by Armadeus Cornelius
[b]Are there any differences between the two technologies?[/b]
When it comes to flat screen TVs the two technologies LCD and Plasma look very alike with almost lifelike images and the ability to be hung on the wall. Although LCD tvs and Plasma tvs panels may look very similar in the stores, there are many differences between the two technologies.
How LCD tvs work
The LCD tv screen is a thin flat display device and is made up of a two clear panels which have many colour liquid crystal filled pixels that are arranged in arrays in between them. When a small voltage is applied to the crystals they twist or untwist repositioning themselves so that light can either pass through or it is blocked. When millions of crystals do this a picture is displayed. Very little power is used in this process. Behind the screen is a back light that illuminates the pixels and displays a colour depending on their colour (either red, blue or green), also areas of light and dark spots are displayed depending on how they are positioned. LCD devices are available in small sizes for items such as a watch up to 108 inches. Most television manufacturers produce LCD television including Hitachi, JVC, LG, Panasonic, Pioneer, Philips, Samsung, Sharp, Sony, and Toshiba.
[b]How does a Plasma screen display a picture?[/b]
A Plasma tv screen comprises of millions of tiny ‘light bulbs’ which are small glass cells filled with inert gases such as xenon and neon. These cells are lit by a current being applied to electrodes in the gas filled cell and its atoms become ‘excited’ to a plasma and emit photons of ultraviolet light. These photons in turn hit a phosphor coating which emits visible light. The colour of the visible light emitted by each cell depends on the three different coloured phosphors - green, blue and red, and can together make billions of colours when combined. As with the LCD tv screen, the millions of cells combine to form the image on the screen. Currently the only manufacturers of Plasma tvs in the UK are Samsung, Philips, LG, Panasonic, and Pioneer. Plasma tvs are available in sizes from 32 inches to 150 inches.
A comparison of the Picture quality and Performance
Screen Brightness
The picture on LCD tv screens can be brighter than Plasma tvs. Depending on where the television is located and what is being viewed will determine how this difference in screen brightness is perceived. However both Plasma tv and LCD tv screens are capable of producing brightness levels that in normal viewing conditions are in excess of what is needed.
Black Levels and the Contrast Ratio
The contrast ratio is a measure of the difference in light and dark tones that a panel can produce - generally 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. Recent advances in technology have created techniques to avoid 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 significant because black encompasses the entire spectrum of colour. Thus the deepest blacks generate 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
colour saturation is a measure, in the presence of grey shades, of colour purity or how accurate the colours are on the screen. The higher the colour saturation the more accurate the colour rendition. With blacks that aren’t as black the grey shades will be higher resulting in a lower colour saturation. Plasma offer better levels of colour saturation due to better black levels because of their ability to switch of pixels that aren’t in use, preventing diffusion of colours by stray light emissions. This results in hues and tints that are more life like colourful.
Colour Gamut
Is the breadth and amount of colours that can be displayed. The most costly models are now claiming to have colour gamut’s getting rather close to the full spectrum for plasma tv and LCD tv. In fact most plasma TVs tend to be better than most LCD tvs apart from the top of the range ones. With the cheapest LCD tv models usually offering a colour gamut that isn’t as good the higher LCD tv models. So as a rule the plasma tv has the best colour gamut.
Resolution
The screen resolution is a measure of the total pixels that makes the screen. The higher the definition, the higher the resolution and greater the detail and sharpness will be in the picture. LCD HDTVs have a resolution 1920 pixels x 1080 pixels or 1080P are, size for size, at a lower price than comparable Plasma tvs. 1080 is the vertical reolution and the P is for progressive scan because it isn’t interlaced. The best Plasma tv and LCD tvs screens above 37 inches are HDTVs with the same 1080P resolution i.e. there is no difference. At sizes of 37 inches down to 32 inches the best LCD tv screens are still 1080P set but the Plasma tvs are lower resolution HD ready 720P (1280 pixels x720 pixels) models. There aren’t any Plasma tvs smaller than 32 inches, but there are many LCD tv models.
Response Time and Refresh Rate
The combination of these two characteristics affects how a screen can handle fast moving pictures without blurring. The gauge of how quickly a screen can change when an input signal is received is the response times. In the past motion blur on LCD tvs was caused by slow response times where the pixels took too long to change state from, on to off, and back on to refresh an image. The latest models of LCD tv screens have improved pixel response times that now mean that actual response time is no longer the cause of motion blur. The main cause is now the frame rate refresh rate.
Less expensive models of Plasma televisions and LCD televisions operate at frame rates of 50 frames per second or 50hz. Recently 100hz tv models were launched on more expensive models which reduced motion blur by creating an extra middle frame which is placed in between the normal frames. This middle frame is created by advanced signal processing software that interpolates what the middle frame would look like. The addition of the middle frame results in moving images that are fluid with little, if any definition loss. Even with 100hz the best plasma tvs still out perform the best LCD tv models but by a narrowing margin because of reduced response times and 100hz and 200hz.
Viewing Angle
This is a measure of how far off axis you can view the picture before it fades. The viewing angle of a Plasma television is between 160 degrees to 180 degrees. On an LCD television the viewing angle is about 100 degrees at which point the picture dulls.
[b]The screen surface[/b]
Plasma tvs have reflective shinny screen surface which can be affected by glare depending on the viewing conditions. Lcd tvs have a matt finish which reduces glare.
[b]Screen Burn-in[/b]
For Plasma tvs With prolonged displaying of non-moving images, graphics or text, such as a menu bar, channel logo, or news scroll a permanent ghost image can be permanently burned on the screen with a darkened appearance. SO even if the image on the screen is changed or removed it can’t be removed for the rest of the plasma tvs life and is always seen as a shadow image. Regardless of how long a static image is displayed on an LCD tv they aren’t affected by burn-in. But the problem of burn-in is exaggerated and unlikely under normal viewing.
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.
[b]Is there any difference in power consumption of LCD tvs or Plasma tvs ?[/b]
The power consumption of an LCD tv is virtually constant because they have a back light that is on all of the time. The back light can be manually adjusted but uses less power on a low setting and more power on a high setting. The modulation of the LCD tvs screen pixels takes very little 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.
Conclusion
Both technologies have benefits. Plasmas tvs typically have a superior subjective picture with healthier black level, higher contrast, and superior colour rendering than LCD TVs. While LCD TVs have higher brightness, and no screen burn-in worries, and are lighter and thinner. This is not always the case sincea good LCD TV could have a better picture than an middling or poor plasma TV. But on balance I think that a plasma tv is the better option with more advantages. Ultimately you get what you pay for so spend as much as you can afford on you preferred format.