How do rear projection screens get burn-in?

[davidbrit2]

From a technical standpoint, what gets "burned" on the projection screen? And why do they seem to get it so much more easily than a CRT? I know the LCD in the projector can't possibly be getting burned. The screen is essentially a plastic of some sort, right?

[digitalpress]

A TV repairman explained to me that it's the lamps ights that fire off blue, red, and green. You can replace the screen but the burn is actually in the projection lamps and very expensive to replace.

So be careful!

[davidbrit2]

Huh... I don't get it. Isn't there just a lamp that displays through a transparent LCD panel onto the screen, or it it something else entirely?

[digitalpress]

Yeah, pretty much. There are three lamps (at least on my Mitsubishi). Ever play with the "convergence" settings on the projection TV? It's aligning those lamps to point at the proper angle at the plastic screen. I guess each lamp has its own color (mine were red, blue, green), but each one projects the picture, and it's on those lamps that the image gets burned if you allow the same display to sit too long.

[davidbrit2]

Odd. I still don't see how something is being burned on the lamp itself if it's just shining through the LCD and onto the screen. How exactly are they constructed, then?

[digitalpress]

I have absolutely no idea. All I know is what TV Repair Guy told me when fixing a brightness issue and I asked him the same question about the burn-in.

[ghsqb]

My uderstanding is that it isnt the coloured lamps that "burn in" but the Cathode Ray Tube (or CRT) "gun" or projector.

I gues sthe best way to explain it is that like anything else, these projectors get "broken in" like anything else.

Your favorite joystick, stiff at first, but after use gets broken in, your ball glove etc...

Same principle with the CRT projectors.

Except that burn in occurs when a static image is projected continuously, causing a specific area of the projector to be "broken in" more quickly than the rest of the area.

This causes "burn in", or a well worn area of the projector, where you can see this image constantly, cause as anything shines through the projector, it passes through the broken in area, causing the image to appear all the time.

People have experienced the same problems watching "letterbox" style DVDs, the image of the black bars on the edge of the screen can get burnt in.

Even older TV's and monitors, not just projection tvs were succeptible to this.

For example, look at almost any old Pac Man arcade machine and the maze will likely be burnt in to the screen, many many hours of play combined with the static image of the maze, caused burn in, so much so, you can actually see the maze on the screen even with the power off.

[YoshiM]

Okay, ever play with a magnifying glass and the sun, burning wood, ants and whatnot? If you direct the beam at a chunk of wood and then take it away immediately, nothing happens. The longer you train the beam, the wood will start to burn.

Same thing with CRT TVs and projection screens using CRT guns. The electron beam in a TV hits a layer of phosphors and illuminates them. Typically in TV shows every thing on screen is in a state of motion (ie not staying on screen for more than a few minutes). Everything is fine. Now put a static image on there. What happens is that the electron beam continues to those phosphors with the same color and intesity constantly and, like the magnified sun's rays on wood, actually deteriorates the phophors after a really long period of time. Since the phosphors are dulled, they never light up as bright as the did before and thus you get that "burned in" image that is imposed over that movie you are watching. However it takes a REAAAALLLLLLY long time of constant static images to burn in. Hours upon hours and days upon days. Hence why we always see scores burned onto the monitors of our favorite classic games in the arcades.

Now a projection screen (front or rear) using CRT guns has it worse. Here we have three separate picture tubes, if you will. They are smaller and (especially in newer TVs) are brighter than a standard TV tube. Same rules apply: an electron beam hits glass covered in phosphors, light's 'em up, and then is projected and then reflected (sometimes) onto a large screen. Since these puppies are actually pumping out more power it's easier to have the burn in happen. Using the sun/magnifying glass example, this would be similar to taking a frickin' laser beam to the wood.

Plasma screens can have the same problem from what I hear. LCD's, at least the newer ones, don't and you shouldn't have any problems with burn in.

Hope that explains it.

[davidbrit2]

Ah, okay, so the rear projection TVs actually use super bright CRTs then, correct? That would explain it completely. I didn't think light bulbs, LCD panels, and plastic screens could experience burn in. It completely makes sense, though, if the "projectors" are actually just really, really intense CRTs.

[digitalpress]

What, you didn't like my "it wuz the light bulb what burnt it" explanation? :P

Thank god for technical-minded people.

[Sylentwulf]

Yoshi nailed it, the phosphors heat up to become illuminated. Heat the same set of phosphors up for too long at a time, and they burn. EVERYTHING get's burnt in on a rear-pro TV, it's just the if it's a static image, that part burns more than the rest. With a constant moving imagine, the whole screen slowly burns in at the same pace, so you never really notice it (Besides having to re-adjust once a year or so)