CurtPalme.com Home Theater Forum

[ecrabb]

The subject came up in another thread about whether anything was truely "burning" in a CRT when it turns brown, so I thought I'd start a new thread on the topic.

Years ago, I read a thread over on the other site that got into the nitty gritty of CRT aging (what we call wear or burn), and I can't find it now. Besides, it would be cool to have it here, anyway. I thought it might be useful for anyone that would like to understand more about the process. Those with more knowledge, please chime in to correct, clarify, or otherwise add to the little I was able to find and digest.

First, let's separate "wear" and "burn". Most of us already know that "burn" is simply uneven "wear". Sometimes burn is also referred to as CRT "image retention". In either case, though the face of the CRT turns brown, and doesn't emit as much visible light, either making the whole projected image darker ("wear"), or in the case of "burn", leaves a visible pattern on the tube face and the projected image. What I'm curious about, though is what is actually happening that's causing either one.

I couldn't find much on the actual chemical process of wear or burn, but as far I as I can tell, they're both slang - which is probably another reason it's hard to find the technical data - because the technical data refers to it by the technical term: "aging"... To be more exact, "phosphor aging by electron beam".

Here's what I could find with a short search:
http://www.ddc-co.com/articles/38%20-%20Phosphor%20Aging.pdf

[Tim in Phoenix]

Guys!

The electron gun may degrade over time due to aging of the filament, or degradation of coatings on the cathode. The vacuum may take on a little air, that sort of thing. Still better than arc lamps that go in a thousand hours.......

[cmjohnson]

The face glass of any CRT used in any projector that WE have ever seen, is made of a type of glass that does NOT brown with age
and X-ray exposure, but the BELL of the tube IS, and this is readily apparent by looking at a well used CRT. The face glass is
water clear while the bell is yellow to brown.

It's been a long time since X-ray reactive glass which browns with exposure has been used on the face of a CRT.

Phosphor browning is a chemical change which, as I've been told, is mostly if not entirely in the binder used to make the phosphor
adhere securely to the glass.

In your high school chemistry classes you learned that there are oxidation reactions and reduction reactions. I believe that the
browning of the phosphor is an oxidation reaction in the binder. *IF* someone formulated both a binder and a phosphor that lacked
an oxidation component, then you might end up with a phosphor layer that never darkens, though it may still exhibit a dropoff in output
as it ages due to the dropoff of the electron work function.

However, I believe that it's possible for even the dropoff in electron work function to be eliminated, if enough thought and
esxperimentation were to go into finding a solution. I say this because not one of the elements of which the phosphor is composed of
is ever changed by being used. Barium is barium, always and forever, and so is Strontium, Potassium, Sodium, Tungsten, Phosphorus,
Beryllium, and so are all other non-radioactive elements, regardless of how much electrical current may pass through them. So I
believe that phosphor ageing and burning can at least theoretically be entirely eliminated.

CJ

[Dave Lister]

[rod]

So Dave, with all the countless hours on gaming. what else exactly do you do?

[ecrabb]

Dave,

I've seen people say before that the older machines like your 1000 were much, much less susceptible to burn and wear because of their relatively low output to begin with. In other words, they're not very bright, and aren't pushing the phosphor even when cranked. No burn, very little wear. The newer, brighter machines are a different story... people have burned DVD menus into the phosphor by falling asleep. Now, if you cranked the contrast down on the same modern machine so that it was outputting a similar amount of light as your machine, it would last forever, too.

My dad had a couple of video-only machines, too - and Advent from the '79 ballpark, and an Infinity RSTV from the '85 timeframe. He was still using the Infinity a couple of years ago, and it had to have close to 10,000 hours on it. Still, the CRTs were still pure white. They just don't drive the phosphor very hard.

SC

[ecrabb]

CJ,

I assumed it was some sort of oxidation. Thanks for that. I forgot about the binders in the phosphor coatings - I actually did read that somewhere. Maybe it was your post back on the other site years ago that I remember - because that makes sense.

Anyway, if what you wrote about non-browning binders is true, then it just pisses me off all the more to think about what CRT projection would be like if big companies like Panasonic, Barco, Sony, NEC and Electrohome were still in the game putting big R&D dollars toward to developing these machines. I know VDC is still pushing the Marquee chassis, but I assume there has been little to no development whatsoever into the tube technology itself for what, the better part of 10 years?

Imagine what a brand new Barco or Sony machine would look like now if R&D had continued from where the G90 and 909 were almost 10 years ago! Damn, the technology just starts to be viable to make wear and burn a thing of the past (or at least less of an issue), and the stupid digital craze renders the whole CRT business a cottage industry.

SC

[cmjohnson]

Another thing to consider is cathode life. Even if you had a perfect, non-browning phosphor and binder that never lost brightness with age and use at a given current level, the cathode, even modern long life dispenser cathode designs, will only last so long. Possibly it could
also be made "permanent" for the same reason as stated above, but now you're up against the hard limit of filament life. As the filament
runs at operating temperature, atoms of tungsten inevitably fly off the filament and deposit themselves elsewhere. That's the silvery spot
seen as a faint ring on the tube neck of some well used tubes, around the back end of the electron gun assembly. Eventually, the filament
will fail from this slow evaporation process.

But there is no technical reason why a tube couldn't include a second filament that's not put into service until the first filament fails to meet
its performance specs.


With enough research and development, I think a 100,000 hour CRT is technically possible. But we'll never see it or be able to afford it.
Especially these days, with digital technologies catching up so quickly.


CJ