– 10 –
10.0 SLOWING DOWN MOTION
As discussed, the primary use of the Pocket-Strobe is to slow down or “freeze”
the apparent motion of moving objects. This allows you to analyze their
run-time performances safely and easily.
To make an object appear to move in slow motion, you need to strobe it at a rate
slightly above or slightly below its actual speed (or any harmonic of its speed as
discussed below). Simply use the COARSE/FINE ADJUSTMENT knob until you
achieve the desired apparent movement.
Helpful Hints:
The speed at which the object appears to move can be determined by subtracting the
flash rate from the object’s actual rate.
Example:
If an object is rotating at 1,000 RPM and you strobe it at a rate of
1,005 flashes per minute (FPM), the object will appear to be moving
at a rate of 5 RPM.
Speed
= Actual Rate minus Flash Rate
= 1,000 –1,005 = 5
=
5 RPM
The direction (clockwise vs. counterclockwise or forward vs. backward) at which the
object appears to move is determined by the flash rate, the object’s actual direction of
movement and the orientation of the stroboscopic beam to the object.
Example:
Assume you wish to visibly slow down the movement of a fan which is
rotating clockwise at 1,000 RPM.
Case 1:
If you stand in front of it and strobe it at a rate of 1,005 flashes per
minute (FPM), the object will appear to be moving at a rate of 5 RPM
in a counterclockwise direction.
Case 2:
If you stand in front of it and strobe it at a rate of 995 FPM, it will
appear to move at a rate of 5 RPM in a clockwise direction.
Case 3:
If you stand behind it and strobe it at a rate of 1,005 FPM, it will appear
to move in a clockwise direction at a rate of 5 RPM.
Case 4:
If you stand behind it and strobe it at a rate of 995 FPM, it will appear
to move in a counterclockwise direction at a rate of 5 RPM.
NOTE:
Typically, stroboscopes are brightest (and can therefore illuminate an
object the best) when the flash rate is between 2,000 and 6,000 FPM.
Often, you can still make an object appear to be frozen or moving in
slow motion within this range because of the effects of harmonics.
This principle is explained section 10.
– 11 –
11.0 HARMONICS
If you continuously increase the flash rate while strobing an object, it may appear to
freeze, slow down, speed up, go forward, freeze again, go backwards, form multiple
images, etc. These images appear at mathematically determined multiples or
harmonics of the object’s actual speed.
Example:
Assume you wish to slow the motion of the fan used in the last example,
but you want it to be brighter.
Technique:
Starting from 1,000 FPM, slowly increase the flash rate. At 1,500 FPM
the image will appear to freeze again. Continue to increase the rate.
The image will appear to freeze again at 3,000 FPM. At this rate, the fan
appears to be very bright. You can now use the FINE ADJUSTMENT
knob to vary the rate above and below’3,000 to make the fan appear to
move both clockwise and counterclockwise.
Helpful Hint:
■
Harmonic images appear at both whole number multiples as well as fractional
intervals of the object’s actual rate. For example, a fan rotating at 1,000 RPM will
appear to be frozen at the whole number multiples of 2,000 (2x), 3,000 (3x),
4,000 (4x) etc., as well as at the fractional rates of 500 (1/2x), 750 (3/4x),
833 (5/6x) and 1,500 (1 1/2x), etc.
■
Some of the harmonic images are “singular” in appearance while others
are “multiple.” This becomes important if you want to determine the objects
actual rate as discussed in section 11
