitability by the following criteria:

1. Economy governed by the amount of addi-tional nonstandard equipment required.

2. Quality of picture, that is, signal-to-noise ratio, resolution, color or black and whiterendition.

3. Complexity of operation.

Previewing systems group themselves in twomain categories, time and spectral multiplexing.

As an example of time multiplexing, onecould utilize the retrace portion of the hori-zontal scanning cycle for the preview.

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Spectral multiplexing systems can be oper-ated within or outside of the visible spectrum.The preview may utilize the existing phototubesor an auxiliary-storage camera associated withthe scanning source.

While it may be argued that for a consider-able number of iestallations, such as those en-countered in small station operation, one scan-ning source will be adequate and no previewproblem exists, we cannot ignore the requirementfor preview facilities for multiple camera opera-tion nor can we fail to accept this challenge toour ingenuity. During the initial phases of thedevelopment of the Vitascan system, there werea number of fundamental considerations concernedwith the production of adequate color pictureswhich occupied our time. Now that these problemshave been satisfactorily resolved, the finalintriguing technical problem, that of the preview,is receiving our close attention.

At the risk of appearing unduly secretive,we feel that it is too early at this junctureto say what final form the preview system willtake. A number of methods have been discarded;among them the horizontal retrace method becauseof insufficient resolution. Others hold consid-erable promise of ultimate success and we expectto be able to report in the near future on thesystem finally chosen. The main purpose ofdiscussing the preview problem in this paper isto bring to your attention the fact that it is aproblem and that considerable effort is beingmade to solve it.

Conclusion

There appear to be three major fields where-in the Vitascan should find application. Theseare:

1. Industry

2. Color TV-Research

3. Color TV Broadcasting

Our experience with the color Vitascan leadsus to be enthusiastic concerning its place as aResearch tool. It has definite advantages overa transparency scanner. It provides excellentcolor TV signals especially in respect to resolu-tion, color rendition and lack of color mis-registry. From an operational standpoint, itis the simplest and most reliable live-actioncolor TV pickup system in existence. Its initialcost is relatively low because of the inexpensivecomponents employed. Operating costs are alsolow because of the long life and smaller numberof components.

in the field of color television broad-casting, the Vitascan has both advantages anddisadvantages. The major disadvantages arerather obvious. It cannot be operated outdoorsor where extraneous illumination exists. Multi-ple camera use will be limited until suitablepreviewingsfacilities are developed.

However, certain of its advantages are notobvious until comparison is made with imageorthicon camera operation. For example, typicalcolor studio operation requires illuminationlevels of at least 100 watts per square foot. Inaddition, the heat generated presents a formidableair-conditioning problem. The Vitascan, there-fore, represents a very significant reduction inoperating power costs.

Perfect colorimetric camera match obtainsat all times since the pickup tubes are commonto all scanners.

Illustration of Vitascan picture sharpness(panchromatic copy from original color positivetransparency).

Because of its excellent color reproduction,no special makeup is required with the Vitascansystem. There is no image burn-in so that ascanner may be left undisturbed on a scene at

fixed focus for any length of time. Finally, thewarm-up and adjustment time required is only afew minutes compared to the hour or longer neededfor the image orthicon color camera.

Presented at the Seventh Region Technical Confer-ence, Salt Lake City, Utah, April 11-13, 1956.

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