DAP Type Liquid Crystal Display With Means for Obscuring Viewing Angle Related Changes in Color

This invention relates to the DAP (Deformation of Aligned Phase of Nematic) type of liquid crystal displays.

Since the DAP type of liquid crystal displays utilizing double refraction of liquid crystals was invented by M. F. Shiekel et al in 1971, research activities have been directed to utilize successfully the peculiar characteristics for practical use which allows freedom in controlling or changing the displayed color by an applied voltage. It is also well known that the above described type has inherent deficiencies in that the observed color varies with different viewing angles (the color is same within a range of the viewing angle of .+-.5.degree.) and thus the viewing angle is very limited. For these reasons difficulties have been encountered in reducing the DAP type of liquid crystal displays into practical use.

It is therefore an object of the present invention to compensate for the above discussed inherent deficiencies of the DAP type of liquid crystal displays. According the present invention, the problem with the prior art that the display changed dependent upon the viewing angle is overcome by cyclic variations in the effective value of an applied voltage across selected electrodes of a liquid crystal display of the DAP type described above.

In addition, the DAP type liquid crystal display embodying the present invention manifests excellent ornamental effects which changes periodically color. The displaying color when viewing in a certain direction changes, for example, red.fwdarw.blue.fwdarw.yellow . . . .fwdarw.red.fwdarw.blue.fwdarw.yellow . . . . While the displaying color differs from one viewing angle to another in a moment, the displaying color from a different direction also changes, for example, yellow.fwdarw.red.fwdarw.blue.fwdarw. . . . yellow.fwdarw.red.fwdarw.blue.fwdarw. . . . , thereby eliminating changes in color with different viewing angles (a certain color is viewable at different points in time with different viewing angles, though). The ornamental effects of the DAP type liquid crystal display embodying the present invention are suitable for displaying in clock use. As long as the applied voltage is varied at an operating cycle of a second, changes in the displaying color are provided with the function of "counting seconds".

For a more complete understanding of the present invention and for further objects and advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation of a DAP typed liquid crystal display construction;

FIG. 2 is a plan view of the construction illustrated in FIG. 1;

FIG. 3 is a block diagram of a circuit for operating the liquid crystal display construction according to one preferred embodiment of the present invention;

FIG. 4 is a detailed diagram of FIG. 3; and

FIG. 5 is a timing chart for explanation of the operation of the circuit of FIG. 4.

Referring now to FIG. 1, there is illustrated in a cross sectional view a DAP type liquid crystal display which includes a nematic liquid crystal 1 having the positive or negative anisotropy, patterned transparent electrodes 2, 3, transparent insulating layers 4, 5 both carried and sandwiched by a pair of glass supports 6, 7. A polarizer 8 and a detector 9 are further disposed outside the cell structure. A sealer and spacer is labeled 10, a light source is labeled 11 and the viewer's eye is labeled 12.

Upon the application of an electric field the molecules of liquid crystals are tilted. Linear polarization through the polarizer 8 takes in the form of elliptic polarization under the influence of the tilted molecules of liquid crystals, thereby enabling the detector to transmit a part of light. In particular, provided that the liquid crystal display is illuminated with monochromatic light, the intensity of the transmission light is varied periodically with the tilt of the molecules of liquid crystals, namely, an applied voltage. The relationship between the intensity of the transmission light and the applied voltage also depends upon the wavelength of the illumination light. For example, if white light is applied, then the transmission light through the detector 9 will be colored due to interference. This is the reason why the displaying color changes with different applied voltage. The tone of color is determined largely by an electrode-to-electrode spacing, optical properties of the liquid crystal layer and the applied voltage.

What follows is a description of a display for clock use according to one preferred embodiment of the present invention. In this case, the effective values of voltages applied to respective segments in the hour and minute display sections are varied at each second and in the sequence of five seconds so that changes in the displaying color provides the function of counting "second". The respective segments are disposed on one of the glass supports for displaying hours, minutes and a punctuation (dot) therebetween, whereas a common electrode is disposed on the other glass support. The display is operated in a static enable mode. In FIG. 2, the above described liquid crystal display is illustrated in a plan view, which includes hour and minute displaying transparent segment electrodes 13, a dot displaying transparent segment electrode 14, and a transparent common electrode 15 positioned vis-a-vis to the transparent segment electrode 13, 14. The remaining structure is same as in FIG. 1.

Circuitry for operating the DAP type liquid crystal display of the above illustrated embodiment is now described with reference to a circuit diagram of FIG. 3.

A clock circuit 16 develops a common signal CS, segment signals SS.sub.1, . . . SS.sub.n reflecting updated time information and a one-second signal P. The clock circuit 16 may be made by a well known technique with the detailed disclosure thereof omitted. A static register 17 receives as an input the one-second signal P and provides in sequence signals P.sub.1, P.sub.2, . . . P.sub.5 from its output leads. A voltage generator 18 is responsive to the output signals of the static shift register 17 to produce the output voltage of which the value is variable at each second in a stepwise manner. A mixer 19 receives as inputs the output of the voltage generator 18, the common signal CS and the segment signals SS.sub.1, . . . SS.sub.n and converts the common signal CS and the segment signals SS.sub.1, . . . SS.sub.n with respect to level in response to the output of the voltage generator 18, thereby providing a common electrode signal CP and segment electrode signals SP.sub.1, . . . SP.sub.n. FIG. 4 shows details of the voltage generator 18 and the mixer 19 of FIG. 3 and FIG. 5 shows the waveforms of signals occurring within the circuit of FIG. 4. In FIGS. 4 and 5, the common signal is denoted as CS, the segment signals are denoted as SS.sub.1, SS.sub.2, . . . SS.sub.n, an on-segment signal is denoted as SS.sub.1 and an off-segment signal is denoted as SS.sub.2.

In response to the rectangular pulses P.sub.1, . . . P.sub.5 (the pulse width: 1 second, the pulse interval: 5 seconds) sequentially provided from the static shift register 17, switching transistors Tr.sub.1, Tr.sub.2, . . . Tr.sub.5 constituting the voltage generator 18 are turned on one at a time so that the voltage level OV developing at the output terminal O of the voltage generator 18 varies stepwise as viewed from FIG. 5. In the mixer 19, the common signal CS and the segment signals SS are converted into ones CP and SP.sub.1, SP.sub.2, . . . SP.sub.n that alternate the output voltage level OV of the voltage generator 18 and the ground level. SP.sub.1 denotes the on-segment electrode signal.

With such an arrangement, the voltage V.sub.on applied across the liquid crystal composition on the on-segments is varied in effective value in a stepwise manner as indicated in FIG. 5. It will be noted that the frequencies of the common signals and the segment signals are depicted as somewhat lower than actual ones.

As stated earlier, according to the present invention, the voltage of a periodically variable effective value is applied across selected ones of the segment electrodes of the DAP type liquid crystal display.

Whereas the present invention has been described with respect to specific embodiments thereof, it will be understood that various changes and modifications will be suggested to one skilled in the art, and it is intended to encompass such changes and modifications as fall within the scope of the appended claims.