How LCD Panels Work

As you may have guessed, LCD panels are a bit more technical than you would first imagine. LCDs do not emit light; rather, they act as "shutters" to precisely control how much light is allowed through the panel. A backlight behind the LCD provides the light, and a color filter in front provides the color. Each "pixel" on an LCD is actually three separately addressable sub-pixels with red, green and blue filters. The amount of light let through each sub-pixel determines the color you'll perceive. For example, if all three sub-pixels let through as much light as possible, the pixel will appear white; if the green sub-pixel allows no light through, the pixel will appear magenta (one of the three subtractive colors used in printing). At normal viewing distances, the individual sub-pixels on an LCD panel aren't discernible, but you can see them easily with a magnifying glass.

An LCD panel used to construct a computer monitor or television consists of five layers, as illustrated below. Each layer plays a specific role in the effort to render the final result. Here are the individual roles of each layer as they pertain to commercial LCD computer monitors and televisions:

Normally, the opposing polarization filters in layers 2 and 5 would block any light coming through the panel– in other words, the default state of the pixel is black. The liquid crystals "rotate" the polarization of the light coming through the first filter so that it aligns with the polarization of the second filter, and the degree of rotation (which varies with the voltage applied to the pixel) determines how bright that pixel will be. While all LCD panels use this technology, manufacturers have developed different techniques to differentiate their products.

You may have heard of polarized sunglasses, which claim to block out unwanted reflected light. An interesting way to appreciate LCD technology is to view a LCD monitor with polarized sunglasses on, whilst slowly tilting your head on its side. Notice how the intensity of the light changes as you do this - that's how the polarization filters function in a LCD, by blocking out unaligned light.

There are three major types of LCD flat panels: twisted nematic (TN), vertical alignment (several varieties, such as multi-domain vertical alignment and patterned vertical alignment, exist), and in-plane switching (ISP). The technical properties of these display types are beyond the scope of this article, but in general:

• TN panels offer fast response times (good for games) and are a mature and inexpensive technology— most of the lower-cost panels you'll see at your local superstore are TN panels. However, their color reproduction isn't as good as the other types of panels, and they tend to have poor viewing angles, especially in the vertical plane.
• IPS panels offer the best color fidelity and viewing angle, but generally have slower response times. They're a good choice for graphic artists and others who require the best color reproduction.
• VA panels were developed to combine the strengths of TN and IPS panels. There are several variations of this technology, and it's a popular choice for higher-end consumer panels.

It can be difficult or impossible to determine the technology used in any given LCD monitor or television, as many manufacturers do not call out the technology used in any specific panel, even on their detailed specifications page. Virtually all monitors list their display or panel type as "Thin-film transistor (TFT) active matrix", but since every LCD monitor or television made is a TFT active matrix, that's not much help (the three panel types discussed above are all TFT active matrix). There are some exceptions: for example, Dell lists their 30" Ultrasharp monitor as having an IPS panel, and their 27" Ultrasharp as having a VA panel, but few of their other monitors show this information. You can sometimes tease the information out of Google by searching for "<manufacturer name><display type>", i.e. "Apple IPS".

Now let's start digging into the features and specifications you should be familiar with when shopping for a new monitor or television...

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