Brief note about LCD displays

Liquid crystal was discovered by the Austrian botanist Fredreich Rheinizer in 1888. ``Liquid crystal'' is neither solid nor liquid (an example is soapy water). Liquid crystals are almost transparent substance; the light passes through liquid crystals is being polarized according to orientation of molecules. Such property belongs to solid state substances, namely crystals. The orientation of molecules changes when voltage is applied to the liquid crystals.

The main idea of LCD displays is to attenuate the brightness by polarisation plane's change. When liquid crystal is placed between two polarisers [1], whose polarisation planes have 90 angle, one can change transparency of the liquid crystals by application of different voltage (see Fig.1). Typical pixel pitch is about 200-300 micrometers.

How to make control the LCD

The segment drive method is used for simple displays, such as those in calculators, while the dot-matrix drive method is used for high-resolution displays, such as those in portable computers and TFT monitors.

Two types of drive method are used for matrix displays. In the static, or direct, drive method, each pixel is individually wired to a driver. This is a simple driving method, but, as the number of pixels is increased, the wiring becomes very complex. An alternative method is the multiplex drive method, in which the pixels are arranged and wired in a matrix format.

To drive the pixels of a dot-matrix LCD, a voltage can be applied at the intersections of specific vertical signal electrodes and specific horizontal scanning electrodes. This method involves driving several pixels at the same time by time-division in a pulse drive. Therefore, it is also called a multiplex, or dynamic, drive method.

How to make colour on LCD

Each pixel is divided into three section - for red, green, and blue part (there is a separate light filter on each one). Degree of angle's rotation of LCD molecules is almost linear to applied voltage in definitive range of voltage. Hence we can obtain about 64 levels of brightness on each element, or 262 144 (18 bit) per pixel of three colour filters.

Inversion

In liquid crystal pixel cells, it is only the magnitude of the applied voltage which determines the light transmission (the transmission vs. voltage function is symmetrical about 0V). To prevent polarisation (and rapid permanent damage) of the liquid crystal material, the polarity of the cell voltage is reversed on alternate video frames.

First scheme is full-frame inversion, when the voltage of each next frame has different polarity (the most simple scheme yet has a drawback of flickering and crosstalk). Second scheme is row inversion - the advantage is absence of crosstalk between neighbour pixels. Third scheme is column inversion. The last and the most tricky is pixel-by-pixel inversion, when the pixel's voltage changes its polarity according to the one of neighbour pixels polarity (the most complicated and hence energy-consuming). Unfortunately it is very difficult to get exactly the same voltage on the cell in both polarities, so the pixel-cell brightness will tend to flicker to some extent at half the frame-rate. If the polarity of the whole screen were inverted at once then the flicker would be highly objectionable. Instead, it is usual to have the polarity of nearby pixels in anti-phase, thus cancelling out the flicker over areas of any significant size. In this way the flicker can be made imperceptible for most ``natural'' images.

Cross-talk

Owing to the way rows and columns in the display are addressed, and charge is pushed around, the data on one part of the display has the potential to influence what is displayed elsewhere. This is generally known as cross-talk, and in matrix displays typically occurs in the horizontal and vertical directions. Cross-talk used to be a serious problem in the old passive matrix (STN) displays, but is rarely discernable in modern active-matrix (TFT) displays. For most practical purposes, the level of crosstalk in modern LCDs is negligible. Certain patterns, particularly those involving fine dots, can interact with the inversion and reveal visible cross-talk. If you try moving a small Window in front of the inversion pattern (above) which makes your screen flicker the most, you may well see cross-talk in the surrounding pattern.

Refresh time

is the rate at which the electronics in the monitor addresses (updates) the brightness of the pixels on the screen (typically 60 to 75Hz). For each pixel, an LCD monitor maintains a constant light output from one addressing cycle to the next (sometimes referred to as ``sample-and-hold''), so the display has no refresh-dependent flicker. There should be no need to set a high refresh rate to avoid flicker on an LCD.

Response time

The transmittance of the LCD pixel is changing according to applied voltage. But any liquid crystal is characterized by viscosity so it takes a time to change orientation of molecules. LCD displays' manufacturers traditionally measures the least time of the monitor's response, namely switch time from black 90% to white 10%. Such measuring technique does not tells anything about real switching time because it is more likely for monitor to switch brightness of pixels gradually. So the molecules need to rotate on smaller angle; but rotation speed is proportional to voltage. Hence switching from black to white is always faster than switching from black to grey.

LCD matrix technology

Higher-priced LCDs (probably using ``In-Plane Switching'' liquid crystal modes) should have colours which are less affected by viewing angles for that application (IPS tends to have a less-good black-state-lower contrast- however). ``Vertically Aligned'' (Multidomain -VA) boast the darkest blacks, equivalently highest contrast, of any LCD technology, but response time and viewing angle are poorer than IPS.

TN+Film-matrix

TN-matrices (additional ``Film'' word means scattering film), ``Twisted Nematic'' - when the voltage is applied, molecules are twisted; the polarisation axis in this case is perpendicular to the one of panel's. (Fig.1.1.1). When no voltage is applied, TN matrix allows the light to pass through both polarisers. That is one of features of TN matrices: when the pixel is damaged, a bright dot appears on the screen.

IPS-matrix

In-Plane Switching - liquid crystals in cells of IPS-panel are located in the same plane and always parallel to the panel's plane (see Fig.1.1.2). When voltage is applied to the pixel, the pixel passes the light through; if not, no light passes. That is why the damaged pixel remains black (in contrast with TN+Film, where damaged pixel passes light) [2]. Both electrodes are located in the same plane, hence the area of electrodes is greater than for TN+Film matrices. Such circumstance leads to decrease of contrast and brightness of the matrix as well as to deterioration of switching speed (about 35 ms). The advantage of IPS matrix is better angles of views than TN+Film and the best colour reproduction. The specific feature is the colour of black: when you look at the IPS monitor aside, the black colour seems a little bit purple.

There were developed several technologies based on IPS such as Super-IPS (S-IPS), Dual Domain IPS (DD-IPS) and Advanced Coplanar Electrode (ACE), A-SFT, A-AFT, SA-SFT, and SA-AFT.

MVA- и PVA- matrix

MVA (Multidomain Vertical Alignment) - it is something easier to draw than to explain (see Fig.1.1.3 fully opened pixel) [3]. The MVA pixel is divided into domains that are rotating synchronously. Liquid crystals are aligned differently in domains (Fig.1.1.3). So there is practically no difference from which side a user looks on the monitor: crystals in different domains are aligned in different angles. As for IPS, damaged pixels looks like a black dot [4].

PVA - Patterned Vertical Alignment - is like a MVA; domains of different orientation of molecules in one pixel allow to reproduce the colour almost independently from view angle.

Bibliography

1

Shin-Tson Wu Fellow IEEE Qi Hong Ruibo Lu, Xinyu Zhu and IEEE Thomas X. Wu, Senior Member.
Ultrawide-view liquid crystal displays.
JOURNAL OF DISPLAY TECHNOLOGY, 1:1, SEPTEMBER 2005.

2

MASAHITO OH-E* and Hitachi Ltd. 7-1-1 Ohmika-cho Hitachi-shi Ibaraki-ken 319-12 Japan KATSUMI KONDO, Hitachi Research Laboratory.
The in-plane switching of homogeneously aligned nematic liquid crystals.
Liquid Crystals, 22:379-390, 1997.

3

Ivan I. Smalyukh Mingxia Gu and Liquid Crystal Institute Kent State University-Kent Ohio 44242 Oleg D. Lavrentovich, Chemical Physics Interdisciplinary Program.
Directed vertical alignment liquid crystal display with fast switching.
APPLIED PHYSICS LETTERS, 88, 2006.

4

Kenji okamoto Yoshio Koike.
Super high quality mva-tft liquid crystal displays.
Fujitsu Sci. Tech., 35:221-228, 1999.

Parts of the text are courteously from techmind.