[eric343]

You need:
A laser pointer
A CRT-based oscilloscope, preferably an old Tek

1. Turn out the lights
2. Use the laser pointer to doodle on the screen by holding it really close and moving slowly
3. Turn off the laser and admire your short-lasting handiwork.
4. Go back to doing something useful, like watching paint dry.

 

[kevin gilmore]

Old tek scope is right.

First go and price a replacement tube for your scope.

Don't dare try this on any analog memory or pmt enhanced
scope the effects will be permanent (24xx especially 2467).

 

[morsel]

I heard someone just tried this with a blue LED flashlight on a computer monitor at a party last week. The host was left with permanent doodles burned in to his brand new monitor.

 

[eric343]

OK, never mind, *don't* try this at home

kevin- Heh, right, if I could find a tube it would cost more than a new scope; my scope is an old plug-in model...

 

[Voodoochile]

Mainframe?

 

[Melchior]

Hrmm, I dont know much about scopes, but it really shouldnt harm any decent computer monitor I've seen in the past 5 years. Maybe the newest ones are using different phosphors or something.

 

[eric343]

Quote:

Originally posted by Voodoochile Mainframe?

Yes. Tek 75something, if I remember correctly.

 

[morsel]

Melchior, you could apply a blue LED to your monitor and report back to us.

Personally, I am inclined to believe the story, as blue LEDs have enough energy to activate many phosphors. Purple/UV LEDs are even more powerful and can cause blindness. Perhaps it was a purple rather than blue LED. The bottom line is that although the tale is rumor and perhaps apocryphal, do you really want to risk your CRT to find out?

 

[eric343]

Oh, BTW, I did this with the 'scope *OFF*.

And I just checked; it's fine...

 

[aos]

Quote:

although the tale is rumor and perhaps apocryphal, do you really want to risk your CRT to find out?

At prices like $100 for a new one? You joking? But of course!

I *AM* joking though... but only because I don't find it amusing.

 

[grinch]

Quote:

Originally posted by morsel The bottom line is that although the tale is rumor and perhaps apocryphal, do you really want to risk your CRT to find out?

hey i heard if you have a drink at an airport before a flight, you'll wake up in a bathtub full of ice hours later with a kidney missing.

anyway, what does this appear to do to the screen anyway? does it leave bright marks on it?

 

[Melchior]

Morsel, I dont recall ever saying the story wasnt true. It's possible that this persons monitor used very poor quality materials. Heres the deal, I speak because I've done this stuff with monitors and TVs before. A florescent blacklight will cause the monitor to glow a muted green color, it you stick it right up to the screen it'll glow for a bit and dim back down after a couple seconds. A Turquoise LED on my monitor doesnt do anything, it doesn't even make a dull glow that a little incandecent pen-light will. A white LED leaves a dull 1 inch diffuse glow that dims to nothing after about 2 seconds. As I said, modern phosphors in computer monitors are supposed to be highly resistant to burning in that fashion. Now, the according to this spectrum graph http://ledmuseum.home.att.net/ledblu2.htm the output of blue leds in the UV bandwidth is similar to any other color led, such as white, green, turquoise, etc. Heres a spectrum graph of the green, http://ledmuseum.home.att.net/ledgrn.htm I have one, it doesnt do anything to the monitor either. . I'll be sure to try this with a high output gallium nitride blue led as soon as possible anyways, you've managed to pique my curiosity. UV LEDs are a different beast, their power output in the UV band is far greater than a florescent blacklight and can easily cause permanent eye damage if you stare at one. Damaging a monitor with one of those wouldn't be surprising, they don't have a visible output that looks anything like a blue led however.

Caveat: Scopes are another matter entirely, and at the price of a good one I cant really recommend doing something that might potentially damage a few thousand dollars worth of sensitive equipment. I'll defer to Kevin Gilmore on those.

 

[jeffreyj]

I've been following this thread with a bit of humor. It seems there will always be an "eric343" out there that tries something that seems fun and harmless only to be told by a "kevin gilmore" that the seventh trumpet of the apocalypse has just been sounded as a result.

The truth, as hinted at by Melchior, is that no battery powered visible spectrum LED is going to be able to generate enough photonic energy to damage the phosphors in a TV, monitor or oscilloscope. Electrons inside the CRT itself are accelerated by a potential difference of 15-30kV, reaching high enough velocities to produce "synchrotron" radiation in the X-ray band; and some little LED with a peak output in the milliwatt range is going to affect them? C'mon, people - this defies all reason and sensibility! Also, unless the faceplate glass of the CRT is quartz, it is going to greatly attenuate UV band radiation, so I doubt the new UV LEDs will cause any problems here as well.

A laser pointed right at the CRT might be a different story, but the high-mode output of semiconductor diode lasers - as found in pen size laser pointers - can barely be collimated, so I doubt even organic phospors, much less the europium oxide ones in TV's and monitors, would be affected by them.

If any of the naysayers (three horsemen.. er... horsepersons of the apocalypse??

) can dig up anything to contradict my view point, though, I'd be very interested in reading it because it really does seem to defy common sense, and things that do that are invariably fascinating (to me, anyway).

 

[eric343]

Quote:

Originally posted by jeffreyj I've been following this thread with a bit of humor. It seems there will always be an "eric343" out there that tries something that seems fun and harmless only to be told by a "kevin gilmore" that the seventh trumpet of the apocalypse has just been sounded as a result.

LOL!

Incidentally, you'll note that Kevin talked about analog memory/PMT enhanced scopes as the ones that would take to your doodles permanently - those have very sensitive phosphors designed to hold their luminance for a long time; as opposed to the standard phosphors which you want to respond quickly (that is, that's how I *think* it works)...

AFAIK his comment about pricing a replacement tube for your scope was a 'just in case' measure, as not all 'scopes will turn permanently into pieces of artwork. (mine didn't.)



Now, morsel, on the other hand...

 

[kheldar]

i remember when i was a kid, we had an ancient color tv, generated so much static electricity, that you could draw on the screen with your finger, and the glow would last for a few minutes =)