Cell gap measurement of liquid crystal displays by scanning white light interferometer

• Main Authors: Shih-guo Cai, 蔡世國
• Other Authors: Sheng-hua Lu
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/82930554864136432480

碩士 === 逢甲大學 === 光電研究所 === 95 === The cell gap of a liquid crystal display (LCD) determines the LCD’s image quality and photoelectric characteristic. This research proposes a scanning white light interferometer (SWLI), based on low coherence tandem interferometry, for measuring the liquid crystal (LC) cell gap. The theoretical model is simple, and no time-consuming library matching is required. The SWLI consists of a white light source, a scanning Michelson interferometer (SMI), and a LC cell. The SMI and LC cell are in tandem and thus can prevent the dispersion error caused by the upper glass of the LC cell. The light is first directed to the SMI. Then a beam splitter divides the output beams of the SMI into two parts. One part is collected by a reference detector. The other part is incident on the LC cell, and the reflection from the cell is received by the measuring detector. The optical path difference of the SMI is continuously changed and the output signals of the detectors are recorded. The normalized reference signal is subtracted from the normalized measuring signal to reduce the influence of the central white light fringe, which is always generated when the path difference in the SMI is zero. After zero-order interference fringes are identified with centroid algorithm, the cell gap is directly determined from the separation between these fringes. Three different cells were measured with the SWLI. The nominal gaps of empty cell, twisted nematic (TN) cell, and multi-domain vertical alignment (MVA) cell are 5 μm, 3.5 μm, and 4.2 μm, respectively. Experimental results show that the deviation is about 0.1 μm for the empty cell, and the deviation for the other two cells is 0.4~0.5 μm。