The most essential passive optical element in liquid-crystal displays (LCDs) is the polarizer. Polarizers are indispensable for the displaying of the information content and important front-of-screen performance parameters, such as brightness and contrast, are strongly influenced by the performance of the polarizer. Currently, the most widely used polarizers for LCD applications are derivatives of the H-sheet polarizer as invented by E. H. Land in 1938.[1] These dichroic polarizers are based on uniaxially stretched poly(vinyl alcohol) that is impregnated with iodine or doped with dichroic dyes. These sheet polarizers show excellent optical performances that can be expressed by the polarization efficiency (PE) in combination with the single-piece transmittance (T sp ; transmission of unpolarized light through a single polarizer) aswhere T p and T c are defined as the transmission of unpolarized light through two polarizers with their transmission axis parallel and perpendicular, respectively, and where A ʈ and A ⊥ are defined as the absorbance parallel and perpendicular to the average orientation of the long axis of the chromophores, respectively. The polarizer performance can also be expressed by a single parameter; the dichroic ratio (N)For high-end sheet polarizers such as those applied in LCD monitors or flat-panel televisions, the performance exceeds a polarization efficiency (PE) of 99.9 % at a single-piece transmission of 43.5 %, corresponding to N exceeding 50. Unfortunately, two triacetylcellulose (TAC) layers are needed in conventional polarizers to protect the stretched poly(vinyl alcohol) film on both sides against moisture and to obstruct relaxation effects under influence of heat and/or humidity by which the PE value would deteriorate. Further, an adhesive layer is needed in order to laminate the polarizer to the display. The necessary use of protective and adhesive layers adds unnecessary thickness to H-sheet polarizers. Finally, these polarizers show limited thermal stability and low resistivity against solvents. Numerous advantages are foreseen when the traditional sheet polarizers are replaced by ultrathin coatable polarizers situated on the inside of the cell (in-cell). Apart from a significant reduction in display thickness and weight, the positioning of the polarizers inside the cell eliminates all parallax-related issues and is beneficial to the robustness of the display. Furthermore, substrates that otherwise would be rejected because of their birefringence, e.g., thin, low-weight, and strong plastic foils or cheap, low-quality glass substrates, can be used when the polarizers are situated inside the LCD cell. The optimal position of the coatable polarizers inside the LCD cells can vary between display designs, however, it is beneficial to place the polarizers beneath the electrodes in order to prevent the need for increased driving voltages of the LCD panels.One possible approach to obtaining thin, coatable polarizers is based on the use of lyotropic liquid-crystalline dyes that form a crys...
Photopolymerization of liquid-crystalline diacrylates is a versatile tool to make optical films for liquid-crystal display ͑LCD͒ enhancement. The constant drive towards LCD's having an improved front-of-screen performance demands optical films with properties that can be adjusted on ͑sub͒ pixel level. Birefringent films made from liquid-crystalline diacrylates allow for the required adjustment of the optical property on ͑sub͒ pixel level. In this paper we report on the composition of the acrylate mixture that results in planarly aligned nematic films usable as optical retarder in transflective LCD's as well as the mass transport phenomena that take place during heating of a mask-exposed birefringent film of liquid-crystalline diacrylates. The mass transport phenomena are studied by interferometry as a function of temperature and time. Upon heating a pronounced surface corrugation arises from the latent image formed during the mask exposure. The surface profile largely depends on lateral feature sizes. For 1 ϫ 1-mm 2 areas the exposed areas rise compared to the nonexposed areas, whereas the opposite is observed for 100ϫ 100-m 2 areas. Finally, the direction of the mass transport depends on the molecular orientation of the liquid-crystalline diacrylate. The protrusion formed by lengthwise diffusion is 1.7 times higher than that formed by sidewise diffusion.
Mixtures of liquid‐crystalline di‐oxetanes and mono‐oxetanes are made for the purpose of making birefringent films by photopolymerization. The composition of a di‐oxetane mixture that forms spin‐coated films of planarly aligned nematic monomers is reported. These films are photopolymerized in air. The molecular order of the monomers can be changed on the microscale to form thin films with alternating birefringent and isotropic parts by using a combination of photopolymerization and heating. The interface observed between the birefringent and isotropic 10 μm × 10 μm domains is very sharp and the films show hardly any surface corrugation. In addition, the polymerized films are thermally stable, making them very suitable for use as patterned thin‐film retarders in high‐performance transflective liquid‐crystal displays (LCDs) which satisfy customer demand for displays that are brighter and thinner and that deliver better optical performance than conventional LCDs with an external non‐patterned retarder.
In-cell retarders can be a major breakthrough for mobile LCDs. When a patterned in-cell retarder replaces the external retarders on transflective LCDs, brighter and thinner transflective LCDs with lower power consumption and wider viewing angle can be obtained. Additionally, when in-cell retarders are applied in reflective LCDs, the thickness of the LCD is considerably reduced without affecting the optical performance of the reflective LCD. This paper presents the technology needed to make in-cell retarders and the performance of reflective and transflective LCDs with in-cell retarders.
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