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4.5.1. Motion during exposure time
During the exposure time of a flash the vocal cords move, often at high speed, and it is
important that the exposure is finished before the motion is observable. Through experi-
ence with ultra-high-speed photography (Ref. 3) it is recommended that the vocal folds
should be filmed at a frame rate 20 to 30 times the fundamental frequency of phonation.
This points to an ideal exposure time of
1 /30
of a phonation period. As seen from Fig.
4.3. the half intensity duration of the flash pulse is max. 80 µs which equals
1 /30
of a
period at 475 Hz. This amply covers the fundamental frequencies of male and female
subjects as well as those of children.
The half intensity duration of the normal stroboscopic flashes is approx. 5µs (see Fig.
4.4).
~
·en
C:
Q)
E
Flash pulse
durati
on
(µs)
Fig.
4.3. Flash light output waveform
5
Flash pulse duration
(µs)
Fig. 4.4. Stroboscopic light waveform
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