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.

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:
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

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

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.

On Sunday, February 8, 2015 at 12:19:13 PM UTC-5, Lloyd E. Sponenburgh wrote:
fired this volley in
:

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

Live and learn, huh?

Yeah, LCDs are more-or-less adversely affected by temperature, at both
ends.

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

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.