Measurements of empty cell gap for liquid-crystal displays using interferometric methods

In order to determine the empty cell gap of a liquid-crystal (LC) cell to better than 1% accuracy, the empty LC cell has been modeled as an optical medium consisting of seven layers with an air gap representing the true cell gap in the middle layer. The given index of refraction and thickness for each layer, optical reflections as a function of wavelength ranging from 500 to 650 nm have been calculated. To simulate the experimental measurement, the apparent cell gaps were deduced from reflection peaks and compared with the true cell gaps in the region of 5–10 μm. It was found that the difference between the apparent and true cell gaps was larger than 0.66 μm using a 0.1-μm polyimide as an aligning layer coated on top of the transparent indium-tin-oxide electrode of 0.04 μm in thickness. This modeling is applicable to three interferometric methods commonly used for the measurement of an empty cell gap.