1. Technical Field
The present invention relates to a method for characterization of a liquid crystal cell, in particular for measuring a cell gap and an average refractive index of the liquid crystal in the cell. The method provided by the invention can also be applied to transparent thin films with properties similar to the liquid crystal layers.
2. Background Art
In the manufacturing process and in research and development of liquid crystal devices it is often necessary to know the liquid crystal cell gap and average refractive index.
The most simple and well-known prior art solution involves measuring the thickness of an empty cell i.e., cells prior to filling and sealing. For example the thickness may be calculated from interference of light reflected by the cell. However, the thickness of the filled cell can be different from an empty one.
A second method (for example U.S. Pat. No. 6,081,337) for determining the thickness of a liquid crystal layer is based on measuring an optical phase difference between the ordinary and extraordinary waves traveling through the liquid crystal layer. The thickness of the liquid crystal layer can then be calculated if the birefringence of liquid crystal is known. However, this method cannot be used to measure the cell gap of the liquid crystal cell if it is filled with the cholesteric liquid crystal. It is also necessary to know the birefringence a priori.
The closest prior art solution is based on the interference measurement of the optical path in the liquid crystal layer (W. Kuczynski and B. Stryla, “Interference Method for the Determination of Refractive Indices and Birefringence of Liquid Crystals”, Mol. Cryst. Liq. Cryst., 1975, Vol. 31. pp. 267-273). According to this method, the liquid crystal located between two sub-mirror substrates is illuminated with light that has a broad spectrum, and the spectrum of reflected or transmitted light is measured. The optical path can be derived according the formula
                              Δ          =                      dn            =                                                            λ                  1                                ⁢                                  λ                  2                                                            2                ⁢                                  (                                                            λ                      2                                        -                                          λ                      1                                                        )                                                                    ,                              λ            2                    >                      λ            1                                              (        1        )            where d is the cell gap, n is a refractive index, λ1,λ2 are the wavelengths of two neighbor interference maxima or minima in the measured spectrum. The drawback of this method is the incapability to independently determine the cell gap d and the refractive index n.