On today’s column, Cecil discusses the possibility of a strobe effect from fluorescent lighting. Way back in those distant days we played music from something called a phonograph record on a machine called, among other things, a turntable. A friend of mine had a paper disk that had a bunch of radial “spokes” and you adjusted the speed of your turntable by playing this disk under a fluorescent light and adjusted the speed until the spokes appeared to stop rotating.
Another way to see the strobe effect is to spin a coin (a quarter works well) under a fluorescent light.
neon bulbs might be part of a turntable with markings on the side of the platter to gauge speed.
you might think that a fluorescent lamp shouldn’t work because of the light being produced by excitation of the phosphor by the gas vapor; the phosphor being very short persistence makes it happen.
I understand strobe effects with lights that flicker as their nature. I even remember hearing from my high school physics teacher that some factories specifically banned fluorescent lighting (due to it’s nature to flicker and the stroboscopic effect, certain machines might appear as stopped, and someone could be injured). I’ve always wondered about the car wheels example though… I’ve seen the effect in direct sunlight. Since that light source is definitely non-strobing, what is causing the stroboscopic effects on the spokes?? I can’t imagine that a little light reflected (from the bug cap, whatever) would be enough to overpower direct sunlight enough to make the effect visible. What gives?
In the case of my turntable (yes, I have a turntable; I still have dozens of vinyl albums that I have no intention of rebuying in a more “modern” format), there is a neon-bulb-illuminated hash mark array (for different nominal RPM rates and for 50hz and 60hz power) which is used to tweak motor speed up and down to get that perfect 33 1/3 RPM.
I don’t get why this effect is impossible in natural light.
There is a concept of “frames” in the brain; there are several places in the retina and the visual cortex where complete images are formed / collated. And there is a finite limit on how frequently this can occur.
Indeed the fact that we can comfortably watch TV is because our brains can interpolate frames and this interpolation exists because it’s necessary for us to have smooth visual normally.
there’s something called ‘persistance of vision’ - which is basically the time it takes your rods/cones to ‘recharge’ between firings. this makes 24 fps tv/movies appear to be a continuous image because the images “bleed” over to the next in our brain. this should have nothing to do with seeing strobe effects in sunlight.
The brain can indeed interpolate motion, and it was this that I was thinking was why 24 fps looks smooth.
But if it’s down to persistence of vision that would explain why it is not vulnerable to strobe effects. And how it is that we can know TV images would not appear continuous to many birds (presumably their rods/cones have a faster recharge time).
The whole thing about turntables comes up because Cecil mentions that strobe effects are rarely seen “in real life”, i.e., other than in movies. Turntables used to have strobe markers on their edges. Here are some pictures.
I think #3 is talking about using human eyes in natural light, no camera involved.
One factor that might be at play is the nature of of the reflection from each spoke - that is, as the wheel rotates, parts rotate into positions where their surfaces are just right to reflect the (more or less) point source illumination directly into our eyes, then they rotate out of that position and a new spoke rotates in - creating a strobe effect in itself.
Another one is when the shadow of a pierced/spoked rotating wheel is viewed through the gaps between the spokes themselves - there’s a moire/interference effect similar to the effect exploited by a mechanical zoetrope.
another way to can experience this effect is to place an electric fan 1-2 meters away from a television screen. you will see an outline of the fan that seems to be slowing down and then rotating in reverse. never knew it was called strobe effect before i read the column though.
That will only work for CRT TVs which display pictures sequentially and only a small part of the screen is on at one time; LCD TVs are always “on”. This also applies to fluorescents on electronic ballasts, which are driven at a high frequency, 10s of kHz (some might be modulated at the line frequency (actually twice due to rectification) but the ones I have seen have filter capacitors). On the other hand, cheap LED bulbs that run LEDs directly from AC power would be highly effective since LEDs have virtually no persistence (which is why they flicker so badly, especially those that use only half the AC cycle; regular fluorescents don’t bother me but I can definitely see a LED on 60 Hz half-wave AC flickering).
Also, when phonograph tables were suggested as a demonstration of the stroboscope effect, my first thought was old floppy drives, which had a pattern that enabled you to set their speed.
