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Among the more unusual high speed cameras is the Beckman andWhitley, or now the Cordin, Dynafax rotating drum framing camera. Theoriginal Beckman and Whitley camera is capable of up to 25,000 picturesper second (the Cordin version goes up to 35,000 pps) making 224 16mmframe size photographs on a 3 foot length of 35 mm film placed aroundthe inside flange of a rotating drum.

This drum rotates and at the same time drives an 8-sidedcounter rotating reflecting glass block at its center. The glass blockrotates14 times faster than the lager, film carrying, drum. Each rotation ofthemirrored block "prints" 112 images on one side of the film and another112 on the other side of the film to make up the 224 total frames.

It does this by reflecting the image from the central 8 sidedglassblock mirror to a pair of deflecting mirror assemblies located 45degrees apart alongthe inside of the camera. Thus after "printing" one image ofthe subject on one side of the film through one set ofdeflecting mirrors it prints the next one after turning 22.5 degreesfurther (and reflecting the light from the objective lens 45 degrees)through the second set of deflecting mirrors. Each facet of therotating mirror block thus prints 2 frames. The two  images arerecorded sequentially 14 frames apart which it the length of filmequivalent to that in 45 degrees of the surrounding drum. Thus 16frames are reorded per revolution of the central block for a total of16 x 14 or 224 frames with (as stated earlier) 112 along one side andanother 112 along the other side of the film strip.

The Dynafax overcomes some of the physical limitations thatput a ceiling on the rateat which film can be transported through a high speed motion picturecamera. To reach higher framingrates than those reached by rotating prism cameras, cameras based on adifferent image motion compensation scheme were devised. The Dynafax isa prime example and can reach speeds to 35,000 pps although itcompromises onimage sharpness due to small amounts of motion between the film and theimage during exposure.

In the Dynafax the image forming rays are deflectedby a rotating multifaceted mirror driven by a rotating drum that alsoholdsthe film during exposure. The mirror rotation rate is locked in twiththehat \of the film carrying drum so it turns 14 times faster than thefilm drum. The two are comnnected by a set of gears that ensure theyremain locked together during operation.

The rotating glass block turns at a rate set by the cameradesign parameters so that the image moves at the same rate as the filmlocated along the inside periphery of the rotating drum. This rotatingmirror also deflectsthe image of a "stop", through which the image forming rays pass. Asthis imagemoves across a physical replica of the stop and because the stop'simageis moved more rapidly than the subject's image, the shuttering of sucha camera is quite remarkable in terms of duration.

The optical complexity of the camera, use of several relaylenses andthe shutter stops determine the operating or effective aperture of theoptical system as well as the shutter factor. The camera can beequipped with a shutter stop (often called a diamond stop due to itsshape) that limits the effective aperture to f/11. f/16 or f/22 eventhough the primary objective (a "C" mount lens of 75mm focal length) isusually used wide open at an aperture of f/2.8. At thesame time, the shutter factor is 10, 20 and 40 respectively.

To put these factors in perspective whenthe camera is fitted with the smallest shutter stop andrunning at the highest speed the effective aperture is f/22 and theexposure time is 1/25,000x40 or 1 microsecond!

As shown in the diagram below in operation the image formed bythe objective lens is brought to a focus at an image plane. Beyond thisimage there is a field lens whose function is to collect all the lightrays formed in the image plane and send them on to the entrance realylens placed 2 focal lengths away from the image plane. The light rayspass through an opening, called a "diamond stop" which is used forshutterinmg the system, and located 1 focal length of the field lensaway from it. The light rays then are deflected towards the center ofthe camera by a first surface mirror placed at 45 degrees to theincoming beam. This relay lens then brings the image plane to a focusjust beyond the surface of the central rotating 8 sided mirror. Therelay lems, being 1 focal length from the diamond stop projects animage of the diamond stop, filled with light, as an unfocused beam ofessentially parallel light.

The fact that the image of the image formed by the primaryobjective is brought to a focus just beyond the surfave of the rotatingglass block menas that as the block rotates it also moves the imagealong with it. The beam of light that is associated with the diamondstop, however, is swept towards edge of the drum at a very fast rate.Much faster than the image of the image plane is moving.

Beyond the central rotating glass block the image forminglight rays encounter a collimating relay lens located 1 focal lengthfrom the moving image near the surface of the glass block. From herethe image forming rays leave this lens as a parallel bundle and proceedthrough the diamond stop until they encounter a lens which brings theserays to a focus at the film plane located 1 focal length away. Becausethe image just beyond the rotating mirror is moving the final image isalso moving. It moves in the opposite direction that the image locatednear the mirror does because the optics reverse the direction of motionof the image forming rays.

On the way to this last lens the light beam associated withthe first diamond stop is brought to a focus by the relay collimatinglens, located 1 focal length of the lens away from the 2nd diamondstop, at the location of the 2nd diamond stop. Here an image of thefirst stop moves across the physical obstruction intoduced by the 2nddiamond stop. The image moves relatively quickly and this gives rise tothe possibility of shuttering the system very effectively. The systemtransmits the maximum amount of light when the image of the firstdiamond stop exactly matches the opening of the 2nd one. Any mismatchand the light level drops. Since the image of the first stop moves veryrapidly the camera has the capability of reaching very short exposuretimes.

Shapes other than diamonds can be used for shuttering but thediamond shape provides the shortest exposure times when these aremeasured from 1/2 peak transmission to 1/2 peak transmission of thesystem.

The principle of operation of image motion compensationand shuttering in cameras such as the Dynafax is referred to, afterits inventor DavidMiller, as the Miller Principle.

Diagrammaticlayout of the Cordin 350 Dynafax rotating mirror and drum high speedcamera.1 Objective lens, 2 Mask,3 Field lens, 4 Cappingshutter, 5 Entrance stop, 6 Entrancerelay mirror, 7 Entrance relay lens,8 Rotating hexagonal mirror, 9 and 9'Collimating relay lenses, 10 and 10' Exit stops, 11 and 11' Imagingrelaylenses,12 and 12' Exit relay mirrors,13 Film

To go further then, as stated above the Dynafax only holdsenough film for 224 separate full-frame16mm pictures but can achieve recording rates ofup to about 35,000 pictures per second at exposure times for each frameofless than a microsecond. Such a relationship obviously means that thecamerais out of film in less than 1/100 second when running at full speed butthe framing rate is uniform along the film and generally no timinglightsare needed. One simply needs to make a record of the framing rate thecamerawas running at during the recording. This is displayed by a simpletachometerreadout.

Unlike most other cameras, drum-type cameras generallydo not need any sort of synchronization scheme between the camera andtheevent as long as the event is self-luminous. It is said that they are"alwaysalert". This is a result of the camera running the same film past theimagegate over and over at the desired framing rate. The drum is simplybroughtup to the desired speed and when an event happens it turns on a brightlight and the event gets recorded by the film. One only needs to closea shutter (or use a light that lasts less than one revolution of thedrum)before the drum has made a complete turn to prevent multiple exposures.If the built-in shutter (called a "capping shutter") is used it needsto be set to a time shorter than one revolution of the drum and in thiscase the camera needs to be synchronized to the event somehow.

The camera's framing rate, and thus time magnificationcapability,is high but the images captured by the cameras can generally not beconvenientlyviewed with a projector. They are, instead, viewed as a series of stillimages or are "animated" by duplication onto standard motion picturestock. The Dynafax is manufactured by the Cordin Corporation, (SaltLake City, UT).

In this application the Dynafax camera is used to determinethevelocity of a .22 caliber high speed long rifle bullet. For thisapplication the camera is fitted with a 20x shutter or diamond stopand to make data reduction calculation convenient the framing ratechosen is 20,000 pictures per second. This makes the time betweenframes on the final record 1/20,000 second.

Because the camera provides an aperture of f/10 with anexposure time of 1/20,000 x 20 or 1/400,000 second per frame atransilluminationlighting scheme is used. In this case this is a single mirror schlierenoptical system. The mirror provides a convenient scale as it is 12inches in diameter. Lacking this, some other scale would have to beincluded in the photograph in the plane of the bullet's flight path sothat eventual data reduction becomes possible.

The general layout of the experiment is shown in the twofigures above.To prevent rewrite or multiple exposure of the film in case the drumturns more than once during the event a flash light source is used thatlasts lest than one revolution of the drum. This light lasts only about1/500 of a second and since at 20,000 pps the drum takes 1/90 second tomake one revolution that ensures that there will be no multipleexposures of the film to the light of the flash.

The drawback to using a flash as a light source if the factthe light level will vary considerably from the beginning of theexposure to the end but this is acceptable for this application. Infact, the resulting images should also give an idea as to what theduration and performance of the output of the flash.

In practice the flash is synchronized with the passage of thebullet in front of the mirror by the use of a sound synchronizer placeda foot or so on the side of the schlieren mirror towards the gun. Thiscauses the flash to turn on just before the bullet arrives at the edgeof the mirror and since the light lasts for about 1/500 second thisshould be ample time for the bullet to traverse the mirror while thelight is "on".

This is the product of theproject. As shown at the left theDynafax camera makes two rows of images along the length of the film(here reduced to a length shorter than the whole length that wasloaded in the camera). The two rows are offset by 14 frames which isdue to the fact that the rotating multifaceted mirror imprints each rowin an alternating basis and when the mirror has turned 11.25 degreescausing an image beam displacement of 22.5 degrees.

On the right there is an enlarged section of the resultingfilm. The sequence has been shortened to the time during which thebullet passed in front of the mirror. The images have been adjusted fora uniform look.

The sequences have also been brought so they are displayedside by side and only offset by 1/2 frame so they can be read,time wise, as 1,2,3,4 when alternating from left row to right.

Data analysis for this event then starts by identifying thefirst image that will serve as the zero or "base" time marker. Weassume that this is the 2nd image on the right. That one almost has thebullet visible. The next image in the series would be the 2nd one onthe left and it was recorded 1/20,000 second after the one we assumedto be time=0.

Now we count images where the bullet clearly can be seenmoving from right to left until we reach the mirror where the bulletseems to have reached the same position as it had int the frame wecalled "0" and that would be the second one from the bottom on the leftrow, the total number of images involved there fore is 14. Since thefirst image is our reference image it does not count and therefore wededuce that the bullet traveled a distance of 1 foot (the diameter ofthe mirror) in 13/20,000 second. Velocity being determined from therelationship of change in distance divided by the change in time, thevelocity of the bullet is 1 foot in 13/20,000 second or 1,540 feet persecond.

If we had estimated that the traverse duration was off by justa single image, so 15 instead of 14, then the determination of thebullet's velocity would have yielded a result of 1428 feet per second.

Another approach would be to divide the width of the mirror in12 parts. This would provide the distance across the mirror for 1 inch.Then we measure the distance that the bullet moved between twoconsecutive images based on the known dimension for 1 inch. Thatdistance divided by 20,000 is the velocity of the bullet in inches persecond and this divided by 12 the velocity in feet per second.

In this case the image of the mirror was made to be 2 inchesonan enlarged print from the negative making 1 inch equal .1643 inches.This scaled inch can then be used to make measurements directly on theprint.

Between the top two consecutive images on the print the bulletmoved from a point located .861inches from the left edge of the firstprint to  .722 inches away from it. Thus it moved .139 inches onthe print. But 1 real inch equals .164 inches on the print so thatmakes the displacement .139 divided by .164 or  ..847 real inches.So, ,847 in 1/20,000 second is 16951 inches per second or 1412 feet persecond.

The discrepancy between the two approaches can be due toerrors in judgement as to where the image of the bullet actually is inthe sequence or to slight errors in measurement between locations ofthe bullet in consecutive pictures.

It may be more accurate to measure the distance traveled overseveral mirrors instead of just two. So let us pick 8 of them. In thefirst mirror where the bullet is clearly seen it is 1.861 inches fromthe left edge of the mirror. In the 8th image it is .813 inches fromthe same edge so it traveled 1.048 inches. But 1 inch is .164 scaledinches so it went 1.048 / .164 or 6.39 real inches over 8 mirrors. 8mirrors minus 1 (the reference mirror) is 7/20,000 seconds and that is1/2857 seconds. The velocity then is 6 inches divided by 1/2857 secondor 17142 inches per second or 1428 feet per second.

All this goes to show that small errors in locating theposition of an image in a photographs for measurement purposes can leadto errors or discrepancies in results depending on various factors.

By assuming that the location of the bullet is exactly thewidth of the mirror in the when counting whole frames errors can creepin if the assumption is not valid. In the case of measuring velocityfrom frame to frame small errors in identifying the position of thebullet in consecutive frames also leads to possibly erroneous results.It is best to measure over a larger distance then over a small one.

As mentioned above, sequences of images made by the Dynafax donot lend themselves readily for projection and analysis in with amotion picture projector but selected frames can be rephotographed andassembled into an animated sequence. This is shown in the example thatyou can see by clicking onANIMATIONwhere a supersonic bullet flies across a schlieren field mirror.

Hopefully you found this project report of interest. TheDyanfax istruly an image engineering marvel. Especially of high framing rate andgood action stopping ability are important. Its drawbacks are poorlight transmission capability and some loss of resolution. Although notof major importance it is cheap to operate and it provides a consistentrecording rate throughout its run eliminating the need for timinglights to keep track of recording rate.