what is the actual cause of backlight bleeding?

Im curious what the ACTUAL cause of backlight bleeding is, specifically when it is only occuring around the edges, and uniformily. I took apart my Toshiba 26" to see if I could reduce some of the edge backlight bleeding. After getting the bezel off, you are left with the LCD glass screen which has a black rubber seal around the edges. It was of high quality and even all the way around. If you lift up the screen, behind it is a white panel, and behind that is the CCFL.

Doing some experimentation I first tested the CCFL and its light distribution was VERY even, and obviously not the cause of bleeding around the edges. I then did some experimenting with electrical tape adding more "seal" to the edges, but this did not have any affect on reducing the bleed.

So this leads me to my question of what actually is causing the bleed around the edges?

my assumption has always been...

it's as simple/complicated as proximity to the light source... the term "bleed" is used because LCD's basically block a white light source to reproduce colors. Light often however isn't completely blocked (I've yet to see an LCD displaying black that wouldn't be a source of light in an unlit room), and will "bleed" through. When it happens excessively in one area is when it's most bothersome.

All this I'm guessing you know, but perhaps one measure to alleviate the situation is if you were to somehow modify the display to allow for more space between the ccfl source(s) and the LCD array? Never tried this myself but that'd be my intuitive guess...

EDIT: another thought... have you checked to see if lowering the screen brightness helps the situation significantly? I prefer to keep my displays tuned down from factory defaults as it's easier on the eyes.

and its light distribution was VERY even, and obviously not the cause of bleeding around the edges

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Well, actually unevenness is the cause. It may look perfectly even without a frame of reference, but the light is brighter the closer you are to the bulb. Or maybe not brighter... I should say it's dispersed more the farther you are from the source. Think of it this way: you have a piece of very slightly translucent black plexiglass or whatever, and you hold it up in front of a lamp across the room. You can't see much/anything through it. Put it on top of the lamp, and you can see light through it. Same general principle with an LCD, except instead of black plastic it's black liquid crystal.

There's really not much you can do about it... it's just one of the inherent flaws in LCD monitors. It may be possible to put some additional/better diffusing material behind the panel, which would probably help, but you're not going to get rid of it entirely without way more effort than it's worth.

sounds like we are thinking on the same page. I am familiar with light spots on an LCD, but its edge bleeding thats really annoying. Whats interesting that is that corner flashlighting or edge bleeding occurs where it does.

I think you may be on to something with distance between the glass and the light source. Possibly the angle at which the light strikes edges/corners is why it shows up more often and more dominate in those areas as opposed the center of the screen. Although, why would edge/corner bleeding various from brand to brand? With some having none to slight and other having moderate to severe?

ashenfang said:

sounds like we are thinking on the same page. I am familiar with light spots on an LCD, but its edge bleeding thats really annoying. Whats interesting that is that corner flashlighting or edge bleeding occurs where it does.

I think you may be on to something with distance between the glass and the light source. Possibly the angle at which the light strikes edges/corners is why it shows up more often and more dominate in those areas as opposed the center of the screen...

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this fella's writeup for a bad case fix sounds similar, but not identical to your efforts w/ electrical tape:

http://www.hardforum.com/showthread.php?t=1032268

another thought might be to try to diffuse the light source more near the lamps - perhaps with baker's parchment paper (somewhat heatproof and translucent)? A more sophisticated approach might be strips of translucent plastic - I'm an architectural engineer so I'm thinking of this in terms of light fixtures - many things you could try from a "diffuse the source better" approach.

...Although, why would edge/corner bleeding various from brand to brand? With some having none to slight and other having moderate to severe?

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I would venture that manufacturers using differing light sources, differing layouts for those light sources, variances in component (lamp) quality, differing LCD technologies (TN vs PV etc...) and housing construction differences all play into why one panel will exhibit different but consistent light bleed characteristics.

(***ACCOMPLISHMENT: run-on sentence of the day!!!***)

A more sophisticated approach might be strips of translucent plastic - I'm an architectural engineer so I'm thinking of this in terms of light fixtures - many things you could try from a "diffuse the source better" approach.

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That's what I was thinking. LCDs already have a sheet of frosted plastic behind the LCD panel for that purpose, however, it's extremely thin and only lightly frosted. You might run into problems by using strips... you could end up with noticeable lines in the display at the edge of the strips, either due to the extra opacity or the slight gap created between the LCD and the backing due to the thickness of the extra material. If you can find a piece of thicker, heavily frosted plastic (or glass, I suppose) to replace the original diffuser, that might work a bit better.

Old Thrashbarg said:

That's what I was thinking. LCDs already have a sheet of frosted plastic behind the LCD panel for that purpose, however, it's extremely thin and only lightly frosted. You might run into problems by using strips... you could end up with noticeable lines in the display at the edge of the strips, either due to the extra opacity or the slight gap created between the LCD and the backing due to the thickness of the extra material. If you can find a piece of thicker, heavily frosted plastic (or glass, I suppose) to replace the original diffuser, that might work a bit better.

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followup suggetion - just occured to me: go to a college bookstore or craft store and ask for a sheet of 26"x36" mylar. it's a super-even and semi-translucent paper with a very smooth surface used for pen-and-ink drafting by hand. You should be able to easily get a huge sheet and have enough left over for re-trys should you measure and cut incorrectly. If I had to build something like this from scratch that's where I'd start. You may find that using this in addition to your existing plastic sheet will do the trick. Not sure how heat-proof the material is though so use with discretion.