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human eye (the ability to remember and image) and its response to bright
light to give an apparent stop motion image. Imagine a shaft rotating at 6000
RPM or one rotation every 1/100 of a second (10 msec). If the strobe flashes
once every 10 msec for a brief moment, the user sees the flash at the same
spot in the rotation of the shaft and the persistence of the eye remembers this
until the next flash making the shaft appeared to be stopped. As the target is
rotating there is some movement evident during the strobe flash. The longer
the flash duration, the more obvious the rotation is and this increases the blur.
5.1 Calculating Blur
Blur can be calculated – if the shaft takes 10 msec to complete one
revolution and the strobe flash duration is 100 µsec (1/100 of a millisecond),
the shaft will turn:
(flash duration/time per rotation) x 360°, which is (.0001/.01) x 360 = 3.6°.
So you will see the shaft move 3.6°. As the flash pulse widens you will
see greater degrees of rotation which results in more blur and a brighter
perceived illumination (the LEDs are on longer so the average light the
eyes see is greater). The trade-off is blur versus brightness. One also has to
take into account tangential velocity (rotational speed) – the further away
the rotating point is from the center axis the faster the tangential velocity
and the worse the blur appears to be – it is always the same number of
degrees of rotation but the physical length of the blur gets bigger as the
point moves faster. The strobe adjusts the width of the pulse automatically
to keep the degree of rotation visible constant.
There are two methods of adjusting the flash pulse width and hence the
brightness and consequently the blur.
5.2 Brightness in Degrees of Rotation
The first method is to adjust the flash pulse width for degree of rotation
visible (blur). The user can set this from 0.2 to 14 degrees out of 360. The
higher the setting the brighter the strobe appears to be but the more blurred