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#143798 - 09/13/05 05:01 AM fluoro lights and screen flicker ?  
kiwi  Offline
Member
Joined: Dec 2004
Posts: 354
christchurch new zealand
Can anyone tell me if fluorescent lights can cause computer screen flickering, ie the "stroboscopic" effect. Do you have to have the lights on different phases in a workplace/office ?

Can fluoro lights with poor electro-magnetic emmission compliance have an effect on adjacent computer screens ?


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#143799 - 09/13/05 06:13 AM Re: fluoro lights and screen flicker ?  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
The short answer is no.
Pauluk is the Electronics guy here, if there is anyone else willing to chime in, do so.
What is the current scan speed of a monitor?.
Kieran,
Are you talking bare tubes or troffer fittings or the "egg-case" fittings?.


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#143800 - 09/13/05 07:11 AM Re: fluoro lights and screen flicker ?  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
Reason I say that kiwi, is because of the distance.
I've never seen until now, a computer monitor affected by a ballast 5-6 foot away on any phase.
What could cause the same effect is a magnetic device on the desk where the monitor is.
Look for anything with an un-shielded magnet in it (like a speaker).
Also look for bright colours on the screen of the monitor itself, that would be your best indication.
It will pull the electron field out from the Cathode ray Tube.
Try it, put a speaker next to your monitor!.


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#143801 - 09/13/05 07:56 PM Re: fluoro lights and screen flicker ?  
aussie240  Offline
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Joined: Jul 2005
Posts: 223
Blue Mountains, NSW, Australia
Scan rates of monitors are typically 60Hz and above. Assuming Windows operating system; start>settings>control panel>display>settings
That will tell what the vertical scan frequency is. The reason why flicker is visible when something electromagnetic running off the mains is near the monitor is because the flicker frequency is the difference between the mains frequency and the monitor scan frequency. So, for a 60Hz monitor, the flicker is 10Hz and therefore very annoying. As the electron beam is responsive to a magentic field, it can also be deflected by external fields as well as those intended from the monitor's own scanning coils.
Incidentally, this is one reason why television vertical scanning frequencies are the same as that of the mains, though no longer actually synchronised. It was purposely done to avoid this problem. The other reason was to minimise the effects of imperfect power supply filtering in the TV sets themselves.
Placing a permanent magnet (eg. speaker magent) near the monitor causes the colours to change as the purity is being upset. Instead of each of the R,G & B electron guns being aimed at their R,G & B phosphors on the screen, the electron beams are shifted onto the wrong colour phosphor. This can also happen if the shadow mask becomes magnetised over time in which case it needs to be degaussed.
It is surprising how affected some monitors are; I've seen even small plugpack transformers say 8" away from a monitor cause a problem. If the transformer, fluoro light choke or whatever can't be relocated, changing the orientation can help as the magnetic field from such devices is directional.
Of course, using an LCD monitor eliminates all these problems.
To get back the the question, Kiwi, I'm wondering if you're talking about an optical effect...ie; the actual light from the fluoro tubes causing an effect. The answer is no because the monitor is generating it's own light. If you were using a mains operated fluoro light as a backlight for an LCD screen, then yes you would see a stroboscopic effect. One reason why the fluorescent tubes used for such purposes run at high frequency (~30-100Khz).


#143802 - 09/14/05 05:45 AM Re: fluoro lights and screen flicker ?  
kiwi  Offline
Member
Joined: Dec 2004
Posts: 354
christchurch new zealand
Trumpy the situation that brought this question up for me is an office where more fluoro lights were added ( batten holder type with a diffuser ) and one computer screen in the office started flickering very slightly but enough to be annoying.

Interestingly it was the oldest computer in the office and no other screen was affected.

There are speakers next to the screen but its not that kind of interference. And it still flickered when the speakers were removed. The screen is about 2m from the nearest light.

Thanks Aussie & Trumpy for your replies. I just had to ask someone else before I tell them to take the screen to an appropriate recycling depot.


#143803 - 09/14/05 12:13 PM Re: fluoro lights and screen flicker ?  
Alan Belson  Offline
Member
Joined: Mar 2005
Posts: 1,803
Mayenne N. France
Years ago my boys and I made a soccer game which used ferrite magnets araldited to sticks to move 'players', ( crude wooden men with a drawing pin base), around the field by manipulating them from under the baseboard. This worked fine till the youngest boy decided to 'see what would happen' if you put the magnet directly on the tv screen. What happens is:
A large multi-colored blob appears, permanently, distorting the image. ( Bad )
Your sons vanish into thin air, and don't ask for money. ( Good )
Mom goes ape. ( Bad: Brand new tv).
Dad solves the problem, ( Good, after realising that the newsgirl hasn't got a fancy new hairstyle), by using another magnet to slowly draw the 'blob' up and out of view at the top of the screen.
Game exits to trashcan. ( Bad, it was fun ).

Alan


[This message has been edited by Alan Belson (edited 09-14-2005).]


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