IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 11, NOVEMBER 2002 1863 A Simple Method for Measuring the Cell Gap of a Reflective Twisted Nematic LCD Xinyu Zhu, Wing-Kit Choi, and Shin-Tson Wu, Senior Member, IEEE Abstract—A simple method is demonstrated for measuring the cell gap of reflective twisted nematic (RTN) liquid crystal (LC) cells. This method utilizes a single laser beam and crossed- polarizer configuration such that Fabry–Perot effect and surface reflection are eliminated. Experimental results agree well with theory. Index Terms—Cell-gap measurement, reflective liquid crystal display. I. INTRODUCTION R EFLECTIVE TWISTED NEMATIC (RTN) liquid crystal display (LCD) is widely used in personal information systems due to its low power consumption, light weight, and good outdoor readability. In a reflective LCD, cell gap plays an important role in determining the performances, such as brightness, contrast ratio, and response time. During device fabrication, the LC cell gap may deviate from its originally designed value. Since the reflector is embedded in the inner side of the bottom substrate, the cell gap of a reflective cell is inconvenient to be measured by the conventional methods developed for the transmissive displays. [1]–[4] Several methods have been proposed to measure the cell gap of reflective LC cells, such as the spectrum-scanning method, [5]–[7], the phase-compensation method [8], and the input-polarization-angle-dependence method. [9] In the spec- trum scanning method, the measured reflectance spectrum is used to fit the twist angle and (where is the cell gap and is the LC birefringence) [5], or to search linear polarization conversion and circular polarization conversion conditions.[6], [7] Owing to the inner surface reflections of in- dium/tin/oxide (ITO) substrates, the spectrum scanning method always exhibits Fabry-Perot effect [10] and hence disturbs the measurment accuracy. The scanning spectrum method requires a spectrophotometer, which is somewhat complicated and costly. Thus, from the measurement accuracy, simplicity, and cost viewpoints, a single wavelength method is preferred. Another factor affecting the measurement accuracy is surface reflec- tion resulting from the refractive index mismatch between air and glass substrate. The crossed-polarizer configuration would eliminate such an undesirable effect [5], [9]. Manuscript received May 13, 2002; revised August 28, 2002. This work was supported by Toppoly Optoelectronics Corporation, Taiwan, R.O.C. The review of this paper was arranged by Editor J. Hynecek. The authors are with the School of Optics/CREOL, University of Central Florida, Orlando, FL 32816 USA (e-mail: swu@mail.ucf.edu). Digital Object Identifier 10.1109/TED.2002.804718 TABLE I THE PREFERED POLARIZER ANGLES TO DETERMINE THE RATIO.THE LISTED AND ARE THE CASE OF RIGHT-HANDEDNESS LC CELL, WHILE FOR LEFT-HANDEDNESS, THE SIGN OF AND SHOULD BE REVERSED ACCORDING TO (1). HERE,APPROXIMATE IS BASED UPON 550 nm VISIBLE LIGHT In this paper, we present a simple cell-gap measurement method using a He–Ne laser and crossed-polarizer configu- ration. By taking the ratio of reflected light intensity at two different polarizer angles, we can measure the value of the reflective cell. If the birefringence of the employed LC is known, then the cell gap can be obtained ( ). Our experimental results agree well with theory. II. THEORY In an RTN cell, the normalized reflectance ( ) under crossed-polarizer configuration has been obtained by the Jones matrix calculus as follows [11]: (1) where , , and is the angle between input polarizer and entrance LC director, and is the twist angle of LC. Here, the counterclockwise angle is de- fined to be positive and the clockwise angle is negative. For LC cell, right-handedness twist angle is positive and left-handed- ness twist angle is negative. After taking into account the interference between the beams reflected from interfaces above and below the LC layer, (1) should be revised as [5] (2) In (2), ; ; 0018-9383/02$17.00 © 2002 IEEE