Subject: PowerBook screens 
From: John A Savage <troc@jhunix.hcf.jhu.edu>
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.......

********************************************************************************
 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 : Troc@jhunix.hcf.jhu.edu ! Wot no disclaimer?                  !
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