The weakness of the LCD technology is the viewing-angle dependence of the formed image. When one looks at an oblique way to the screen the color and brightness seems to vary. This is a big drawback in applications as television where there are several viewers at different positions. The origin of this problem lies in the use of birefringent materials where the polarization of the propagating light is very critical. Many investigations and improvements have already been done, but there are still lots of things that need further investigation.
Two manners to deal with the problem are the use of compensation foils and using completely different technologies to produce LCDs. The aim of the research is to investigate the viewing-angle dependence at one-dimensional birefringent layers and study the diffraction that occurs at displays with microscopic lateral variations.
The black items on the figure below show the typical elements of a LCD : a light source, two polarizers (one of them is called the analyzer) and a liquid crystal layer. Liquid crystal molecules have the property to align parallel or perpendicular to the electric field. An other important property is the birefringence. The refraction index differs when one works in a direction parallel or perpendicular to the axes of the liquid crystal molecules. By changing the applied voltage, the direction of the molecules is varied and the polarization of the outcoming light is being adapted. Due to the direction dependent refraction indices, the characteristics of the LCD vary with the viewing angle.
To minimize the variation of color and brightness with the viewing angle, one adds one or more compensation foils (red layers on figure). These foils are also birefringent and compensate the dependence of the retardation on the viewing angle.
In the left figure a classical LCD is shown. Typical are large electrodes on both glass substrates. Due to the large surface of the electrodes a one dimensional geometry is accomplished.
In more recent types e.g. In Plane Switching LCDs both electrodes are formed as interdigitated finger patterns on one glass substrate. Due to the microscopic lateral variations in the structure and molecule orientation incoming light beams suffer from diffraction.
|Classic type of LCD's|
|In-plane switching LCD|
The picture below shows a microscopic view of the electrode structure in a realistic IPS cell. The width of the electrodes and the gap between them is 4µm. The picture is taken with a microscope at a magnification of 500.
More detailed information about the results we have obtained in our research can be found in our publications.