Chromatic parameter space representation of LCD operating modes

A chromatic parameter space (CPS) representation is proposed to represent the operation of LCDs. Both chrominance and luminance are visualized in the CPS diagrams. All the usual display modes, both transmissive and reflective, such as TN, ECB, OMI, STN, and SBE are shown clearly on the diagrams. Two designs with the help of CPS are proposed and demonstrated: multicolor STN and a novel fully-compensated reflective STN with high brightness and almost no chromatic dispersion. Introduction Optical modeling is very important in designing and optimizing liquid crystal displays (LCDs) . In previous work , we proposed a new systematic technique called parameter space (PS) representation of all nematic LCDs. This PS approach is very helpful in understanding the physical operation of various LCD modes and their relationships. With the help of the PS, we also analyzed the various different reflective LCD designs and developed several new reflective LCD operating modes . However, in our previous work, the PS is calculated only for a single wavelength, normally 550nm. Thus it does not show the appearance of the LCD under white light illumination. Even though the monochromatic PS can provide an idea of the chromatic dispersion behavior of the various LCD modes, it is still desirable to devise a scheme where the chromaticity of the display can be shown more clearly. It is particularly so for LCDs with considerable chromatic dispersion, such as STN displays. In this paper, a chromatic parameter space (CPS) representation is proposed and demonstrated. Both chrominance and luminance are visualized in the same diagram with the help of color representation. The real appearance of the LCDs in the static state (voltage-off or nonselect state) can be shown in the CPS diagram, both in terms of brightness and color. Using this new CPS, we analyzed the chromatic dispersion of various display modes, both transmissive and reflective. Furthermore, in this paper, we shall show two examples of the application of this CPS in designing new LCDs with good performance: (1) Multicolor STN (2) Fullycompensated reflective STN with high brightness and almost no chromatic dispersion. It is interesting to note that in the first case, we make use of chromatic dispersion to achieve multicolor display. In the second case, we do our best to eliminate chromatic dispersion. These two cases provide a good demonstration of how CPS can be used for LCD design and novel device development. Chromatic parameter space The reflectance or transmittance of an LCD is determined by four variables: input polarizer angle α, output polarizer angle γ , twist angle φ , and retardation value d∆n. Any pair of these variables can be used to generate a contour map which is called the PS diagram. The calculation assumes a fixed wavelength λ. The idea of the CPS is to repeat the PS calculation for all λ in the visual range ( 380nm ~ 780nm), and obtain the luminance and chrominance of the output assuming a standard white light input source. In this paper, CIE D65 is used as the incident light. From the output spectrum, we can obtain the CIE 1931 XYZ values. The XYZ color coordinate is transformed to the standard color space: sRGB, which is proposed by Hewlett-Packard and Microsoft . The sRGB tristimulus values can be computed using the following relationship: