What sort of light source to use a strobe disc?

In a light hearted discussion on a woodworking forum what sorts of a light source do you need to use a stobe disc or indeed not to use near rotating machinery?

one chap ( a very respected member) insists a standard filament lamp will suffice?

I say anything other than a filament lamp....

Tube if it's a 50Hz disc

What sort of illuminators were used in record turntables?

Mains Neons.

Well this guy insists he uses a desk lamp with a filament lamp in...has he got superhuman vision so he see a bulb cooling and heating 50 times a second?

and also in factories he wired filament bulbs across the 3 phase to avoid strobing...which again I’ve never heard of..other than to balance the load... fluorescents yes but not GLS bulbs

Edited By Ian Parkin on 24/09/2020 17:52:40

You do get some variation in intensity at 100 hz with a filament lamp, which may be enough, but a neon is better. Or these days an led but without smoothing. If you had 3 filament lamps one on each phase that would have much lower total intensity variation, but not a single bulb between 2 phases.

LEDs will work, depending on how bright or how large an area needs illuminating. But I've use printed strobe disks to check the speed of my mill against multiples of the 50Hz mains.

Bill

Despite all the scepticism, back in the '70s my Dad had printed card discs that were used to check the speed of record turntables.

They worked under a filament lamp but were best with some fluorescents - depending on the persistence of the phosphor I guess.

It's nothing to do with superhuman vision. Persistence of vision actually helps the effect.

Neil

And this graph from Wikipedia gives an explanation:

Basically at ~100Hz you need only a very low depth of modulation between bright and dark(er) for the effect to be visible.

More at: en.wikipedia.org/wiki/Stroboscopic_effect

Edited By Neil Wyatt on 24/09/2020 21:32:16

+1 for John's suggestion thermal inertia of filament lamps is to great for the eye to detect, neon lamps are the best but suffer from intensity, xenon tubes are better as used in industrial strobe units.

Gren.

I built a light beam communicator some 55 years ago. The DC of a torch bulb was modulated with an audio signal. I used a microphone and small power amp for this. The torch beam was demodulated by using a cadmium sulphide cell and amplifier. With good torch optics, ranges of several hundred yards were possible and speech was very intelligible. I have no difficulty in believing that light output from a GLS lamp is modulated by mains frequency and that this can be detected, without much difficulty.

Andrew.

I'll go with that. You aren't trying to see the flicker of the bulb but the contrast created on the disk. Eyes are logarithmic so lower intensity will help. Human retinas will respond to approx 4 photons minimum and frogs can do single photons so I am told. It's not enough to generate a neural response at that level but the molecular machinary is active down to that sort of photon count. As Niel says the pigments take a little while to switch off again so there is some integration of coherent images (static black lines on your disc) which would increase the contrast. Interesting thoughts.

regards Martin

You can get apps for smartphones that act as a variable frequency strobe light, so you can do away with the strobe discs and separate light source completely if you wish.

I've been using this one on an iphone that gives an RPM equivalent

https://apps.apple.com/us/app/strobe-light-tachometer/id978182913

Of course anyone who still has a filament bulb handy can do the test for themselves...

I just looked up 'blub smash slow mo' on you tube and in the first example you can clearly see the reflection of the filament (the filament itself is over exposed) pulsing at mains frequency. It doesn't go out but dims noticeably.

Neil

I sometimes notice 50Hz flicker on old bulbs out of the corner of my eye. The effect is more obvious on 40W bulbs than 100W bulbs.

I've read somewhere than machine lamps are run at low voltage partly for safety reasons and partly because thick high current filaments in low voltage lamps cool down more slowly than the thin filaments in high voltage bulbs of the same wattage.

Day full of appointments but whilst in my Sunday best waiting for the next one I programmed an Arduino to flash a yellow LED. (I believe these don't have any phosphor, just light off the junction.) I can just see a trace of flicker with 2mS pulses (500Hz). Distinct flicker at 150Hz and no doubt at all at 50Hz. Anything below 25Hz is sick making annoying until individual flashes appear at about 5Hz. If it has to flash, 1Hz is bearable!

Whoops, time to hit the road again!

Dave

Here’s a nice strobe disc to play with:

**LINK**

https://soundsclassic.com/pix/33-78.pdf

MichaelG.