Subject: PowerBook screens
From: John A Savage <
[email protected]>
Date: Tue, 8 Dec 1992 17:58:24 -0500
Hi
This is a copy of the report I made about PowerBook LCD displays a week or two
back. Due to numerous requests, I have decided to submit it to you, for the
reports directory, or the digest or whatever is most apropriate. I have never
really done this before, so I hope all goes well.......
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This is a more detailed description of how an LCD display works. I will not
go into detail about the difference between Active/Passive etc. This is
purely to explain the ScreenSaver effects and things.
For all Chemists out there, I realise this discussion isn't totally accurate
but it is 'correct' and I can't be bothered to give a lecture course on nematic
crystals!! :-)
1. An LCD consists, basically, of two polaroid materials (get those sunglasses
out!) which are 'crossed' That is, no light passes through them as they cancel
each other out. To demonstrate this, put two pairs of polaroid sunglasses at
right angles, no light. Between these polaroids is a 'liquid crystal' This is
a chemical that 'bends' or 'rotates' light, the more of it there is, the more
the light is bent. The inside surfaces of the polaroids are coated with a
chemical that makes the liquid crystal (nematic) line up, with all the crystals pointing in the same direction. These chemicals ensure that 'at the polaroids
the nematic lies in the plane of the polaroid. Now, because they are crossed,
the nematic rotate by 90 degrees as you go from one polaroid th the other.
----------- ..... Polaroid
--> --> --> ..... nematic
-> -> -> ->
| | | | .... nematic at right angles to other nematic
-----------
Now, before with no nematic, no light got through but, because the nematic
rotates the light which passes through it, the light is rotated by 90 degrees
by the time it gets through and passes through the bottom polaroid. This is
'OFF' for an LCD and is white (clear, or whatever)
2. Another property of nematics is that they line up in an electrical field
apply an current and they all point along it. Well, if we do that to the
cell above, so that all the nematics are pointing upwards, then they don't
rotate the light at all, and the LCD goes dark, due to the crossed polaroids
Well that's it the difference between Active and Passive is down to how the
different pixels are turned on/off.
Ok, so what about screen savers, well first you dont need one, as there is
nothing to burn in, no chemical reaction (or otherwise) occurs, these things
last for ever (nearly) Second, it is possible that a screen saver that blacks
the screen could reduce the life of the display as that requires current to
flow.
This system relies on the fact that these crystal tend to line up. But at high
temperatures, they don't line up so well, and bend the light all over the
place, causine the screen to go black. All you do is let it cool down and they
behave again.
If a display is left on for a while, the picture may 'lock' this is because
once the current has been turned off, the crystals have to move back to where
they were before, and this can take some time. Don't worry they realign
eventually.
Liquid Crystal Displays require very small amounts of power and hence produce
no 'emissions' to worry about (no powerful beams of electrons to Zap you!!!)
Any questions, don't hesitate to ask
I hope that helps, anybody near a good library, just find a decent Chemistry
Textbook on the subject!
John
:-}
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! John Savage :
[email protected] ! Wot no disclaimer? !
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