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15.4/ X. J. Yu 15.4: Reflective LCD Cell Gap Measurement by QHQ Compensation Method X. J. Yu and H. S. Kwok Center for Dis y Research, Department of Electrical and Electronic Engineering University of Science and Technology Clear Water Bay, Kowloon, on the laser source. So method [3] is not too reliable A phase compensation method is presented for measuring either. the cell gap of reflective LCD. We use the so-called QHQ compensator that is made up of two quarter-wave tes In this paper, we will report a new cell gap measurement (Q) and one half-wave te (H). This method is different technique using QHQ compensation. This method has a from other compensation methods in that we can change simpler procedure and a higher accuracy than those both the orientation angle and retardation value of the reported. compensator by rotating the quarterwave and halfwave retarders. As a result, the Q ompensation method has 2. Theory a higher accuracy and a simpler procedure. The experimental set-up is shown in Figure.1. 1. Introduction In reflective LCD design, the retardation value dn  = is most important. It makes a critical influence  to the performance of the LCD. But its value is quite often not known exactly in a finished LCD s e the final cell gap can deviate from the designed value due to processing inaccuracies. So it is desirable to determ he