Technical Articles

The Inner World of LCD TVs - Part II


Let There Be Backlight

Regardless of the type of LCD technology, all LCD TVs must include a backlight located behind the LCD panel. White light from the backlight passes through the liquid-crystal material, and each red, green, or blue subpixel blocks more or less of it from emerging and reaching viewers' eyes based on the electric field applied to that subpixel. The more light that emerges, the brighter that subpixel appears on the screen.

LCD TVs used to employ CCFL (cold-cathode fluorescent) backlights, which consisted of several horizontally oriented fluorescent tubes, much like the tubes found in office light fixtures, only smaller. A diffuser placed in front of the tubes dispersed the light, distributing it much more evenly across the entire back of the LCD panel.

Fig.5: LCD TVs originally used thin fluorescent tubes as backlights.

These days, virtually all LCD TVs use LEDs (light-emitting diodes) as the backlight, because they use less power and are more environmentally friendly, since they contain no mercury. In addition, LEDs offer a greater degree of dynamic control—they can be quickly dimmed and brightened to enhance the apparent contrast of the image.

In most modern LCD TVs, the LEDs are mounted along one or more edges of the panel; this is called edgelighting. The LEDs can be placed along the top, bottom, left, and/or right sides. Special light-guide panels diffuse the light and bend it to pass through the LCD panel.

A few LCD TVs have LEDs mounted in an array directly behind the LCD panel; this is called direct or full-array backlighting. A diffuser is still required to spread the light evenly across the entire back surface of the panel, but it needn't bend the light to pass through the panel.

Fig.6: In LED-backlit designs, an array of LEDs is placed behind the LCD panel.

Direct or full-array LED backlighting offers several distinct advantages over edgelighting. For example, the illumination is more uniform across the screen, which is particularly evident in dark scenes.

Another advantage of direct LED backlighting over edgelighting is the possibility of a feature called local dimming. The TV's processor analyzes the image and dims the LEDs behind the dark areas while brightening the LEDs behind the bright regions. In essence, the LEDs form a low-resolution, monochrome image that corresponds to the high-res, full-color image on the screen, as shown in Fig. 7 below. This greatly increases the perceived contrast in the image, though it's not without its own problems, which I'll describe shortly.

Fig.7: In LCD TVs with full-array backlighting and local dimming, the LEDs behind the LCD panel form a low-resolution, black-and-white version of the image on the screen, darkening the dark parts and brightening the bright parts.

So why don't more LCD TVs use LED backlighting? For one thing, it means the TV can't be made as thin as it can with LED edgelighting—and thin seems to be a top priority with consumers these days. Also, LED backlighting with local dimming is more expensive to implement, so it's normally found only in high-end sets. (Vizio recently bucked this trend by announcing that its entire 2014 line of LCD TVs will use LED backlighting with local dimming.)

A growing number of companies are introducing low-cost LCD TVs with backlighting instead of edgelighting. However, these sets have no local dimming, and there are fewer LEDs than in sets with local dimming. To distinguish them from full-array local-dimming (FALD) sets, the backlight technology is often referred to as "direct backlighting," which still has the advantage of more uniform screen illumination than edgelighting. Go to Page 6: Perennial Problems