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Choosing the Best LCD Panel Technology E-mail
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Written by Olin Coles, David Ramsey & Vito Cassisi   
Wednesday, 15 September 2010
Table of Contents: Page Index
Choosing the Best LCD Panel Technology
LCD Technology
How LCD Panels Work
Contrast Ratio, Viewing Angle, and Surface Finish
Color/Bit Depth
LCD Resolution and Scaling
LCD Panel Image Processing
3D LCD Technology
Connections and Content Protection
LCD Summary and Recommendations

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:

Layer #1: White backlight from the LCD lamp provides illumination.
Layer #2: Polarizing filter ensures light waves from the backlight are directionally aligned.
Layer #3: This layer works as a filter, only allowing light through one of the three colors. Power signals controls the "twist" which directly effect the amount of light allowed to pass beyond the filter.
Layer #4: Liquid Crystal cells are in front of a fine grid of wires that can be addressed by x and y coordinates. Your graphics adapter signals the appropriate coordinates to each command.
Layer #5: A second polarizing filter that is perpendicular in direction to layer #2.

lcd_layer_construction.jpg

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".

NOTE: "LED" televisions

Confusingly, some manufacturers have started marketing "LED" televisions as distinct models from their LCD and plasma televisions. It's important to realize that in this case the term "LED" only means that the television uses white LEDs for backlighting, instead of the fluorescent tubes most LCD televisions use.

LED backlights use less power and produce less heat than standard backlights, and reach full brightness instantly; also, televisions and monitors with LED backlights can be made thinner than those with standard backlights.

True LED televisions use colored LEDs to produce the actual image; currently, the only true LED television on the market is the Sony XEL-1, an 11" (yes, eleven inch) television with a 3 millimeter thick screen that sells for $2,500.00. At the 2010 Consumer Electronics Show in Las Vegas, several manufacturers showed prototype 15" OLED (organic light emitting diode) televisions, but these are still years away from volume production. At this point in time, OLED is becoming popular on smaller devices such as smartphones.

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



 

Comments 

 
# RE: Choosing the Best LCD Panel TechnologyDoug 2010-09-15 22:47
A very good overview and more. I would suggest these two links to better clarify TN and IPS and it's variations and to check which panel your monitor or prospective monitor has:

Panel Types and Benefits:
TFT Central (##tftcentral.co.uk/articles/panel_technologies.htm)
Panel Type (##tftcentral.co.uk/articles/panel_technologies.htm)
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# SED and FEDSteve 2010-09-16 09:20
One day SED and FED TV's and monitors will dominate, if only cannon (SED) and sony/AU Optronics (FED) can get manufacturing prices and legal issues out of the way.
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# Great summaryBernardP 2010-09-17 06:11
... and objectively written too. May I add that any large screen TV buyer should make sure to also look at plasma sets? ;-)

Also, regarding monitors, NVidia video drivers allow for creation and proper scaling of custom resolutions. This gives very good results and gets around limitations in the monitor's built-in scaler. Unfortunately, with ATI/AMD, resolution choices are limited to those the monitor natively supports.

Being able to display and properly scale a lower resolution is useful for no-longer-young eyes that have trouble with the high pixel pitch on, for example, a 23-inch 1920x1080 monitor.
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# 720P and 1080iJames 2010-09-24 11:40
You stated "given the fact that 720P and 1080i images have different aspect ratios"

This is not true, the ATSC television standard is 16:9 aspect ratio, also known as 1.78:1. 1080i is no different than 1080p except that with interlace, every other scan line is drawn per 1/60th of a second - in progressive scan the entire frame is drawn every 1/60th of a second. All the HD/ATSC formats are the same aspect ratio. The overall frame shape remains the same among them all. Some wide-screen computer panels (and a few odd tvs) come in 16:10 aspect ratios, which may be the source of your confusion, but in all cases the correct aspect ratio for HDTV is 16:9, and it is the only aspect ratio specified by ATSC.
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# RE: 720P and 1080iJames 2010-09-24 11:56
Tvtechnology.com is the leading trade magazine for people in the industry, so if you don't believe me, check around. But a calculator can also solve this. 1280/720 = 1.78. 1920/1080 = 1.78.
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# Time for an update versionNehemoth 2012-05-18 16:52
What about if you update this amazing post?
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# Update, due to a DIY Project(or)Medenyx 2012-07-13 12:57
Can't agree more with Nehemoth.
Me, ... , well, I am trying to build a 3D (shutter!) DIY projector, and I am in a trouble to find 120Hz 15.4" (or max 15.6") LCD with its corresponding LCD Controller, with a proper DVI input, that comes in a dual link version, not to mention HDMI, VGA & the other possible (older, analog) inputs, all in the same separate / independent panel. An RF digital TV tuner, and a remote would be nice, too. ;-D
nVidia's solution is almost perfect!, but where to find a WUXGA 1920x1200 (6->8 bit) LCD 3D 120Hz Display?
I am open to any positive suggestion. Especially if it is not the most expensive one!
If you have any, thanks in advance!
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