August 2017 11
FSTN- and FSTN+ (F is for filtered) are variants of STN that use an
additional layer between the STN material and the rear polarizer to
get better contrast and sharpness. And while you didn’t mention it, I
wanted to bring up IPS and TFT displays.
IPS means In-Plane Switching, where the electrodes are to the
sides of the pixel so that the electric field is parallel to the glass. This
significantly increases the viewing angle, and thus the color consistency
for color displays. This is because the electrodes are not covering the
transmissive area, and because of the way the molecules are moved by
the field. They also don’t react nearly as much to your finger distorting
the electric field — an important characteristic for touch displays.
TFT stands for Thin-Film Transistor, and is often combined with
IPS. These displays include transparent FETs on the glass for row and
column control. That allows much more current to be moved around
to change the electric field quickly. Since the electrodes form a parallel
plate capacitor, there is charge there that must be moved to change
the field strength, and we do that with current: the time rate of change
I’ve always found it amazing that we are looking through huge
numbers of transparent transistors when we stare at our computer
screens (Figure 5). NV
n FIGURE 5. Transistors in a TFT LCD (bottom) (source Wikipedia, CC BY-SA
3.0, user Gabelstaplerfahrer).
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