|Re: Help with a general LCD problem in HP 11C (Voyager series)|
Message #2 Posted by Diego Diaz on 14 May 2008, 2:23 p.m.,
in response to message #1 by Chris Falk
Sorry about the bad news, but there isn't such a cure for LCD's "seal broken" desease.
What has been repeatedly described a number of times in this forum as: "stains", "worm paths", "hair lines", (and a very interesting set of the most ingenuous descriptions) is nothing but air.
Originally, LCD displays are sealed, containing a very small amount of "liquid crystal" between two glasses. Transparent conductive electrodes (made of an Indium salt compuond IICR) are etched on the inner surface of both glases.
Most of LCD's used in calculators use the transflective method and twisted nematic Liquid Crystal, requiring a reflective surface back layer behind the bottom glass. The reflected light is polarized and this polarization must be "twisted" by the crystals.
On top of the upper glass all LCD's are covered by a polarised filter. Reflected polarized light comming from the back panel will pass thru the crystals if the segment is not "active".
In fact what we "see" as an *ON* segment is usually the segment which polarisation (produced by the electrodes) blocks the path of light, by twisting its crystals in the opposite direction to which the polarized upper filter is oriented.
When shock, vibration or just *age* damages the sealing, a small portion of air find its path between the glasses. Of course, air does not twist the light in any way so it cannot pass thru the filter, thus the *black* stain.
Hope this claryfies the issue, and sorry not being of much help.