White Paper: LCD Technologies Explained and Compared

Posted 02/04/2010 at 9:00am | by Michael Brown

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Discover what separates the IPS and VA LCD monitors from inferior TN models

The performance of an LCD monitor ultimately depends on how its liquid crystals are manipulated to channel light. We’ll examine the three most common technologies: Twisted Nematic (TN), In-plane Switching (IPS), and Vertical Alignment (VA).

Each of these three technologies creates a pixel using a cell of liquid-crystal molecules controlled by a thin-film transistor. Liquid crystals are used because they’re capable of effecting light as though they’re a solid, while exhibiting the malleability of a fluid. In a color LCD, each pixel is subdivided into three cells, or subpixels, which are colored red, green, and blue, respectively, by additional filters. These cells are arranged in a matrix of rows and columns sandwiched between two panes of glass, with a polarizing film on the exterior side of each pane.

A light source, such as a cold cathode fluorescent lamp or an LED grid, is placed behind the first glass panel. Light waves from the backlight follow the alignment of the liquid-crystal molecules, but they must pass through the two polarizing filters before reaching the surface of the display. Light waves must be oriented perfectly parallel to the first filter to pass, but since the second filter is oriented perpendicular to the first, no light will pass unless it’s reoriented first.

Twisted Nematic LCD

In a twisted-nematic (TN) display, an electrically conductive polymer is applied to the two glass substrates on the sides opposite the polarizing filters. Horizontal grooves are pressed into the polymer material on one panel and vertical grooves are pressed into the other. The rod-shaped liquid-crystal molecules are sandwiched in between, so that the molecules adhering to the panel with vertical grooves possess a north-south orientation while those closest to the panel with horizontal grooves are oriented east-west. The molecules in between are forced into a helix-like pattern.
 
As the light waves follow this helix, they twist 90 degrees to become parallel to the second polarizing filter, passing through it to create a white pixel. A TN panel has a very low power requirement because it produces a white pixel in its “off” state (meaning no voltage is required to reach that state). When voltage is applied, the liquid-crystal molecules realign vertically, so the light waves are no longer twisted. In this state, light cannot pass through the polarizing filters and a black pixel is created.

Twisted-nematic LCDs have become the most common consumer display because they’re relatively inexpensive to manufacture and they feature very low response times (response time, measured in milliseconds, describes how quickly a display can reorient its liquid-crystal molecules and register that change on the screen. The faster a display’s response time, the less likely it will suffer from motion-blur and ghosting artifacts.).

TN LCDs have a number of shortcomings, with their inability to display a full 24-bit color palette (256 shades each of red, green, and blue per pixel to produce 16.7 million colors in total) at the top of the list. TN displays use only six bits per color (64 shades each of red, green, and blue per pixel to produce 262,144 colors in total). TN displays rely on either dithering (combining adjacent pixels to produce a shade a single pixel can’t produce) or frame rate control (rapidly cycling a pixel through a series of shades to trick the eye into perceiving a given color) to simulate shades they can’t produce natively. TN displays also have narrow viewing angles because the vertically oriented molecules tend to scatter light waves.

In-Plane Switching (IPS) LCD

An in-plane switching (IPS) LCD applies voltage to both ends of the liquid-crystal molecules, which moves the crystals in parallel to the display panel, rather than perpendicular to it, as a TN panel does. This requires two transistors for each pixel, which increases the panel’s manufacturing cost. The panel’s increased transistor count also blocks more of the transmission area, so a more powerful backlight is needed to compensate. The stronger backlight consumes more electrical power, which renders this technology a poor choice for most notebook computers running on battery power.

The parallel orientation of the liquid-crystal molecules in IPS displays scatters much less light, which endows these monitors with a much wider viewing angle than TN models. IPS monitors represent color using eight bits per component, which enables them to produce a true 24-bit color palette with 16.7 million shades without resorting to tricks such as dithering or frame rate control.
 
IPS displays typically suffer from slow response time, which renders them inappropriate for fast-paced applications such as gaming. But manufacturers have focused on this issue and there are several IPS models on the market with response times as low as six milliseconds. Variations on IPS technology include Super-IPS (S-IPS), Advanced Super-IPS (AS-IPS), and Horizontal IPS (H-IPS).