What is the difference between the flat screen technologies ?



Both LCD tvs and Plasma tvs appear to give similar levels of performance. So is there any real difference? Both are slim and can be hung on the wall. Many people will incorrectly refer to small screens as plasma tvs when they are LCD tvs. However both technologies work and perform very differently. What is the difference between the two technologies?

How LCD tvs work

LCD is an abbreviation for Liquid Crystal Display. A LCD tv screen is made up of two clear panels between which is a matrix of colour liquid filled pixels. These pixels respond to a small voltage which makes them change state by twisting or untwisting which allows light to pass through them or to be blocked depending on the state of the pixel. This process of twisting takes very little power. The pixels are either red, green or blue and they are structured in a matrix of millions of pixels to make a picture. The coloured pixels of the panel are illuminated from Behind with a back light to show their colour, and areas of light and dark, depending on how the pixels are twisted. Lcd screens are available in small sizes for watches up to 108 inches for large televisions. Manufacturers such as LG, Samsung, Hitachi, Panasonic, JVC, Pioneer, Sharp, Toshiba, Philips and Sony have a range of lcd televisions available.

How does a Plasma television work ?

The screen of a Plasma tv is made up of millions of microscopic gas filled glass cells which have electrodes in them. The cells are filled with inert gases including xenon and neon and form each pixel. When a current is applied to the cell electrodes the gas atoms become 'excited' and become a plasma emitting photons of ultraviolet light. When the photons strike the phosphor coating that is inside the glass cells light is emitted. The light colour emitted is determined by which of the three coloured phosphors are used - Red, green or Blue. These colours can be combined to produce billions of colours. When millions of the pixels are combined they form an image on the screen like LCD televisions. Plasma tvs are only manufactured by Panasonic, Philips, Pioneer, LG, and Samsung and are available in range of sizes from 32 inches through to 150 inches.

[b]How does the Picture quality and the performance compare?[/b]

Screen Brightness

LCD TVs can be brighter than plasma TVs. This perception of this brightness is of course reliant on where the TV is located and what is being viewed. In reality both technologies are capable of producing a level of brightness that is in excess of what is required in normal viewing.

Black Levels and the Contrast Ratio

The contrast ratio is a measure of the variation in light and dark tones that a panel can deliver - in the main a high ratio means greater details can be discerned. Historically LCD tv panels have had a lower contrast ratio than plasma tvs because the backlight bleeds through darkened pixels and lightens the image. New advances in technology have created techniques to prevent light leakage, and improvecontrast ratios so that they are more comparable with Plasma tv, but still not as good. Because each cell on a plasma tv is able to be switched off rather than blocking the light as per LCD tvs the blacks are blacker. For a television picture, the black level is very important because black encompasses the entire spectrum of colour. And so, the deepest blacks create the richest colours and thus more realism. Without deep black levels the colours look more 'pastel' like. Typically the black on an LCD tv screen is more 'grey' when compared to the plasma tvs black.

Colour Saturation

This is a measure of the correctness of the colours on the screen based on the existence of grey shades - the higher grey shades results in lower colour saturation. Plasma TVs have high colour saturation due to the way they emit light. The capability of Plasma tv pixels to be switched off when they are not in use prevents the emission of stray light that diffuses colour. This is why tints and hues on Plasma TVs are noticeably more vivid and vibrant

[b]Colour Gamut level[/b]

The colour gamut is the set of possible coulors within a colour spectrum that a screen can display. The top end models of Plasma tv and LCD tv manufacturers are claiming that the colour gamut is near to the full spectrum. On comparable priced LCD tv and Plasma tvs, the Plasma tv again out performs the LCD tv on all but the top end models. The reason for this is that for the same manufacturers the colour gamut isnt as good on the lower models. On plasma tv models the cheaper models don't have such a difference compared to more expensive models from the same manufacturer.

TV Screen Resolution

This can be defined as the amount of information shown on a screen. The more information that is on a screen the higher the quality of the image. The higher resolution usually means that the screen can display a higher quality image with more detail and sharpness. The resolution is measure in pixels. LCD HDTVs and Plasma HDTVs (High Definition Televisions) have a resolution of 1920 pixels x 1080 pixels or 1080P. Where '1080' is the vertical resolution and 'P' is progressive scan meaning that the image isn't interlaced. Usually LCD HDTVs are cheaper than the same sized Plasma's. HD plasma are available in sizes of 42 inches upwards whereas LCD HDTVs are available from 32 inches upwards. Therefore at sizes 32 inches to 37 inches LCD tvs have the advantage of offering 1080P HD compared to the 720P (1280 pixels x720 pixels) HD Ready Plasma tvs. At sizes less than 32 inches there aren't any plasma tv models.

Refresh Rate and Response Time

These two attributes in combination influence how fast a moving picture a screen can reproduces without the image blurring. Response time is a measure of how quickly a screen can change when an input is received. Historically LCD tv panels had slow response times which was the limiting factor causing motion blur because each pixel has to go from an on state, to an off state, and back to on in order to refresh an image. Improvements in pixel response times on LCD tv screens means that the actual response time isn't the main cause of motion blur, its mostly caused by the refresh rate, or the frame rate.

All LCD tvs and Plasma tvs have until a short time ago had frame rates of 50 frames per second or 50hz. The more costly LCD tv and Plasma tv models now have 100 Hz to compensate for any motion blur. Since the regularsignal is only 50 Hz the added frames are created with signal processing software which interpolates what the extra middle frames should look like. The end result is moving pictures that loose hardly any definition and are more fluid. The best Plasma tv screens nevertheless respond better to rapid motion than LCD tv but the variation has narrowed as the best LCD tvs motion response time has improved and 100hz and 200hz has been introduced.

[b]The Viewing Angle of the screen[/b]

Pictures on plasma TVs are nevertheless clearly viewable at about 160 degrees and on some models up to 180 degrees. But the picture dulls beyond about 100 degrees.

[b]The screen surface[/b]

LCD tv screens are available with matt finish screens which reduce glare whereas Plasma tvs have a reflective screen.

Image Burn-in

This is characterised by a 'ghost image' of a previously displayed static image that had been displayed for a prolonged period of time and is still present. Even if the screens image is changed from the original 'ghost image' or removed it is present for the rest of the plasma tvs life. For LCD tv screens burn-in isn't a problem no matter how long the static image is displayed. Nonetheless the burn-in risk is overstated and improbable in normal viewing conditions.

Image retention

image retention is often confused with burn-in which has similar characteristics. However image retention is a 'ghost image' that appears on a plasma tvs after a still picture has been displayed for an extended period of time, but then disappears when a bright new image is displayed. Or it vanishes after a few seconds. By having a 'break in' period of about 100 hours from new, this can be minimised. Whist watching programs during this break in period care should be taken to watch programs that don't have any static bright images like station logos and scrolling news headlines at the bottom of the screen. Also try to watch programs that fill the whole of the screen. It recommended that during this period the brightness and the contrast is kept at about a mid level. These characteristics of image retention and burn-in have been greatly reduced by technical improvements to reduce the possibility of burn-in or image retention.

How does the Power consumption of LCD tv and Plasma tv compare ?

The backlight on an LCD tv is constantly on so the power consumption is almost constant. By manually adjusting the brightness of the back light up or down the power consumption is either increased or decreased. The twisting and untwisting of the LCD pixels consumes hardly any power.

Because Plasma TVs charge the gas in the individual pixels to a plasma to create the light, and the more light that is required on a scene the more energy is used. So pictures with low brightness in them use less energy, and high brightness more energy i.e. the power consumption varies.

The energy requirements that are quoted for Plasma tvs are for full brightness settings. The effect of this is that lcd tvs seem to use less power than plasma tvs. But the power consumption of a plasma tv varies depending on the amount of dark and bright areas on the displayed image. Studies have found that when the viewed programs are mainly dark or on movies the amount of power consumed for plasma tvs will be lower than an lcd tv. However when the viewed programs are sports and cartoons the power consumption for plasma tvs is greater than on lcd tvs. When varied programs are viewed the power consumption is similar for LCD tvs and Plasma tvs. Future plasma tvs are going to consume less than half the amount power consumed at present.

Life expectancy of Plasma televisions

Many 'experts' claim that plasma tvs don't last very long. But in reality manufactures are publishing life expectancy figures of 100000 hours, this equates to 11.5 years of non stop use. Clearly there is no reason for concern.

[b]Conclusion - which should you buy?[/b]

Both lcd tvs and plasma tvs are able to offer pros and cons. The lcd tvs have higher brightness and no chance of any screen burn. They are also available in thinner designs and they are lighter. When comparing picture quality the plasma tv have more life like colours, higher contrast and blacks that are blacker than lcd tvs. This is not necessarily the case for all plasma tvs compared to lcd tvs. A budget plasma tv will be out performed by a top of the range LCD tv. On balance when the pros and cons are considered a plasma tv is the obvious choice but spend as much as to can on one.