"Daniel60" wrote in message
...
They probably are fairly crude. I know they flicker, for example if I
use my cordless drill, the chuck appears to spin the wrong way under
the LED lighting.
I remember seeing that with a washing machine (under fluorescent
lights). As the tub was slowing down, the row of holes around the tub
would appear to reverse direction. Same thing with (spoked) wagon
wheels
in movies.
... and, in real life, the Mag wheels of some cars seem to be spinning
backwards, dependant on the speed at which the car is travelling!!

That's not what I'd call flicker, which is when an object is illuminated
sufficiently infrequently that you can see each pulse of light, even when
there's no motion in the scene. CRT TVs, especially European 625/25 as
opposed to US 525/30, were prone to flicker, especially if you saw one out
of the corner of your eye which seems to be more sensitive to flicker. CRT
computer monitors on 50 Hz progressive flicker more than on 60 Hz, and 72 or
90 Hz are better still. Interlaced scans are smoother for the same refresh
rate, but suffer from "twitter" where alternate lines flicker even though
the picture as a whole looks constant.

What you are describing is a stroboscopic effect of viewing a moving
contrasty object by intermittent light. That can happen at any intermittent
rate, even those which are far faster than the eye perceive as flicker. If
you let your eyes pan across an LED, you will see several images of it for a
fraction of a second - it's noticeable with some car rear lights or some
traffic lights, if there is relative motion (you are driving past the car or
traffic light).

In real life? I assume you mean under streetlighting. That effect can't
occur with a steady lightsource such as the sun.

No, I'm sure I've noticed it in sunlight, as well.

If you are seeing stroboscopic effects under DC light or sunlight, which are
constant not intermittent light level, then there's something very odd going
on.

And my 'real life' was to distinguish from the mentioned "wagon wheels in
movies"!

My dad's old record player had two sets of black and white stripes round the
edge of the turntable, so you could set the correct speed stroboscopically
under 50 Hz or 60 Hz mains light. What was surprising was that it worked
even with filament lights which have a long thermal inertia, so the light
doesn't change instantly from on to off, as with a LED or fluorescent tube
(*), but decays gradually. Despite this, the stripes were still fairly sharp
and not blurred. Of course, any setting of the turntable speed is only as
accurate as the mains frequency at the time, which can vary by up to +/- 0.5
Hz (http://mainsfrequency.uk/fm-home). I think the strobe markings were
calibrated for 33 1/3 rpm, with the assumption that the "gearing" (sprockets
or friction wheels) for the other speeds was exact, so if you calibrated at
one speed, it would be correct by definition at the other speeds.



(*) OK, with a fluorescent you see two images: a bluish one caused by the
very rapid on-to-off transition of the mercury discharge and a yellowish one
caused by the more gradual decay of the phosphor, though this decay is still
a lot quicker than the light from a filament bulb.