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Types of HDTVs: LCD, plasma, projection

By The Vann’s Editorial Team
Last revised December 11th, 2009

LCD

Liquid. Crystal. Display. LCD. It’s all in the name. Just as the name implies, LCD screens use liquid crystal as the foundation of their image reproducing technology. An LCD’s screen consists of an array of hundreds-of-thousands to millions of square pixels arranged into a grid. When different levels of electricity are applied to a layer of liquid crystal sandwiched between each pixel’s two light filters, the molecules that make up the liquid crystal arrange themselves in a particular way so that only a certain amount of the light being produced behind them is allowed to pass through and be seen by the viewer. While the liquid crystal is involved only in regulating the amount of light, a color filter placed between the light filter nearest the viewer and the viewer is responsible for determining the color of the light that passes through the screen. To create blacks, enough energy is applied to the liquid crystal so that its molecules arrange themselves so as to block almost all of the light.

Because of its small scale, LCD technology can be housed in very thin flat-panel displays that are easily and cleanly mounted on walls. Also, LCD technology has extremely high longevity — the average panel has a potential viewing life of 60,000 hours. Finally, LCD displays are often best suited to use in areas with higher levels of ambient light because their screens are less reflective than those of plasmas.

LED

LED stands for Light Emitting Diodes, and it’s quickly becoming the “next big thing” in TV tech. However, so-called “LED TVs” are really not a new “class” of TV — they’re a different type of LCD. What makes LED LCDs different from other LCD TVs you’ll find is the lamp. Instead of the standard incandescent bulbs used to light most LCD panels, LED panels are lit by . . . LED bulbs. This new lamp technology offers several advantages over traditional LCDs. First of all, LEDs allow for a brighter picture with more vibrant colors. Secondly, due to their nature, LEDs consume quite a bit less electricity than older LCDs or plasmas of the same size.

It’s important to know that there are two different types of LED lighting used in LCDs. Backlighting refers to the use of thousands of small LED bulbs behind the viewing screen for lighting. This brings one especially cool feature with it, known as local dimming. Some bulbs can be turned on while others are off for a more lifelike contrast than on typical LCDs with darker blacks than ever. The other type of LED LCD TV is an edge-lit screen. With this type, LED bulbs are placed all around the screen rather than behind it. Edge-lighting is cool because it allows for thinner LCD screens than ever with some models just over an inch thick.

Other than the features mentioned, LED LCDs are really the same as any other LCD!

Plasma

A whole new state. Elementary science class reminds us that there are four states of matter: solid, liquid, gas, and plasma. Just as with LCD panels, with plasma panels, it’s all in the name. Plasma screens also consist of an array of hundreds-of-thousands to millions of pixels arranged into a grid. While LCD pixels sandwich liquid crystal between light-filter panes, plasma pixels position a gas-filled cell between two glass plates. When electricity is applied to a pixel cell, the gas is ionized, becoming plasma, and the movement of the gas ions results in the emission of photons, or particles of light. While the ionization of the gas is responsible for the production of light within a plasma display, both the amount of electricity applied to a pixel and the use of phosphor coating on cells result in the production of color.

The compact nature of plasma display technology also allows for a flat panel design perfect for mounting on your wall and freeing up space in your living room. Plasma displays also have a potential life of 60,000 hours — nearly 30 years at five hours of use per day. Plasma screens are better suited than LCDs to use in situations where there is a lower level of ambient light, for two reasons. The first reason has to do with screen materials. While the slightly reflective nature of plasma screens that can result in unwanted glare in high-light situations, this is not an issue under low-light conditions. The second reason concerns the production of black levels. Black levels, or the ability of a display to reproduce blacks, is a key component in the reproduction of high-definition images. More blacks mean more contrast and more detail. Plasma displays completely shut pixels off to achieve deep, high-detail black levels. Because this method reduces the luminosity of the panel, plasmas are better suited to low-light conditions. LCD displays, on the other hand, reproduce black levels by blocking the passage of light through a pixel. Because LCDs can not completely block all of the light, lower black levels are attained and these panels may appear overly bright in low-light situations.

As a final note, there is some concern about the risk of “phosphor burn-in” with plasma displays. This refers to a phenomenon by which the prolonged display of a stationary image on a plasma display can cause a “ghost-like” residue of that image to remain, permanently. Phosphor burn-in occurs because the phosphor coating on the individual pixels can lose luminosity with use. When the same image is displayed constantly (even on just a small part of the screen), the pixels involved in displaying that image begin to fade or dim (i.e. lose their luminosity) and can be visible to the eye. Phosphor burn-in occurs most commonly when a television station that displays its logo in the corner of the screen is left on for extended periods of time. Also, menu screens on video games and DVDs can have a similar effect if left on for long periods. While phosphor burn-in can happen in these extreme cases, many manufacturers have developed technologies that greatly reduce the possibility of burn-in. Look for pixel orbiting, image washing or screen-saver modes.

Projection

Three modes, one principle. Projection televisions operate by magnifying a small image from a video signal onto a larger, viewable screen. While there are a variety of technologies for magnifying the video image, there are basically two projection formats: rear projection and front projection. In rear projection setups, the image source is located behind the screen, within the television unit. On the other hand, in the case of front projection televisions, the image source is an independent component positioned in front of the screen, which is often nothing more than a large hanging screen, much like in a movie theater.

There are three principle image reproduction/magnification technologies associated with the projection television format. First, CRT or Cathode Ray Tube technology — the same technology used in standard televisions is exclusive to rear projection televisions and uses a much smaller tube than do standard TVs so as to reduce the unit’s cabinet size. Generally CRT televisions operate by firing a beam of electrons onto a phosphor-coated screen, which emits light when energized by the electrons. Modulation of the electron beam delivers the control necessary to cause a detailed image to be displayed. Some CRT units use one tube and different-colored phosphor screen coatings or a spinning color wheel to produce color, while other units use multiple tubes, each for a separate primary color. CRT projectors achieve excellent black levels and contrast ratios — central in reproducing high levels of detail — and have long life times and wider viewing angles than other formats. However, CRTs tend to be heavy and large, are susceptible to phosphor burn-in and can have focus problems.

LCD projection televisions, which can be either front or rear projection, operate on the same principle as LCD flat-panel televisions. Light is sent through a panel containing thousands or millions of individual pixels that control the passage of light by regulating the arrangement a layer of liquid crystal. The only difference is that projection LCDs use a small LCD chip while LCD flat-panels use a large, screen-sized LCD. Most commonly, the light coming from the light source is refracted into three separate primary-color beams that are then each sent through a distinct LCD chip. A lens then recombines the three beams and magnifies the single beam into a viewable image on the screen. Because of the use of three LCD chips, some projection LCDs are often referred to as 3LCD. Projection LCDs are smaller in size than CRTs and are not susceptible to phosphor burn-in, but have poorer black levels and infrequently experience the “screen-door effect,” in which individual pixels become visible on very large screens.

A final projection television technology is DLP or Digital Light Processing. Projection DLPs create a visible image with the use of a digital micromirror device or DMD. On its surface, a DMD chip contains a matrix of microscopic mirrors, each of which corresponds to a single pixel in the final image. Light from the light source (generally a lamp or LED source) is directed at the DMD and the mirrors are rotated in order to control the light that they reflect. In projection DLPs, a color wheel is placed in between the light source and the DMD, so that the light is colored before it reaches the mirrors which then are responsible for determining the brightness of the light. In some instances, similar to 3LCD, three DMDs are used and a prism is used to split the light into three primary-color beams each of which is directed to a separate DMD, before being recombined and magnified to viewing size by a lens. Projection DLPs achieve excellent black levels, are not susceptible to phosphor burn-in and do not experience the screen-door effect. They may, however, experience the “rainbow effect”, in which subtle flashes of colored light may be slightly visible. This effect is, however, exclusive to single-chip DLP projectors, and thus not found on three-chip DLP displays.

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