Optically compensated birefringence

In LCDs various LC modes of operation are applied like twisted nematic (TN) [5], super twisted nematic (STN)[6], vertically aligned nematic (VAN) [7,8], optically compensated birefringence (OCB) [9] and in plane switching (IPS) [10-12]. The LC mode used depends on the demands of a specific application, like the viewing angle, power consumption and manufacturing cost. 

The compensation birefringence measurement is very easily coupled to optical microscopy in the transmission and reflection modes, thus allowing characterizing orientation with a spatial resolution of a few hundreds of nanometers [14]. Polarizing microscopes are widely available and are often used for birefringence studies even if spatial resolution is not required. Objectives specifically designed for cross-polarized microscopy are necessary to avoid artifacts. 

Wang, Z., Folkenroth, J., Zhou, W., Zhang, Y., and Shi, X. (2012). Cellulose ether polymers as optical compensation fUms for LCDs-high birefringence and tunable optics, IDW/AD 12, Kyoto, Japan, pp. 499-502. 

H. Mori, Y. Itoh, Y. Nishima, et al.. Performance of a novel optical compensation film based on negative birefringence of diseotie eompound for wide-view-angle twisted-nematic liquid crystal displays, Jpru J of Appl. Phys., 36, 143 (1997). 

A coloured backgrormd caused by optical interference is the most serious disadvantage of the STN-LCD because it degrades the colour reproduction of an LCD display. To solve this problem and produce an achromatic black-and-white bacl rormd, the double-layered STN-LCD (D-STN-LCD), which comprises a liquid crystal optical compensator mounted on the STN-LCD, has been developed [4]. Figure 4.3.5 shows the principle of the D-STN-LCD. The compensation layer, which is placed ready fabricated on the driven cell, compensates the optical dispersion caused by the birefringence effect and decolourizes the STN-LCD as long as the following three conditions are met ... 

There are as yet ho accepted standard materials for discotic nematic liquid crystals. It is expected that the birefringence of these materials will be negative, and they have already found application as optical compensation films for liquid crystal displays [23]. In order to illustrate an example of the optical properties of discotic nematic liquid crystals, we give in TABLE 10 measurements of the ordinary and extraordinary refractive indices of hexakis[(4-octylphenyl)ethinyl]benzene (Tm = 100 C). These are taken from [24] and were measured for a wavelength of 589 nm. 

Lagorceix, H., Reynaud, R, 1995, Squeezing highly Birefringent Fiber for an Accurate Nondestructive Determination of Principal-Axis Orientation along the Waveguide application to Fiber Babinet Compensator Implementations Optical Fiber Technology 1, 171... 

If the birefringence is low as in asbestos minerals, or if the particle is thin, the color seen will be a low order white or gray. A retardation plate or compensator added to the optical path can add or subtract retardation. Whether retardation is added or subtracted depends upon whether the slow ray of the com-... 

This principle of compensation enables the microscopist to measure the thickness of a particle if the birefringence is known or to measure the biregringence of a particle if the thickness is known. The birefringence of a particle is useful for particle identification by consulting tables of optical constants. 

There is an almost infinite potential to combine different kinds of compensation films in many different ways to achieve a given effect, e.g. compensation layers of negative birefringence with the optical axes in the plane of the cell situated on either side of a TN-LCD improve the viewing angle dependence in the horizontal plane, but not in the vertical plane. However, if the optical axis of the compensation layer is tilted with respect to the plane of the cell, then superior optical properties in the vertical direction are also achieved. Compen-... 

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