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6.0 STROBE BRIGHTNESS
The strobe’s brightness depends on how wide the strobe’s fl ash pulse is; the
wider the pulse, the brighter the fl ash from the LEDs appears to be. There is
however, a downside to the wider pulses. All strobes work by giving short
bursts of light (the pulse width) at a rapid repetition rate (the fl ash rate).
Strobes rely on the persistence of the 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 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.
6.1 Calculating Blur
Blur can be calculated– if the shaft is turning at 6000 RPM, it takes 10
msec to complete one revolution. If 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 appear to 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. The further away the rotating point is from
the center axis the faster the tangential velocity and the worse the blur
appears to be.
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