Displays

Displays come in three types, Cathode Ray Tube (CRT), Liquid Crystal Display (LCD) & Plasma Display Panel (PDP). CRT is the oldest technology and looks much like traditional televisions and has a very deep footprint. LCD technology allows the displays to be much thinner while offering a better graphics quality. LCD displays are now the dominant technology with CRT displays all but obsolete. 

A plasma display panel (PDP) is another type of flat panel display which is more common within the home entertainment market. Plasma screens work by turning gas, held in tiny cells between two plates of glass, into plasma by introducing and electric current.

Liquid Crystal Display (LCD)

LCD refers to the technology behind the display and were first used on laptop computers before the technology improved enough to make the jump to desktop, server and home entertainment displays. An LCD display consists of five layers: a backlight, a sheet of polarized glass a "mask" of colour pixels, a layer of liquid crystal solution responsive to a wired grid of x, y coordinates, and a second polarized sheet of glass. By manipulating the orientations of crystals through precise electrical charges of varying degrees and voltages, the crystals act like tiny shutters, opening or closing in response to the stimulus, thereby allowing degrees of light that have passed through specific colour pixels to illuminate the screen, creating a picture.

As LCD technology evolved, different techniques for producing colour emerged. Active-matrix or TFT (Thin Film Transistor) is currently the most popular technology and produces some of the sharpest colour and images in the display market and is generally considered superior to any previous passive-matrix technologies.

LCD’s are not emissive i.e. they do not generate their own light. The choice of backlighting is therefore an important consideration in design and construction of the panels. There are three different technologies used for backlighting with LCD’s as follows:

Cold Cathode Fluorescent Lamp (CCFL) – This is the most commonly used form of LCD backlighting currently used due to exceptional brightness and full spectral output. A CCFL is a thin glass tube with an internal vacuum. A pressurized mix of Neon/Argon is sealed within the fluorescent tube and energised with an electric current. It is the electrons striking the fluorescent material within the tube that then generates a bright white light.

Light Emitting Diodes (LED) – LED backlighting is a popular method for small and medium LCD’s. The advantage of LED backlighting is low cost, long life, immunity to vibration, low operational voltage, and precise control over its intensity. LED backlights come in a variety of colours however white is becoming a cost effect and popular option.  LED backlights offer a long operating life of 15,000 hours minimum for white and much longer for other colours. To ensure the best spread of light the LED’s are used in conjunction with a light guide and can be mounted behind the display or in some cases along the edge which can offer a thinner solution.

Electroluminescent Panel (ELP) – ELP backlights use a solid state phenomenon based on coloured phosphors to generate light when an AC voltage is applied to the panel. ELP backlights are very thin, lightweight and provide an even distribution of light without the need of a light guide. They are available in a variety of colours with again white being the most popular colour. Whilst power consumption is fairly low ELP backlights have a limited life of 3000 – 5000 to half brightness (half-life). This limited brightness and lifetime has made them unpopular so they are rarely used to backlight LCD’s for graphical applications.