A simple optical method of measuring the tilt bias angle of liquid crystals

Abstract

A simple optical method based on refractive indices mcasnrcment for dcterrniriing tllc tilt bias angle of liquid crystals is described. The required refractive indices n,, n, and (10 are measured by (i) Abbe's refractometer and by (ii) optical wedge method. The tilt bias angle by this technique can be determined to accuracy of f0.2" in the high tilt angle rangc (20" t o 40"). This method is rather simple and quick and dotLs not, require any extrinal magnetic field and/or knowledge of other physical constant of liquid crystals, as arc necessary in some of the earlier reported met,hods. Liquid crystal multiplexed display devices which hare come into great prominence during the last decade, depend critically for their electro-optic performance characteristics on the tilt bias angle (GOODMAN et al.) (the angle which the uniformly aligned liquid crystal (LC) molecules make a t the bounding glass surface). .A very simple, accurate arid quick method based 011 the refractive indices measuremcnt is proposed herewith. It is well known that the nematic liquid crystals are hirefringent and when a linearly polarized beam of light incidents on a thin uniformly aligned nematic slab with its E vector either parallel or perpendicular to the preferred director's direction, it experiences refractive index n, or no, respectively. However, when the IAC molecules make a finite angle (tilt bias angle) with the Founding glass surfaces, the polarized light with i t s E vector making an angle 0 with the liquid crystal