Liquid Crystal Displays - Ernst Lueder

Скачать книгу

is depicted in Figure 3.12(a) (Uchida, 1999; Lueder et al., 1998). The polarizer transmits linearly polarized light again at an angle of π/4 to the x-axis in Figure 3.12(b). The illumination is provided either by ambient light or by an external light source above the polarizer. After having travelled through the cell in Figure 3.12(a) with half the thickness d/2 of a transmissive cell, the light is reflected at the mirror usually made of Al and finally exits through the same polarizer. Thus, the reflective cell saves one polarizer. The reflective cell can be designed according to the general principles, which will be outlined now (Lueder et al., 1998). We first recall the operation of a Freedericksz cell with parallel polarizers as depicted in the left column of Figure 3.12(c). We know that linearly polarized incoming light in the direction α = π/4 to the x-axis is blocked at z = d for wavelength λ0. Due to Equation (3.80), the thickness is d = λ0/2Δn. We determine at which value z the light is circularly polarized.

      With Equation (3.45) this happens for the first time for δ = π/2, reflecting in

      As sin δ = sin(π/2) > 0 the light is right-handed circularly polarized seen against equation images, as indicated in the middle column of Figure 3.12(c). The highly conductive mirror in the middle column in Figure 3.12(c) reflects this light which is drawn with solid lines in two positions from 1 to 2 for increasing time. Since the mirror cannot sustain an electric field as the high conductance shortens the fields, the field vectors 1 and 2 have to be compensated by vectors 1′ and 2′ of the reflected light shown with dashed lines. The reflected light propagates in the direction of the wave vector equation images, and it represents left-handed circularly polarized light seen against kr. The upwards travelling reflected light in the right column in Figure 3.12(c) images the downward moving light in the left column, and is blocked in the polarizer as linearly polarized light, in the same way as the downward travelling light is at its lower polarizer. The described imaging of the downward wave by the reflected wave can no longer take place if the mirror is not placed at z = d/2. Therefore, many reflective cells are constructed according to the principle discussed. If a voltage V is applied to the reflective cell, the LC molecules orient themselves in parallel to the electric field for Δε > 0. The linearly polarized light travels downward, is reflected, reaches the polarizer unchanged apart from a phase shift and passes the polarizer. Hence, the reflective cell exhibits the same electro-optical performance as the transmissive cell. The performance is given for the normally black Freedericksz cell with parallel polarizers defined by α = π/4 and Equations (3.73) and (3.85).

Schematic illustration of the reflective Fr eedericksz cell. (a) Cross section; (b) top view; (c) explanation of the operation of a reflective cell in the field-free state.

      The surface of the mirror and the lower edge of the LC material in Figure 3.12(a) are supposed to be located at z = d/2, which is not exactly feasible because of the presence of the ITO and the orientation layers. As both layers are very thin, around 100 nm each, this does not show up in the performance of the cell.

       3.2.4 The Fréedericksz cell as a phase-only modulator

Schematic illustration of an L C D used as an SLM operating as a multiplier.

      Equations (3.40) and (3.41) yield, for γ = α (that is, for parallel polarizers), the Jones vectors Jzξ and Jzη measured in the ξ−η coordinates in Figure 3.4(a)

      (3.89)equation images

      The Jones vector Jz% of the light passing through the analyser parallel to the polarizer is, for a = 0, n and a = n/2 and for no voltage applied,