Abstract:
In order to provide a cholesteric liquid crystal display element of a structure capable of forming a color in the background by means of a low cost electrode structure and burying an Off display color completely in the background color, the color of the display part and background part of the display element is formed by a color reflection state and a transparent state. This configuration makes it possible to form a color in the background so as to bury the Off display color completely in the background by employing a low cost electrode structure, thereby improving a suitability to an industrial design and accomplish a display of a good visibility.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of international PCT application No. PCT/JP2005/005036 filed on Mar. 18, 2005. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a display apparatus using a cholesteric liquid crystal with an image memory function that the apparatus is capable of retaining a display state without requiring electric power (noted as “power” hereinafter), and particularly to a background color forming method for a cholesteric liquid crystal display (LCD) element. 
     2. Description of the Related Art 
     Recent years have been witnessing a use of a cholesteric LCD element capable of retaining a display state without requiring the electricity, that is, capable of performing an image memory display, as a segment type display apparatus, et cetera. The cholesteric liquid crystal noted here is precisely defined as a selective reflection type cholesteric liquid crystal which has two stable states, i.e., an interference reflection state (i.e., a color reflection) for reflecting only a specific color and a transparent state (i.e., black) for transmitting light and which has a characteristic, namely a bistability, of being capable of retaining respective display states without requiring a power after being electrically driven. The cholesteric liquid crystal has a spiral molecular structure and a property of selective reflection reflecting only a light of a wavelength corresponding to the pitch of the spiral in the interference reflection state. Therefore, a use of a cholesteric liquid crystal having a spiral of a pitch corresponding to a wavelength of a color to be desirably reflected enables a desired color display. 
     Meanwhile, the cholesteric liquid crystal also has a property of mutual transition between the interference reflection state and transparent state which is caused by an external pressure and/or a thermal environment. Because of this, there is a problem of being very difficult to revert back to an initial state if an external action results in a change of states in a part in which a counter electrode does not exist, such as an outer circumferential part of a segment and in between electrode wirings. 
     Therefore, in an LCD apparatus, such as a conventional segment type display apparatus, a background color has been limited to black even though the segment part is a color display as shown in  FIG. 1 . In the technical fields, such as clock, marker display, et cetera, putting emphasis on an industrial design, however, a colorful background is highly desired as shown in  FIG. 2 . 
     The following is a description of reason for a background color being limited to black in a conventional display element using a conventional cholesteric liquid crystal by referring to  FIGS. 3 and 4 . 
       FIG. 3  is a diagram of a conventional cholesteric liquid crystal display element  10  when looking at it from the display surface side.  FIG. 3  shows an example of a segment display of the number “35” in color with black as the background. Also shown are segment lead wires  12  for applying a voltage to each segment electrode  11 . 
       FIG. 4  shows a cross-section of the conventional cholesteric LCD element  10 . 
     What are shown here are, from the display surface side, a layer of segment electrodes  11  and light-shield film  13 , a liquid crystal unit (i.e., a cell)  14  filled with a cholesteric liquid crystal, a light absorption layer  15 , a common electrode (i.e., a flat type electrode)  16  and a glass substrate  17 , while segment lead wires  12  are omitted. The segment electrodes  11  are transparent, while the light-shield film  13  and light absorption layer  15  are black. 
     In the conventional cholesteric LCD element  10 , a part which is a background part other than a segment part where there is no counter electrode pair part such as the segment electrode  11  and common electrode  16  is made black by forming the light-shield film  13  shielding a cell  14  of which the state can possibly be changed by an external action so as to make a reflection color of a segment as an On display and a black color of a transparent state, which is the same as the background color, as an Off display. This accordingly limits the background color to black. 
     It is of course easily possible to conceive a method for making a colorful background by coloring the light-shield film  13  to form a background color, and reversibly displaying negative (On) and positive (Off) displays; which, however, requires a segment reflection color to be exactly identical with a background color in order to bury the display color completely in the background. The reflection color of a segment is an interference reflection color of a liquid crystal expressing a special shade dependent on a view angle and it is therefore very difficult to make it identical with the background color which is colored with a pigment or dye. 
     Meanwhile, it is possible to form a background color by using a liquid crystal by comprising a background-use electrode; an inter-electrode space of tens micrometers needs to be formed for insulating a border with a display pattern electrode, however. Because of this space, it is not possible to bury the display color completely in the background. 
     As an example, each of the following reference patent documents 1 and 2 notes an LCD element forming a background color by comprising a background-use electrode; either of them, however, has not been able to bury the display color completely in the background since there is a gap, although it may be very small, between the electrode for the display pattern and that for the background. 
     As a configuration colorizing a background, the following reference patent document 3 notes a display element filling display cell with a liquid crystal, in which a transition from a cholesteric phase to a nematic phase is caused by applying an electrical field, added with a multi-color dye, and controlling a hue by applying an electrical field, thereby performing a display. This display element, however, has no image memory function and therefore a constant application of the electrical field is required for maintaining a display state. 
     Patent document 1: Japanese Published Patent Application No. H11-337672 
     Patent document 2: Japanese Published Patent Application No. 2002-229051 
     Patent document 3: Japanese Published Patent Application No. S53-35564 
     SUMMARY OF THE INVENTION 
     The problem for the present invention to be solved is to provide a cholesteric LCD element of a structure which is capable of forming a color background by using a low cost electrode structure and burying an Off display color completely in a background color. 
     For that purpose, the present invention is contrived to form a display part and a background part of the display element by means of a color reflection state and a transparent state of the cholesteric liquid crystal. The background is fixed to the color reflection state by employing a mechanical pressure for example. An alternative configuration is to make an electrode for the background and that for the display part as two-layer structure and allow no gap between both of the aforementioned electrodes when viewed from the front face of the display element. Furthermore, a configuration is to form a film of a pattern feature for displaying on one of counter electrodes within a cell and give a difference of drive voltages for changing states of the liquid crystal in regions of the pattern and on the outside thereof, thereby turning On-Off the display only in one zone of the pattern and outside thereof. 
     The adoption of the above described method makes it possible to form a color so as to bury an Off display color completely in the background color by using a low cost electrode structure in a display of a cholesteric liquid crystal, thereby improving a suitability to an industrial design and accomplishing a display of a high visibility. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing a conventional display element of which a background is black; 
         FIG. 2  is a diagram exemplifying a display element colorizing a background; 
         FIG. 3  is a diagram of a conventional cholesteric LCD element viewed from the front; 
         FIG. 4  is a cross-sectional diagram of a conventional cholesteric LCD element; 
         FIG. 5  is a front view diagram of a cholesteric LCD element according to a first embodiment; 
         FIG. 6  is a diagram describing an electrode pattern of a cholesteric LCD element according to a first embodiment; 
         FIG. 7  is a diagram describing a transparent state, a reflection state generated by applying an electrical field, and a reflection state generated by a mechanical action, in a cholesteric liquid crystal; 
         FIG. 8  is a front view diagram of a cholesteric LCD element according to a second embodiment; 
         FIG. 9  is a diagram showing a structure of an outer circumference electrode of a cholesteric LCD element according to the second embodiment; 
         FIG. 10  is a diagram showing a cross-section of a cholesteric LCD element according to the second embodiment; 
         FIG. 11A  is a diagram showing a cross-section of a cholesteric LCD element according to a third embodiment; 
         FIG. 11B  is a diagram of a cholesteric LCD element, viewed from a display surface side, according to the third embodiment; 
         FIG. 11C  is a diagram showing a color combination of a pattern and a flat type pattern and of a cholesteric LCD element according to the third embodiment; 
         FIG. 12  is a diagram describing a method for driving a cholesteric LCD element according to the third embodiment; 
         FIG. 13  is a diagram describing an example of performing a segment display by using a cholesteric LCD element according to the third embodiment; 
         FIG. 14  is a diagram showing a cross-section of a cholesteric LCD element according to a fourth embodiment; and 
         FIG. 15  is a diagram describing a method for driving a cholesteric LCD element according to the fourth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following is a description of the preferred embodiment of a cholesteric LCD element (sometimes noted as “display element” hereinafter) according to the present invention by referring to the accompanying drawings. 
       FIGS. 5 through 7  are diagrams for describing a first embodiment of the present invention. 
       FIG. 5  is a front view diagram of a display element  100  according to the first embodiment, in which the liquid crystal at a background part is fixed to a color reflection state by using a mechanical pressure including a part on a segment wiring. Therefore, an Off display color is buried in the background. The mechanical pressure is applied to the entire surface of the display surface by using a roller apparatus (i.e., a laminator apparatus), or a press apparatus, after injecting the liquid crystal into the cell during the production process of the display element. 
       FIG. 6  is a diagram describing a segment electrode pattern  110  and a common electrode pattern  160  according to the first embodiment. The common electrode pattern  160  is configured to be opposite to the segment electrode pattern  110  and so as to prevent a display change in the area on the wiring of segment lead wires  120  as shown in the drawing. Therefore, a counter electrode does not exist in the background including a part on the segment wiring, thereby making a state of the background part staying in a color refection state, which is fixed during the production, even after a usage start. 
     Note here that there is no longer a necessity of the light-shield film  13  equipped in the display element  10  of a conventional example noted in  FIGS. 3 and 4 , for all embodiments put forth herein. 
       FIG. 7  is a diagram describing a transparent state, a reflection state generated by applying an electrical field, and a reflection state generated by a mechanical action, in a cholesteric liquid crystal. As shown in the drawing, the reflection state generated by applying an electrical field has a little difference in the state of the liquid crystal from the reflection state generated by a mechanical action. This sometimes makes a hue of the background different from that of the display color (i.e., an Off display color), causing a little dissatisfaction in the visibility. 
     The next is a description of a second embodiment by referring to  FIGS. 8 through 10 . 
       FIG. 8  is a front view diagram of a display element  200  according to the second embodiment. The second embodiment is configured to use an outer circumference electrode  280  (shown in  FIG. 9 ) for a background color use, and therefore  FIG. 8  differs from the front view of  FIG. 5 , showing the first embodiment, where there is an outer circumference electrode lead wire  285 . 
       FIG. 9  is a diagram showing a structure of the outer circumference electrode  280 . The structure of the outer circumference electrode  280  is a rectangle for example and is a result of coring out the part of a position, where a segment electrode exists, from an electrode having no particular pattern (i.e., a flat type electrode). 
       FIG. 10  is a diagram showing a cross-section of a display element  200  according to the second embodiment. A common electrode  260  is formed on a glass substrate  270 , and a light absorption layer  250  is formed on the common electrode  260 . Formed on the display surface side is an electrode pattern of a two-layer structure, i.e., a layer comprising a segment electrode  210  and a lead electrode  215  and a layer of the outer circumference electrode  280 , by way of an insulation film  290 , and a gap with the common electrode  260  side is featured with a liquid crystal unit  240  which is filled with a cholesteric liquid crystal. On the glass substrate  270  of the common electrode  260  side, a flat type electrode pattern is generated. The state of the liquid crystal is transparent (i.e., black) at the time of an On display of the segment, and is the same reflection color as the background color at the time of an Off display. In the second embodiment, an Off reflection color and background color of the segment are completely identical, thereby improving a visibility. And a reversing changeover between negative and positive displays can easily be implemented by choosing an electrode to apply an electrical field between the segment electrode ( 210 ) and outer circumference electrode ( 280 ). Because the outer circumference electrode  280  is in the inner layer of the lead electrode  215 , the background is not affected by the lead electrode. 
     Therefore, a various application can be conceived, such as reversing negative and positive displays at a set time or between the morning and afternoon. 
     The next is a description of a third embodiment by referring to  FIGS. 11A through 13 . 
     The third embodiment is configured to form a film of a pattern feature to be displayed on one of counter electrodes within a cell in a display element using a cholesteric liquid crystal, which changes a state based on an applied electrical field. The pattern feature gives a difference of electric field strength using a drive voltage under which a liquid crystal changes a state in regions of the pattern and outside thereof, and cause a transition from the color reflection state to transparent state, or vice versa, either only in the pattern or outside thereof. Therefore, this enable an On-Off the display either only in the pattern or outside thereof. 
     The next is a description of a configuration of the display element according to the third embodiment by referring to  FIGS. 11A through 11C . 
       FIG. 11A  is a diagram showing a cross-section of a display element  300  according to the third embodiment. Layered are, from the display surface side, an upper substrate  310 , an upper flat type electrode  320 , a liquid crystal unit  330 , a pattern  340 , a flat type pattern  350 , a lower flat type electrode  360 , a lower substrate  370  and a light absorption layer  380 . 
       FIG. 11B  is a diagram of the display element, viewed from a display surface side, according to the third embodiment, exemplifying a key mark as a pattern  340 . 
       FIG. 11C  is a diagram showing a combination of color tones of the pattern  340  and the flat type pattern  350 . A combination of both black of (1) shows a clear contrast, enabling an elimination of a light absorption layer  380 . In terms of design, however, four combination between black and transparent can be adopted, enabling a utilization of an oriented film or insulation film as a transparent layer. The transparent layer may be configured by not forming a film, or, if both of the pattern  340  and periodical pattern  350  are transparent, either one of them may be configured by not forming a film. 
     The next is a description of a drive method, and a display state, of the cholesteric LCD element  300  according to the third embodiment by referring to  FIG. 12 . The initial state prior to an application of a voltage is assumed to be a color reflection state. The graph noted as “single layer BK” shows a relationship of an applied voltage with a reflectance of a liquid crystal in the part of the flat type pattern  350  constituted by one layer of black, and the graph noted as “two-layer BK” shows a relationship of an applied voltage with a reflectance of a liquid crystal in the part constituted by two layers of black with the pattern  340  being protruded. 
     When an applied voltage is at V 1 , the part of the single layer BK is transparent (i.e., black) and the part of the two-layer BK is a reflection color, thus becoming a positive On state; and when the applied voltage is at V 2 , both of the part of the single layer BK and two-layer BK is transparent (i.e., black), thus becoming a positive Off state. When the applied voltage is at V 3 , the part of the single layer BK is a reflection color and the part of the two-layer BK is transparent (i.e., black), thus becoming a negative On state and displaying a key mark in the color background. When the applied voltage is at V 4 , both of the part of the single layer BK and two-layer BK is a reflection color, becoming a negative Off state. 
     The above embodiment has been described by assuming the mark display part as two layers and the background part as one layer; it is, however, apparent that a configuration of the mark display part being one layer and the background part being two layers makes it possible to display the same. In the case of configuring the transparent layer as not forming a film, a mark display part becomes one layer or a background part becomes one layer. 
       FIG. 13  is a diagram describing an example of performing a segment display in seven segments by using a cholesteric LCD element according to the third embodiment. The use of the cholesteric LCD element according to the third embodiment for each segment enables a segment display as shown in  FIG. 13 . The flat type electrode can be configured to conceal a segment border by making a two-layer structure likewise the segment electrode  210  and outer circumference electrode  280  of the second embodiment. 
     The next is a description of a fourth embodiment of the present invention by referring to  FIGS. 14 and 15 . 
       FIG. 14  is a diagram showing a cross-section of a cholesteric LCD element  400  according to the fourth embodiment. As compared to the cholesteric LCD element of the third embodiment shown in  FIG. 11A , the difference lies in printing a pattern on the flat type pattern in multiple stage thicknesses.  FIG. 14  exemplifies the case of three patterns, i.e., the pattern A 410 , pattern B 420  and pattern C 430 . 
       FIG. 15  is a diagram describing a method for driving the cholesteric LCD element  400  according to the fourth embodiment configured as described above. Since the present configuration is printed with three kinds of patterns A, B and C, eight kinds of display patterns can exist in terms of mathematics as combinations of On and Off; in actuality, however, the display pattern  3  in which only the pattern B 420  is turned Off can not be implemented, and the display pattern  6  in which only the pattern B 420  is turned On requires some devising. 
     Assuming that the initial state prior to applying a voltage is a color reflection state, a drive starts with the state of the display pattern  1 . Increasing the applied voltage initially constitutes the state of the display pattern  5  as a result of the pattern A 410  becoming transparent, that is, turned Off. Then, as the applied voltage is increased, the display patterns transits from the pattern  7  to  8  to  4  to  2 , followed by returning to the display pattern  1 . In order to carry out a display of the display pattern  6 , the power is turned off in the state of the display pattern  2 , followed by applying the voltage in the state of the display pattern  2 , in which only the pattern C 430  is transparent, as the initial state. This turns the part of the pattern A 410  into transparent ahead of the part of the pattern B, and the display pattern  6  in which only the pattern B 420  is a color reflection state can be achieved. 
     Therefore, a positive display can be achieved by switching over three ways of display patterns by using the display patterns  4 ,  6  and  7 . 
     As described above, the present invention is contrived to make it possible to bury an Off display color in a colorized background, thereby enabling a positive response to a requirement of an industrial design suitability by utilizing a display element according the present invention in industrial fields putting emphasis on an industrial design suitability such as clocks, marker displays, et cetera, which require a colorful background as background colors. 
     The first embodiment is configured to be able to fix a background color by using a mechanical pressure, thereby eliminating a necessity of an electrode for a background color. This accordingly eliminates a necessity of controlling a voltage of a background color-use electrode. 
     The second embodiment is configured to make it possible to drive a background part including a part on the segment wiring, thereby making an Off reflection color of the segment completely identical with the background color and improving the visibility. Also a reversing changeover between negative and positive displays can easily be implemented by selecting an electrode for applying the electrical field between the segment electrode and outer circumference electrode. 
     The third embodiment is configured to make an electrode pattern as a flat type pattern and make it possible to form a display pattern such as a mark by employing a simple process for forming a flat type pattern and a pattern on the aforementioned flat type pattern of the electrode. 
     The fourth embodiment provides benefit of obtaining a more complex display effect in addition to the benefit of the third embodiment.