The present invention relates to a quarter wavelength plate which is particularly advantageous in enhancing the contrast, and an optical unit using the same for use in a reflection type liquid crystal display device.
A reflection type liquid crystal display device using a reflex liquid crystal display element for displaying images is disclosed, for example, in JP-B-7-38050.
FIG. 1 is a schematic diagram illustrating an optical unit in the reflection type liquid crystal display device disclosed in the above-cited document. The optical unit herein referred to is assumed to be an optical display device which comprises a light source, an illumination optical system, a video display element, and a projection lens. The video display element is irradiated with light from the light source through the illumination optical system. A light intensity modulation is performed for converting the irradiated light to the contract of each pixel on the video display element. A resulting image is enlarged by the projection lens for display.
In FIG. 1, the optical unit comprises a light source 1, a polarization beam splitter 2, a quarter wavelength plate 3, a reflex liquid display element 4, and a projection lens 5.
Light from the light source 1 is split by the polarization beam splitter 2 into an S-polarization component which is reflected by the beam splitter 2 and a P-polarization component which transmits the beam splitter 2. The S-polarization component reflected by the polarization beam splitter 2 transmits the quarter wavelength plate 3 and impinges on the reflex liquid crystal display element 4. The S-polarization component is converted to a P-polarization component by the reflex liquid crystal display element 4. The resulting light for producing a bright display again transmits the quarter wavelength plate 3, and is projected onto a screen by the projection lens 5 along a light path which extends through the polarization beam splitter 2. The light not converted by the reflex liquid crystal display element 4 again transmits the quarter wavelength plate 3 and travels back to the light source 1 along a light path which is bent by the polarization beam splitter 2.
Here, the quarter wavelength plate 3 is used with the intention of improving the contrast which would be degraded by incident light on the polarization beam splitter, tilted with respect to the optical axis, in the reflection type liquid crystal display device, as disclosed in detail in JP-B-7-38050.
Materials suitably used for forming the quarter wavelength plate include, for example, a stretched polycarbonate film adhered on a glass substrate (hereinafter called the “phase difference film”), a crystal having optical anisotropy such as quartz.
FIG. 2 is a schematic diagram illustrating a quarter wavelength plate which is formed of a stretched polycarbonate film adhered on a glass substrate. In FIG. 2, the quarter wavelength plate comprises the glass substrate 6, the stretched polycarbonate film 7, and a phase delay axis 8 of the quarter wavelength plate.
FIG. 3 in turn is a schematic diagram illustrating a quarter wavelength plate formed of quartz, which is for use in a reflection type liquid crystal display device. In FIG. 3, the quarter wavelength plate 12 comprises a reference axis 9; a first optically anisotropic crystal 10 having a phase delay axis 131 which is parallel with the reference axis 9; and a second optically anisotropic crystal 11 having a phase delay axis 132 which is perpendicular to the reference axis 9. The quarter wavelength plate 12 is formed by adhering the first optically anisotropic crystal 10, the phase delay axis 131 of which is parallel with the reference axis 9, to the second optically anisotropic crystal 11, the phase delay axis 132 of which is perpendicular to the reference axis 9. The quarter wavelength plate 12 has a thickness t which is conventionally set to approximately 1.0 mm from a viewpoint of workability.
In the optical unit for use in a reflection type liquid crystal display device, a retardation value required to a quarter wavelength plate is in a range of 100 to 180 nm because visible light has wavelengths ranging from 400 to 700 nm. The retardation value used herein refers to the product of a difference between the refractive index in the direction of the phase delay axis of the quarter wavelength plate and the refractive index in the direction of the phase advance axis perpendicular to the phase delay axis (hereinafter called the “birefringence difference”) and the thickness of the quarter wavelength plate. However, for providing the above-mentioned retardation value, for example, with a quarter wavelength plate formed of quartz chosen from among optically anisotropic crystals, the quarter wavelength plate must be made to have a thickness of approximately 15 μm or less because quartz has the birefringence difference of approximately 0.009. However, a quarter wavelength plate having a thickness of 15 μm or less is virtually impossible from viewpoints of the yield rate and strengths of parts in the manufacturing. To solve this problem, two optically anisotropic crystals having a thickness of approximately 1.0 mm are adhered with their phase delay axes oriented orthogonal to each other, as illustrated in FIG. 3. By ensuring that a difference between the retardation value of the first optically anisotropic crystal, the phase delay axis of which is parallel with the reference axis, and the retardation value of the second optically anisotropic crystal, the phase delay axis of which is perpendicular to the reference axis, falls within a range of 100 to 180 nm, it is possible to achieve the retardation value required for a quarter wavelength plate for use in an optical unit of a reflection type liquid crystal display device.