In a Plasma TV, there are many tiny little cells between two glass plates. Tese cells are filled with a mixture of the noble gases neon and xenon (some manufacturers add helium, too). Three cells each for red, green and blue together represent a pixel. The RGB colors are created using differently colored luminescent materials that the forward inner sides of the cells are coated with.
The functional principle is as follows: The mixture of noble gases is ignited, and during this very process the mixture changes its state of aggregation from gaseous to an ionized gas (=plasma). This plasma emits particles and radiation, while in plasma TVs, the ultra violet radiation is used for stimulate the luminescent material of phosphor in such a way that it illuminates.
In order to not only have an On/ Off state - it wouldn't be possible to create large color spectrums with this - the cells are ignited in very quick intervals. The interval length decides about the amount of brightness the RGB colors light up with and, in doing so, it is possible to create millions of color nuances by blending ground colors in an additive process.
The advantage of Plasma TVs is that they can be produced basically up to any given screen size.
The shortcomings of Plasma TVs are the gradually occurring and after some significant loss of brightness (after approximately half of its service life, it has dropped to about 50 percent of the initial brightness of when it was new) because the luminosity of the phosphor material decays. What is more, contents that don't move, like channel logos for example, can burn in over the years, which will then permanently appear as shadows. The pretty high power consumption is another drawback of Plasma technology.
An LCD TV uses TFT technology (TFT is short for Thin Film Transistor) the so called panel is composed of two plates with a liquid in between that contains the liquid crystals. There also are polarization and color filters embedded between both of the plates. The liquid crystals are aligned to two alignment layers. In order to change the alignment of the liquid crystals, a voltage is impressed on the alignment layers by means of the Thin Film Transistors. Consequently, an electrical field aligning the liquid crystals is generated.
One pixel consists of three subpixels for the colors RGB. Since each sub pixel is controlled by a transistor it is called an activematrix. The alignment of the crystals decides about how much of the backlight's brightness reaches the color filters and thereby about the intensity the subpixel will illuminate with. The millions of color nuance are achieved by the additive color blending of the ground colors RGB.
There are various panel technologies which differ in certain aspects of their structure and have their own pros and cons each. TN (Twisted Nematic), IPS (In Plane Switching also available as Dual Domain In-Plane Switching und Super Inplane Switching)), MVA (Multi Domain Vertical Alignment) and PVA (Patterned Vertical Alignment).
Most of the time, the backlight is made from several mercury-vapor lamps. Then there is the reflector and the diffuser which distribute the brightness of the backlight evenly across the panel area. It has been recently announced that LCD TVs with LED (Light Emitting Diode) backlight will enter the market soon. We could see such LCD TVs at some manufacturers' booths as early samples at CeBit 2005.
The benefits of LCD TV are the low power consumption, the low weight and generally high service life of the backlight.
A disadvantage of LCD TV is that they can not be manufactured at arbitrary screen sizes. Furthermore, an LCD panel is more expensive at the same size. But LCD TV prices constantly keep on falling and are consequently about to come down to the levels of Plasma TVs.