White Paper: LCD Technologies Explained and Compared

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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.

Voltage causes the liquid-crystal molecules in a twisted-nematic display to reorient vertically to block the passage of light. In an in-plane switching display, the molecules block the passage of light until voltage reorients them parallel to the panel.

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).

Vertical Alignment (VA) LCD

The liquid-crystal molecules in vertical-alignment LCDs are naturally aligned perpendicular to the substrate. In the absence of voltage, light waves from the backlight pass uninterrupted through the liquid-crystal molecules but are blocked by the second polarizing filter to produce a black pixel. When voltage passes between the two polymer layers, the liquid-crystal molecules reorient themselves so they’re horizontal to the substrate. The light waves are now twisted parallel to the second polarizing filter, so a white pixel is produced. VA LCDs are cheaper to manufacture than IPS displays because, like TN models, they require only one transistor per pixel.

A variation on this technology, Multi-Domain Vertical Alignment (MVA), delivers more consistent brightness over a range of viewing angles. MVA LCDs divide each cell into four regions (domains) and use protrusions on the glass substrate to pre-tilt the liquid-crystal molecules in the desired direction. Less-expensive VA and MVA panels deliver six-bit color and use dithering or frame rate control to simulate larger color palettes; upscale models deliver true eight-bit color. Other variations on vertical-alignment technology include Patterned Vertical Alignment and Super Patterned Vertical Alignment.

Spec Speak

Here are some other LCD specs you should take into account when shopping for a new panel:

Backlight

All LCD monitors require a source of illumination, with cold-cathode fluorescent lamps (CCFL) being the most common (every display in this roundup uses one). White LED backlights are one alternative solution, found most commonly in mobile displays. Some high-end displays use RGB LEDs, which enable them to deliver a wider color gamut. CCFL and both types of LED backlights have drawbacks: CCFL backlights deliver a narrower color gamut, while LEDs can age at different rates, causing color and white-point shifts over time.

Color Depth

Color depth indicates the number of bits the panel uses to represent the color of one pixel. A display that uses eight bits each for the red, green, and blue channels (28) can produce 256 shades of each color for a total of 16,777,216 colors (256x256x256). Most LCD monitors based on twisted nematic (TN) technology, however, cannot transition eight bits per pixel quickly enough to compensate for fast motion, resulting in unacceptable blurring and smearing while displaying movies and games. To get around this problem, mass-market LCD panels use six bits per pixel (26) to represent the RGB color space. Since this reduces the total number of displayable colors to just 262,144 (64x64x64), many panels use frame-rate control (a dithering method) to have each pixel display a slightly different shade with each successive screen refresh. Frame-rate control can enable a six-bit panel to simulate 16,194,277 colors.

Color Gamut

Color gamut describes a subset of a defined color space that a display is capable of producing. For the purposes of this comparison, we asked each manufacturer to report its display’s color gamut as a percentage of the NTSC color space. Most of the manufacturers claimed their displays delivered 72 percent of the NTSC color space.


The triangle in the center of this chromacity diagram represents the NTSC color gamut, used to measure the color output of LCDs.

Contrast Ratio

Contrast ratio is supposed to measure the relative magnitude between the brightest (white) and darkest (black) colors the display can produce. Unfortunately, the manufacturers’ propensity for using different methodologies and unstated variables in their measurements has effectively rendered this specification meaningless. The industry has further muddied the waters by introducing entirely new variations of this measurement, such as dynamic contrast ratio. We recommend you ignore this spec when comparing LCD monitors.

Inputs

Nearly all the monitors in this roundup support the two most common digital video interfaces, DVI and HDMI (with HDCP copy protection, so you can watch Blu-ray movies at full resolution using either one). None of them, however, use the DisplayPort digital interface. In terms of analog display interfaces, every monitor has an old-school VGA port, but the Samsung P2370HD is the only monitor to also feature composite and component video inputs (useful for connecting such analog sources as VCRs and older set-top boxes and DVD players). None has an S-Video input.

Response Time

Response time measures how long it takes an LCD monitor’s pixels to transition from one state to another and is measured in milliseconds. A monitor with a low response time will display fewer motion artifacts with movies and games. In order to make apples-to-apples comparisons, we asked each manufacturer to report its display’s gray-to-gray response time, because that is the most common real-world transition.

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