Page 327 - Understanding Automotive Electronics

2735 | CH 9 Page 314 Tuesday, March 10, 1998 1:24 PM

9 AUTOMOTIVE INSTRUMENTATION

Nonpolarized light can be polarized by passing the light through a polarizing
material. To illustrate, think of a picket fence with narrow gaps between the
pickets. If a rope is passed between two of the pickets and its end is whipped up
and down, the ripples in the rope will pass through the fence. The ripples represent
light waves and the picket fence represents a polarizing material. If you whip the
rope in any direction other than vertically, the ripples will not pass through.
Now visualize another picket fenced turned 90˚ so that the pickets are
horizontal. Place this fence behind the vertical picket fence. This arrangement is
called a cross-polarizer. If the rope is now whipped in any direction, no ripples
will pass through both fences. Similarly, if a cross-polarizer is used for light, no
light will pass through this structure.
The configuration of an LCD can be understood from the schematic
drawings of Figure 9.16. The liquid crystal is sandwiched between a pair of glass
plates that have transparent, electrically conductive coatings. The transparent
conductor is deposited on the front glass plate in the form of the character, or
segment of a character, that is to be displayed. Next, a layer of dielectric
(insulating) material is coated on the glass plate to produce the desired alignment
of the liquid crystal molecules. The polarization of the molecules is vertical at the
front, and they gradually rotate through the liquid crystal structure until the
molecules at the back are horizontally polarized. Thus, the molecules of the liquid
crystal rotate 90˚ from the front plate to the back plate so that their polarization
matches that of the front and back polarizers with no voltage applied.

Figure 9.16
Typical LCD
Construction

FPO

314 UNDERSTANDING AUTOMOTIVE ELECTRONICS