However, I get a lot of questions as to how new TV technologies (Plasma, LCD, and DLP) actually work to produce images. Here is an overview that should shed some light on the difference between CRT and current TV technologies.
CRT Technology
CRT televisions employ an electron beam scans rows of phosphors on a line-by-line basis in order to produce an image. The electron beam originates from the neck of a picture tube (essentially a large vacuum tube). The beam is deflected on a continuous basis that so it moves across the rows of phosphors in a sequential manner.
Depending on the type of signal, the phosphor rows can be scanned alternately, which is referred to as interlaced scanning or sequentially, which is referred to as progressive scan. For more details on the difference between interlaced and progressive scanning, check out my article: Progressive Scan - What You Need To Know
Plasma Technology
Plasma televisions, on the other hand, although employing phosphors similar to a CRT, the phosphors are not lit by a scanning electron beam. Instead the phosphors in a Plasma television are lit by superheated charged gas (similar to a Fluorescent light). All the phosphor picture elements (pixels) can be lit at once, rather than having to be scanned by an electron beam as is the case with CRTs. Also, since a scanning electron beam is not necessary, the need for a bulky picture tube (CRT) is eliminated, resulting in a thin cabinet profile.
For more technical details, check out How Plasma TV Works (How Stuff Works).
LCD Technology
Taking another approach, which also results in a thin cabinet profile, unlike a traditional CRT televisions, the images on an LCD television are also not "scanned" by an electron beam. The picture elements (pixels) of an LCD Television are merely turned off or on at a specific refresh rate.
In other words, the entire image is displayed (or refreshed) all at once every 24th, 30th, 60th, or 120th of a second. Actually, with LCD you can engineer refresh rates of 24, 25, 30, 50, 60, 72, 100, 120, 240, or 480 (so far). However, the most commonly used refresh rates used in LCD TVs is 60 or 120. Keep in mind that refresh rate is not the same as frame rate.
For more specifics on what refresh rate is, how it works, and how it is different that frame rate, check out my article: Video Frame Rate vs Screen Refresh Rate.
It must also be noted that LCD pixels do not produce there own light. In order for an LCD television to produce a visible image the LCD's pixels have to be "backlit". The backlight, in most cases is constant. What happens in this process is that the pixels are rapidly turned on and off depending on the requirements of the image. If the pixels are off, they don't let the backlight through, when they are on, they let the backlight through.
For a more technical look at how this process works, check out: How LCD Works (How Stuff Works).
It is important to note that there are new backlight technologies which enhance the pixel on/off process, such as Global Dimming and Local Dimming. These dimming technologies employ an LED-based backlight (either full array or edge light system) rather than traditional Fluorescent backlighting.
Global Dimming can vary the amount of backlight hitting all of the pixels for dark or bright scenes, while Local Dimming (not applicable to edge light systems) is designed to hit specific groups of pixels depending on which areas of the image need to be darker or lighter than the rest of the image. For a detailed look at Local Dimming and LED use in LCD TVs, check out an informative article from Home Theater Magazine.
DLP Technology
Still another technology used in televisions (rear projection televisions, that is) is DLP (Digital Light Processing), invented, developed, and licensed by Texas Instruments.
The key to DLP is the DMD (Digital Micro-mirror Device), in which every chip is made up of tiny tiltable mirrors. This means that every pixel on a DMD chip is a reflective mirror.
The video image is displayed on the DMD chip. The micromirrors on the chip (remember: each micromirror represents one pixel) then tilt very rapidly as the image changes.
This process produces the grayscale foundation for the image. Then, color is added as light passes through a high-speed color wheel and is reflected off of the micromirrors on the DLP chip as they rapidly tilt towards or away from the light source. The degree of tilt of each micromirror coupled with the rapidly spinning color wheel determines the color structure of the projected image. As the amplified light bounces off the micromirrors, it is sent through the lens, reflected off a large single mirror, and onto the screen.
For further technical explanations on DLP, check out my article: Rear Projection Televisions: DLP as well as the Texas Instruments DLP Website.
Fixed Pixel Displays
Despite the differences between Plasma, LCD, and DLP televisions, there is one thing they share in common.
Plasma, LCD, and DLP TVs have a finite number of screen pixels, thus they are "fixed-pixel" displays. Input signals that have higher resolutions must be scaled to fit the pixel field count of the particular Plasma, LCD, or DLP display. For example, a typical 1080i HDTV broadcast signal needs a native display of 1920x1080 pixels for a one-to-one point display of the HDTV image.
However, since Plasma, LCD, and DLP televisions can only display progressive images, 1080i source signals are always either deinterlaced to 1080p for display on a 1080p TV, or deinterlaced and scaled down to 768p, 720p, or 480p depending on the native pixel resolution of the specific TV. Technically, there is no such thing as a 1080i LCD, Plasma, or DLP TV.
Digging Deeper
If this overview has wetted your appetite for more details on TV tech, read Display Myths Shattered: How Monitor & HDTV Companies Cook Their Specs posted by Maximum PC.
Also, as a supplement to this article, watch our Home Theater Video Tip: Proper Distance for Viewing Different TV Sizes.
