Handbook of High Speed Photography - IET Labs, Inc.

Figure 1-2. A .22-calibre rifle bullet
photographed in flight. The faint gray
streak extending in front of the
bullet along its path is exposure
resulting from flash-larnp afterglow.
A single, high-intensity flash of the
Strobotac was triggered with a micro
phone to stop the projectile's 1100-
feet-per-second velocity. (Polaroid
Type 47 film, ASA 3000, 3-microsecond
exposure at f/22. The reflector
was removed from the lamp.)
Suppose, for example, a photograph is required of a .22-calibre
rifle bullet in flight. The bullet travels about 1100 feet per second.
During the 1-millisecond exposure produced by a mechanical shutter
the bullet travels about 13 inches. During the 80-microsecond exposure
of a high-speed movie camera, it moves about one inch, still too far
for good photographic resolution. Needed for this and similar applica
tions is an exposure time of a few microseconds. In 3.0 microseconds,
for instance, the bullet travels only 0.04 inch (see Figure 1-2). An ex
cellent example of what is possible using a single-flash of a few micro
seconds duration is the series of photographs of a milk drop splashing
on a hard surface shown in Figure 1-3. For other examples, refer to
the photo section near the end of this book. For explanation of the
methods used to produce such pictures refer to Sections 4 and 5.
1.2 MULTIPLE-EXPOSURE MOTION STUDIES WITH A STROBO-
TACHMETER.
Often the study of the relationship between events occuring in
rapid succession calls for a means of making a series of high-speed
photographs in a very short time. Usually a series of single flash
photographs is prohibitively difficult to obtain or will not really show
what is happening. A method which is easy and which results in clear
representation of many high-speed actions is the multiple-flash expo
sure of a single film frame.
For example, suppose an engineer wants to determine the angular
velocity and acceleration of a rotating shaft so that he can calculate
the transient characteristics of the load being driven by the shaft.
This data would be easy to collect if the engineer had a source of
short-duration light flashes (short enough to stop the motion of the
shaft) that would repeat at equally spaced and known time intervals.
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