to the birefringence, the components of the electric field vector of the light in parallel and perpendicular to the directors travel with different speeds, which depend upon the wavelength. They superimpose along their path between the two glass plates first to elliptically polarized light, in the distance d/2 from the input to circularly polarized light, then again to an elliptic polarization, and if
they reach the analyser again linearly polarized, but with the polarization plane rotated by 90°. If the analyser is crossed with the polarizer, the light can pass the analyser. The pixel appears white. This operation is termed the normally white mode. If the analyser is rotated by 90°, a parallel analyser, the light is blocked in the analyser. The pixel is black. This is called the normally black mode. A useful visualization of what happens to the light while travelling through the cell is as follows: the planes of the various polarizations follow the twist of the helix. This is, however, only true if Equation (2.15) holds. The explanation is also only true for light travelling and viewed perpendicular to the plane of the substrate. If viewed under a different angle, light perceived by the eye has travelled in a different path with different angles to the director and a different cell thickness d.
Figure 2.10 The structure of a TN-LCD (a) while light is passing, and (b) while light is blocked, a: polarizer; b: glass substrate; c; transparent electrode; g; orientation layer; e: liquid crystal; f: illumination
Figure 2.11 LC molecules with pretilt angle a0 on top of the orientation layer
If a voltage VLC of the order of 2 V is applied across the cell, as shown in Figure 2.10(b), using the two transparent ITO-electrodes 100 nm thick, the resulting electric field attempts to align the molecules for Δε > 0 parallel to the field. This holds independent of the sign of the vector of the electrical field, as already pointed out in Section 2.1.2. Hence, the following effects are not dependent on the polarity of VLC. Due to the anchoring forces, a thin LC layer on top of the orientation layers maintains its position almost parallel to the surfaces. A threshold voltage Vth is needed to overcome intermolecular forces before the twisted molecules start to rotate. A uniform start over the plane of the panel is favoured by a pretilt angle around 3°, which seems to avoid strong differences in the anchoring forces. Only at a saturation voltage Vmax several times Vth with a value around 10 V have all molecules besides those on top of the orientation layers aligned parallel to the electric field, as depicted in Figure 2.10(b). In this state the vector of the electrical field of the incoming light oscillates perpendicular to the directors, and encounters only the refractive index n┴. Hence, no birefringence takes place and the wave reaches the crossed analyser in the same linearly polarized form as at the input. The analyser blocks the light and the pixel appears black. This is an excellent black state as it is independent of the wavelength, resulting in a blocking of the light. This black state is gradually reached from the field-free initial state by increasing the voltage VLC from OV over an intermediate voltage up to Vmax, which is also gradually rotating the molecules in Figure 2.12 from the initial twisted state with directors parallel to the surfaces (Figure 2.10(a)) over an intermediate state with the director already tilted down with tilt angle a (Figure 2.12(b)) to the final state with directors parallel (a = 90°) to the electric field. The transmitted luminance, also termed transmittance, of the light is shown in Figure 2.13 for the normally white mode discussed so far. In the normally black mode, the analyser is parallel to the polarizer and allows the light to pass at the voltage Vth ≤ VLC ≤ Vmax. For this mode the transmitted luminance is also depicted in Figure 2.13. Only in this mode is the threshold voltage Vth visible, as in the normally white mode a small change in luminance at a high value of the luminance cannot be perceived by the eye.
Luminance is the correct term for ‘brightness’. The physical meaning and dimensions of luminance and other display-related units are explained in Appendix 2.
The blocking of light in the analyser as described by Equation (2.15) is only valid for one wavelength for which, as a rule, yellow light with λ = 505 nm is chosen. As other wavelengths can still pass the analyser, the black state is not perfect. As a rule, it has a bluish tint. The imperfect black state can be improved by compensation foils, as discussed later.
Figure 2.12 Change in the position of the LC molecules with increasing voltage
Figure 2.13 Transmitted luminance versus the reduced voltage VLC across the LC cell for the normally white and normally black modes
The measurement should be performed without the interference of reflected ambient light, i. e. in darkness. If the black state in the denominator of Equation (2.16) is increased by the imperfect blocking of the light, contrast falls in any case. This is the case in the normally black state, whereas the normally white state described above yields an excellent contrast due to a much lower value of the denominator in Equation (2.16).
Grey shades of a pixel are controlled by the voltage VLC in Figure 2.13, which modulates the luminance from a full but imperfect black up to a full white. Luminance differs when the display is viewed under angles different from perpendicular to the glass plates. Contrast decreases the more oblique the angles become.
The TFT addressing circuit will be placed on the glass next to the backlight in Figure