TV Technology Demystified

CRT, Plasma, LCD, DLP, and OLED TV Technologies Overview

Woman Standing In Electronics Store Facing TVs
Woman Standing In Electronics Store Facing TVs. Image provided by Getty Images - Glow Images, Inc - Collection: Glow - 142547173

Buying a TV can be very confusing these days, especially when trying to sort out what type of TV technology you want or need. Gone are the bulky CRT (picture tube) and rear-projection sets that dominated living rooms in the second half of the 20th century. Now that we are well into the 21st century, the long-awaited wall-mountable TV is now common.

However, a lot of questions remain as to how newer TV technologies actually work to produce images.

This overview should shed some light on the difference between past and current TV technologies.

CRT Technology

Although you can't find new CRT TVs on store shelves anymore, a lot of those old sets are still operating in consumer households. Here is how they work.

CRT stands for cathode ray tube, which is essentially a large vacuum tube—which is why CRT TVs are so big and heavy. To display images, a CRT TV employs an electron beam that scans rows of phosphors on the face of the tube on a line-by-line basis in order to produce an image. The electron beam originates from the neck of a picture tube. The beam is deflected on a continuous basis that so it moves across lines of phosphors in a left-to-right motion, moving down to the next needed line. This action is done so rapidly that the viewer is able to see what appear to be complete moving images.

Depending on the type of incoming video signal, the phosphor lines can be scanned alternately, which is referred to as interlaced scanning, or sequentially, which is referred to as progressive scan.

DLP Technology

Another technology, used in rear-projection televisions, is DLP (digital light processing), which was invented, developed, and licensed by Texas Instruments. Although no longer available for sale in TV form since late 2012, DLP technology is alive and well in video projectors. However, some DLP TV sets are still being used in homes.

The key to DLP technology is the DMD (digital micro-mirror device), a chip made up of tiny tiltable mirrors. The mirrors are also referred to as pixels (picture elements). Every pixel on a DMD chip is a reflective mirror so small that millions of them can be placed on a chip.

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 gray-scale foundation for the image. The color is then added as light passes through a high-speed color wheel and is reflected off the micromirrors on the DLP chip as they rapidly tilt toward 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 it bounces off the micromirrors, the amplified light is sent through the lens, reflected off a large single mirror, and onto the screen.

Plasma Technology

Plasma TVs, the first TVs to have a thin, flat, "hang-on-wall" form factor, have been in use since the earlier 2000s, but in late 2014, the last remaining plasma TV makers (Panasonic, Samsung, and LG) discontinued manufacturing them for consumer use.

However, many are still in use, and you may still be able to find one refurbished, used, or on clearance.

Plasma TVs employ an interesting technology. Similar to a CRT TV, a plasma TV produces images by lighting phosphors. However, the phosphors are not lit by a scanning electron beam. Instead, the phosphors in a plasma TV 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 details on plasma TV technology, check out our companion guide.

LCD Technology

Taking another approach, LCD TVs also have a thin cabinet profile like a plasma TV. They are also the most common type of TV available. However, instead of lighting up phosphors, the pixels are merely turned off or on at a specific refresh rate.

In other words, the entire image is displayed (or refreshed) 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 in LCD TVs is 60 or 120. Keep in mind that refresh rate is not the same as frame rate.

It must also be noted that LCD pixels do not produce their own light. In order for an LCD TV to display a visible image, the LCD's pixels have to be "backlit." The backlight, in most cases, is constant. In this process, 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, and when they are on, the backlight comes through.

The backlight system for an LCD TV can either be CCFL or HCL (fluorescent) or LED. The term "LED TV" refers to the backlight system used.  All LED TVs are actually LCD TVs.

There are also technologies used in conjunction with the backlight, such as global dimming and local dimming. These dimming technologies employ a LED-based full array or edge backlight system.

Global dimming can vary the amount of backlight hitting all of the pixels for dark or bright scenes, while local dimming 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.

In addition to backlighting and dimming, another technology is employed on select LCD TVs to enhance color: quantum dots. These are especially "grown" nanoparticles that are sensitive to specific colors. Quantum dots are either placed along the LCD TV screen edges or on a film layer between the backlight and the LCD pixels. Samsung refers to their quantum-dot-equipped TVs as QLED TVs: Q for quantum dots, and LED for LED backlight—but nothing that identifies the TV as an actual LCD TV, which it is.

For more LCD TVs, including buying suggestions, also check out our Guide to LCD TVs.

OLED Technology

OLED is the newest TV technology available for consumers. It has been used in cell phones, tablets, and other small screen applications for a while, but since 2013 it has been successfully applied to large-screen consumer TV applications.

OLED stands for organic light-emitting diode. To keep it simple, the screen is made of pixel-sized, organically based elements (no, they are not actually alive). OLED has some of the characteristics of both LCD and plasma TVs.

What OLED has in common with LCD is that OLED can be laid out in very thin layers, enabling thin TV frame design and energy efficient power consumption. However, just like LCD, OLED TVs are subject to dead pixel defects.

What OLED has in common with plasma is that the pixels are self-emitting (no backlight, edge-light, or local dimming is required), very deep black levels can be produced (in fact, OLED can produce absolute black), OLED provides a wide undistorted viewing angle, comparing well in terms of smooth motion response.

However, like plasma, OLED is subject to burn-in.

Also, indications are that OLED screens have a shorter lifespan than LCD or plasma, especially in the blue part of the color spectrum. In addition, current OLED panel production costs for the large-screen sizes needed for TVs are very high in comparison to all other existing TV technologies.

However, going with both the positives and the negatives, OLED is considered by many to display the best images seen so far in a TV technology. Also, one stand-out physical characteristic of OLED TV technology is that the panels are so thin that they can be made flexible, resulting in the manufacturing of curved-screen TVs. (Some LCD TVs have been made with curved screens as well.)

OLED technology can be implemented in several ways for TVs. However, a process that LG developed is the most common one in use. The LG process is referred as WRGB. WRGB combines white OLED self-emitting subpixels with red, green, and blue color filters. LG's approach is intended to limit the effect of premature blue color degradation that seems to occur with blue self-emitting OLED pixels.

Fixed-Pixel Displays

Despite the differences between plasma, LCD, DLP, and OLED televisions, they all share one thing in common.

Plasma, LCD, DLP, and OLED 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, DLP, or OLED 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, DLP, and OLED 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, DLP, or OLED TV.

The Bottom Line

When it comes to putting a moving image on a TV screen, a lot of technology is involved, and each technology implemented in the past and present has advantages and disadvantages. However, the quest has always been to make that technology "invisible" to the viewer. Although you want to be familiar with the technology basics, along with all the other features you desire and what will fit in your room, the bottom line is whether what you see on the screen looks good to you and what you'll need to make that happen.