AVForums explains how a plasma TV works.
With all the recent excitement surrounding the launch of Organic Light-Emitting Diode (OLED) TVs at CES this year and the continued success of Liquid Crystal Display (LCD) TVs - especially the ones using Light-Emitting Diodes (LED) as a backlight - it is easy to forget that Plasma remains the display technology of choice for the enthusiast. A plasma panel still offers the highest performance and the best value of any display technology on the market but what exactly is a Plasma TV?
Well, a Plasma Display Panel (PDP) is a flat panel display technology that uses small luminous cells that contain electrically charged gases, similar to those found in fluorescent lamps. A plasma panel consists of millions of such cells that are essentially tiny neon lamps which are compartmentalised in a space between two sheets of glass. When an electrical charge is applied to these cells, the gas in the cells becomes hot and begins to glow which is how a plasma illuminates itself. In physics this heated form of gas is called plasma, which is where the technology gets its name from. In order to add colour, the back of each cell is coated with a fluorescent compound called a phosphor and the light emitted by the plasma in the cells excites these phosphors and they create a specific colour. Since the colour in a TV picture is composed of combinations of red, green and blue, those are the three phosphor colours that are used.
A high definition plasma panel has a resolution of 1920 x 1080 which results in over two million individual pixels and each pixel is made up of three separate cells or sub-pixels, each with a different coloured phosphor. One sub-pixel has a red light phosphor, one sub-pixel has a green light phosphor and one sub-pixel has a blue light phosphor and when combined, these colors create all the colours in a TV image. Plasma panels use Pulse-Width Modulation (PWM) to control the amount of electricity in each cell and hence the brightness of each cell. By varying the pulses of current flowing through the different cells thousands of times per second, this control system can increase or decrease the intensity of each sub-pixel color to create the image. These pulses can be seen as noise in the image when looked at up close but is not usually noticeable at any sensible viewing distance.
There are a number of advantages to plasma technology, especially when compared to LCD panels, and because a plasma panel is self illuminating it is capable of producing much deeper blacks and a far superior contrast ratio which is the difference between black and white. Plasma panels have much wider viewing angles than LCD panels and they don’t suffer from image degradation when viewed from the side or from above. Compared to an LCD TV, a plasma panel also has far higher refresh rates, which is the number of times an image can be shown per a second, and much faster response times, which is the time it takes for a pixel to change, resulting in both superior motion handling and better 3D performance. All these elements combine to produce an incredibly realistic image and a really good plasma panel is capable an astounding level of picture quality.
Of course, no technology is perfect and even plasma panels have their issues, the first of which is that since the panels are made of glass, they can be quite reflective. The manufacturers of plasma panels have tried to address this by using an anti-glare filter material that both improves the perceived black levels and rejects ambient light. In older plasma panels the phosphors took longer to disappear and as a result moving objects on the screen would leave phosphor trails behind their images but with the introduction of faster decaying phosphors, this is far less of an issue. One of the areas where plasma panels have received a considerable amount of criticism in these eco conscious times is their power consumption. However, the manufacturers of plasma panels have made a concerted effort to reduce the energy consumption of their panels and in reality, a properly set up plasma will use far less energy than is shown in the specifications.
Perhaps the biggest worry consumers have about plasma displays is the danger of image burn-in, which occurs when the same picture is displayed for long periods of time. This causes the phosphors to overheat, losing some of their luminosity and producing a ‘shadow’ image that is visible on the screen. Early plasma panels were plagued with burn-in, making it impossible to use them with video games or anything else that displayed static images. Plasma manufacturers have tried various ways of reducing burn-in such as using grey pillar boxes, including ‘pixel orbiters’ that move the entire image faster than is noticeable to the human eye and scrolling images to remove any possible burn-in. However, nothing to date has eliminated the problem entirely and all plasma manufacturers continue to exclude burn-in from their warranties. In reality though, as long as you are careful with your plasma and avoid displaying static images for too long, especially during the first few months after purchase, you should have no problems with image burn-in.
Plasma panels also exhibit another issue which is sometimes confused with image burn-in damage, which is called image retention. In this case, when a group of pixels are run at high brightness, such as when displaying white, a charge build-up in the pixel structure occurs and a ghost image can be seen. However, unlike burn-in, this charge build-up is only temporary and usually self corrects after the image that caused the effect has been removed or a long enough period of time has passed. With modern plasma panels, image retention has become less of an issue and, thanks to much of the technology developed to prevent image burn-in, it is rarely seen these days.
There has been a continuing myth propagated by certain retailers, that a plasma needs ‘re-gassing’ after a specified period of time. This is simply not true and is in fact ludicrous, after all how would anyone go about replacing the gas in over six million minuscule plasma cells? Whilst it is true that earlier plasma panels did have phosphors that lost luminosity over time, resulting in a gradual decline of absolute image brightness, newer models are advertised as having life spans exceeding 100,000 hours. Or to put it another way you could watch 8 hours of TV on your plasma panel every day for over 30 years before needing to replace it.
Although the arrival of OLED TVs offers the potential of a picture that is as good as, if not better, than a plasma panel, the initial cost of these new displays will preclude them from the majority of consumers. So for the time being at least, plasma panels still offer the best choice for both enthusiasts and consumers who are looking for uncompromising picture quality at a sensible price.
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