Handbook of High Speed Photography - IET Labs, Inc.

1.3 USING THE STROBOSCOPE WITH MOVING FILM.
It is impossible to study some complex motions using the multi
ple-exposure technique. This is especially so when the motion is con
fined to a small area or is characterized by a change of shape rather
than a translational motion, that is, the subject does not "get out of
its own way" to permit the photographing of separate and discreet suc
cessive images. For example, the changing stress patterns in a sample
of material (see Figure 1-6). Studies of this type can be made by means
of a high-speed motion picture camera, such as the Hycam, Fastax,
etc., and a stroboscope. These cameras can move film past the camera
lens at a high rate of speed so that thousands of frames per second can
be exposed. The series of pictures produced by this method will dis
play successive phases of a single cycle of the studied motion.
The stroboscope is also a very useful light source for use with
high-speed movie cameras. The short duration flash of the stroboscope
stops motion that would normally be blurred even when photographed by
the fastest of movie cameras, as would the motion of a high-speed pro
jectile. This improvement is especially advantageous where frame-byframe
analysis of the film must be made. For a more detailed descrip
tion of the use of a stroboscopic light sources with moving-film devices,
refer to Section 6.
The stroboscope can also be used to advantage in photographing
heat-sensitive subjects, e.g., biological specimens. The stroboscope,
due to its extremely low duty cycle or "on" time, is a relatively cool
source of light.