BRUEL & KJAER 4912 Instruction Manual page 13

order to maximise the strobed patch brightness. Light falling on the patch from other di-
rections will not be reflected into the eye.
The large diameter of the INTERNAL GENERATOR wheel enables the 4912 to be oper-
ated with one hand, when it is used in this mode. The wheel may be set with the thumb
of the hand holding the handle.
The direction of rotation or linear motion of objects may be determined by offsetting the
INTERNAL GENERATOR frequency slightly from a position of synchronism. If the wheel
is off-set anticlockwise (lower flash rate), the direction of apparent motion is the same
as the direction of actual motion.
The frequency range of the internal generator is 5 to 1 25 Hz (flashes per second), corre-
sponding to 300 to 7500 revolutions per minute. Objects having higher motion frequen-
cies may be observed readily; more than one cycle of motion will then take place be-
tween flashes.
If the highest stationary single image is obtained with a flash frequency of more than
62,5 Hz (3750 r/min), it is possible that the motion frequency is higher than this. It
could be two, three or more times the flash frequency. To find out whether this is so,
and if it is, determine the true motion frequency, proceed as follows:
1. Record the highest flash frequency (in Hz) at which a single stationary image is ob-
tained.
2. Continue turning the INTERNAL GENERATOR wheel anti-clockwise, reducing the fre-
quency until another single stationary imagine is obtained (if none is obtained when
the flash frequency has fallen to half the frequency recorded in step 1, then the true
motion frequency is the same as the flash frequency recorded in step 1 ).
3. If another single stationary image is obtained, record the flash frequency (in Hz) at
which it occurs.
4. Locate the highest flash frequency (from step 1) on the left-hand scale in Fig.3.2.
5. Locate the second-highest flash frequency (from step 3) on the centre scale in
Fig.3.2.
6. Lay a straight edge across Fig.3.2 so that it passes through the two points located
in steps 4 and 5.
7. The true motion frequency may be read off the right-hand scale, in Hz or r /min, at
the point where the straight edge intercepts the scale.
Example
Highest flash frequency
=
75 Hz (left-hand scale)
Second-highest frequency
=
50 Hz (centre scale)
Follow broken line to right-hand scale
True motion frequency
=
1 50 Hz
=
9000 r /min.
Fig.3.2 is obtained from the equation:
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