Touch-sensitive screens (touchscreens)

Touch-sensitive screens (touchscreens)
Touchscreens are another method of allowing the user to point and select objects
on the screen, but they are much more direct than the mouse, as they detect the
presence of the user’s finger, or a stylus, on the screen itself. They work in one of
a number of different ways: by the finger (or stylus) interrupting a matrix of light
beams, or by capacitance changes on a grid overlaying the screen, or by ultrasonic
reflections. Because the user indicates exactly which item is required by pointing to
it, no mapping is required and therefore this is a direct device.
The touchscreen is very fast, and requires no specialized pointing device. It is
especially good for selecting items from menus displayed on the screen. Because
the screen acts as an input device as well as an output device, there is no separate
hardware to become damaged or destroyed by dirt; this makes touchscreens suitable
for use in hostile environments. They are also relatively intuitive to use and have
been used successfully as an interface to information systems for the general public.
  The computer
They suffer from a number of disadvantages, however. Using the finger to point is
not always suitable, as it can leave greasy marks on the screen, and, being a fairly
blunt instrument, it is quite inaccurate. This means that the selection of small
regions is very difficult, as is accurate drawing. Moreover, lifting the arm to point to
a vertical screen is very tiring, and also means that the screen has to be within about
a meter of the user to enable it to be reached, which can make it too close for comfort. Research has shown that the optimal angle for a touchscreen is about 15 degrees
up from the horizontal.