Bleeding 7-segment LCD displays, explained

A talk given by a local expert on LC devices, Matthew Proctor, Southampton University, from the Q&A after the talk, my query

I've brought along , what I often see in repairing electronic stuff, black bleeding within a 7-segment LC display. All I know is the problem seems to develop from being excessively cold or hot and I'm assuming the 2 glass plates cleave apart , then the LC bleeds across but I've never found out why the numbers fail to register anything although there is LC everywhere? All such LC displays have polarizors in them , light going in through one and out through another . If we remove the LC from the situation entirely all you get is something that light cannot go through. You can only get this birefringent effect with 2 glass plates , normally coated with some kind of plastic , stick them close to one another , the gap can go up to about 0.5mm if your lucky. In a TV they are usually about 5 micron or so. As you say , in this numeric LC display, the plates have moved apart slightly and the LC is no longer ordered properly. You get something that is cloudy again, its not birefringent. You have small area htere where the LC is lined up but lots of these microdomains where there is no order as to the direction of the domains and the light is scattered . The LC is no longer being aligned by the polymer and what you have instead is something that is not doing anything significant to the light and you have 2 polarizors and so black. I forgot to say how thick this layer is . It has to be carefully controlled depending on what colour you want to come through. If you change the thickness of that layer a bit, then you'll change the colour of the light that comes through it. If you poke a LCD with a finger and see rainbow fringing that is what is happening there, you are flexing the display, changing the thickness , and changing the wavelength of light its letting through, as you have RG and B light behind it. With large flat pieces of glass on laptop screens , how can they guarantee absolutely rigorous separation, until you poke your finger at it, anyway, it seems impossible? They are very very good engineers, very good at what they do. The things we make in the lab , tend to sag in the middle , so about 2 microns thinner than at the edges. Something we ar einvestigating as its often ignored in hte literature. Really thin, tiny electrodes , so carefully designed to get these things working. Generally on new monitors these days, you will not get a single pixel breaking for years.

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N_Cook
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Bleeding 7-segment LCD displays, explained

On Sunday, February 8, 2015 at 9:35:56 AM UTC-5, N_Cook wrote in sci.electronics.repair:

I never knew that excessive cold (absent moisture) could hurt electrical functions.

Reply to
mogulah

Well, and on another note, I guess the car battery is rather weak starting up the car when its cold, too.

Reply to
mogulah

And a second query answered

Is there any long term degradation problems with LC. If you have a TV display or an instrumental 7 segment display , it requires a certain amount of voltage for a certain amount of contrast. If the voltage step stays the same , 20 years time, will it produce the same contrast?

You do get a degradation. Known as image sticking . Its not actually LC fault . We like to think that we've only put molecules in there that we want , but we process them in a factory , transpported , stored them and we introduce impurities. Some of these impurities are going to be charged within the LC. I apply a voltage so a positive voltage over here and negative over there that attacts charged material to either side. Bits of rubbish here and bits of rubbish there . Generally if you turn it off it will all relax back intio the system and no problem except for something called charge-screening?. However you have a polymer on the surface and you can get a chemical interaction wiht the rubbish and stay s locked in when turned off, that contributes to inmage-sticking. So switched off there is still an electric field . Especially in the early days , if I displayed a still picture and leave it on for just 3 or 4 hours then the image will stay there even when turned off.

Reply to
N_Cook

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