Monarch Instrument Pocket LED Strobe - Manual (Page 8)

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image. Imagine a shaft rotating at 6000 RPM or one rotation every 1/100
of a second (10 msec). If the strobe fl ashes once every 10 msec for a brief
moment, the user sees the fl ash at the same spot in the rotation of the
shaft and the persistence of the eye remembers this until the next fl ash
making the shaft appeared to be stopped. As the target is rotating there
is some movement evident during the strobe fl ash. The longer the fl ash
duration, the more obvious the rotation is and this increases the blur.
This blur can be calculated – if the shaft takes 10 msec to complete
one revolution and the strobe fl ash duration is 100 µsec (1/100 of a
millisecond), the shaft will turn:
(fl ash 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 fl ash 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 fl ash pulse width and hence the
brightness and consequently the blur.
1.2.1 Degree of Rotation Adjustment
The fi rst method is to adjust the fl ash pulse width for degree of
rotation visible (blur). The user can set this from 0.1 to 18 degrees
out of 360. The higher the setting, the brighter the strobe appears
to be but the more blurred the target is. Optimal setting to stop
motion is 1 to 3.6°. The number of degrees is a proportional amount