The Principles
Plasma technology has some similarities to conventional CRT sets. Both use phosphors to emit coloured light, in combinations of red, green and blue, to create an image featuring millions of colours.
The differences lie in how the light-emitting phosphors are ‘excited’. In a CRT set an electron gun bombards the phosphors with electrons (see our Guide to CRT TVs), while a plasma set is comprised of tiny chambers containing inert gases - neon and xenon.
When these gases are subjected to an electrical charge they briefly turn into plasma and emit ultraviolet light. It is this ultraviolet light that excites the phosphor in the chamber and produces controlled bursts of coloured light.
A group of three chambers, producing red, green and blue light, make up a single pixel on the screen, and the gas chambers are encased between two sheets of glass.
How it works
Plasma is neither solid, liquid nor gas. It is a ‘fourth state’ that comes into being when normal gas atoms are stimulated to release electrons. The loss of the negatively charged electrons means that the atoms become positively charged and the gas is said to be ‘ionised’. When this process has reached a certain level the gas has become plasma.
Let’s look more closely at how the gas in each chamber is converted to plasma. A grid of electrodes encases the chambers. To the rear is a series of vertical electrodes (known as the address electrodes), while at the front of the chambers is a series of horizontal electrodes (the display electrodes). The display electrodes are transparent so you are not aware of their presence just behind the protective glass screen.
Each pixel is effectively at an intersection, where the vertical and horizontal electrodes cross paths. In this way it is possible to target a pixel with pin-point accuracy, but it takes a computer to do it.
A current, applied between the two electrodes at a given intersection, introduces a stream of free electrons into a gas chamber, which collides with gas atoms and knocks free more electrons. The gas becomes plasma, particles collide and energy is given off in the form of photons.
It is these photons (most of them ultraviolet and therefore invisible to the human eye) that in turn excite the phosphors and make them generate energy as light – but this time the light is visible.
Depending on the characteristics of the original electrical charge the photons will emit more or less intense light, and along with the combinations of red, green and blue cells there is enough material to create a TV picture with millions of colours.
Although this all sounds rather cumbersome, the transitions from gas to plasma and back to gas take place so rapidly that the pixels can easily be manipulated quickly enough to produce a smooth, highly watchable image, but there is a problem with the process…
Poor contrast has been an Achilles heel for plasma screens that is, thankfully, being addressed. The problem stems from the need to keep the gas chambers ‘primed’, that is, to keep a low current running through them constantly.
Without this priming, the process of activating the cell would take too long. The time it takes for a cell to go from inactive to active and back to inactive is known as the ‘response time’ and a response time that is too slow will impact on picture quality by introducing blurring. Consider how long it takes a fluorescent light to illuminate when switched on and you will have an idea of the potential effect of a poor response time.
Priming increases the responsiveness of the gas chambers – in effect it keeps them in a state of readiness - but it also results in them emitting a low level of light even when they should be inactive. This impacts on contrast. Early plasma screens had contrast ratios as poor as 70:1, while some manufacturers now claim ratios of 15000:1, with corresponding improvements in image quality.
Glossary
CRT – Cathode Ray Tube. The conventional TV format, where electrons are fired at a phosphor-coated screen to generate images
DVI – Digital Visual Interface, a digital video-only input or output
HDCP – High-bandwidth Digital Copy Protection. Intended to stop us from running off high-quality pirate copies of HD programming
HDMI – High Definition Multimedia Input – also known as the ‘digital Scart’ it carries video and audio signals
Phosphor – A substance that generates light when excited by an energy source
Pixel – A ‘picture element’, the little dots that make up an image on a screen
Plasma – A state of being distinct from gas, where the electrical characteristics of a gas have been changed due to the application of heat or other energy source
Priming – The application of a low-level current to a gas cell to speed up response time when that cell is activated
Response time - The time it takes for a cell to go from inactive to active and back to inactive. The lower the figure the better
