Display Technologies

The displays used in handsets are based on liquid crystal diode (LCD) technology
that has been used in consumer products for some time. Liquid crystal materials
have some special properties that are exploited to create displays; most importantly,
the crystals have a twisted structure and the amount of twist can be altered by
applying a voltage to the crystal material.

LCD Display Structure
A basic LCD display is a sandwich of layers through which light passes.
One of these layers is the liquid crystal that is situated between two layers of glass
that contain electrical connections. By altering the signal to these connections, the
crystals can be made to alter their twist, which has an effect on the polarization of
the light and can be used to create the dark/light contrast necessary for a display.
The specific LCD technology found in many displays is super twisted nematic
(STN), which relates to the special form of liquid crystal that is used. Displays can
be characterized as being either reflective or transmissive. A reflective display relies
on incident light from the front of the display, passing through all the layers to a final
reflective layer where it is reflected back to the front of the display. It is possible to
provide front-lighting or back-lighting to reflective displays. A transmissive display
uses backlight from within the display. The use of back- or front-lighting will increase
the energy consumption of the display; and when used in handsets, the lighting has
an associated sleep circuit to switch off the light after a few seconds of user inactivity.
There are a number of variants of the twisted nematic (TN) display, although
they all generally employ the same principles of operation.

Color Displays
Adding color to a display is relatively simple. Each pixel in the display
has three separate filters associated with it: one red, one green, and one blue.
Therefore, each pixel is effectively divided into three sub-pixels. The filters can be
activated so that only light of a particular color can pass through for that pixel. The
three sub-pixels can be manipulated to create a range of colors.

Display Types
The way pixels in a display are addressed, so that they can be switched between states,
has led to two main display technologies, the so-called passive and active displays.
In a super-twisted nematic display, a passive display (often called STN,
the individual pixels are addressed by row and by column signals, one
pixel at a time, and thus the display is relatively slow because it takes time to build
up an image pixel by pixel.

On the other hand, active displays add another component in the form of a
transparent transistor at each pixel, hence these displays are referred to as thin film
transistor (TFT). Using the active technology allows a whole row (or column) of
pixels to be addressed at once, which means that creating an image is much more
rapid than in an STN display.

The disadvantages of STN displays are their relatively slow operation, and
there are also issues relating to brightness and angle of view. However, they are
cheap to manufacture and use less energy than a TFT display,
which corrects the major problems of the STN format. In handsets with two
displays, where a simple display is used for phone functions and a higher specification
display is used for viewing videos and playing games, it is common to
find both technologies deployed — STN for the simple display and TFT for the
high-quality display.

Other display technologies are being developed; one in particular, the organic
LED (OLED), is receiving a lot of interest. OLED are emissive devices and thus
do not require a backlight, they can also be created on very thin layers of polymer
(almost like printing), and they consume much less energy than LCD displays,
which makes them ideal for handsets and other power-constrained devices.
OLEDs are being demonstrated in consumer products and much research is
being conducted into these components to overcome some of their limitations, at
which point widespread deployment would become a reality. For example, the lifespan
of blue OLED elements is only a few thousand hours, which means their use
in a TV or phone display is not yet commercially possible.