Special cameras and flash lamps for high-speed underwater photography

HYDRODYNAMICS LABORATORY

PASADENA

^ROHItr

PUBLICATION NO. ® ^

Reprinted from Uie Joutnal of tbe Soctety of Motion Plctue .Baginecra, „ - . • V o l . ' * Ï . N o . l ( J i i I y , l M 7 î , P p . f t 4 ^ . • ' •.. • •

FECIAL CAMERAS AND FLASH LAMPS FOR HIOH-SPEEP

ROBERT T. KifAPP*^

.Summary.—The eguipmeiü described was developed'for atiaXyziiig underwater 'molton ôf spUd hodin. The experiment demanded a High rate of ptclure taking nnd that the subject studied should be in Ihe.jidd of xit least two cameras at all times. Èdgerton4ypè flash,lamps instead cf shtüto' mechanisms were adaf4td', an endless jOm- belt giving a one-second exposure was developed, and. a film speed of

approxi-mately 35 ft per sec used.

. The ProW.em.-r—For the past few yeaxs mudh. of the work of the Hydrodynamics Laboratory of the. Cahforpia Institute of Tech-nology has beeii:m connection wit^ projects ^ Division 6-o£ the N^-tionat Defense Research Council and for liie Bureati of Ordnwce of the Ui. S. Navy^; One of these projects required for.its study the devdcxpment of metiiods for making detailed *measurèmeçLts of the path and the orientation of a rapidly moving underwater body. .Consideration of the project showed that the wjork could be cajçrïed out ina h<^izontal, (yUiujn(^:tank abgutj^ ftJong, a d^th oî

yrsAxx of apprps^m^it^y 10 ft with a few feet of air space above the water surfajce. The deared range of experimental conditions im-posed the necessity of vaxying'the pressure from a positive value-c^ .two or three atmo^heres äown to a vacuum of a small fraction.of an .atmosphere., This ineans that the tank had to be a closed pressure

vessel and that very large windows would be both *atpensive and hazardous. In spite of the difficulties mvolved, detailed study of the problem indicated that the photograi^ m ^ o d of measuring the performance of the l^dy the most promising".

.Carelul Emal}i^s the probable éxpmmental nee<^of the progtam indicated that to secure the desired information,, measuriiig points would have to be obtamed at a mayimnm rate of from 1000 to 3(XX)

•^Presented Oct. 22^ 1946. at the ^ P E Convention in Hollywood.

" Hydrodynamics Laboratory,'California Instituteof Technology, Pasadena, Calif ' . , '

-64

July 1947 • I ^ H - S P E B D U N S ^ W A T B R PHOTOGSA]?HY 65 per sec, depending upon the .speed of the imdmrater body that was

be-ing c^jserved. However, ânce ihe size <^ the tank limits the length of the path without regard to speed, the maximum number of pcnhts required for any one set of measurements was raïntlated to be about 3000. If these measurements w ^ to be obtained by the use ci motion pictures, these requiremoits meant that camera equipment would have to be developed that could take up to 3000 frames at a speed ci from 1000 to 3000 per sec. Fiuthennore, an analysis of tbe required accuracy of measurements shoved that very littie dis-tortion could be tolerate. This indicated that the rotating prism-l^pe of high-q)eed camera would not be suitable. It was finally dedded that the best pcœsibîHties appeared'to lie in the direction of a camera using a constantiy moving film strip with no shutter mecha-nism, operating in conjtuurtiaa with a battoy of hifj^-speed flash lamps wHch could act both as a source df îQunûnatîon and as a shutter. The iihderwater bodies to be shidied were of a type wfait^ would eater the water from the äir at one end of tfae tank with a relatively high vdodty. From the point of entrance on, thejr would be free bodies and thus could move to any point in the tank, depending on the resultant fcnx:es that açte4 upon them. This meant that the photographic measuring tedmique had to be able to determine the position of the body in space. In other words, it would be neces-sary to obtain the horizontal and votical cp-on&nates of tiie body on a plane normal to the axis of the reccnrdixq; camera, but it al£K> would be iiecessary to obtain the horizontal distance of the body from the camera. The best way to do this appeared to be by tfae ose of the stereoscopic eSect, wläch meant ti^t for each point recorded, the body would have to be photographed by at least two cameras. To ^^ure this conditi<m, a design spediGcation was established that in the nominal plane of foqis, the sd: of cameras shcmld be so spaced that thdr fields of view would have a 60 per Cent overiap. As a result of tfais^ together with tiie other e}q)aimciitäl requiremmts, the recording equipment .developed into a bank of five cameras,

spaeed at 4Vrft centers and operated from a common drive shaft. Tiàs ^ve& a coverage ova: the entire experimental region cS tifê tank

The caititd 20 ft of the 30-ft tank has a two-camera coverage over the effective width and d^>th, with single-camera coverage on the two 5-ft end sections. Fig. 1 (a) and 0} shows the diagrammatic arrangement of the cameras in the tank with thdr overiapping fidds of view.

66 K N A P P Vol 49, No. 1

aiB THj..i!eTOHT copHFr-rmr. wirtoow

wtm WflTER LEVEL

M U L T r m a M l a m p

RECORDER

(a)

m TWAJECTCHT MMERA IS 5WWWI1

t-AUWawiC W.AHE,, MULWFCASH L«MM ft P t B TUBT) IMNSMtCNT TUBES

M W M MOTOR

M H W E T I N S W H n O »

Pio. 1. Diagnum showing arrangement of cameras and their overlapping fields

July 1947^ H I G H - S P E B D U N D B R W A T B R P H O T O G ^ i F H Y 67

Detailed Requirements of Cameras and Light Sources.^In the design and development of a catoera system of t ï ^ type just out-lined, it is necessary always to bearin mind the extreme importance of the relationship between the camo^ and the Ughts because the lights function as an essential part of the camera, since they-replace the normal shutter mechanism. In fact, the characteristics of the

Ûash lamps control mudh <^ the dedgn of the camera ItseK.

There-fcoe, these rdationships will be discussed first in analyzing the de-tailed requirenients of the ovar-all design.

The most important charactoistic of the flash lamp itself is prob-ably the eff«tive duration of the flash. ' TTiC' mtnimum available flash duration controls the maximum usable film ^peed. it must be remembered that in this type of camera the film moves constantiy. Tiierefore, the flash diuation of the lamp müst be short enougfa to "stop" the motion ofthe film; otherwise, the recorded image wiU be blurred. One reasonable criterion of the nmnmum usable film speed is that the allowable amoimt <4 motion during one flash should not be greater than the drde oî confusion of the lois of the camera. For most applications, this4s a mnxùi more severe requirement than tiie one derived from a consideration of the possible image blttr catised by the movouent of the object being pfaotogmphed.

This difference can be seen eadly by oonadering a set of typical conditions for the installation imder d^cusdon. The maximum g)eed antidpated for the body to be photographed is in the ndgh-biwhood of 200 to 300 ft per sec Its average distance from the camera will be about 100 to 150 focal lengths. Thus the average speed of the inmge in the focal plane <^ the camera 'VKLLI be only about

2V2 ft per sec. Tiierefore, a filtn speed of 2^/2 ft per sec wotdd cause

the same amount of blur for a given flash duration as would the move-ment of the object itself. It will be remembered, however, that espoimental reqtdrements call for a camera whicfa can take from 1000 to 3000 frames per sec. Obviously, it wotild be imposdble to take 3000 pictures per sec on a film faaving a speed of 2Vi ft per sec dnce tfae fiïime hei^t would be less than Va» <^ an inch. Tfais means tfaat a fifan speed of at least 10 times this vahie must be obtained. Obviously, any Kg^t wMcfa faas a sfaort enoüj^ flasfa duration to dimi-n?.te blur caused by fflm motion at tfais faigfaer film speed will have no (HfiSculty in stepping the motion of objects going at much faigher speeds than those that will be encountered in this apphcation.

68 K N A K » Vd 49. No. r

be obtained from spedally designed flash lamps mdicated tfaat 1 to 2 microseconds was tfae shortest flash that could be antidpated. Tfais dtuation permits a film speÊd of from 25 to 40 ft per sec to be used safdy in tfae camera. Chi tfais bads tiie value of 35 ft per sec was sdected as tfae di^gn objective.

-It will be seen tfaat there is no possibility of gettmg 3000 full-sized

frames xm a 35-ft strq> of .35-mm film. In fact, the permissible frame

fadgfat is only about Vs ii^- instead of in- On the otfaer hand, tfae dimensions <^ the tank and the possible camera installation in-dicated that it would be very desirable to have as large a frame fad^t as possible m order to cover the working depth of the water. The obvious way to solve these conflicting requirements appeared to be td use one ci the standard tricks long employed with flash-lamp il^ lumination, i. e., to provide a blade background and take multiple exposures. This is permisdble in the present applicatton dnce the speed of the body under study will never be high enougfa to cause images from successive flashes to fall on top of one anotfaer if tfae film moves as much as Vs ill'between exposures.

The second hnportant characteristic of the flash lamp for such use is the intaidly of the illiimination. It is appareat that this intensity must be extretnely high to produce an image-of reasonable density in tiie very short flash time available. The magnitude of the problem, can be seen more graphically if tfae operation of this ^pe of equips ment is compared to that of a hypothetical motion picture camera of the standard type using a normal shutter, but operating at a speed of 3000 pictures pdr sec. S t ^ a camera would probäHy have a shtitter openiug of about 180 deg, which corr^ponds to an effective exposine time of ^/sooo of a second. Such a sfaort exposure would cer-tainly require very-high-ihtendty illumination if a reasonably good nq^ative were to be secured. However, V«M of a second is à Httie over 160 nucroseconds. This meaps that, for a flash duration of one microsecond, the intensity must be at least 160 times as gr^t as would have be»i necessary witfa a l^pothetical camera usu^ a normal shutter. • ,

The phrase "at least as great" was used advise^y, since there is some evidence to indica.te that the reriprocity law breaks down for such extremdy short ecposuies with the result that even higher-iptendty illumination may be reqiured to obtain a satisfactory image on the film This particular installation imposés some extria demands en the îQnmînatùm because the pictures must be taken under water

J d y 1947 H l G H ^ S ^ B D U N D E R W A T E R PHOTOGRAPHY 69

and the long underwater Ught path m^ns large Hght absorption. T a ^ n togeth^, these conditions aU point to tfae fact tfaat there is no possibflity (rf securing suffident illumination from a single lamp

capable of flashing, as rapidly as is necessîuy to meet our

require-ments. TherefMe, a synchronized battery oi lamps is reqoired, wfaich, altfaoc^ it offers tiu po^biUty of solving tfae xnoblem of siâà-dent intendty illumination, adds änotha- difficulty of its own. If tfae flasfa duration is to be only 1 to 2 microseconds, tfaen, if tfae lamp battery is to be effective, tfae lamps must be ^mduronized to flasfa simultaneoudy within a limit of about Vio of a microsecond.

It may be of interest to consider tfae physical dgnificance of tfae duration of tbe iUuminatk)n of t h ^ flash lamps. Tlie. speed of ligfat is approximatdy 186^000 nules per sec. Thus, in one microsecond, ligfat can travd something less than ^/u of a mile, or roughly 1000 ft.

In other words a one-microsecond flash of light is only 10(X) ft long,

and if two such flashes are ^nchronized to ^/u of a microseœnd, t h ^ wfll run along neck and n«ik with thdr noses not over 100 ft a p ^ .

Lens Selection.—As previously stated, the fact that the tank must withstand both pressure aitd vacuiim predud^ tfae use of large windows, faence, tiie cameras must be mounted very dose to the tank. Likewise, the tank must be kept as small as possible to avoid undue expense in its constmction and operation. This means that wide-angle lenses nmst be employed if tiœ mmiber of cameras reqtrired to cover the experimental area is to be kept tritfain reason.

At the time the de^n was started, lenses of one-inch focal lei^th were tfae widest angle lenSes available for tfae of 35-mm film. Calculations showed that t f a ^ lenses would cover tfae entire vertical fidd, assuimng no r^i^ction at tfae transition from air to water. However, wfaen refraction was conddered, the angle of view was redœied to the place that would necessitate two banks of camenä, one above the other, to cov^ the total depth of the water. It was obvious that two banks of cameras would introduce very serious com-plications, such as expulse of caostruction, eonn^lication of operation and maintenance, and increased difficulty in analyzing r^ults. Tfaerefore, a more acceptable solution of tfae p r o b l ^ was sougfat. Tlie obvùnis metfaod oi attadc was to find some way of diminating tfae reduction in the field caused by the refraction at the air-water iot^eif ace, aiace without Ûàs refraction a single bank of cameras woold be sufficient to do tfae job.

70 K N A P P Vol 49, No. l which all the H ^ t rays will pass through tfae interface at sn angle of 90 deg and tfaerefore suffer no refraction. Tfais pos^biHty was re-ferred for analysis to Dr. Leonard M . Ross, consultant îor Mt. Wilson Observatory.

A series of careful calcuIatKnis diowed tfaat spfaerical windows were feasible and tfaat the optical distortion would be no worse than in air if a satssfactory radius of oirvature were employed and if the camera lefls w^e mounted so that ite front nodal p<ùnt was at tfae center of curvature of tfae window. This cocnbinatkKn results in a sUght decrease in the firfd of view of tfae lens, dnce tfae additiœi of. tfae spfaerical window is eqmvalent to adding anothù dement to the lens system. Its effect is to decrease the apparent distance between the ori^ginal lens and the object. Tfaus, for esuunpbe, if the distance from the lens to the object is actually 12 ft, the lens must be set for a focal distance of about 2 ft when used with tiie spfaerical window. For a one-^incfa lois, this decreases tfae fidd of view aboiit 4 pa- cent, whidi is not very serious.

ÎÏÏm-Magazine Requirements.—Tfae normal type of film maga-zine for a motion picture camera is, of course, one which uses two spools, a supply spool for the tmexposed film and a ta^-up spool for the exposed film. However, tfais system is not wdl. adapted to film speeds as faigfa as 35 ft per sec. To Obtain sucfa a speed starting witfa the film a t ^ s t requires a leader many feet lox^, even tfaougfa ;tfae spools are accelerated as rapidly as is possible witfaout fihn break-age. Decderating tfae film at tfae end of tfae exposure is also a prob-lan if fraying of tfae fHrn and the conséquent fiUmg of the camera with small film fragments is to be avoided.

The mstallation contemplated faere offers the unique feature that the cameras ate looldng hxto a completdy dark tank until tfae flasfa illumination is started. Tfais makes posdble tfae use of an ^dless- ' bdt type of magazine into ii^hicfa tfae required amount of film for one run can be loaded and cemented into a continuous strip. With sudi an arrangement tfae film can be run tiirough tfae camera over and over again, thus maldng it posdble to accdemte tfae film at a com-pletdy safe rate tmtil the desired speed is reached. This speed can {>e set exactiy and fadd witfaout variation during tfae time of the ex-posure, after which tfae film can be decderated and brought to a stop with no danger of damage- Saxih a system appeared particularly desiratde for use with a bank <k cameras, since it would not only re-duce the fihn consumption, but also Ihe'dang^ of fihn breakage and

July 1947 H I G H - S P B E D U N D E R W R T T B R P H O T O G R A P H Y 71 dmilar troubles, and tims would increase the possibUily of

obtain-ing succe^ul' records from all cameras.

The main reqtiirements for the fihn drive have already been in dicated, t.«^ smooth accderation and decderation-to prevent dam-âge to the film. However, the fact tfaat this film was to be used for predse measûronents made it d^irable to maintain a constant film speed during tfae a s s u r e and also a fixed rdationsfaip between tfae film speed and tfae number of'fladies. This latter reqifirfioient is, of course, equivalent to tfae spedfication of a constant frame dze, and, if it can be made to work satisfactorily, it offers a posdbility for the devdopment of a rdativdy dmple projector to present the record in the form erf ultra-dow-motion movement.

Since these cameras are to be used as me^urihg mstnimente, they must be as carefully aligned and as pmnanentfy fixed to the windows of tfae tank as would be necessary for'the i ^ of any <ïtixer optical -measuring instrument. This means that the magazines most be of tfae dayligfat-loailing type since the cameras cannot readily be removed from the tank and taken into the darkroom for rdoading.

Obviously, if a dayHgfat-loading type of magazine is to be used, each strijp of film wül contain a small exposed portion which can also be made to contain the spUce. To obtain a complete record of the expériment, measuremente must be started at the beginning of the entire unexposed lengtfa erf fihn, or, in otfaer words, just after the closed portion, induding tfae spHce, has passed through the camera. If tfais conditipn is to hold simultaneously for aU of the cameras in the battery, then the lengths of the film in all of the magaànes must be identical ; they must have the same number of sprocket holes per film bdt. If this condition can be otained« t h ^ all of tfaie splices in the erposed portions of the fifan can be set at the same rehitive podtibn and will maintain this relation during the entire run.

To secure identical lehgtfas of film in eacfa niagazine requires a preddoh-loadÙQg tedinique, a pierequisite of wfaich is an exact means for measoring the fifan being loaded. Furthermore^ m endless-belt t3rpe <rf magazine demands a fixed predetermined pattem of threading wfaidi must be maintained during the loadix^ and unloading cydes. The best w^y of meeting tfaese rather complicated demands appear to be throu^^ the constxucti<m of a magazine loader wfaidi would incorporate a film-nœasnring device, a ^Ucer, and film supply and take-up rolls of suffident dze to load tiie œtire camera battery a number <rf times. Smœ this auxiliary piece of eqmpment is necessary

72 K NA P P Vol 49. No. 1

for the succesfui operatiosL of the camera battery, it can be coii-sidered as a necessaiy part of the camera equipment, and tiierefore a description-crf.it wiU be induded.

Description of the Camera.—The camera itself is badcally a very simple deviœ.. Inherentiy, it has a fipced focus since witfa the lens wide ox>en the deptii of focus is suffident to cover the entire operat-ing portion of the tank. The camera oondsts essentially of two elements, tfae lens and its mount, including the spfaerical window, and the film drive 'vriiich incorporates tfae spedal gate at tfae focal

pl&jih. Fig. 2 shows a sectional devation of the camera itself.

• n u l l fmim "1 r™'

W M W I U3HT lOM

Fro. 2. Secdonal elevation of high-speed camera.

The 1ms system can be seen in this figure. It wül ht observed tiiat tfae lens is mounted in a barrel having a &ie pitch tln«ad for initial focal adjustment The mounting for the spfaerical window has been given ä great deal of condderation. It diotdd be remembered that water is always in direct contect with tfae convex dde of this window. Tlie concave side, as well as the camera, is in air and re-mains at atmo^hfidc pressure. On the convex dde the water pres-sure may vary from about 14 per sq in. bdow^atmosphdric prespres-sure to 45 lb per sq in. above it. Since tiiis spfaerical i ^ d o w is actuidly an

July 1947 HIGH-SPEED UNDERWATER PiaproGRAPHY • 73 auxiliary lens for the camera, precise aUgnment must be maintained

at all times. This is ensured by installing tfae gla^ in direct contact witfa a mdal mounting flax^e. Leakage is prevented by the use of a rubber-ring gasket öf the unsupported-area type which works equally well for dther positive or negative pressures. Fig- 3 shows the ap-pearance of this spherical window as seen from the inside of the tank. Tfae camera lens is viable in the center of the window.

The film .path' throu^ the camera is very dmple, as m à y be seen in Fig. 2. It enters tfae camera near the top, passes over a roller which guides it into the focal j^ane, goes down through the gate to another guide roUer, wfaich féeds it to the drive sprocket. Tfae film faas a 90-deg wrap on the drive sprocket and leaves it with a correct

Fio. 3. Spherical window as seen frcan Fzo. 4. Detail of roller guides for ^ocal

izisiüé .of tank. plane. alignment to go directiy to the first idler spool in tfae magazine. Tfae

^te itsdf is of spedal design to operate at the faigfaest ffim speed that is used. It was believed that it wouH be extremdy difficult, if not impossible, to prevent film damage if rubbing contact were permitted :at any point,in the camera witii, a film speed of 35 ft per sec. At the same timey exact podtkining of the film in tfae focal plane was required

ia order to secure quantitetive measurements^from tfae records.

The system of roHër guides^ seen in Fi^. 2 and sfao^ in ä&taaX in

Fig. 4, Tiras devdoped for this purpose. It will ibe noted that tfae roflers are "relieved between the sprocket fades so tiiat tfaey do not touch the fifan in the active area.. AU of the rollers in the camera and maga^the are so!relieved. Edge rollers are provided to define

74 K N A P P Vol 49. No. 1

ïwrecisdy tiie lateral position of the fihn. A double pair of rollers at entrance and exit was found necessary to flatten the filni properly as it passes through the focal plane. Tests showed, however, that this construction alone was not suffident to ensure complete flatn^ of the film tmder all conditions dnce a sli^t lateral curvature tended to perdst. Tlie otdy successful method yet devdoped of eliminating tiiis curvature has beäi throu^ a. rough control of the hunudity by use of faumidifying pads in tfae magazine. Fig. S is a side view of tfae camera showing the loading door. The opening and closing of this door also moves interlocking pins wfaich operate the light locks so as to prevent acddental exposme of the film.

Fig. 6 shows the extemal view of the film m a r i n e . It wiU be noted tfaat it seems dispropor-tionatdy large compared to the dze of the camera, Tfais is, of course, because of the f ^ tfaat. tfae film is laced as an endless bdt in tfae magazine. Fig. 7 (o) and (b) diows the external con-struction of the film rack witfa the film laced in ^ace. Tfae run on wfaich the film èntö^ and leaves the camera is a straight vertical run, whereas most of the othera are diagonal. The altemate use of large and smaE spools on botfa tfae upper-and lower sfaaîte ehminates tfaè possibility of tfae film's toucfaing and rubbing on tfae crossover pointe. The shaft which carries the lower set of spools is loaded both by gravity and by an auxiliary spring in order to secure tfae desired film tendon. The film lacing shown in K g . 7 utilizes the m E œ r à i u m capadty of the magazine. Tfais lacing can be modified to accommodate' shorter lengths in case a fuU-FiG. 6. Erteroal view of film magazine, length film strip is not necessary.

FIG. 5. Sde view of camera showing

July 1947 HIGH-SPBED UNDERWATER PHOTOGRAPHY

FIG. 7. External construction of film rack with film laced in place.

(a) Front. (ft) Back. .

Camera Drive.—Fig. 8 is a general view of tfae bank of five cameras moimted on the dde of tfae tank. Tfae common drive shaft can be seen paä'süig tfarough all of the cameras. This drive shaft was designed for faigfa tordonal rigidity to dinmmte any ap-preciable displacement angle between tfae (frive sprockets of the different cameras^ To prevent any binding at the b&irin^, fl^blc universal jointe of the metallic-disk type have been provided at each dde of each camera. Wfaen the battery is bemg opiated, witfa tüB magazines running at the maxxnmm speed of 35 ft per sec, tfae angular displacement erf tfae drive sprockets between the first and last cameras in the ban^ is less than 0.04 deg, which is eqmvalent to 0.001 in. of fihn travd. For most concQtions of operation, this introduces an o T o r in the measmements too

^nall to be dgnificant. However, it can be corrected very ^siJ^ by adjusting tfae corresp<mding-pro-jectors to have the same angular displacement with respect to the first ramera in the bank. Tfaesyn-clironous drive motor can be seen on tfae extreme left of tfae drive

diaft. Fig.9isamoredetailedview YIO. 9. ^chrofious ânve motor.

7 6 K N A P P ' , Vol 49, No.1

of this motor. It will be seen that there is an dectric brake on each end of it.' These Inukes are i ^ d to secure the low and ^ootfa accd-erations and decelaccd-erations, required to prevent fihn damage. It wâl be obs^red in Fig. 9 that the ^tire motor is mounted on trmmmn bearings so tfaat it can rotete; The power supply is broügfat to it throng^ a set of three slq) migs mounted on tfae oûtdde of tfae motor frame. The elédirîc brake on the right-hand end operates on the motor shaft; whereas the one on the left-hand end works against tiie motor frame, Tfae operation of this system can best be seen by fol-lowing throûgh a complete cyde, as follows:

Conddtf tlmt the corneras are all loaded witfa full magazines and that everything is stetionary but ready for a normal photographic run. Tfae motor-diaft brake is then clamped and tfae motor-frame brake released. Tlie starting switch is then dosed, applying power to the motor. Since the totor and drive shaft are clamped, the motor frame starts to revolve on its trunnion bearing. It acMlerates up to normal speed and syndironizes. Next tfae diaft brake is rdeased, making it possible for the rotor and drive sfaaft to turn. The frame brake is thai apphed ffrädxtaSly at a predetermined rate. This dowly brings the motor frame to a stop while the rotor with the drive' sfaaft, camera sprockete, and fifan magazines all come up to full synduronous

speed. The cameras are now ready to record the residts of the

experi-ment. This is done by launching the body, the motion of whicfa is to be recorded, and dmultaneoust^ tnmmg on tfae battery of flash lamps. W h ^ the exposnre is made and tfae reccnd (^Ttained, tfae film is brought to a gradual stop by a reveraal of the above procedtite; i.e., first the frame brake is rdeased so that the frame is free to tmn, then tiie diaft brake is appfied at a gradual rate, and tfae ffim in all the cameras brought to a standstill. By means of a series of inter-locking rdays, all of these operations are performed automatically after the sterting or stopping switch is dosed by tfae operator.

One fortfaer refinement is incorporated ni tiie procedure. The cam-era drive shaft is connected to a sinrple gear train wlüch acts as a counting device to keep track of the location of tfae fflm splices. This, counter is interlocked with tfae main contr<d of the experiment itsdf so that tfae experiment can be started and tiie fiadi lamps energized only immediately after tfae film spHce and tfae ei^sed section of the filrn faave ^ne tfarou^ the camera. Tfais interlock ensures that fuU record oi the phenomenon under investigation will aX^ys be d i t ^ e d at each run.

Jxäy 1947 HIGH-SPEED UNDERWATER PHOTOGRAPHY 77

Magazine Loader.—^The film magazines are all detachable from the cameras and may be taken to tfae darkroom for loading. How-ever, tfaey are large and bulky and quite heavy, even though made of almninum. As explained previously, a spedal loading device is necessary in order to preserve the threading pattern ahd also to en-sm^e that each magazine is loaded with exactly the same length of fflm. A little study showed that it would be simpler to dedgn the magazine load^ so that it could be brougfat to th& m a r i n e , ratha: tfaan to take ths magazines into tfae darkroom for rdoadtng. To fadlitete tfais operation, a special bracket was installed at each cam-era to permit the magazine to swing down in a horizonal podtion

. FIG. 10. Magame in loading position.

bdiind the camera. Hg. ,10 diows one of the magazines in tfais loading position witfa tfae toatkx moimted in place on it. . A standard Mitcfaell 1000-ft camera magazine is used for tfae unexposed and ex-posed filni stora^. The lower section of the loader contains the sprot^t-facde counter and tfae spUcer.

The operation of the loader is as foUows : It is first placed uppn the fnagazine üdng the same locatii^ dowels that position the magMÎne on tfae camera. The loading-ffim loop containing the spU<x projecte out of the magazine into the loader. This loop is now broken. One end of it is spHced to tfae end of the exposed fifan going to the take-up :^pool. The other end is spHced to tixe unexposed film coming from the supply spool. The loader door h now do8ed> wfaicfa opens tfae

78 K N A P P _{Vol 49, No. 1} Ught locks to tfae magazine and to tfae supply and take-up ^KMIS. Tfae operating Crank is tfaen tumed until tfae exposed Sbn faas been drawn out of tfae magazine and a rdoad of unexposed film drawn in. Tfae correct amount of film is indicated by-tiie reading of the sprocket-hole counter. The loader door is then unlodced, wfaidi doses tfae Ugfat locks into tfae camera and film magazine. Tfae fihn is then cut

from the feed and .take-up spools and a spUce made to form the end-less belt in the camera magazine. Tfa^ splice is made at the exact sprocket hole indicated by the ooimter. It wiU be^ seoi that all of tfae spfadng and cutting can be done in daylight since tfae only film exposed is that section which it is necessary to use in any case during tfae threading of tfae loop through the camera gate and drive sprockets. '

Flash Lamps.—The flasfa lamps are straight gas-filled tubes approximatdy 8 in. long and are made of quartz. Quartz tubes are necessary because of the tremendous amount of energy that ia fed into them to produce the flashes. The t^e of quartz makes

FIG. 11. Flash lamp.

posdble mucfa longer nms than could be obtained by Pyrex or any other type of glass tube- Tlie diaracteristics of these tubes limit the lengtfa of record that can be secured whai bemg operated at flash speeds of 1000 per sec or greater. Their effective Ihnit is about 3000 flasî^ At this ppint they become so hot that the dmration and

in-tensity of the flasfa is serioudy affected. Continued operation beyond 3000 flashes quickly re-sidts in softehiag and final col-lapse of the tube. F:^. 11 sfaows oneof these tubes.

-Fig. 12 is a time-intendty record of the flasfa produced. This record was obtained by observing the flash with a photo-ceU driving ä synchroscope. It

Fie. 12. Hme-intensity record of flash ' . . produced.

Horizontal time scale: 2 divisions per microsecond.

July 1947 HIGH-SPEED UNDERWATER PHOTOGRAPHY 79 wiU be seen tfaat tfae duration at peak intoisity is approximatdy one microsecond and the effective photographic duration between 1 and 2 microseconds. The energy input to the tube is approxi-mately one joiile per flasfa; Expressed ia other units, this means tfaat at à fladnng rate of 300Ô per sec, each tube requires an amount erf energy equivalent to a continuous input of 3 kw. If the duration of each flasfa is assumed to be one microsecnifd, the U ^ t is on V m and off '"/TO of the total time. Tlius tiie rate of energy input to one tube during each flash is about one me^watt (1000 kw).

The j^sh tube iis made in the

FIG. 13. Flash tube mounted in reflector.

form of a line source because the area to be iUuminated is rec-tangular. To utiUze the Ught as efficientiy as possible, each tube is mounte<l in a spedal-re-flector as shown in Fig. 13. Tfaese reflectors are constmcted of Ludte, cemeniid togetfaer and then aluminized by f the vacuum-sputtering technique. The cross section of tfae reflector is so de-dgned that the Ught is dis-tributed as uniformly as possible

over the depth <rf w(»-ktng section in the water tank. Tfaese lamps are normaUy used ni banks, jointed togetfaer as shown in Fig. 14. This construction makes it possible to i n s ^ tfaem into Ludte tubes of about S in. in diameter. These: tubes run longitudinaUy throng tfae tank and are provided with stuffing boxes at each end to pre-vent leakage. Thus, tfae Ughts themsdves are in qir at atmospheric pressure at aU times, but so far as iUummating the tank is concemed, they act as if they were xmder water.

À detaUed description of the power supply and control drcuits for these lamps is beyond the scope of tfae present paper. They' are badcaUy the product of Prof. Harold E. Edgerton and his asdstents

- t - r l i f e

80 ^ A P P Vol 49, No. 1

FIG. Ifî. Battery of six control

units.

àt Massadiusette Institute of Tedi-nology, wfao are pioneers of many years standing in this fidd. Ràtiier ext^ive modifications of tiie normal eqii^ment were inrorporated in tins development for the Hydrod3fnatmcs laboratory, which shortened condder-ably the duration erf tfae flash and also made it possible to operate tfae lamps witii cables <rf varying lengtfas up to 100 ft connecting tfaem to thdr re-spective contrd units. Measurements taken dming operation , indicate that the circuite employed in the control

mats showed devotions in flash time

of 0.1 microsecond or less. This means tfaat the flash duration of tfae entire battery of lamps was not over 10 per cent longer tfaan that of any individmd tube. Each lamp require a separate conti'ol circuit. Fig. 15 diows a battery of six such wiits with the cabinet door open to give a view o^ the interior. In aU, seveii batteries faave been uistaUed, giving a maximum of 42 lamps that can be operated in

synchronism-The fundamental prindple of opera-tion is very simple. Eadi unit con-tains a faigfa-voltage capadtor, wfaicfa

acts as a power reservoir to supply

tfae necessary energy to opoate tfae Ugfat during tfae flash period. Power is poured into tfais capadtor duiii% tfae refetivdy long period between flashes. At tbe time of the flash, the entire amoimt of energy stored in the capacitor is disduuged tfarough ,tiie lamp tube throng à high-capadty

July 1947 HIGH-SPEED UNDERWATER.PHOTOGRAPHY 81 control switch. Tlie power source for diargmg oU tfae capadtors of

tile lamp battery is a l20-kw three-phase, faigh-voltege rectifier wluch supplies d-c power at any desired voltege up to 6000. Tbe oontrol circuite incorporate voltege doublera, thus the Ughte them-sdves can be operated at any dedred potential up to 12,000 v, de-pending on the ligfat intendty desired. Needless to say, drcuite operating with d-c voltages and capadtors of tfaese magnitudes re-quire safety precautions for tfae operating personnd.

Operating Experience-—Tfae canieras tiiat ha,ve been described and tfaeir assodated equipment are now coming into fuU operation. It is felt tiiat tfae mstaUation wiU prove to be a very keen tool fox the study of high-speed hydrodynamic phenomena. It may be of interest to know that one- of the major difficulties that has had to be overcome in obtaining satisfactory resulte witfa this equipment has been tfae seeming and mainteimng of a supply of water faaving satisfactory darity. It wiU be remembered that the length of tiie Ug^t path from tfae lamps to the object under study and back to the fflm in the canœra is about 25 feet.

Measiiremente made with dust-free double-dótiUed water, purified with the utmost care, indicates that a layer of such water one foot thick win transmit 99 per cent of Ught that falls on ite face. At first tfais sounds weU, but it must be remembered that the light path is 25 ft long. To calculate the Ught transmitted tfarougfa 25 ft of water, it is necessary to raise the coeffident of Ught transmission to the 25i^ power: 0.99 to tfae 25th power is 0.78. In other words, one fottrth of tfae %ht wiU be absorbed and three fourths wiU reach the camera. The corresponding absorption for a 25-ft air path is immeasurably

smaU. Thus it wiU be séoi tfaat the very best water obtainable ab-sorbs an appredable amount of Ugfat.

Hô'wevêr, ti^ Laboratory has made Comparative measuremente of a number of different samples of water, aU of whidi appeared very dear to the eye when viexSed in'5-gal containers. It .was found that carefuUy ffltered de-aerated àty Tfrater stored in glass until the time of measurement had a tran^sdon coeffident óf 0.95. Identical wat^ stored in a rtibber^ed tank for two wedcs diowed a transmission coeffident of about 0.65. Anotfaer of the same samples stored for tfae same lengtfa of time in à tank painted with faigfa-grade zinc cfaromate showed a transtmsdon coeffident of about 0.85, but, in addition, it was found tiiat the transmission in the blue was com-. pletdy diminatedcom-.

82 K N A P P V d 49, No. i . Table 1 sfaown a comjùirison <rf t h ^ various water samples on tbe bads of a 25-ft Ught path. An examination of this teble makes only too evident tfae absofaite neeesdty erf securing a dear wat^ supply

for underwater pfaoto^^^^y which, recpiires an appreciable light

path. A major step in the solution of tiie problem for the Hydro-dynamics Laboratory tank has been to Une the tank compl^dy ^tfa Koroseal of a type that was first sub j«:ted to rig^>rous teste to prove that it did not affect tfae water stored in it in any way,

T A B L B l

Feisaitags Treannniwon otZJsht , Coeffidents Trammltted per Water Sample per ^ßoot 2S-Ft Path Filtered distilled 0.99 78 FEtëred dty supply 0.95 28 Zinc Chromate paint container 0.85 2 Rubber-lined coatama 0.70 0.01