Читать книгу Liquid Crystal Displays - Ernst Lueder - Страница 20

Figure 2.9 depicts the top view of a display panel with the conducting rows and columns terminating in the contact pads. The rectangular pixels can only be electrically addressed from those contact pads.

A colour VGA display, as used in laptops, has 480 rows and 3 × 320 columns forming triple dots for the three colours red, green and blue. An NTSC TV display has 484 rows and 3 × 450 columns corresponding to 653 400 pixels, whereas an HDTV display has 1080 × 3 × 1920 = 7320 800 pixels. For more standardized formats, see the table in Appendix 1.

Table 2.2 Properties of nematic LC materials with a wide temperature range

	MLC-1380000	MLC-13800100	MLC-1390000	MLC-13900100
Transition temp, smectic-nematic	< −40°C	< −40°C	< −40°C	< −40°C
Clearing pt Tc	110 °C	111°C	110.5 °C
Rotational viscosity, 20 °C	228 mPas	151 mPas	235 mPas	167 mPas
Δε 1 kHz 20 °C	+ 8.9	+ 5.0	+ 8.3	++ 5.2
n0= n┴	1.4720	1.4832	1.4816	1.4906
ne= n||	1.5622	1.5735	1.5888	1.5987
Δn	+ 0.0902	+0.0903	0.1073	+ 0.1081

Figure 2.9 Top view of the rows, columns, pixels and contact pads of a display panel

Figure 2.10 shows a pixel of a transmissive twisted nematic LC cell with no voltage applied. The white backlight/passes the polarizer a. The light leaves it linearly polarized in the direction of the lines in the polarizer, and passes the glass substrate b, the transparent electrode c out of Indium-Tin-Oxide (ITO) and the transparent orientation layer g. This layer, made of an organic material such as polyimide, 100 nm thick, is rubbed to generate grooves in the direction of the plane of the polarized light. In these grooves the rod-like LC molecules are all anchored in parallel, but, as shown in Figure 2.11, with a pretilt angle a₀ to the surface of the orientation layer. The sequence of layers is the same on the second glass plate. A typical thickness of the cell in Figure 2.10 is d= 3.5 μ to 4.5 μ. The grooves on the second plate are perpendicular to those on the first plate. This forces the liquid crystal molecules to twist on a helix by β = 90° from one plate to the other without the addition of chiral compounds. All twist angles are called β.

Due 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

(2.15)

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 V_LC 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 V_LC. 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 V_th 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 V_max several times V_th 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 V_LC from OV over an intermediate voltage up to V_max, 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 V_th ≤ V_LC ≤ V_max. For this mode the transmitted luminance is also depicted in Figure 2.13. Only in this mode is the threshold voltage V_th 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 V_LC across the LC cell for the normally white and normally black modes

(2.16)

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 V_LC 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 2.10. In a colour display, the glass plate facing the viewer carries the pixellized colour filter, shown in Figure 2.14. The pixels for red, green and blue are covered with a compound which absorbs all wavelengths originating from the white backlight besides red, green and blue, respectively. The saturation of the colours is individually controlled for each pixel by the voltage V_LC in the same way as for grey shades.

The geometrical arrangement of the colour pixels in triangles in Figure 2.14(a) and along diagonals in Figure 2.14(c) are recommended for moving TV pictures, whereas the colour stripes in Figure 2.14(b) are preferred for computer displays often presenting rectangular graphs.

The cross-section of a colour filter in Figure 2.15 contains the colour materials for R, G and B, an absorptive layer with a low reflection in between the pixels, a so-called black matrix, an overcoat layer and, in the case of TFT addressing, an unpixellized ITO electrode over the entire display area. For other addressing schemes, the ITO layer is no longer unstructured. The ITO electrode on the TFT-carrying plate is pixellized. The black matrix prevents light between the pixels, which is neither controlled by the voltage V_LC at the ITO electrodes nor exhibits the desired colour, from seeping through the cell. This light would lighten up the black state, and would thus degrade contrast and the saturation of colour. A suitable material for a black matrix is an organic material with carbon particles exhibiting a reflectivity of only 4 percent, whereas the previously used Cr-oxide has a reflectivity of 40 percent. The overcoat layer (e.g. out of a methacrylate resin solution) equalizes the different heights of the colour pixels and protects them.

Figure 2.14 The geometrical arrangement of colour pixels for red R, green G, and blue B (a) in triangles, (b) in stripes and (c) in diagonal form

Figure 2.15 Cross-section of a colour filter for TFT addressed LCDs