Plasma Tv or LCD TV?

In the old days cathode ray tube (CRT) televisions were mainstream. You still can buy CRT TVs, they are cheap now but rare. Plasma TVs came by about in 1998. In the beginning they were very expensive. Now take a look at how they work.

Compared with the largest CRT televisions, plasma TVs have a wide screen. Plasma screens are about 6 inch (15 cm) thick. The TV set lights up tiny dots (called pixels) with a high-energy beam of electrons based on the information in a video signal. Most systems feature 3-pixel colors (red, green, and blue), which are evenly distributed on the screen. The TV set can produce the entire color spectrum by combining these colors in different proportions. Plasma displays illuminate tiny fluorescent lights to form an image. Each pixel is made up of 3 fluorescent lights - a red light, a green light, and a blue light. The plasma display varies the intensities of the different lights to produce a full range of colors.

The central element in a fluorescent light is a plasma, a gas made up of free-flowing ions (electrically charged atoms) and electrons (negatively charged particles). Under normal conditions, a gas (or any material for that matter) is made up of elecrically neutral atoms. In other words, gas atoms contain equal numbers of electrons and protons (positivley charged particles). When an electrical voltage is applied to the gas, that is you introduce many free electrons into it, the free electrons collide with atoms, knocking loose other electrons. Atoms loose an electron and thus their balance. So the atoms become ions. In the electrically charged plasma, electrons rush toward the positively charged area of the plasma, and positively charged particles rush toward the negatively charged area. This way particles are constantly bumping into each other in the plasma. Collisions excite the gas atoms in the plasma, causing them to emit ultraviolet (UV) light photons that are invisible to the human eye. These UV photons are used to generate visible light photons.

Inside the display, the gas is contained in hundreds of thousands of tiny cells. The cells are positioned between two plates of glass. There are long electrodes between the glass plates, on both sides of the cells. These are either address electrodes which are positioned behind the cells along the glass plate, or they are transparent display electrodes, which are positioned above the above the cells, alond the front glass plate. The transparent display electrodes are surrounded by a dielectric layer and a protective layer.

The address electrodes are arranged in vertical colums accross the screen, while the display electrodes run horizontally across the screen. I have described above how the cells are caused to release UV light. There is a phosphor material coated inside the walls of the cells. Every pixel is made up of 3 subpixel cells, each with differently colored phosphor (red, green, and blue). When the UV light hits the phosphor, visible light is emitted. The plasma display varies the intensities of the different colors to produce a full spectrum of colors. This is how plasma TVs display the picture.

LCD (liquid-crystal display) screens work by blocking light. A solution of twisted nematic (TN) crytal liquid is positioned between two perpendicularly aligned panes of polarized glass. Electrical voltage is introduced into the liquid crytals. The crystals will untwist depending on the charge running through them. This way it becomes possible to manipulate the intensity of light that is able to pass through the second polarized pane. LCD displays can switch between light states (where the crytals are fully twisted) and dark states (where the crytals are fully untwisted), or somewhere along the greyscale in between.

From the back (the inner side of the screen) to the front (the surface of the screen), the LCD display is made up this way: there is a fluorescent backlighting, then comes a mirror, then the vertical polarizing filter, then the rear glass plate. After the rear glass plate there is a negative electrode plate, then the liquid crytal layer, then a positive electrode plate, then the color filter layer, then the front glass plate. Then comes the horizontal polarizing filter, and finally the front plate. The screen is divided into tiny pixels. The polarizing filters have red, green and blue channels. Each pixel contains all 3 channels.

You can see that unlike plasma TVs, LCD TVs need a backlighting. There's a process here called addressing. This means that based on the video signal, the display turns certain pixels on (which disables the passage of light) and switches other pixels off (which enables the passage of light). The light source for the backlighting is either a series of fluorescent bulbs (cold cathode fluorescent lamp - CCFL) or LED (light emitting diode). LED is a more modern technology, and you can see TVs using this thechnology being described as LED TVs, but basically they are still LCD TVs so this term is a little bit misleading.

The backlighting emits white light. This light passes through the crystals, then through the polarizing filter. Since all wavelenghts (of light) can pass through, the full spectrum of light can be manipulated to create the desired color. This is how LCD screens create the picture.

The production of LCD televisions dates back to about 2001.

CONTRAST AND BLACK LEVELS

Plasma TVs have high contrast ratios. Some manufacturers brag about a contrast ratio of 10,000:1. Plasma displays use internal algorythms to block the power to particular pixels in order to render a pixel dark or black. Plasma TVs achieve impressive blacks this way, although this can limit the television's grey scaling. Plasma TV sets use the most power when they are producing full whites. LCD TVs utilize electric charges to twist and untwist liquid crystals, which causes them to block light and thus, to emit blacks. The higher electrical voltage is applied to the crytals in a given pixel, the more effectively blocks it the light. This makes the pixels darker. Contrary to plasma displays, LCDs use the most power when they are producing blacks or a dark image. This is a difficult process, and only the best LCD displays have achieved a contrast ratio of 500:1. In this field, clearly the plasma technology is the winner.

COLOR ACCURACY

In plasma displays, each pixel contains red, green and blue elements. This way billions of colors can be produced. This way color information is very accurately reproduced. Color saturation and color richness are remarkable. The colors look very naturally. LCD displays reproduce colors by manipulating the wave lenght of the light and subtracts colors from white light. Color accuracy and vibrancy is inherently difficult to maintain this way, though most LCDs still have an outstanding performance. When comparing LCDs and plasmas with similar pixel counts, plasmas are more impressive. Plasmas have an advantage in this field, too.

VIEWING ANGLE

With plasma technology, each pixel produces its own light, instead of light being spread across the screen from one central source. Therefore, plasma TVs have a viewing angle of aBOUT 160 degrees. LCDs utilize one central source of light, thus, they do not give as great a viewing angle as plasmas. The best LCDs have a viewing angle of about 120 or 130 degrees. Therefore, plasma technology is the winner here.

DEPTH PERCEPTION

Plasmas have a deeper cell structure. Depth perception is therefore better with plasma TVs.

BRIGHTNESS

As LCDs produce white naturally with untwisted crystal and uses the least power when displaying whites, LCDs are naturally brighter than plasmas. LCD is the winner here.

FAST-MOVING IMAGES

LCDs have a longer response time, therefore, motion blur my be a problem with LCDs, especially with the cheaper models. A slight motion response lag may occure with fast-moving images. Plasma TVs have an excellent performance with fast-moving images. Plasma technology is the winner here.

The new television broadcasting system is the High Definition Television, or simply HDTV. This offers higher image resolution and better sound system than previous systems such as PAL, NTSC and SECAM. Now HDTV signals come in 4 variants: 720i which is 1280x720 interlaced, 720p which is 1280x720 progressive scan, 1080i which is 1920x1080 interlaced, and 1080p, which is 1920x1080 progressive scan. HD Ready (or 720p HDTV) sets are capable of accepting HD signals, but they are only have a maximum display resolution of 1280x720. Full HD (or 1080p HDTV) sets are capable of accepting HD signals and displaying them at afull resolution of 1920x1080. HD Ready TVs have to scale the resolution down to 720i/720p to fit their screen when they receive a 1080i/1080p signal. In other words, you lose image resolution and therefore, clarity.

For the same screen size, Full HD sets are considerable more expensive than HD ready sets. If money is not a problem, go for the Full HD. But if you are to save money, go for the HD Ready. Also important to note that with TVs smaller than 42 inch, the visible resolution difference is meaningless between Full HD and HD Ready sets. Therefore, if you buy a small TV, it is advisible to spare money and get a HD Ready television.

Plasmas use a lot of electricity to light each pixel on the screen. LCDs use fluorescent backlighting, thus they consume considerably less power. LCD displays consume about half the power plasma displays do.

A plasma TV is essentially a glass substrate envelope wiht gases compressed therein. In a low air pressure environment, therefore, an air-pressure differential will occur. Due to this, plasma TVs at high altitudes will give a sound similar to the buzzing of an old neon sign. LCDs are insensitive to changes in air pressure. This is why LCDs are far more often used on airplanes.

LCD diplays are not prone to it because of the properties of the "twisting crystals". Static images may "burn in" with plasma displays. For various plasma models, the time it takes for the picture to burn in varies greatly. It all depends on the anti-burn technology of the manufacturer. For the latest plasma models it can take as long as an hour to burn in.

Plasma displays utilize slight electric impulses to excite the gases contained therein. This process will wear down the phosphoric elements contained in the plasma display over time. Then the plasma screen will glow half as brightly compared to when the TV was new. Some manufacturers claim a hal-life of about 100,000 hours.

LCD manufacturers claim that LCD sets will last fro about 100,000 hours. However, LCD screens are basically a prismatic substrate which light passes through. Therefore, in reality, LCDs can never be worn out. An LCD will last as long as its backlight does - and the backlight can sometimes be replaced. But there's a catch - the backlight can change colors as it ages and that will ruin the white balance. When this occurs, you will have to recalibrate your TV, replace the backlighting if you can, or throw the whole unit out if you cannot raplace it. And it is not easy to tell if the backlighting can be replaced or not. Manufacturers usually don't tell it for various reasons.

Having introduced the two systems, I hope this will help you in making the decision when you buy a TV set.