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 Tubes

These 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.

CRT Advantages

  • Bright colours, highest contrast available
  • Can cater for a wide variety of digital image resolutions
  • Mass production over the years has enabled this to become the cheapest of all display technologies.

CRT Disadvantages

  • Weight and bulk - Large screens require large tubes and high voltages. Transformers and safety screening add to the weight.
  • Flat screens difficult to achieve without picture distortion.
  • Susceptible to settings drift and colour fade over time
  • Affected by the presence of magnetic fields.
  • CRT computer monitors require high frequency refresh rates if irritating flickering is to be avoided.
  • "Burn in" can occur but is not so likely in the latest generation of tubes.
LCD - Liquid Crystal Displays are now the second most common type of display. Invented in 1971 and based on Liquid Crystals discovered over 100 years ago they have been in use on digital watches and calculators since the mid seventies. Colour LCDs are now common on new mobiles and cameras, just two of the ever growing applications. It is foreseeable that the importance of LCD will be increased dramatically with the fast popularity of Flat Panel Displays

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.

Exploded TFT LCDAdjacent 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.

Liquid Crystals Normal AttitudeThe 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.

LCD - Advantages

  • Flat panel Technology is successfully employed - wall mounting is possible- Space saving monitors
  • Light Weight
  • Sharp well defined pictures available in bright colour at native resolution.
  • Long life - no screen burn effect
  • No magnetic interference.
  • Low power requirements - very green

LCD - Disadvantages

  • LCD more expensive
  • Motion blurring
  • Resolutions other than natural blurred
  • Screen Door effect on some models -(the lines between pixels show)
  • Narrow viewing angle - The original change of horizontal to vertical of the Liquid Crystal orientation was very restrictive on viewing angles but using the method explained above this is overcome!
  • Stuck and weak pixels do occur but are less and less common.
  • The contrast ratios are not as good as on CRT's.
  • Total blackness is virtually impossible to achieve.
Similar LCD techniques are also used in the manufacture of the relatively powerful projectors that are becoming more and more economically viable. Another method for these projectors is DLP (Digital Light Processing) using a DMD Digital Mirror Device. The DMD when put under a microscope is revealed as thousands of mirrors with a tilt on and tilt off position. When a strong light beam is applied to it, this can be controlled to give you moving pictures on as large a screen as you would require.

Plasma - Displays

Plasma 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.Plasma Cell 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.

Plasma -Advantages

  • This provides large flat screens - mainly used for television (80ins)
  • Colour and brightness similar to CRT's
  • Sharp images on TV
  • Excellent viewing angles
  • Good life expectancy

Plasma - Disadvantages

  • Though thin fairly heavy
  • Fragile
  • Susceptible to screen burn-in
  • Cannot produce deep black
  • Use a lot of power
  • Small screens not available(42 ins)
  • Expensive

Summarising

The 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.

Marcus Palmén
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