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V=
100° 1000
1000 • N
160
jooo , in frames/sec. (2)
For example: With 8 frames in i10oo = 5 timing units, a film velocity of 1600 frames/sec results. The corresponding film velocity v, measured in ft/sec, as well as the relationship between v and V, depends on the type of film.
25 • N 1.5-N .
= — , m ft/sec, (3) iiooo leo
with the relationship
V = 40 • v, in frames/sec. (4)
With 8 frames in 5 ms, a film velocity of 40 ft/sec results.
8-Mm Film: v =
12.5 12.5 • N
12.5 • N 0.75 • N
-, in ft/sec, (3')
iicoo
leu
with the relationship
V = 80 • v, in frames/sec. (4')
With 8 frames in 5 ms, a film velocity of 20 ft/sec results.
Frame time and film velocity are related values. They always satisfy relationships (2), (3) and (3'). Both values are also used advantageously for special evaluation tasks, such as determining operating time of a machine part, cyclic rate of a mechanism, etc.
What is the operating time of a machine part, when the former comprises N frames on the film?
The beginning and ending of the operation have been marked and N frames have been counted for this time interval. First, the frame time, ti, has to be determined according to (1), by evaluating a short time interval, consisting of a few timing units in the middle of operating time. Then the operating time, t,
is:
ii ms.
(5)
For example: With the above frame time of 0.625 ms/frame, an operation comprising 80 frames lasts 50 ms. For a long operating time, the frame time, ti, may be taken at the beginning and at the end of operation and the average value thereof may be used for calculations.
What is the cyclic rate of a gun (or other mechanism) when N frames are counted for the cycle time, t? Using frame time ti, the cycle time is given by Eq. (5) and the cyclic rate follows:
60,000 60,000 . n = — — = -j^2— JT, in rpm. (6)
For example: With the above frame time of 0.625 ms/frame and 80 frames per cycle, the result is a cyclic rate of 1200 rpm.
What was the approximate total running time of the film, T, in seconds?
To answer this question, the average film velocity over the total running time, v or V, should be known. Figure 2 illustrates the acceleration characteristic of film run, that is, the increase of film velocity during film run in the camera. The average film velocity, V, is somewhat lower than the maximum film velocity near the end of the film run Vmax. How much lower it is depends on the type of camera and, eventually, on the voltage used. For one type of high-speed motion picture camera the proportion
V v
was found to be approximately 0.8. To have more accurate data available, the acceleration characteristics should be determined for all types of cameras and voltages used. Based on a film length of 100 ft, the total running time then is:
100 -= sec.
(7)
When introducing the film velocity in frames/sec :
June 1951 Journal of the SMPTE Vol.^56