Sep 24, How-it-Works: Flat-panel LCD Display Technology

LCD - short for Liquid Crystal Display devices, have become an important part of our everyday life. Their use range from wristwatches, calculators, and mobile phones, to more demanding high-resolution applications in test instrumentation, computer monitors and high definition LCD Televisions. And the use of LCD panels is growing at an incredible rate. Suffice to say that in the world of HDTV, LCD is already the dominant non-CRT HD display technology; sales of LCD HDTV sets had already exceed 4million units by end 2006 in the US alone.

20 DisplaySearch press release: “Global LCD TV Shipments Reached 146M Units in 2009, Faster Growth Than 2008,” Feb 22, 2010,

Further more, in a press release dated Feb. 22, 2010, DisplaySearch stated that global shipments for LCD TVs reached 146 million units by end 2009.

LCD panels are common because they offer a few important advantages over other display technologies.

These LCD displays are slimmer. LCD Televisions hardly exceeds 3 inches in dept, and are much lighter than a cathode ray tube (CRT) TV or even a plasma television. Further more, they draw less power than plasma TVs; typically, LCD panels use 20 to 30 percent less power than the latest energy-efficient plasma displays, while the latest LED LCD TVs consume close to 50% the power of similar size plasma TVs.

In addition, LCD display panels do not emit harmful electromagnetic radiation. To top-up this whole pro list, some of the latest LCD panels have a rated lifetime of around 100,000hrs of use!

Mind you, liquid crystal displays do not represent the perfect display technology; LCD displays have their drawbacks as well. In particular, viewing angle and display response time may be issues of concern especially with LCD panels from 2nd tier display manufactures. And price may also be an issue especially as one moves towards the larger screen sizes in excess of 50-inch diagonal - this apart from the high price tag associated with the latest LED LCD HDTV sets.

In this article, we look at the underlying technology that makes LCD display panels work in order to represent numbers, words, and high-resolution images.

We explain the complexities involved in the physical setup of an LCD display and how this impacts the production of LCD displays. In the process, we discuss the issue of 'bad' pixels, and why manufacturers never guarantee that LCD panels are 100% free of bad, stuck or dead pixels.

LCD displays consist primarily of two sheets of polarized glass plates with some liquid crystal solution trapped between them. The type of liquid crystals used in LCD panels have got very specific properties that enable them to serve as effective 'shutters' that close or open to block or otherwise, the passage of light. This blocking takes place in a  perpendicular manner to the passage of light on the application of an electric current.

This current through the liquid crystals is controlled by a voltage applied between the glass plates through the use of transparent electrodes that form a grid - with rows on one side of the panel and columns on the other - representing the picture elements or pixels.

Though the three most common states of matter are solid, liquid, and gaseous, yet some substances can exist in a totally odd state that is a sort of liquid and a sort of solid at the same time.

Equally odd is the behavior of their molecules when substances are in this state, since these tend to maintain their orientation, like the molecules in a solid, but at the same time, they also move around to different positions, like the molecules in a liquid.

This means that liquid crystals are neither a solid nor a liquid, even though from a behavior perspective, these are closer to a liquid state than a solid.

There is a variety of liquid crystals - each with different properties. The liquid crystals used in LCD panels are referred to as Nematic Phase liquid crystals. These have their molecules arranged in a definite pattern.

One type of nematic liquid crystal, called twisted nematic (TN), has its molecular structure naturally twisted.

The orientation of the molecules in the nematic phase is based on the 'director'; this can be anything from a magnetic field - say resulting from the application of an electric current due to an applied voltage across the glass plates holding the liquid crystal solution, to a surface that has microscopic grooves in it.

Basic Operational Principle of an LCD Display Panel (Click on image to enlarge)

In the later, the molecules at the various layers of the liquid crystal will gradually align themselves till the molecules at the layer adjacent to the surface will be exactly in line with the direction of the microscopic grooves on the surface.

Microscopic grooves in LCD display panels are applied on the surface of the glass plate that does not have the polarizing film on it, to help align the molecular structure of the liquid crystals as these approach the glass surface in line with the polarization filters on either side of the LCD panel.

Now, the polarization filters on either side of an LCD display are set at 90 degrees to each other (ref. to above diagram). This means that the crystal lineup will go through a 90 degrees twist from one panel surface to the other.  When a light shines on the glass surface of the first polarization filter, the molecules in each layer of the liquid crystal solution will guide the light they receive to the next layer. In the process, they will also change the light's plane of vibration to match their own angle. When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. If the final layer is matched up with the second polarized glass filter, then the light will pass through.

When an electric current is passed through these liquid crystals, they will untwist to varying degrees, depending on the current's voltage. This untwisting effect will change the polarization of the light passing through the LCD panel. As the polarization changes in response to the applied voltage across the glass plates, more or less light is able to pass through the polarized filter on the face of the LCD display. 

Unlike CRT or plasma displays, LCD displays require an external light source to display the picture. The least expensive LCD displays make use of a reflective process to reflect ambient light over to display the information. However, computer and LCD TV displays are lit with an external light source, which typically takes the form of built-in micro fluorescent tubes - often a few millimeters in diameter - above, besides, and sometimes behind the LCD. A white diffusion panel is used behind the LCD to redirect and scatter the light evenly to ensure a uniform display.

Latest developments in LCD backlight has also brought about the use of LED-based backlight systems. LED-based LCD displays can be either edge-lit or full array with local dimming; the latter are capable of exceptional picture quality while making use of less energy requirements. But LED backlights have their cons as well. More information on LED-based LCD backlighting systems is available in an article appearing on our site here; it discusses the latest technological developments in LCD displays.

There are two main types of LCD displays - passive matrix and active matrix.

Passive Matrix: These are the type of LCD display panels that rely on the display persistence to maintain the state of each display element (pixel) between refresh scans. To a certain extent, the resolution of such displays is limited by the ratio between the time to set a pixel and the time it takes to fade.

To operate, passive-matrix LCDs use a simple grid to supply the charge to a particular pixel on the display. The grid is made up of conductive transparent material - usually indium-tin oxide - over two glass layers (called substrates) housing the liquid crystal solution, with one substrate taking the columns, and the other the rows. The rows or columns are connected to integrated circuits that control when a charge is sent down a particular column or row. The point of intersection of the row and column represents the designated pixel on the LCD panel to which a voltage is applied to untwist the liquid crystals at that pixel to control the passage of light.

A display can have more than one pixel 'on' at any one time because of the response time of the liquid crystal material. Pixels have a short turn-on time during which the liquid crystal molecules will untwist to control the passage of light. Once the voltage between the respective electrodes addressing a pixel is removed, the pixel behaves similar to a discharging capacitor, slowly turning off as charge dissipates and the molecules return to their twisted orientation.

Because of this response time, a display can scan across the matrix of pixels, turning on the appropriate ones to form an image. As long as the time to scan the entire matrix is shorter than the turn-off time, a multiple pixel image can be displayed.

Passive matrix LCD displays are simple to manufacture, and therefore cheap, but they have a slow response time - in the order of a few hundred milliseconds - and a relatively imprecise voltage control. These characteristics render images that are somewhat fuzzy and lacking in contrast. Passive matrix LCD displays are therefore unsuitable for most of today's high speed, high resolution video applications.

Active Matrix LCD display panels depend on thin film transistors (TFT) to maintain the state of each pixel between scans while improving response times.

TFTs are micro-switching transistors (and associated capacitors) that are arranged in a matrix on a glass substrate to control each picture element (or pixel). Switching on one of the TFTs will activate the associated pixel.

The use of an active switching device embedded onto the display panel itself to control each picture element helps reduce cross-talk between adjacent pixels while drastically improving the display response.

By carefully adjusting the amount of voltage applied in very small increments, it is possible to create a gray-scale effect. While most of today's LCD displays support 256 levels of brightness per pixel, yet some high-end LCD panels used in HDTV LCD televisions support up to 1024 different levels of brightness. This results in improved gray scale performance and therefore improved picture detail in those areas of the image that are primarily all dark or all bright.

For an LCD display to show color, each individual pixel is divided into three sub-pixels with red, green and blue (RGB) color filters to create a color pixel. This is somewhat similar to the way CRT and Plasma use different phosphors to glow red, green, or blue to create color.

With a combination of red, blue and green sub-pixels of various intensities, a pixel can be made to appear any number of different colors. The number of colors that can be made by mixing red, green and blue sub pixels depends on the number of distinct gray scales (intensities) that can be achieved by the display.

If each red, green and blue sub-pixel can display 256 different intensities of their respective color, then each pixel can produce a possible palette of 16.8 million (256x256x256) colors.

Color TFT LCD TV displays require as many controlling transistors as the number of sub-color pixels forming the display.

This means that the manufacturing process associated with the production of color LCD display panels involves also the production of an enormous number of thin film transistors etched onto the glass substrate to control each and every sub-pixel. Simple mathematics shows that a typical wide screen 1080p panel with a screen resolution of 1920 x 1080 pixels would require over 6.2 million transistors!

Any faulty transistor during the manufacturing process cannot be replaced - leading to what are know as 'bad pixels' - mainly visible only during static displays. A bad pixel is referred to as 'dead pixel' and will show up as a black spot if it remains always off; it is referred to as 'hot pixel' and will show up as a white spot of light if it is permanently on; and it is referred to as 'stuck pixel' and will show up as a colored spot of light if one of the sub-pixels is damaged. This also explains why some refer to bad pixels as 'bad', 'hot', or 'stuck' pixels.

If the number of 'bad' pixels is above normal, the whole LCD display panel will have to be rejected. In the process, some 30 per cent of all manufactured LCD TV panels have to be rejected because of bad pixels. This relatively low yield is the primary reason behind the high cost of LCD panels, as the price for 'good' panels will have to make  up for the manufacturing costs of all rejected screens.

The large number of TFT's on LCD display panels also means that even brand new LCD panels may contain a few bad pixels. It is unfortunate here that most large manufactures of electronic gear do not have clear policies when it comes to replacing LCD display panels with bad pixels - rather, they consider the presence of a few 'bad' pixels not as a sign of some malfunction, but rather, as something inherent within the production process itself.  It is as if you are buying a brand new car, but then it is OK to have a few dents on its sparkling paintwork!

It is interesting to note that way back in 2001, the International Organization for Standards had come up with its set of guidelines - referred to as ISO 13406-2:2001 - that represents a set requirements for electronic visual displays and therefore, which is also applicable to LCD panels. These were then revised in 2008 by ISO 9241-303. These standards list four pixel fault classes, while defining three types of defective pixels for each class:

Type 1: a hot pixel (always on)

Type 2: a dead pixel (always off)

Type 3: a stuck pixel as a result of one or more sub-pixels being always on or always off - the result is a colored pixel that is either red, green, or blue, or one of the secondary colors.

The standard recommends a maximum number of defective pixels per million pixels for each class as further shown in the table below.:

Cluster with more than Type 1 or Type 2 faultsWhile as of 2007, most LCD display panel manufactures started specifying their pixel fault rate as ISO 1304-2 Class II compliant, yet the standard per se is nothing more than a guideline and is not mandatory. Worst still, the standard leaves too many loopholes on how a display manufacture may interpret the requirements specified by the standard - both with respect to the positioning of the faulty pixels - in this case on the LCD display panel - as well as how to interpret the numbers.

Unfortunately, some manufactures are just using the ISO standard as an excuse. You see, a TV maker would still be abiding by the standard if he does replace a one million pixel resolution LCD display if it has say three Type 1 or Type 2 damaged pixels but then does not replace the same faulty one million pixel panel if it has two Type 1, two Type 2, and five Type 3 damages pixels, for a total of 9 damaged pixels!

Luckily, some manufactures are realizing that what they may consider as an inherent aspect of the LCD display panel manufacturing process, may eventually turn out to be of great concern to end customers. For this reason, we are starting to experience a shift by top flat-panel display manufacturers, towards a 'zero bad pixel' policy; Samsung and Viewsonic were among the first to have moved in this direction.

If you are in the market for an LCD HDTV, we advise to visit the amazon storefront; the vast choice of products and the various buying options available from the different retailers are among the best online. 

However, prior to any decision, we recommend to do some research of your own. Look at what other customers had to say about their LCD following their purchase. This will give you extensive insight about the product of interest; a good starting point is the customer feedback posted on the amazon site. 

You can search the amazon storefront for LCD HDTVs without leaving our site by using the amazon search box below; your search results will appear here under.

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