White Paper: LCD Technologies Explained and Compared

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

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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.