Powerbook Lcd Displays

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 p…