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TIME-MILLISECONDS
Fig. 1. Curve showing flame front
travel in train of unwrapped black
powder pellets.
line had existed through the opening and the condition of the powder dust typical of the operation was simulated as nearly as possible.
A time-displacement curve of the powder flame front is shown in Fig. 1.
Next, the assistance of high-speed photography was requested in connection with one of the Company's textile fiber packaging operations. To obtain the type of information needed, it was necessary to develop some method of frame-by-frame analysis from which time-displacement curves could be plotted for the elements under investigation. A Bell & Howell Filmosound projector was purchased and fitted with suitable handcrank and frame counter. A heat-reducing filter of sufficient cooling capacity permitted the examination of any single frame for an unlimited period. A projector so equipped can now be purchased as a standard item. This unit proved invaluable in the textile studies, all measurements being made directly from the projected images. A vertical drawing board equipped with a drafting machine was used as a projection screen because of the large number of measurements required. It was found that this arrangement facilitated the measurement of angular rotation as well as linear dimensions, the straight edge
being laid tangent to a line on the image and the angle read directly and accurately, regardless of change in position of the rotating member in the field from frame to frame.
One problem which became evident as the use of the high-speed camera was extended, particularly in the textile field and in the study of sprayed materials, was the need for improved time resolution. The Fastax camera as manufactured has an exposure ratio of 1 to 3; that is, at 5,000 frames/sec the exposure duration is 1/15,000 sec, or 66 jusec. In photographing filament action in many yarn-handling operations, calculations showed transverse filament velocity in terms of filament diameter to be high enough so that blurring of the image of the filaments would occur during the 66-jusec exposure. Test runs corrobated these conclusions. Since the rear opening in the camera's aperture box (the stationary box around the rotating prism) serves as the optical equivalent of a focal-plane shutter, it appeared that by varying the vertical height of the opening, the exposure duration could be adjusted independent of the frame rate. Three additional aperture boxes were fabricated, one having a rear aperture one half the normal height; one, one-quarter the normal height; and one having an opening 0.010 in. wide. Using these, exposure durations of 33, 17, and 4 jusec, respectively, were realized at 5,000 frames/sec with some loss in frame height. In order to maintain full frame height, it was necessary to widen the front opening of each aperture box slightly.
A marked reduction in motionproduced blurring was accomplished (Fig. 2); however, the time distortion common to all focal-plane shutter cameras was noted. Of course, a proportional increase in light intensity was necessary.
In order to effect the split-second synchronization of light, camera and
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November 1953 Journal of the SMPTE Vol. 61