WP's HP: HOME | SloMo: Links ~ FAQ ~ Info ~ Tips ~ Data ~ Clips ~ HYCAM ~ [Freq.] ~ f-Calculator ~ Trigger ~ Image RAM | ?Help? | Deutsch

WP's Scanning Frequency (Nyquist-Shannon-Kotelnikov Theorem)


Why illumination flickers

Effective or nominal mains voltage of 115 V or 230 V alternates 50 or 60 times per second between the square root 2 higher peaks ±163 V and ±325 V, resp. Illumination devices directly driven by AC mains are pumping more or less with this mains frequency of 50 Hertz or 60 Hertz, resp. (1 Hz (Hertz) = 1/sec). Usually a light bulb is too sluggish to show these changes. And of course, our eyes are too slow to perceive these variations. Also a video camera usually does not take notice of these flickers.
Even so-called DC lamps, where electronics rectify or smoothen the ac voltage, can show a certain ripple or oscillation of higher frequencies a movie or a video camera does not note, but a high-speed camera of sufficient frame rate does.

Nyquist-Shannon-Kotelnikov theorem
Sequence and flickering illumination

That's the cause for a frequent fault report: »The camera sequence flickers and jitters!«.

This is, however, not a fault of the high-speed camera. The reason is connected with the so-called Nyquist-Shannon-Kotelnikov theorem of sampling, refer to the figure on the left. If one measures a changing quantity with at least double of its frequency, one will be able to reconstruct its curve shape.

In the figure on the left the video camera runs almost with half the frequency of the ripple of the illumination source, whereas the high-speed camera is factor four to five faster again. The gray areas give the amount of light per frame. The white areas show the read-out time or inactive phases.
It is clearly to see how ups and downs of the intensity compensate themselves more or less, the video camera is integrating over them. Usually this is independent of the phase shift.
The frames of the high-speed camera, however, are exposed with varying values. The sequence will flicker, when replayed in slow motion. Something one can often notice in slow motion sequences of sports event broadcast on TV.

Only if one (strictly) reduces the time of exposure of the video camera, narrower gray and wider white areas in the figure above, one will be able to make it flicker as well. With clever synchronized frequencies, however, it may not necessarily happen.

Using a frame rate of some 100 frames/sec one is able to shoot the pumping of fluorescent tubes. Therefore this kind of illumination is not very suitable for high-speed cameras.
Merely lamps driven by a battery (DC voltage) are safe from high-speed cameras. But usually halogen lamps with 50 or 60 Hz due to their inertness are sufficient for usage in technological set-ups.


Coach wheel optical illusion

Effect well-known - the coach moves forward, its wheels turn backward - is a consequence of the sampling theorem as well. Between two succeeding movie images the wheel rotates by slightly less than one full turn. Or by some full turns whereas the last turn is not completely finished. That is a special case of the stroboscope effect where one tries to capture a still image of a rotating object by shooting integer multiples of a single turn. One's brain interprets these images then in the wrong way as pseud movement or just as no movement.
You can perceive the same effect with rotating propellers or helicopter rotors.


Jutter replay

Except for technical defects, there are two causes. At first the replay rate can be so low that a fluent sequence is not given. One must offer ones eyes 14 frames/sec at least in order to prevent them from visualizing the single frames and to keep up the illusion.
On the other hand a very short shutter time (time of exposure; in each frame x shown in the figure above just one single small gray column) compared with frame rate or movement, resp. can cause the movement appearing choppy because too much location change happens from frame to frame.




©WP (1998 -) 2012
Update: V8.4, 2012-03-02