Hardware Reference
In-Depth Information
one with a pen, pencil, paper clip, or gloved finger and you will see that nothing
happens. The human body is good at storing electric charge, as anyone who has
shuffled across a rug on a cold, dry day and then touched a metal doorknob can
painfully testify. Plastic, wooden, and metal instruments are not nearly as good as
people in terms of their capacitance.
The ''wires'' in a touch screen are not the usual copper wires found in normal
electrical devices since they would block the light from the screen. Instead they
are thin (typically 50 micron) strips of transparent, conducting indium tin oxide
bonded to opposite sides of a thin glass plate, which together form the capacitors.
In some newer designs, the insulating glass plate is replaced by a thin layer of sili-
con dioxide (sand!), with the three layers sputtered (sprayed, atom by atom) onto
some substrate. Either way, the capacitors are protected from dirt and scratching by
a glass plate placed above, to form the surface of the screen to be touched. The
thinner the upper glass plate, the more sensitive the performance but the more frag-
ile the device is.
In operation, voltages are applied alternately to the horizontal and vertical
''wires'' while the voltage values, which are affected by the capacitance of each
intersection, are read off the other ones. This operation is repeated many times per
second with the coordinates touched fed to the device driver as a stream of (
x
,
y
)
pairs. Further processing, such as determining whether pointing, pinching, ex-
panding, or swiping is taking place is done by the operating system. If you use all
10 fingers, and bring a friend to add some more, the operating system will have its
hands full, but the multitouch hardware will be up to the job.
Flat Panel Displays
The first computer monitors used
cathode ray tubes
(
CRT
s), just like old tele-
vision sets. They were far too bulky and heavy to be used in notebook computers,
so a more compact display technology had to be developed for their screens. The
development of flat panel displays provided the compact form factor necessary for
notebooks, and these devices also used less power. Today the size and power bene-
fits of the flat panel display have all but eliminated the use of CRT monitors.
The most common flat panel display technology is the
LCD
(
Liquid Crystal
Display
). It is highly complex, has many variations, and is changing rapidly, so
this description will, of necessity, be brief and greatly simplified.
Liquid crystals are viscous organic molecules that flow like a liquid but also
have spatial structure, like a crystal. They were discovered by an Austrian botan-
ist, Friedrich Reinitzer, in 1888 and first applied to displays (e.g., calculators,
watches) in the 1960s. When all the molecules are lined up in the same direction,
the optical properties of the crystal depend on the direction and polarization of the
incoming light. Using an applied electric field, the molecular alignment, hence the
optical properties, can be changed. In particular, by shining a light through a liquid
