Principal of LCD and importance of Spacer in LCD application



Basic structure


Figure 1 shows the general basic structure of a liquid crystal display. It is constructed with 2 pieces of glass substrates with sphere or rod shape spacers which determine the cell gap d. Liquid crystal is filled in between two surface alignment layers which determine the orientation of the liquid crystal directors at the boundaries. Transparent Indium Tin Oxide (ITO) layers attached on the glass are used for conduction since the liquid crystal display is electrically driven. The function of the polarizers outside of glass is to modulate polarization of the light that pass through the liquid crystal display so as to realize light switching. Figure 1 The basic structure of a liquid crystal display. Besides the above stuff, a general liquid crystal display may have some other materials not shown in the above figure such as dopant for liquid crystal, retardation film, light diffuser and backlighting system.
Figure 1 The basic structure of a liquid crystal display.
Besides the above stuff, a general liquid crystal display may have some other materials not shown in the above figure such as dopant for liquid crystal, retardation film, light diffuser and backlighting system.



Basic operating principles


Figure 2 The operation of Twisted Nematic (TN) liquid crystal display.
Figure shows an example of operation of the most common Twisted Nematic (TN) liquid crystal display. The twist angle of the liquid crystal molecules is 90o and two polarizers are perpendicular to each other. The incident unpolarized light becomes polarized when it passes through the input polarizer. Then it goes into the liquid crystal cell.

In the absence of an electric field, the cell is 90o twisted as the boundaries define. The light polarization vector follows strictly to the twist direction and, consequently, the structure rotates the polarization plane up to the twist angle. When the light reaches the output analyzer, it totally passes through without loss in principle. Thus the liquid crystal display is operated in bright state.

When a strong electric field is applied across the cell, the liquid crystal molecules align along the initial preferred direction but reorient along the electric field perpendicular to the surfaces. Therefore, the light polarization direction almost does not change and, accordingly, the light is blocked when it sees the analyzer. Thus, the liquid crystal display is operated in dark state.



Basic Optics of Liquid Crystal Displays


Liquid Crystal Displays are electro-optic devices, so it is important that we have a good understanding of optics. Figure 33 shows clearly a simple optical structure of a Liquid Crystal Display.
Figure 3 Optics of Liquid Crystal Display
Usually, it is convenient to set input liquid crystal director to be the x axis of the system. The angles of transmission axis of input and output polarizers are and , respectively. The total twist angle of the liquid crystal is , which is defined as the angle that liquid crystal is twisted counted from top boundary to the bottom boundary.

Liquid crystal materials are dielectrically anisotropic. One of the consequences of the dielectric anisotropy is optical birefringence. A birefringent material has two refractive indices: the ordinary index and the extraordinary index . Birefringence is defined as the difference between the ordinary index and the extraordinary index follow:
When light travels in a direction such that it makes an angle with the optic axis (the extraordinary axis), the effective refractive index will be:
Thus, the birefringence becomes a function of the angle . This phenomenon is also called double refraction. The wave along the fast axis seeing the extraordinary index is called e-wave; the wave along the slow axis seeing the ordinary index is called o-wave.

When light passes through a birefringence material, the e-wave and the o-wave accumulate different phase delay. This is called phase retardation. The retardation value of a retarder is defined as
where d is the cell gap of liquid crystal cell or thickness of film. is the wavelength of incident light.



Why spacer is so important to LCD?


The total retardation change of the light passing through LCD is Δnd, which Δn is the refractive index from liquid crystal (LC) and d is the cell gap (the distance between 2 glasses). In order to maintain a uniform cell gap between 2 glasses, spacer should be sprayed during manufacturing process.

The uniformity of total retardation change is highly related to the optical performance of the LCD product including the following area
i) Viewing angle
ii) Background Color
iii) Contrast
iv) Driving dynamic

As a result, the uniformity of the performance LCD product will be strongly related to the uniformity of spacer.

In probability theory and statistics, the coefficient of variation (CV) is a normalized measure of dispersion of a probability distribution. It is defined as the ratio of the standard deviation to the mean. The smaller the CV value, the better the performance. The common CV value of spacer in the market is 4%, while the LCD products of which using those materials are still suffering from un-uniform background color and contrast. Based on the research in Hong Kong University of Science and Technology, for high resolution LCD products (duty > 32) to get uniform background color, viewing angle and contrast, the CV should not exist 3%.