Technical & Editorial

The Inner World of LCD TVs - Part I


(Source: Oleg Lavrentovich, Liquid Crystal Institute, Kent State University)

The Big Picture

When you go shopping for a flat-panel television these days, the vast majority of available models are LCD TVs. Today's LCDs are super-slim, super-bright, and super-colorful thanks to many improvements made since Sharp introduced the first commercial LCD TV—with a screen measuring a whopping 14 inches—in 1988.

But exactly how does an LCD TV work? What improvements have been made over the last quarter century that led to this technology's overwhelming dominance in the flat-panel market? Let's begin with a broad overview...

Like all video displays these days, LCD TVs form an image in an array of tiny pixels. In most cases, the array is 1920 pixels horizontally by 1080 pixels vertically, though lower-end sets are sometimes 1280x720 or 1366x768. The pixels in an LCD TV are not emissive—that is, they do not emit light on their own as they do in plasma and OLED TVs. Instead, they allow more or less light to pass through them. As a result, LCD TVs are sometimes called transmissive displays because they transmit light through the pixels.

The light in an LCD TV comes from a backlight at the back of the screen. In the past, this backlight consisted of small fluorescent tubes called CCFLs (cold-cathode fluorescent lights), which are much like small versions of the fluorescent lights in a typical office. However, in modern LCD TVs, this type of backlight has been mostly replaced by LEDs (light-emitting diodes), because they have no mercury content, use electricity more efficiently, and can be controlled to a greater degree than CCFLs. In most cases, the LEDs are placed at the edges of the screen in order to make the entire TV thinner, though a few models have an array of LEDs directly behind the screen.

By the way, an LCD TV with LED backlighting is often called an "LED TV" by its manufacturer, a confusing marketing misnomer that leads consumers to think it's a different display technology than LCD. It's not—an LED TV is simply an LCD TV with an LED backlight, which is why I normally refer to them as "LED/LCD TVs."

No matter what type of backlight is used—CCFLs, LEDs on the edges, or LEDs directly behind the screen—the light is diffused by a special layer of plastic to make it as uniform as possible across the entire area of the screen. The light then passes through the pixels, each of which can be independently controlled to pass more or less light through it as well as filter the light into any desired color.

Fig. 1: Layers of an LCD TV (Source: Sony Electronics)

In Figure 1, from Sony, you can see the various layers of an LCD TV using HCFL (hot-cathode fluorescent light, similar in principle to CCFL) and LED backlights behind the screen and on the edges. In this example, Sony uses red, green, and blue LEDs behind the screen, but virtually all such backlights use white LEDs these days.

The TV's electronics control the brightness and color of each pixel and process the incoming video signal. For example, if the number of pixels in the signal does not match the number of pixels on the screen, the processor scales the image, adding or subtracting pixels to match the resolution of the screen. If the signal is interlaced—that is, only half of each video frame is sent at a time—the processor deinterlaces it so the TV can display an entire video frame.