Handbook of High Speed Photography - IET Labs, Inc.

pickup is used). Figure 5-6 illustrates the operation of these pickups.
The strength of the signal generated by these devices depends on the
subject speed and distance from the pickup. There is strong physical
attraction between the pickup and the subject. The widely available
proximity device is best suited to applications where it may be per
manently mounted at an accurately controlled distance from the subject.
An iron patch mounted on a nonferromagnetic rotating shaft such as the
one in Figure 5-6a is an example of this type of application. A variable
reluctance pickup could be mounted near an iron or steel gear, whose
passing teeth will generate a signal. For low speed applications, it
is possible to build satisfactory pickups, such as the one in Figure
5-6b, using parts from sensitive electrical relays. Commercially made
proximity pickups may be obtained fromAirpax Electronics Incorporated,
Seminole Division, Fort Lauderdale, Florida or from Electro Products
Laboratories, Inc., Chicago, Illinois 60648.
IRON PATCH
MAGNET
TRIGGER
SIGNAL
- ROTATING
_A_ PART
TRIGGER SIGNAL
Figure 5-6. Triggering by proximity pickups.
5.2.6 ACOUSTIC TRIGGERING. The sound associated with the event
to be photographed may be detected by a microphone to provide syn
chronization. This type of sensing is reliable for loud noises and easy
to set up, and it does not load or otherwise interfere with the subject.
The separation between the microphone and the sound source may be
adjusted to vary the trigger delay. This delay, approximately 0.9 milli
second per foot of separation, is quite stable and is adjustable over a
very wide range. An amplifier is usually required to increase the micro
phone output to a usable level. (Any amplifier, for instance, a highfidelity
amplifier might work.) The sound of the event must contrast
sharply with any background noise to prevent spurious triggering. To
effect this, the amplifier gain, normally controllable over a wide range,
can be set to a level at which the noise is rejected and only the signal
causes triggering. Figure 5-8 shows a typical acoustic triggering setup.
To minimize the amount of fixed delay introduced into the system
and to achieve maximum delay stability the microphone should be ar
ranged so that positive pressure on the microphone's diaphragm causes
the correct output trigger polarity. The photographer will have to deter-
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