Retardation foil

In this section a prototype transmissive LCD will be discussed to demonstrate the effectiveness of photoalignment as a multidomain method in combination with a retardation foil. Prototyping of new LCD designs is an elaborate process and therefore it is often preceded by computer simulation to find the optimum configuration. For this purpose the optical properties of various combinations of multidomain LC layers and retardation foils were simulated using 2x2 Jones matrix modelling [24-26]. 

Figure 9.24 shows an example of a temperature patterned retardation foil viewed between crossed polarisers. In this example the transmissive part does not have any birefringence, which is demonstrated in Fig. 9.24b. The black...

A modelhng sthdy by Karman et al. [30] has shown that the combination of multidomain TN LCDs with patterned retardation foils improves the viewing angle properties of LCDs over multidomain LCDs with uniform retardation foils. In such combinations of a patterned LC layer with a patterned retardation layer, the LC layer acts to decrease the gray scale inversion, whereas the retardation layer improves the off-axis contrast. It is of course important to find an optimum in the number of domains in both the LC layer and retardation foil, but also to keep in mind that increasing the complexity of the system will increase the production costs. It will depend on a specific application whether it is a feasible approach. 

Fig. 9.12. Action of two retardation foils for a 90° TN LCD. (a) Configuration, (b) viewing direction w.r.t. display as shown in (c) example of viewing angle improvement by use of the two foils when looking to the display obliquely from the right hand side. On the left hand side of the photograph the foils are attached whereas on the right hand side the image without foils is presented.

Fig. 9.13. Configurations of the studied LCDs. Top view of a monodomain 90° TN LCD (a), and dual domain 65° TN LCD (b) and cross section of a dual domain 65° TN LCD (c). The light path through the LCD is from left to right. Compensator denotes the retardation foil.

The dual domain display has a polariser with its transmission axis parallel to the LC orientation at the top substrate (continuous arrow) and an analyser with its transmission axis crossed to the polariser. It can be seen from Fig. 9.13b that the LC orientation at the bottom surface is neither parallel nor perpendicular to at least one of the polarisers. This means that the optimum contrast (obtained when a 90° TN LCD is placed between crossed polarisers) cannot be obtained. Therefore a retardation foil needs to be inserted in the light path to optimise the contrast of the display. 

As a result, an ultra-thin birefringent layer is obtained that exhibits a good chemical, thermal and photochemical stability. By varying the coating conditions, as well as the concentration, retardation values up to 600 nm are attainable within 2 % accuracy. The high stability of the films allows for the deposition of ITO conductive layer sputtering or for the fabrication of stacks of retardation films as used in for example wide band A retardation foils. 

Ultimate LCD Performance by Local Director Variations in Both LC Layer and Retardation Foils... 

Figure 38. Setup of a color display with retarding foil (after reference [94]).

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