Randy Hoffner / 11.10.2004
LCD Displays: Fixing the Problems
This column recently discussed several advanced television displays, including liquid crystal displays (LCDs). As is true of all display types, LCDs have positive and negative attributes, but enough are positive to justify efforts to fix the problems.
Principal shortcomings, we will recall, include narrow viewing angles and slow response times. Let's look at how one shortcoming is being addressed. In the previous column, I described the operation of the thin film transistor (TFT), twisted nematic (TN) LCD, the type most commonly used for computer monitors and television displays, both direct-view and projection types. Let's review.
Liquid crystals exist in mesophases--phases that fall somewhere between liquids and solids. One of the mesophases of liquid crystals is the nematic phase in which the molecules, shaped somewhat like sausages, prefer to line up parallel to one another, but in no particular positional order. This preferred order, called the nematic director, can be easily influenced by the application of weak electrical, magnetic or optical fields.
In a twisted-nematic LCD, nematic liquid crystal is introduced between two substrate surfaces, one on the side of the display where the backlight is located, and the other on the viewing or projecting side. Each substrate surface has been prepared to establish the initial positional orientation of the liquid crystal molecules.
Alignment preparation is accomplished by brushing the substrate surface in the desired direction with a cloth, causing microscopic scratches along which the adjacent liquid crystal molecules align themselves. When the alignment preparations of the two substrates are perpendicular to one other, the molecules and their nematic director rotate in a helical fashion from one surface to the other along the twist axis, creating the so-called 90-degree twisted-nematic phase.
If we envision this, considering the molecular alignment adjacent to the input substrate to be the "reference" molecular orientation, it becomes apparent that the degree of twist of a given molecule is dependent on its distance from the input substrate.
That is, the molecules adjacent to the input substrate are not rotated at all, while the molecules adjacent to the output substrate are rotated a full 90 degrees relative to the input substrate.
Each of the two substrates has a polarizing filter covering its outside surface, and the plane of polarization of each filter is parallel to its respective alignment preparation.
When polarized light is passed through the crystals under these conditions, the plane of the light's polarization is propagated along the nematic director's helical axis, following its twist. The result is that light polarized in the vertical phase, for example, by the input polarizer, ends up polarized in the horizontal plane when it arrives at the output polarizer.
Because the output polarizer's plane of polarization is also horizontal, the light passes through it and emerges from the output surface of the display device.
CREATION OF COLOR
When an electrical signal is applied to such a liquid crystal cell, the molecules and their nematic director "untwist" to a degree proportional to the applied voltage. If the voltage is sufficient, the nematic director completely untwists, causing the plane of the light's polarization to arrive at the output surface perpendicular to the output filter's polarization, preventing it from passing through the viewing or projecting surface.
When the applied voltage is modulated by a video signal, the degree of nematic director twist dynamically varies, resulting in a dynamically varying light output from the liquid crystal cell.
This generates a grayscale display at the output of the LCD device. Each pixel of a color LCD screen has three subpixels, covered with red, green and blue filters, respectively. These generate red, green and blue grayscales that, when combined by the human visual system, produce a color display.
In simpler monochrome LCD displays, the liquid crystal pixels are directly driven with electrical currents.
In the more complex color LCDs used for television, thin film transistors or TFTs, etched onto the liquid crystal substrate surfaces, are used to drive the pixels.
One of the principal shortcomings of traditional TN LCD displays is a limited viewing angle. This is not a great problem on laptop computers, which are typically viewed straight-on from a close position. But, it can be a big problem for a television screen or a projection device because television displays are typically viewed from greater distances and at greater offaxis angles than laptop screens.
Poor off-axis viewing manifests itself in several ways, including reduced brightness and contrast ratio, color distortions and, in the extreme case, grayscale reversal, which makes portions of the image appear to be photographically negative.
As previously discussed, in the traditional twisted-nematic LCD cell under conditions of no electrical stimulation, the degree of twist of an individual molecule depends on its distance from the input substrate-- molecules adjacent to the substrate have no twist, while those a little further away from the substrate are twisted slightly. The degree of twist increases progressively until the output substrate is reached, at which point the molecules are twisted a full 90 degrees with respect to those at the input substrate.
There is an electrode on each substrate within the cell, but only the electrode on the output substrate is switched; the electrode on the input substrate serves to "anchor" the molecules adjacent to it. Thus, when the switching current is active, the molecules adjacent to the input substrate remain stationary or anchored; those adjacent to the output substrate are untwisted up to a full 90 degrees, while those in between the two substrates are untwisted to a lesser degree determined by their distance from the switching-signal electrode on the output substrate. It is the effect of these variable degrees of twisting, and the resultant variations in polarization angles at varying depths within the cell, that are at the root of the small viewing angle problem.
One solution to the LCD viewing angle problem is called in-plane switching, or IPS. In an unswitched
liquid crystal cell in an IPS device, all the molecules and both substrates' alignment preparations are oriented horizontally--parallel to one another--rather than being twisted.
Both switching electrodes are mounted on the output substrate in the same plane. When the electrodes are stimulated with switching signals, all the liquid crystal molecules in the cell rotate up to 90 degrees, always remaining parallel with each other and the two substrate surfaces.
Thus, the fully switched condition rotates all molecules to the vertical orientation, while in the fully unswitched state, they are all in the horizontal orientation. This eliminates the problem of the light-angle restriction that is found in the twisted nematic cell, greatly increasing the viewing angle of the display to about 140 degrees in all directions.
This is not, of course, a perfect world, and there are some tradeoffs for this widened viewing angle. The presence of two switching transistors rather than one on the output substrate obscures more of the substrate's surface, requiring stronger backlighting to achieve a good level of brightness.
This makes IPS displays inappropriate for battery operation, but acceptable for mains-powered devices.
Because molecules in an IPS display cell must be switched through the full rotational angle rather than the reduced switching angle, IPS crystal switching speeds are inherently slower than those of TN crystals. This problem is being addressed in various ways by interested developers and manufacturers. It is thought by the industry that the problems of IPS display devices are minor with respect to the potential of these devices.