The Stroboscopic Effect, as mentioned earlier, is another category

of TLA (Temporary Light Artifact). But contrary to the insidious

nature of Flicker, Stroboscopic Effect can consequently bring

shocking, catastrophic outcomes.

You may have seen a similar phenomenon in the movies.

It's

called the "wagon wheel effect" - as the wagon speeds along, the

spokes of the wheel appear to stop or reverse directions.

Heli

copter blades in movies also exhibit this in movies. This is NOT

categorized as a TLA, but the mechanisms are the same.

If you're still unclear, please see the video below - you can see the

wagon wheel effect at the 10 second mark.

The "wagon wheel effect" is where the spokes of wheel appear to

stop or even reverse directions.

A video camera does not record

a continuous stream of motion, but rapidly takes a series of pic-

tures (or frames) at intervals of usually 24/30 frames per second

(in older ﬁlms) -

when these frames are displayed rapidly one

after the other, they appear as one continuous stream, like a ﬂip

page animation.

But

because the camera captures only 24/30 frames per second, it cannot capture all

the ﬁne degrees of spoke rotation around the center.

So when the camera snaps consecutive frames,

it sometimes captures the spokes at the same locations every time (even though it might be a different

spoke).

Please see the short clip below:

In this clip (above link), the box on the left represents what is happening in the real world (clock with sec-

ond hand rotating).

In the second box, you can imagine the camera snapping a frame every second, indi-

cated by when the box background lights up white.

But you'll notice that it takes a snapshot only when the

hand is at 12 o'clock.

This means that when you string all

the pictures together, the second hand will appear stationary

in that single position.

Now imagine that camera is replaced by your eye and the

white background ﬂash indicates a one ﬂash per second

ﬂickering light shining on the clock - what you will see is

only the second-hand at the 12 o'clock position, because the

ﬂash of light is synced with the rotating second-hand as it

lands on the 12 o'clock position.

But it's a little more complicated than that.

You have to

consider many factors before you get stroboscopic effect

with ﬂickering light (e.g. light intensity, duty cycle, modu

lation depth, frequency, how your eyes takes in information

etc.).

And now here's the point of the story.

Let's imagine ourselves in a noisy factory.

Their is a circu-

lar saw on one side of the table.

Out of the corner of your

eye, you reach for the ruler near the circular saw, as you

some busy work in front of you.

It appears, in your periph-

eral vision, that the saw teeth are safely stationary, but in

fact they are spinning at deadly speed.

Get it? Shocking?

The Stroboscopic Effect

Basically, the saw teeth appear stationary because of ﬂickering overhead factory lights

(wagon wheel effect), and therefore your digits are in extreme danger.

https://www°youtube°com/watch?v=6XwgbHjRo30

https://en°wikipedia°org/wiki/Stroboscopic_effect#/media/File:Strobe_2°gif

John Vetterli CC-SA 2°0

The Flicker Handbook