Patent ID: 6538754
Filing Date: 2003-03-25
Classification: G01B

Abstract:
A method of measuring a thickness of a cell gap in a reflective liquid crystal display, comprising:providing a light source to produce a light beam incident onto an optical system, wherein the optical system comprising: a rotating table, on which a reflective liquid crystal display is disposed, the reflective liquid crystal display comprising a front liquid crystal director and a back liquid crystal director; an incident polarizer, to receive the light beam incident from the light source, and to polarize the light beam; a beam splitter, to receive the light beam polarized by the incident polarizer and to split the light beam coming from the incident polarizer into two split light beams, wherein one of the split beams is incident onto the reflective liquid crystal display and reflected thereby; an output polarizer, to receive and polarize the light beam reflected by the reflective liquid crystal display, wherein the incident and output polarizers are located at two sides of an optical path of the reflective liquid crystal display; and a photodiode, to receive the light beam transmitting through the output polarizer, and to convert the light beam into an electric current signal; defining a beta angle as an angle between the incident polarizer and the front liquid crystal director; obtaining a formula between a reflectivity RâŠ¥ and a beta angle &bgr; as: Râ€ƒ&UpTee;&it;â€ƒ&it;â€ƒ=â€ƒ&it;(Î“&it;â€ƒ&it;sin&it;â€ƒ&it;XX)2&it;â€ƒ&it;(sin&it;â€ƒ&it;2&it;â€ƒ&it;Î²&it;â€ƒ&it;cos&it;â€ƒ&it;X&it;â€ƒ-â€ƒ&it;Ï†X&it;â€ƒ&it;cos&it;â€ƒ&it;2&it;â€ƒ&it;Î²&it;â€ƒ&it;sin&it;â€ƒ&it;X)2,&Ggr;=2&pgr;d &Dgr;n/&lgr;, X={square root over (&phgr;2+(&Ggr;/2)2)}, d is a thickness of a cell gap of the reflective &Dgr;n is the birefringence of the liquid crystal, &phgr; is a twisted angle the liquid crystal, and &lgr; is a wavelength of the light source;differentiating the formula of RâŠ¥ by the beta angle obtaining a relationship between a reflectivity RâŠ¥ and a beta angle &bgr; as a second formula: &dd;R&UpTee;&dd;Î²=0&Implies;Î²max=tan-1&af;(-XÏ†&it;â€ƒ&it;t&it;â€ƒ&it;tan&it;â€ƒ&it;X)2,â€ƒwherein &bgr;max is a function of d, &Dgr;n, &phgr; and &lgr;; turning the rotating table to measure &bgr;max; and substituting the values of &bgr;max, &Dgr;n, &phgr; and &lgr; into the second formula to derive the value of d.