Current TV & VDU Display Technologies
Which visual technology will provide our home display screens in the future? We now have a multitude of different techniques available to produce moving images in the home projected or by live screen.
This is a résumé of the current methods employed, namely Cathode Ray Tubes, Liquid Crystal and Plasma displays together with summarised advantages and disadvantages.
CRT - Cathode Ray TubesThese have been in our homes since television moved in and have provided us with moving pictures for over half a century. Black and white in the early days followed by colour. Projectors using three tubes, one for each colour have been in common use by airlines on passenger flights.
The dominance of the CRT is now being challenged by several differing techniques.
Flat Panel Liquid Crystal Displays are nowadays often called TFT (Thin Film Transistor) displays. A description of the construction of these LCDs and of how they work may be of interest and is as follows.
Adjacent is a simplified view of the exploded device. At the top there is a polarising filter and one from the bottom situated above the fluorescent light source there is another which is 90 degrees out of line with the top one. Without any items between the two filters no light would be visible from above, due to the orientation of the filters.
Next to the top there is a colour filter to provide the necessary Red Green and Blue colours for the display. Three colours for each pixel.
Below this is the Liquid Crystal container which consists of a translucent solid top and bottom with a vacuum filled space between into which the Liquid Crystal is introduced. The inside top of the container has grooves in the same orientation as the top polarising filter and the inside bottom has grooves in the same direction as the bottom polarising filter. This causes the crystals to arrange themselves as a spiral between top and bottom of the container as shown below.
The light from the source beneath is modified itself by this twist pattern of the crystals and moves through 90 degrees and is now in line to emerge through the top polarising filter.
The top of the LC container forms an electrode and each colour pixel has its own electrode in the bottom of the container. Voltages applied will distort the shape of the twist and the relative strength of the coloured lights can be varied from no light to full light.
The voltages are controlled by the Thin Film Transistor layer immediately below the LC containing capacitors and transistors. The voltage signal is in the form of an AC square wave causing high speed switching of the light.
Plasma - DisplaysPlasma displays work much in the same way as fluorescent and neon lights - that is, they use electricity to illuminate a gas. In the case of the plasma display, the gas is between two glass plates with transparent electrodes.
When voltage is applied to one of the electrodes, a surface electrical discharge produces ultraviolet rays that excite the coloured phosphors coated inside the opposite plate of glass, emitting light through the glass plate to create an image. Because the phosphors are red, green, and blue, the image produced is in colour.
A plasma TV is sometimes called an "emissive" display — the panel is actually self-lighting. The display consists of two transparent glass panels with a thin layer of pixels sandwiched in between. Each pixel is composed of three gas-filled cells or sub-pixels (one each for red, green and blue). A grid of tiny electrodes applies an electric current to the individual cells, causing the gas (a mix of neon and xenon) in the cells to ionise. This ionised gas (plasma) emits high-frequency UV rays, which stimulate the cells' phosphors, causing them to glow the desired colour.
Because a plasma panel is illuminated at the sub-pixel level, images are extremely accurate, and the panel's light output is both high and consistent across the entire screen area. Plasma TVs also provide very wide horizontal and vertical viewing angles. Picture quality looks sharp and bright from virtually anywhere in the room. Because plasma TV screens do use a phosphor coating (like CRT-based TVs), the potential for screen burn-in exists, so it's important to follow the manufacturer's recommendations on day-to-day use.
SummarisingThe future lies with LCDs. CRTs and the stop gap Plasma Displays are on the way out once larger size LCD's become available.
SEDs ("Surface-conduction Electron emitter Display") which are nothing more than CRT's with a gun for each pixel and which can hence work with a short path, lower voltage and a fixed beams and flat screen are not yet in production and have not been considered.
Different manufacturers employ slightly different Liquid Crystal techniques in their display units and it is well worth knowing what weaknesses to look for, but over the last couple of years the improvements achieved bode well for the future.