Plasma

A plasma TV is sometimes called an "emissive" display — the panel is actually self-lighting. The display consists of two transparent glass panels with a thin layer of pixels sandwiched in between. Each pixel is composed of three gas-filled cells or sub-pixels (one each for red, green and blue). A grid of tiny electrodes applies an electric current to the individual cells, causing the gas (a
mix of neon and xenon) in the cells to ionize. This ionized gas (plasma) emits high-frequency UV
rays, which stimulate the cells' phosphors, causing them to glow the desired color. Each individual plasma cell is switched on and off by its own electrode. An HDTV-capable plasma TV can have up to 3 million of these cells.

Because a plasma panel is illuminated at the sub-pixel level, images are extremely accurate, and the panel's light output is both high and consistent across the entire screen area. Plasma TVs also provide very wide horizontal and vertical viewing angles, especially when compared to conventional rear-projection TVs. Picture quality looks sharp and bright from virtually anywhere in the room. Because plasma TV screens do use a phosphor coating (like direct-view and projection CRT TVs), the potential for image burn-in exists, so it's important to follow the manufacturer's recommendations on day-to-day use. Until recently, all flat-panel TVs were progressive displays — at any given moment all of the pixels are illuminated. But plasma TVs based on innovative AliS technology (Alternate Lighting of Surfaces) are proving that a non-progressive picture can look outstanding. Typical plasma panels have a strip of electrodes for each horizontal row of plasma cells, while ALiS panels share an electrode strip between two rows of cells. At any given instant only half the panel's pixels are turned on. It's somewhat similar to interlaced-scanning on a CRT-based TV — in fact, ALiS technology was developed as a way to make a simpler, lower-cost plasma panel capable of displaying interlaced HDTV signals (1080i).

Two HDTV-capable plasma panel designs. The ALiS panel (left) is a simpler design that uses a "trough" structure instead of the conventional "cell" structure, while also streamlining the drive electronics (fewer electrodes). Bottom line: Both panel designs can deliver outstanding picture quality.

Because there is less screen "real estate" taken up by electrode strips, there's more illuminated area, and as a result, ALiS panels provide a seamlessly clear, bright image. Other ALiS benefits include high-efficiency — they require only half the voltage of conventional plasma drive systems — and extended panel life.

 

LCD (Liquid Crystal Display)

An LCD TV is sometimes referred to as a "transmissive" display — light isn't created by the liquid crystals themselves; a light source (bulb) behind the panel shines light through the display. A white diffusion panel behind the LCD redirects and scatters the light evenly to ensure a uniform image.

The display consists of two polarizing transparent panels and a liquid crystal solution sandwiched in between. The screen's front layer of glass is etched on the inside surface in a grid pattern to form a template for the layer of liquid crystals. Liquid crystals are rod-shaped molecules that bend light in response to an electric current — the crystals align so that light cannot pass through them. Each crystal acts like a shutter, either allowing light to pass through or blocking the light. The pattern of transparent and dark crystals forms the image. It's the same display technology behind your digital watch but way more sophisticated.
The multi-layered structure of an active-matrix LCD panel. Because they use red, green and blue color filters in place of phosphor dots, LCD panels are completely immune to image burn-in.

LCD TVs use the most advanced type of LCD, known as an "active-matrix" LCD. This design is based on thin film transistors (TFT) — basically, tiny switching transistors and capacitors that are arranged in a matrix on a glass substrate. Their job is to rapidly switch the LCD's pixels on and off. In a color TV's LCD, each color pixel is created by three sub-pixels with red, green and blue color filters.

One of the biggest challenges for LCD TV manufacturers has been speeding up the "pixel response" time (how fast an individual pixel's color can change without blurring) to ensure that fast-moving objects don't exhibit "motion lag" or ghosting. It's especially critical for larger-screen LCD TVs where much of the viewing will be DVD movies and/or HDTV.

An important difference between plasma and LCD technology is that an LCD screen doesn't have a coating of phosphor dots (colors are created through the use of filters). That means you'll never have to worry about image burn-in, which is great news, especially for anyone planning to connect a PC or video game system. LCD TVs are extremely energy-efficient, typically consuming 60% less power than comparably-sized tube-type direct-view TVs.