Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-08-30
2003-07-15
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S018000, C349S078000
Reexamination Certificate
active
06593985
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a color shutter for field-sequentially separating a white light into RGB components, and a display apparatus for displaying a color image by field-sequentially mixing the color components.
A system for displaying a color image includes a spatial division display for displaying an RGB image by dividing the RGB image into the RGB components for pixels and a field-sequential additive color mixing display in which an RGB image is displayed with time and an RGB color filter is switched in synchronization with the image.
The field-sequential additive color mixing display is superior to the spatial division display in fineness because the pixel need not be divided into RGB components in the field-sequential additive color mixing display. In the field-sequential additive color mixing display, the method of rotating a disc-like filter divided into three color regions of RGB in synchronization with each RGB image display is most widely known to the art.
A method of switching the displayed color without using a mechanical rotation mechanism is disclosed in, for example, U.S. Pat. No. 5,387,920 to Bos et al. Specifically, proposed is a so-called “liquid crystal color shutter system” in which a color polarizer is arranged on each of the front surface and the rear surface of two liquid crystal cells, and the polarizing plane of the light is controlled by the on/off switching of the liquid crystal cells so as to select the wavelength of light absorbed by the polarizer and, thus, to achieve an RGB display.
In the liquid crystal color shutter disclosed in this prior art, a plurality of color polarizers differing from each other in the color phase are arranged on an optical path such that the absorption axes are rendered perpendicular to each other. For example, a yellow color polarizer, which transmits green and red lights and absorbs a wavelength region of blue, and a blue color polarizer, which absorbs a wavelength region of yellow, are arranged such that the absorption axes of these two polarizers are rendered perpendicular to each other. Similarly, a red color polarizer and a cyan color polarizer are arranged such that the absorption axes of these two polarizers are rendered perpendicular to each other. Further, a liquid crystal cell is arranged between these two sets of color polarizers, and an achromatic polarizer, which is a linear full wavelength polarizer region, and a liquid crystal cell are added so as to select the axis of polarization of the incident light or leaving light.
It should be noted that a red display can be achieved by using a polarized light transmitting through the absorption axes of the yellow color polarizer and the red color polarizer. Also, a green display can be achieved by the combination of the yellow color polarizer and the cyan color polarizer. Further, a blue display can be achieved by the combination of the blue color polarizer and the cyan color polarizer.
The liquid crystal color shutter is advantageous in that a mechanical operation is not involved therein, and its space saving because the area of the display screen can be made equal to the area of the color shutter.
In the liquid crystal color shutter, a colorant-based color polarizer prepared by impregnating a PVA (polyvinyl alcohol) substrate with a dichroic colorant, followed by applying an orienting treatment by stretching to the impregnated PVA substrate is used as the color polarizer. As shown in
FIG. 1
, the colorant-based color polarizer is a partial polarizer capable of absorbing a specified wavelength of a polarized light having a polarizing plane in the direction of the absorption axis.
The liquid crystal color shutter using the particular colorant-based color polarizer is defective in that the transmittance is markedly low. For example, the absorption axis transmitting characteristics of the colorant-based color polarizer are shown in FIG. 6 of a literature “Proceedings of the SID” Vol. 26/2 (1985), 157-161.
As apparent from
FIG. 6
of the literature quoted above, curves of the transmittance characteristics are sharp and the dichroic ratio is sufficient in the red polarizer and the yellow polarizer. However, the characteristics of the blue polarizer and the cyan polarizer are markedly inferior. Therefore, where a liquid crystal color shutter is formed by using these colorant-series color polarizers, the transmittance is markedly lowered.
Under the circumstances, a PRS (Polarizer Retarder Stack) is proposed in recent years by Sharp et al. as a color polarizer performing the function similar to that performed by a dichroic color polarizer and used in a liquid crystal color shutter in place of the dichroic color polarizer (U.S. Pat. No. 5,751,384). The PRS is formed by laminating a plurality of birefringent retardation films (i.e., at least about 5 films) on an achromatic polarizer in a phase axis direction of a predetermined angle.
By setting the retardation and the phase axis direction of the birefringent retardation film appropriately in the PRS, it is possible to allow the white light incident on the side of the achromatic polarizer to be emitted from the polarizer at different angles relative to the optical axis of the achromatic polarizer depending on the wavelength region of the light, as shown in FIG.
2
. For example, the light having the wavelength region of blue is emitted at 0° and the light having wavelength regions of green and red (yellow) is emitted at 90° relative to the optical axis of the achromatic polarizer. It follows that this example is equal to the case where dichroic colorant polarizers of blue and yellow are arranged such that the absorption axes of these two polarizers are perpendicular to each other. The PRS does not include an absorption medium other than the absorption axis of the achromatic polarizer and, thus, has a high transmittance, compared with the dichroic colorant polarizer.
The liquid crystal color shutter employs the system that the transmitting color is switched by controlling the polarizing plane of the incident light. Therefore, where an unpolarized natural light is assumed to be the incident light, one polarized component is absorbed during conversion from the unpolarized light into a polarized light. Thus, the liquid crystal color shutter is essentially lower in its transmittance than the color filter. Naturally, it is important to improve the transmittance of the liquid crystal color shutter.
The optical characteristics of the color polarizer constituting the liquid crystal shutter greatly affects the transmittance of the liquid crystal color shutter. When it comes to the two color polarizers of the dichroic colorant polarizer and the PRS, the PRS system is advantageous in transmittance because the absorbing member is not included in the members other than the achromatic polarizer. On the other hand, in the PRS system, the incident light is separated into mutually complementary colors such as blue/yellow or cyan/red in the axes of the polarized light perpendicular to each other. It follows that it is impossible to cut the undesired light, with the result that the component of the intermediate wavelength region in the boundary region between blue and green and between green and red is allowed to be contained in any of the color display of the RGB displays. Such being the situation, it is difficult to improve the color purity in all of RGB colors.
As an example specifically showing the above-noted problem, the construction of the conventional liquid crystal color shutter using PRS, the transmittance characteristics at each PRS, and the RGB color reproducing region in the CIE1976UCS chromaticity diagram are shown in
FIGS. 3
to
9
.
Specifically,
FIG. 3
shows as an example the construction of a LCCS (liquid crystal color shutter) using PRS. Polarizing rotators
103
,
104
consisting of liquid crystal cells are inserted between achromatic polarizers
105
and
106
and between the polarizers
106
and
107
, respectively. By controlling the voltage applied to each of these polarizing
Saishu Tatsuo
Suzuki Kohei
Taira Kazuki
Tsuda Ryoji
Uno Shigeki
Kabushiki Kaisha Toshiba
Kim Robert H.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Schechter Andrew
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