1 MAKSTLaND BETA CHAPTER TAU Bi^TA PI THE DEVELOPMENT AND USES OP HIGH SPSiJ) PHOTOGRAPHY BOWEN W. SHAW SUMMARY This paper deals with the development of speed in pho- tography from the latter part of the nineteenth century up to the present time . The material is divided into two parts, the first dealing with the mecnanica-± of obtaining very short exposures, and the last part with the development of speed pnotography through the use of flashes of light of very short duration. The applications of each camera are dealt with in the dis- cussion of the camera. — ff*< THE BBVBLCPMENT AMD USES OF HIGH SPEED PHOTOGRAPHY During the past fifty /ears photography has developed from an un- reliable to/ to an integral part of engineering ana r-e»eaT"bh-. There are sev- eral reasons for this growth. First, there is the laot that a photograph furnishes a permanent record of a large number of details without requiring the time neoessary to maKe a detailed drawing. Seconal/, a photograph proper- ly made records things as they actually are as against the eye which tanas to observe ouiy those things which the eye is directed to look for by the mind. Thirdly, a photograph of a moving object shows dearly the position oi the ob^eot at an instant of time (sometimes less than one millionth of a seoond in duration) , whereas the eye sees only a blur oi continuous motion. In this paper the latter subject will be discussed, that is, tue development oi speed in photography. There are four factors which limit the shortness of the exposure in a oaniera. They are the speed of the lens; the intensity and quality of the light on the subject; the spaed of the emulsion on the ixlm; and a method of exposing the rilm for a very short period of time. In the very early experi- mental days of photography the emulsion and the lens w*e so slow that exposures of several seconds or minutes were required and obtaining a suitable meoham- jfecal shutter was not a prooiem. However toward the latter part of the nine- teenth century the emulsions and the lenses were improved to such an extent that the ordinary mechanical shutter became useless for very high speed wori. TYPES JF MECHANICAL SHUTTERS Mechanical shutters may De divided into two general o las s if i cations; those whioh operate at or near the optical center 01 the camera; ana those which operate near the focal plane oi the camera. Eaoh of these ty^es has its own advantages and disadvantages. -3- The shutter whioh operates at the optical oenter of the lenses is advantageous in tht-t all portions of the plate or sensitive lilnt are exposed through the saiae periou oi time* However as this type operates by having an opening in a flat piece of metal move across the film or by causing two or more leaves to move outward exposing the iilm and then inward dosing oil the light, at high speeds the camera is not operating at full aperture throughout the exposure and hence the effective speed of the lens is less than indicated. In the focal plane type oi shutter the plate is exposed by allow- ing a pieoe of metal or cloth containing a narrow slit to move rapidly across the film a short distance in front oi it. The exposure may oe oontroled by varying the width 01 the slot and the speed with which it travels across the plate. A high speea ana a narrow slot will give a short exposure, and con- versely a low s^eea and a wide slot will cause the exposure to be relatively long. For instance, if the slot has a width oi one tenth oi an inch and trav- els at a s^eed of fifty inohes per second every point on the plate will be exposed for a period of one -five hundredth or a second. Ihis camera has the advantage that the camera is operating at the same aperture throughout the exposure. However all of the portions oi the plate are not exposed at tlie same instant and ii the subject is moving rapidly its image will be distorted. In the case cited aoove the exposure was one-iive hundredth oi a second but if the plate were seven inches long one side of the plate woula be exposed fourteen-hundredths of a seconc alter the other siae . DAYLIGHT PH0TU3RAPHY «ITH A MECHANICAL SHUTTER One of the first means oi' obtaining photographs oi very short dur- ation by a mechanical shutter was perfected by Charles Francis Jensins who began his experiments in IdsG. In these experiments Jenkins located a number of lenses on the periphery 02 a wheel. The image irom the leos was reilected oy a prism onto a strip ji moving film and the lenses moved at a speod such -3- that the velocity of the i'ilci and the tangential velocity ox the lens as It passed the prism ware the same. Due to this the image was stationary with respeot to the iilm ana the exposure was nepenoent on the speed of the revolv- ing wheel. There existed a very slight blur in the negative caused by the iaet that the lens moved in an aro rather than in a straight line. However with a wheel of fairly large uiaraater this effect was negligible. with tiiis camera JenJtins was ubie to obtain up to three thousand two hun- dred images per seoond with exposures of as little as one-twenty thousandth of a seoona. The camera was valuaole in Lhat it took a series of pictures oi rap- idly moving objects such as projectiles rather than one single picture as is the case with, some of the methods to he treated later. Another aavantage j&E *as the fact that the camera wat, complete in itself ana no special laboratory conditions such as special lighting or spars discharges were necessary to obtain a series of photographs. Due to tno nuture ol the camera it has a splenaid rati a 01 time oi exposure to time oi operation, as it taices three thousand two hundred pictures a secona ana each exposure is one -twenty thou- sandth oi tt second the total time of exposure is sixteen-hunaredths oi a sec- ond per secjna. Another type of daylight oamerta was uevised in l~yl7 in Germany which is in reality a modified moving picture camera. The shutter of the ord- inary moving picture oamera is a circular metal uisc with a slot cut in it as shown in Figure 1 * - The film is stationary while the slot is in iront oi the film and the film is being exposed. During the interval when the lilm is not being exposed it is advanced one irame. The numoer oi pictures ta^en ana the Of i exposure given each pijture is ^seatr-oled- by the speed with which the disc revolves. 7he regular value is sixteen irames per second, however this is not suitable lor photographing projectiles, ana even ii the speed is boosted to one hunured and twenty irames per second the results are not satisfactory. At this speed oontinuous operation is not Batisiaotory and even if it were the -o- that the velocity of the film and the tangential velocity oi the lens as It passed the prism were the same. Due to this the image was stationary with respect to the lilm and the exposure was Dependent on the speed of the revolv- ing wheel. There existed a very slight blur in the negative caused by the; iact that the lens moved in an aro rather than la a straight line. However with a wheel oi jiairly large diameter this effect was negligible. „ith tiiis camera Jen&ins was able to obtain up to three thousand two hun- dred images per second with exposures of as little as one-twenty thousandth of a second. The camera was valuaole in that it took a series of pictures ot rap- idly moving objeots auoh as projectiles rather than one single picture as is the case with some of the methods to be treated later. Another advantage w as the fact that the camera wat, complete in itself ana no special laboratory conditions such as special lighting or spar^ discharges were necessary to obtain a series oi photographs. Due to the nature oi the camera it has a splendid ratio oi time oi exposure to time 01 operation, as it ta*ces three thousand two hundred pictures a second ana each exposure is one-twenty thou- sandth of a second the total time of exposure is six teen -hundredths of a sec- ond per seeona. Another type of daylight camera was aevised in iyl7 in derinany which is in reality a modified moving pioture camera. The shutter of the ord- inary moving picture oamera is a circular metal disc with a slot cut in it as shown in Figure 1 • The I'iLn is stationary while the slot is in iront oi the ^ilm and the film is being exposed. During the interval when the iilm is not being exposed it is advanced one irame. The numoer oi pictures ta^en ana the exposure given each pioture is controled by the speed with which the diso revolves. The regular value is sixteen iraraes per seeona. However this is not suitable lor photographing projectiles, ana even ii the speed is ooostea to ona hunared and twenty irames per second the results are not satisfactory. At this speed oontinuous operation is not satisfactory and even if it were the / e v^<*>' a / /vy*v<? £ / r /yti*'& & -4- projeotile Is blurred and the interval oetween frames is so great that the projectile might pass entirely through the field of the camera while the shutter is closed. Because of this the shutter was changed to the type shown in Figure 2 in which the single opening is replaced by three narrower ones . When this type is used three exposures are made on eaoh frame thus giving the relative position of the projectile at three instants as shown in Figure 3. This facilitates the calculation of the trajectory of the shell and any deviation of the axis of the shell from its path at that instant* With this type of shutter the projectile may be photographed three times while it is in the field of the camera and the distance between the suoce sk- ive positions of the shell may be no greater than twioe its length* When it is desired to obtain pictures with a phase difference less than in the oase above the camera is modified further so that the shutter is as it appears in Figure 4* Each of these four staggered slots is behind a separate lens, and in operation four pictures are made across the width of of the film with a phase difference of one - twelve thousandth of a second. Figure 5 illustrates the type of picture which is obtained by this system and it may be seen that the projeotile moves very little between the sucoe st- rive pictures. This camera is especially useful in obtaining pictures of large projectiles in flight, as suoh pictures oould hardly be made in a laboratory under controlled light. In order for the shell to be distinct it should be photographed against a brilliant background suoh as the sky or snow. Still another method of stopping a rapidly moving objeot was de- vised by H. R. Curtis, W. H. Wadleigh. and A. H. Sellman in 1924. A diagram of their camera is shown in Figure 6. The film drum contains one loop of film on the surface and the film drum rotates at suoh a speed that the surfaoe of the film on whioh the Image is east is moving in the same direction and S r ta&/'e 4 ^/pase J" 13- _ /lreu9c fitrit ■£.C/7S f*Atoa J¥r*r/7* *> ^"ffvts'e £ -o- at tha same speed that the Image ia moving* The fooal plane shutter is another cylinder which fits closely over the film drum and contains narrow slots parallel to the axis* The shutter revolves in the opposite direotion from the image and at a speed several times as great* Sinoe the film is moving at the same speed that the image is moving, the linage is stationary with respaot to the film. The slot passing in front of the lenses control* the length of the exposure* The faot that the image is stationary on the film means that a longer exposure may be obtained without blurring than oould be obtained if the image were moving* Of course the approximate velocity of the moving object must be known in order to adjust the speed of the film drum* The main shutter is placed behind the lens for the purpose of exposing the film at the proper instant and then closing the aperture after one complete revolution of the film drum. When it ia desired to make a series of pictures with a very short time interval in between them, the camera is modified by replacing the single lens by a number of lenses and the single slot in the fooal plane shutter by the same number of staggered slots, as shown in Figure 7a. If five lenses are used the camera will take five times as many pictures in a given length of time, and there will be five rows of pictures around the film as shown In Figure 7b* The field in whioh this oamera might be used would be rather specialized as the velooity of tha subject must be known and the motion must be in a straight line* Like the above cameras it requires no speoial light- ing conditions, and may be used under natural light. The features of the oamera make it especially useful in work with projectiles. Mith It it is possible to study the yaw of tha projectile, its velooity, the speed of ro- tation, and the blast. A method similar to that above was developed in France in 1932. -6- In this camera 35 millimeter film ran through the camera In the manner that it does in an ordinary moving picture camera except that the film mores continuous 1/ through the camera rather than stopping at each frame • The 7 focal plane shutter is of the same type shown in Figure 3, there being a large number of rows of slots rather than Just one. When the oaaera was made with four concentric rows of slots there were four lenses, and four rows of images were formed on the film staggered similarly to the pictures shown in Figure Ta* The Individual pictures are quite small , the space ordinarily occupied by one image contains 12 smaller ones. By varying the speed of the shutter, up to 12,000 piotures a second may be obtained* This oamera was used in France largely for the purpose of studying the flight of inseots* Despite these accomplishments in mechanically controlled photography it is still difficult to obtain a fair sized clear picture of a rapidly moving object at the precise position that the picture is wanted. In order to gat the subject at a certain point it is necessary to photograph It in a whole series of positions. THE DEVELOPMENT CF LIGHT CONTROIBD PHOTOGRAPHY Over fifty years ago it occurred to experimenters in photography that a plate might be exposed to light by leaving the oamera In a dark room with its shutter open and then flashing a light off and on in the room* The length of the exposure would then be the length of time that the light re- mained on. ELECTRIC SPARK PHOTOGRAPHY In 1393 C. V. Boys delivered a talk telling of his accomplishments in the field of spark photography. He had patterned his apparatus somewhat after that of Professor E- Mach's. The eleotrioal circuit of Professor Maoh's apparatus consisted of two spark gaps and a oondenser in series. /^/grttre 7~teJ _ 2 7 _ / & - 5 - 4- H 3 ^/payc 7&) -7- One of the spark gaps was located on a point In the path 01 the projectile end the other was looated in front of a lens so that its light would be oonoentrated on the projectile. On the other side of the first spark gap was the camera focused on the first spark gap. The condenser would be charged to such an extent that the voltage would be almost enough to cause a spark to jump the two gaps* Then with the shutter of the camera open the gun would be fired* The bullet reaching the spark gap in its path would short circuit It and cause a spark to jump the second gap thus giving a brilliant light of a short duration. The flash of light would accomplish the same result as a shutter being oppenned for the same length of time, and the bullet would be photographed. Boys' apparatus is shown in Figure 3* It consisted of a condenser constructed from a square foot of tinfoil placed on either side of a piece of glass, in series with two spark gaps E and E* * G f is a small capacity ijeydon jar. S is another spark gap in the path of the bullet and the dotted line represents a string wetted with calcium chloride. The system is charged until both of the spark gaps are about to break down and the gun is fired. When the bullet short circuits the gap S. The voltage is able to break down the gap B*. 'with this gap broken down the large condenser oan now discharge through E oausing a brilliant spark lasting about one millionth of a second. Very little of the charge goes through S as the condenser C is quite small and the string is practically a non-conductor during the discharge. The spark generated at E being small casts a sharp shadow of the bullet on the plate. With this apparatus Boys conducted a series of experiments to determine just how the apparatus should be constructed to obtain the maximum illumination with a minimum duration* The original apparatus was set up with ordinary wire for the conneot- -8- iotis and the spark tips at E made of magna slum. The projectile was travel- ing 2,100 feat per seoond and the spark lasted long enough for the projectile to travel half an inoh, blurring the image on the negative. Upon investi- gation it was found that while the main spark lasted one millionth of a second, the magnesium tips glowed for about seven millionths of a seoond. it was oonoludad that a less volatile metal suoh as platinum should be used at the spark gap and this substitution was made. It * as found that the time of dis- charge could be materially reduced by replacing the wires carrying the main discharge by heavy bands of copper made as short as possible. With these changes made it was found that the whole spark was extinct in less than one millionth of a second, and the first blaie which supplies most of the light was extinot in less than one ten millionth of a second* The bullet traveled one four hundredth of an inoh in this time . While the images obtained by this method are shadows they are re- markably olear as the source of light is small and the time is very short. The Images show the projectile, the wires whioh initiate the discharge and In the case of projectiles moving over eleven hundred feet per second the air waves set up at the nose and tail of the bullet are clearly visible. Today this apparatus has changed somewhat into several specialized types of apparatus* Prank S. tfyle, a student at the Massachusetts Institute of Technology in Cambridge constructed a high speed camera which represents one of these types. His apparatus uses one hundred and ten volt alternating current step lng it up to 3,000 volts, and then converting it into direct current. A bank of oondensers is charged with this currant. This charge is discharged through a tube filled with krypton at the same Instant that a high voltage is passed through a spark ooil into a wire around the tube. The eleotrioal oharge passing through the gas oauses a five million oandlepower flash whioh lasts -y- only one-fifty thousandth of a seoond* The light may be timed either by an eleotrioal contact or by the impulse from a miorophone plaoed near the source of a sound connected with the motion to be photographed* When the miorophone was used Wyle was able to delay the flash by moving the miorophone away from the source of the sound* On the following pages some of the results obtained with the camera are illustrated* Figure 9 illustrates the set-dip used in investigating a defect in the operation of a drop hammer* The drop hammer was forming a duralumin air scoop for an airplane and part of the metal was torn away* Examination of the photographs showed the oause and it was corrected. Figure 10 shows how a fine stream of milk appears when photographed by this method* In Figure 11 at the top one sees a bullet which has passed through two glass bulbs and is about to pass through a third. At the bottom the action of a light bulb on being struck by a hammer is shown. A variation of this apparatus was constructed by Francis Behn Riggs, Jr. of Harvard University and an example of its work is shown in Figure 12* The three pictures show the bullet about to enter the bulb, entering it. and leaving the bulb* The bullet was from a forty-five caliber revolver and was traveling at a speed of eight hundred feet per second when photographed* Perhaps the most recent and complete apparatus is that constructed by K. V. Germeshauser, H. JS. Grier, and Harold Eugene Bd gar ton of the Massa- chusetts Institute of Technology* In this apparatus the film moves contin- uously through the camera and the light source is similar to that used in the stroboscope. The camera is known as a strobosoope oamera* The light flashes on and off 6.000 times a second and each flash lasts for one -hundred til Thavsctfi^ of a second* It is the most complex of the apparatus which has been mentioned so far, indeed one whole room is occupied by apparatus concerned with pro- ' rtf. •arc & /^tq&re /a /^/pMre // '/fare /£ -10- duoing the suddenly released high voltages whioh produce the photographio sparks* The camera is linked to the stroboscope and runs at closely regu- lated speeds so that aooe la ration and velocity may be measured on the films. The instrument is extremely useful in clocking projectiles, whirling engines and propellers, and other meohanioal devices. The strobo- scope camera is used in studying the dangerous shiver of high speed maohlnes and is even used in medioine to study action of high spaed micro-organisms* JCNOLUSItHS Several types of cameras have been discussed, but each has its field in which it Is extremely useful. The mechanical cameras which are able to take plotures in natural light cannot be replaced by the ones using artificial light* ill the other hand if a series of clear plotures of rapidly moving objeots which oan be photographed in a laboratory are desired the stroboscope camera is the logical one to use* For very high speed pro- jectiles it might be best to use Boys* apparatus whloh has an exposure time of less than one millionth of a second. The value of this high speed photography oan not be over emphasised* The illustrations show but a few of its uses* Whenever it is desired to study fractures, deformation, or motion occurring at high speeds the camera is the most effective and reliable means of conducting this study. BIBLIOGRAPHY Baker, Thomas Thorne, The Klngc otn of the Game ra . London; Bell & Sons Ltd., 1934. Boys, J. V., "Spar* photographs oi" Flying Bullets," Nature . XLVII (March, 1893), pp 415-421, 4*0-446. i&gnan, A., "Glnematographie Juaqufe 12,000 Vues Par Second," Actualities boientii'jques et Indus triallas . (.1932) . Mueller, Grover J., "Stop Action Photographs';" Popular Soienoe Monthly , Vol. 134 No. 1, pp 80-83. Von Jles, Hildebrand P., "Motion Pictures of Projectiles in Plight in Day- light or by Artifioial Light," Journal of the pnitea States Artillery. 48 (1917), 91ff. "A Camera for studying Projectiles in Plight," Teohnolo^io Pap.ers of t he Bar- r'fau of Sfondards . No. 22o, Vol. 18, (1924). •Photo Amateur Snaps Bullet Striding Lamp," Moaern Meohanix. iXX (April, 1933), p56» Soiance . Vol. 89 Ho. 2299 (1939), p 156. "Stroboscope Photography," Spientjiio Monthly . 1LVI Ho. 5 (1939), p. 455. £img. (January, 1939), p. 97.