Liquid crystal display panel

Abstract: A TN type liquid crystal cell (14) is interposed between a pair of polarization plates (11 and 15). A first and a second retardation films each having slow axes orthogonal each other are disposed between one of the polarization plates and the liquid crystal cell (14). The product Δn₁ × d₁ of the refractive index anisotropy Δn₁ and the thickness d₁ of the first retardation film (12) on the side of one of the polarization plates is set greater than the product Δn₂ × d₂ of the refractive index anisotropy Δn₂ and the thickness d₂ of the second retardation film. The viewing angle of the liquid crystal display panel is expanded by this arrangement.

Description: BACKGROUND OF THE INVENTION The present invention relates to a TN type liquid crystal display panel, and more particularly, to a transmission type liquid crystal display panel of which viewing angle characteristics are improved by interposing two retardation films each having slow axes nearly orthogonal each other between a polarization plate and a liquid crystal cell. In a TN (Twisted Nematic) type liquid crystal display panel, as shown in Fig. 1 for both of prior art and the present invention, a pair of polarization plates 11 and 15 between which a TN liquid crystal cell 14 is interposed is provided. In order to improve the viewing angle characteristics, a TN type liquid crystal display panel wherein two retardation films 12 and 13 each having slow axes nearly orthogonal each other are interposed between one of the polarization plates, 11, and a liquid crystal cell, 14 is proposed in the Japanese Patent Application Laid Open No. 2-125224. The direction of slow axis 3a of the retardation film 13 adjacent to the liquid crystal cell 14 is set nearly equal to the direction 4a of liquid crystal molecular orientation on the surface of the liquid crystal cell 14 facing to the retardation film 13. In the display panel disclosed in the above patent application, each product Δn × d (referred to as retardation) of refractive index anisotropy Δn and thickness d for retardation films 12 and 13 is equally set each other. The viewing angle characteristics for both left-right directions (i.e. horizontal directions) and upper-lower directions (i.e. vertical directions) can be improved to some extent by this prior art. However, the viewing angle characteristics for left-right directions are more important than that for upper-lower directions for a display used in a cockpit of an aircraft. Therefore, strong requirements for a liquid crystal display panel of which viewing angle characteristics for left-right direction are further improved have been raised. SUMMARY OF THE INVENTION It is an object of the present invention to provide a liquid crystal display panel of which viewing angle characteristics for left-right directions are further improved in order to respond to those requirements. According to the present invention, in a liquid crystal display panel, a TN type liquid crystal cell is interposed between a pair of polarization plates, and a first and a second retardation films each having slow axes nearly orthogonal each other are further interposed between one of the polarization plates and the liquid crystal cell with the second retardation film being positioned on the liquid crystal side, the product Δn 1 × d 1 of the refractive index anisotropy Δn 1 and the thickness d 1 of the first retardation film is set greater than the product Δn 2 × d 2 of the refractive index anisotropy Δn 2 and the thickness d 2 of the second retardation film. In the present invention, it is desirable that the product Δn × d of the refractive index anisotropy Δn and the thickness d of TN type liquid crystal cell is set in the range of 470 ± 200 nm. In the present invention, it is desirable that the difference between the product of the refractive index anisotropy and the thickness of the first retardation film and the product of the refractive index anisotropy and the thickness of the second retardation film, Δ R= Δn 1 × d 1 - Δn 2 × d 2 , is set in the range 0 < R ≦ 50 nm. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an oblique viewing diagram showing an embodiment of the present invention and a prior art arrangement; Fig. 2 is a graph showing an example of the viewing angle characteristics in left-right directions in the embodiment of Fig. 1; Fig. 3 is a graph showing an example (corresponding to Fig. 2) of the viewing angle characteristics in upper-lower direction in the embodiment of Fig. 1; Fig. 4 is a graph showing another example of the viewing angle characteristics in left-right directions in the embodiment of Fig. 1; and Fig. 5 is a graph showing another example (corresponding to Fig. 4) of the viewing angle characteristics in upper-lower directions in the embodiment of Fig. 1. DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENT The TN type display panel is arranged, like a prior art, as shown in Fig. 1. The duplicated explanation for Fig. 1 is omitted. A feature of the present invention is to set the product Δn 1 × d 1 of the refractive index anisotropy Δn 1 and the thickness d 1 of the first retardation film greater than the product Δn 2 × d 2 of the refractive index anisotropy Δn 2 and the thickness d 2 of the second retardation film. By such a setting, as shown by the contrast of vertical axis versus the viewing angle of horizontal axis in Figs. 2 and 4, the viewing angle is expanded and the characteristics are improved compared with the prior art example where the retardation Δn × d is set to the same for two retardation films 2 and 3. In Fig. 2, the cases of ΔR= Δn 1 × d 1 - Δn 2 × d 2 =270-270=0 , ΔR=280-260=20 nm and ΔR=290-250=40 nm are shown. In Fig.4, the cases of ΔR=300-300=0 and ΔR=320-280=40 nm are shown. Regarding the viewing angle characteristics in left-right directions, the case of Δ R=40 nm is the best and the case of Δ R=20 is the second best compared with the conventional case of Δ R=0. On the other hand, regarding the viewing angle characteristics in upper-lower direction, as shown in Fig. 3 (corresponding to Fig.2) and Fig. 5 (corresponding to Fig.4), the case of Δ R=20 nm is about the same level as or slightly worse than the case of Δ R=0 and the case of Δ R=40 nm is slightly worse than the case of Δ R=0. Actually, from many experiments, it has been confirmed that the viewing angle characteristics in left-right directions improves as ΔR increases while the viewing angle characteristics in upper-lower direction inversely deteriorates as ΔR increases. Practically, it is appreciated that ΔR of 0 < ΔR ≦ 50 nm is desirable. Incidentally, it is desirable that the product (retardation) Δn × d of the refractive index anisotropy Δn and the thickness d of TN type liquid crystal cell 14 be set in the range of 470 ± 200 nm although this is not specified in a prior art example. The reason is that this range of ΔR indicates the best optical characteristics including viewing angle characteristics. Incidentally, this has been well known. As mentioned above, in the present invention, by setting the retardation ( Δn × d) of the first retardation film 12 interposed between one polarization plate 11 and the liquid crystal cell 14 greater than that of the second retardation film 13, the viewing angle characteristics in left-right direction can be improved compared with the prior art case where the same retardation value is set for both retardation films. Therefore, an LCD preferable for display used in a cockpit of an aircraft can be provided by the present invention.

Claims: A liquid crystal display panel wherein a Twisted Nematic type liquid crystal cell is interposed between a pair of polarization plates and a first and a second retardation films each having slow axes nearly orthogonal to each other are interposed between one of the polarization plates and said liquid crystal cell such that said first and second retardation films face to said one of the polarization plates and said liquid crystal cell respectively, characterized in that the product of the refractive index anisotropy Δn 1 and the thickness d 1 of said first retardation film, Δn 1 × d 1 , is set greater than the product of the refractive index anisotropy Δn 2 and the thickness d 2 of said second retardation film, Δn 2 × d 2 . The liquid crystal display panel according to claim 1, characterized in that the product Δ nd of the refractive index anisotropy Δn and the thickness d of said TN type liquid crystal cell is set in the range of 470 ± 200 nm. The liquid crystal display panel according to claim 1 or 2, characterized in that the difference between the products of the respective refractive index and thicknesses of said first and second retardation films, ΔR= Δn 1 × d 1 - Δn 2 × d 2 , is set in the range of 0 < ΔR ≦ 50 nm. The liquid crystal display panel according to claim 1 or 2, characterized in that the direction of the slow axis of said second retardation film is arranged to match with the liquid crystal orientation direction on the surface of said liquid crystal cell facing to said second retardation film.

Inventors: TAKIGAWA MITSURU, UKAI YASUHIRO, UNATE TAKAO, TAKIGAWA, MITSURU, UKAI, YASUHIRO, UNATE, TAKAO
Applicants: HOSIDEN AND PHILIPS DISPLAY CO, HOSIDEN AND PHILIPS DISPLAY CORPORATION
Application year: 1995
Main classification: G02F1
IPC codes: G02F1