Eliminating light leakage in a liquid crystal display

Liquid crystal displays (LCDs) are widely used for various applications. Most smart phones and tablet PCs use LCDs with in-plane switching and fringe-field switching modes. These modes exhibit the widest viewing angle because the liquid crystals (LCs) are initially homogeneously aligned before rotating within a plane parallel to the substrates when an in-plane field is applied.^{1, 2} However, further improvement is still needed to prevent light leakage and improve the quality of dark images.

The major cause of light leakage in the in-plane switching and fringe-field switching modes is a change in the effective angle between the absorption axes of two crossed polarizers when viewed from oblique directions. Light leakage increases as the polar viewing angle increases because the angle between the two crossed polarizers deviates away from 90^° (see Figure 1).

Figure 1. Photographs of two crossed polarizers viewed from (a) 0^°, (b) 30^°, and (c) 60^°.

Several compensation schemes have been proposed using uniaxial A and C films—for example, in +A/+C, +A/–A, and +A/+C/+A configurations—to eliminate the light leakage associated with crossed polarizers.³ An A plate has an optic axis that is parallel to the plate surface, while a C plate's optic axis is perpendicular. Meanwhile, a positive wave plate has an extraordinary index greater than the ordinary index, while the opposite applies for a negative plate. Although a 100:1 iso-contrast contour can cover the entire viewing cone at an optimized wavelength of 550nm, light leakage at other wavelengths, such as those corresponding to red and blue, remains very severe. The light leakage caused by the wavelength dependence of the phase retardation cannot be removed using additional films or by changing the film configuration and continues to be a problem for LCDs.

Using negative-dispersion films and optical birefringence proportional to the wavelength can reduce the light leakage for blue or red light so that it becomes as small as that at the design wavelength of 550nm.⁴ Using two biaxial films can also be much more effective than using uniaxial films.⁵ However, the cost associated with fabricating negative dispersion or biaxial films can be much higher than that of uniaxial films with normal dispersion. We should note, therefore, that a trade-off exists between the performance and cost of the compensation films.

Our research team has designed achromatic compensation configurations for a perfect dark state in LCDs.^6–8 To cancel out the effect of wavelength dispersion, we replaced each A plate in the conventional uniaxial configurations with a pair of orthogonal A plates, as shown in Figure 2. The +A/+C, +A/–A, and +A/+C/+A configurations can be modified to the proposed +A/+A/+C, +A/+A/–A/–A, and +A/+A/+C/+A/+A configurations, respectively. We are able to optimize retardation and wavelength dispersion, choosing the first A plate with high retardation to have weak wavelength dispersion but the second A plate with low retardation to have strong wavelength dispersion.

Figure 2. Schematic diagram of the proposed +A/+A/+C configuration in which an A plate in the traditional +A/+C configuration is replaced by two orthogonal A plates above the liquid crystal (LC) cell. φ_T: Absorption axis of polarizer. φ_+A: Optic axis of +A film.

We calculated viewing-angle-dependent dark-state light leakage using a numerical calculation (see Figure 3). Conventional uniaxial configurations gave more than 0.01% light leakage, at a polar angle of ±30^°. However, the light leakage in the +A/+A/+C and +A/+A/–A/–A was less than than 0.01% at a polar angle of ±55^°, while it was under 0.01% over the entire viewing cone in the +A/+A/+C/+A/+A configuration. These results indicate that the dark-state light leakage for white incident light can be eliminated over the entire viewing cone by replacing each A plate with two orthogonal A plates, using films with normal dispersion.

Figure 3. Viewing-angle-dependent dark-state light leakage of (a) +A/+C, (b) +A/–A, (c) +A/+C/+A, (d) +A/+A/+C, (e) +A/+A/–A/–A, and (f) +A/+A/+C/+A/+A configurations.

The numerical results do not take into account the effect of the angle of LCs in their initial state, before they are tilted up or down by applying an electric field. However, we know that a non-zero pre-tilt angle increases light leakage (see Figure 4). Since the pre-tilt angle of LC cells fabricated using a rubbing method—the LC alignment method for most commercially available LCD panels—is usually higher than 2^°, it may be very difficult to completely eliminate light leakage using the conventional rubbing method. A near-zero pre-tilt angle, which is essential for complete light leakage elimination, can be obtained using a photoalignment method or by modifying the conventional rubbing method.⁹

Figure 4. Viewing-angle-dependent dark-state light leakage when the pre-tilt angle is (a) 0^°, (b) 1^°, and (c) 2^°in the +A/+A/+C/+A/+A configuration.

In summary, we have proposed achromatic compensation schemes using films with normal dispersion to eliminate dark-state light leakage. Replacing each A plate in the conventional uniaxial configurations with a pair of orthogonal A plates can minimize the effect of wavelength dispersion and effectively eliminate dark-state light leakage. All the films that we use here have normal wavelength dispersion, and can easily be fabricated at a relatively low manufacturing cost. We currently have a choice of three configurations, depending on the application. We are now working on new optical compensation configurations to reduce the number of films while maintaining performance. For this, we aim to make a film that has the same effect as a pair of orthogonal A plates.