Abstract:
A Liquid Crystal Display (LCD) uses a tunable mirror in place of a partially reflective mirror. The tunable mirror has a controllable reflectivity and transmitance which allows the mirror to primarily reflect light when the LCD is operated in a reflective mode, and to primarily transmit light from a backlight when the LCD is operated in a transmissive mode.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is concerned with liquid crystal displays (LCDs) and particularly with transflective LCDs that achieve lower battery usage and higher contrast. 
     2. Description of the Related Art 
     Conventional transflective LCDs have a partially-reflective partially-transmissive mirror element (also known as a transflector) which reflects ambient light received from the viewing screen back through the LCD, and transmits backlight emission which is switched on when ambient lighting is low. 
     In conventional transflective LCDs, the utilization of light is inefficient because light is both reflected and transmitted at the same time, by the partially-reflective partially-transmissive mirror. Additionally, the transmission and reflection have fixed values. Since at any given time, the sum of the transmission and reflection of a mirror can not exceed 100%, the partially-reflective partially-transmissive mirror sacrifices efficiency by simultaneously operating as a reflector and a transmitter. Typically such mirrors have a 70-90% reflectance and a 10-30% transmission. 
     To compensate for such inefficiency, more battery power must be used to increase the backlight emission when operating in the transmissive mode in low ambient light. Likewise, contrast is lost when operating in the reflective mode, at high ambient light. 
     SUMMARY OF THE INVENTION 
     This invention provides a system and method which improve the efficiency of conventional transflective LCDs by replacing the partially-reflective partially-transmissive mirror with a tunable mirror. A tunable mirror is any device having controllable degrees of transmission and reflection. 
     The advantage of using a tunable mirror is that it can be switched between “reflective” and “transmissive” modes to primarily reflect light when ambient lighting is high, and to primarily transmit light when ambient lighting is low and backlighting is needed. This saves battery life by reducing the amount of backlighting needed when operating the LCD in the transmissive mode, and increases contrast and brightness when operating in the reflective mode. 
     The electrochemical reversible mirror (REM) is a suitable type of tunable mirror for use with this invention. Additionally, tunable mirrors may be constructed from a plurality of optical elements, at least one of which has an electrically switchable optical property. 
     One suitable construction for the tunable mirror includes a cholesteric liquid crystal reflector with a quarter-wave (λ/4) retarder. In this combination, the liquid crystal reflector is switchable between reflecting and transmitting states of operation to give the mirror its tunable characteristic. 
     Another suitable construction for the tunable mirror includes a reflective polarizer with a zero to half-wave (0-λ/2) tunable liquid crystal retarder. Such retarder is and is switchable between λ/2 and 0λ states of operation to give the mirror its tunable characteristic. 
     A third suitable construction for the tunable mirror includes a cholesteric reflector with a negative quarter-wave to positive quarter-wave (+/−λ/4) tunable liquid crystal retarder. Such retarder is switchable between +λ/4 and −λ/4 states of operation to give the mirror its tunable characteristic. 
     There are various possibilities for controlling the mirror and backlight. For example, mirror and backlight control systems may be employed which operate the mirror and backlight in tandem, such that when the backlight is switched on, the mirror is set to the transmissive state. Another possibility is to set the mirror and backlight controls automatically responsive to the level of ambient light. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an LCD in accordance with the present invention LCD operating in the reflective mode. 
     FIG. 2 is a schematic diagram of the LCD of FIG. 1, operating in the transmissive mode. 
     FIG. 3 is a schematic diagram of a tunable mirror comprising a liquid crystal reflector and quarter waver retarder, operating in the reflective mode. 
     FIG. 4 is a schematic diagram of the tunable mirror of FIG. 3 operating in the transmissive mode. 
     FIG. 5 is a schematic diagram of an LCD with the tunable mirror of FIG. 3 operating in the reflective mode. 
     FIG. 6 is a schematic diagram of the LCD of FIG. 5 operating in the transmissive mode. 
     FIG. 7 is a schematic diagram of a tunable mirror comprising a reflective polarizer with a zero to half-wave (0-λ/2) tunable liquid crystal retarder, operating in the reflective mode. 
     FIG. 8 is a schematic diagram of the tunable mirror of FIG. 7, operating in the transmissive mode. 
     FIG. 9 is a schematic diagram of an LCD with the tunable mirror of FIG. 7, operating in the reflective mode. 
     FIG. 10 is a schematic diagram of the LCD of FIG. 9, operating in the transmissive mode. 
     FIG. 11 is a schematic diagram of an alternate construction for an LCD with the tunable mirror of FIG. 7, which includes a λ/4 retarder. 
     FIG. 12 is a schematic diagram of an alternate construction for the LCD with the tunable mirror of FIG.  8 . 
     FIG. 13 is a schematic diagram of a tunable mirror comprising a cholesteric reflector with a negative quarter-wave to positive quarter-wave (+/−λ/4) tunable liquid crystal retarder, operating in the reflective mode. 
     FIG. 14 is a schematic diagram of the tunable mirror of FIG. 13, operating in the transmissive mode. 
     FIG. 15 is a schematic diagram of an LCD with the tunable mirror of FIG. 13, operating in the reflective mode. 
     FIG. 16 is a schematic diagram of the LCD of FIG. 15, operating in the transmissive mode. 
     FIG. 17 is a schematic diagram of an alternate construction for the LCD with the tunable mirror of FIG.  13 . 
     FIGS. 18 a - 18   c  are perspective views illustrating the appearance of electronic apparatus of the present invention, in which FIG. 18 a  is a cellular telephone, FIG. 18 b  is a watch, and FIG. 18 c  is a laptop computer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention replaces the partially-reflective partially-transmissive mirror used in prior art transflective LCDs with a tunable mirror. There are many ways to make transflective LCDs. While a particular type of LCD is shown in these figures, the invention is applicable in general to any transflective LCD in which the partially-reflective partially-transmissive mirror can be replaced by a tunable mirror. 
     FIGS. 1 and 2 are diagrams illustrating an LCD  100 , according to the present invention, operating in the reflective and transmissive mode, respectively. 
     The LCD  100  includes a liquid crystal cell  102 ; a tunable mirror  104  behind the cell; a backlight  106  which is located behind the tunable mirror  104 ; a first polarizer  108  in front of the cell  102 ; a second polarizer  110  between the cell  102  and the tunable mirror  104 ; a control  118  for the tunable mirror  104 ; and a control  120  for the backlight  106 . 
     The liquid crystal cell  102  may be a Supertwisted-Nematic (STN) cell, an Active Matrix Twisted-Nematic (TN) cell or a Passive TN cell. The construction of the liquid crystal cell typically includes a liquid crystal layer sandwiched between a pair of segmented electrodes. 
     In a normally white mode of operation, the cell  102  can be either in an ON (light emitting) state or an OFF state, wherein voltage is applied across the electrodes of the cell in the OFF state and no voltage is applied across the cell in the ON state. The first and second polarizers  108  and  110  are oriented such that when the cell is in the ON state, light entering through one polarizer is rotated upon transmission through the cell, such that the polarization direction is approximately parallel to the transmission axis of the other polarizer, and exits through that polarizer. When the cell is in the OFF state, light entering through one polarizer is rotated by the cell, such that the polarization direction is approximately orthogonal to the transmission axis of the other polarizer, and is therefore blocked or absorbed by that polarizer. In the example of a 90° TN cell operating in the normally white mode where the cell rotates light by 90° in the ON state, the polarizers have mutually orthogonal planes of polarization. 
     In an STN cell the planes of polarization may not be orthogonal. A normally black mode may also be used, in which the voltage is applied in the ON state and no voltage in the OFF state. In the example of a 90° TN cell operating in the normally black mode the polarizers are parallel (For a reference on STN cells see T. J. Scheffer and J. Nehring, “Supertwisted Nematic LCDs,” Society for Information Display Seminar Lecture Notes, Vol. 1, M-12, May 15, 2000, and the references therein.) 
     The tunable mirror  104  can be switched between reflective and transmissive modes, for primarily reflecting or primarily transmitting light, respectively. More detailed examples of tunable mirrors will be described below. 
     The backlight  106  can be switched between emissive and non-emissive states of operation, for emitting light in its emissive state when ambient light entering the liquid crystal cell from the opposite side is too low, and is therefore below a “viewability threshold”. Some examples of a suitable backlight assembly can be found in Okumara, (U.S. Pat. No. 6,008,871). Additionally, manufacturers of suitable backlights for transflective displays are Durel Corporation and Eltech. 
     While the liquid crystal cell  102 , tunable mirror  104 , polarizers  108  and  110  are illustrated as being separated, this is for convenience of illustration; in practice these elements would normally be bonded together with adhesives having compensatory indices of refraction. The construction of these individual elements is well known in the LCD art. For example, see Okumara, (U.S. Pat. No. 6,008,871); and transflective LCD units having these components (with a partially-reflective partially-transmissive mirror) are sold by companies such as Seiko-Epson and Optrex. 
     Besides those illustrated, other optional elements are left out of the illustrations because they are not necessary to describe the present invention. These include compensation (retardation) films, which can be located on each side of the liquid crystal cell  102 , and are used with an STN cell. Such elements are well known. 
     Additionally, a light diffusing element may be added to produce a diffused image. Such light diffusing element can be made of embossed plastic plate, or a plastic plate dispersed with beads. In addition, diffusing beads can be mixed into one of the adhesive layers adjacent to any of the above-described elements (See Okumura). Also, it may be possible to produce a tunable mirror with a matte surface for obtaining a diffused surface, and some mirrors may have an inherent light diffusion quality so an additional light diffusing element is not required for diffusion, as will be mentioned in more detail in the discussion regarding tunable mirrors. 
     Moreover, in conventional transflective LCDs, the backlight is switched to the emissive state when operating in a transmissive mode. In the present invention, both the backlight and tunable mirror need to be controlled. There are several options for this. The mirror and backlight controls  118  and  120  can be either manually set by the user or automatically responsive to the level of ambient light. The user may also be given the option of setting the controls at either manual or automatic ambient light responsive modes. In a system which is automatically responsive to ambient light, an ambient light sensor can be used to switch the backlight to its emissive state and set the mirror in its transmissive mode at low ambient light, and at high ambient light, switch the backlight to its non-emissive state and set the mirror in its reflective mode, for example. Additionally, the two controls  118  and  120  may either operate independently of each other, or in tandem. Where both controls are operated by applying a source of electrical potential, two control circuits operating in series or parallel may be utilized. 
     FIG. 1 illustrates the operation of the LCD  100  in the reflective mode in which there is high ambient light, and the tunable mirror  104  is switched to operate in the reflective mode, indicated by the shading  128 . Additionally, the backlight  106  is switched to the non-emissive state of operation  129 . For the purpose of illustration, the liquid crystal cell  102  is assumed to be a 90° TN cell operating in a normally white mode, and in its ON state, wherein it rotates the plane of polarization of linearly polarized light by 90°. Additionally, for the purpose of illustration, polarizers  108 ,  110  have mutually orthogonal planes of polarization in the vertical and horizontal directions, respectively. Although this assumption may not be true for all displays, as in the case of an STN cell, or a normally black mode display, it is adopted for the purpose of simplifying the illustration and does not limit the invention to TN cells or white mode displays. 
     First, randomly polarized ambient light  126 , indicated by unpolarized light vectors  130 , travels through the first polarizer  108 , where it is linearly polarized, as indicated by the polarized light vector  132 . The light then travels through the liquid crystal cell  102 , where it is rotated to have a plane of polarization approximately parallel to the transmission axis of the second polarizer, indicated by the dot  134  representing a light vector coming out of the plane of the paper. The light then passes through the second polarizer  110 , maintaining its linear polarization  136 , and is primarily reflected by the tunable mirror  128 . The reflected light then follows a reversed path with successive polarizations  138 ,  140 ,  142  and  144 , to emerge as an LCD output  145 . (When the cell  102  is in the OFF state, the light entering through the first polarizer  108  travels through the cell  102 , where it is rotated to have a plane of polarization approximately orthogonal to the transmission axis of the second polarizer and is blocked.) 
     FIG. 2 illustrates the operation of the LCD  100  in the transmissive mode, in which the tunable mirror  104  is switched to operate in the transmissive mode, indicated by the absence of shading  200 . Additionally, the backlight  106  is switched to the emissive state of operation  204 . As in the description of FIG. 1, the liquid crystal cell  102  is again assumed to rotate the plane of polarization of linearly polarized light by 90°. First, randomly polarized backlight emission  202 , indicated by unpolarized light vectors  206 , is primarily transmitted through the tunable mirror  104 . The resultant unpolarized light, indicated by vector  208 , then travels through the second polarizer  110  and is linearly polarized  210 . Its polarization is then rotated by the liquid crystal cell  102  to a linear polarization  212 , approximately parallel to the transmitting axis of the first polarizer  108 . Finally the light exits the first polarizer  108  with linear polarization  214 , to emerge as the LCD output  216 . (When the liquid crystal cell  102  is in the OFF state, the light exiting the cell  102  has a polarization direction orthogonal to the transmitting axis of the first polarizer  108  and is blocked.) 
     TUNABLE MIRRORS 
     For purposes of this invention, a tunable mirror is defined as any device having a controllable transmission and reflection. This includes a transmission/reflection which can be switched among either discrete or continuous states. An electro-optic device is one whose optical properties change with an electric signal. This invention contemplates the use of any electro-optic or other device which acts as a tunable mirror for a transflective LCD. 
     Tench (U.S. Pat. No 5,923,456) discloses a suitable reversible electrochemical mirror (REM) having controllable reflective and transmissive modes. The REM can be controlled by applying a source of electrical potential which has reversible polarity and adjustable potential. The REM takes about 1 second to switch between reflective and transmissive modes. Additionally, the REM may have a matte surface to produce a diffused reflectance. The REM can achieve a transmittance of up to 60% due to light absorption, and therefore has limited efficiency. 
     Additionally, tunable mirrors may be constructed by combining a plurality of optical elements, at least one of which is an electro-optic device having switchable optical properties. FIGS. 3-17 illustrate examples of such tunable mirrors, and LCDs incorporating these mirrors, according to this invention. 
     FIGS. 3-6 illustrate a first tunable mirror assembly  300 , and an LCD  500  incorporating the tunable mirror  300 . The tunable mirror  300  includes a switchable cholesteric liquid crystal reflector  302  with a quarter-wave (λ/4) retarder  304  and a control  305  for the liquid crystal reflector  302 . The liquid crystal reflector  302  is controllably switchable between reflecting and transmitting states of operation, for reflecting circularly polarized light of a particular rotational direction in the reflecting state, and for transmitting light in the transmitting state. Such Cholesteric liquid crystal devices are commercially available from Kent Displays and Advanced Display Systems (ADS). Preferably, the cholesteric liquid crystal reflector  302  should be custom designed to have a single addressing pixel, which has the same dimensions as the overall LCD. Such liquid crystal reflector has a diffuse reflectance in the reflecting state and thus will produce a diffused image without the addition of a light diffusing element. When a voltage (approximately between 20 and 80 Volts) is applied to the reflector  302  (i.e. in the transmitting state) it changes from a homeotropic state, in which light is transmitted without a change in polarization. (For a reference see D. K. Yang, J. L. West, L. C. Chien, and J. W. Doane, “control of Reflectivity and Bistability in Displays using Cholesteric Liquid Crystals,” J. Appl. Phys.  76, 1331 (1994 )). 
     The λ/4 retarder  304  is an element used for conversion of light between circular and linear polarization forms. It converts horizontal or vertical linearly polarized light to right-handed or left-handed circularly polarized light, depending on the orientation. Conversely, it will convert right-handed or left-handed circularly polarized light to linearly polarized light, and is substantially transmissive to randomly polarized light. Such λ/4 retarder can be either birefringent crystal or oriented polymer film and are manufactured by Fuji Film, Nitto Denko, and Meadowlark Optics. (For a reference see Polarization Manipulation with Retarders, Meadowlark Optics, Product Catalogue, 1999-2000). Furthermore, the liquid crystal reflector  302  and λ/4 retarder  304  are oriented with respect to each other such that in the reflective mode, the reflector  302  reflects light received from the retarder  304 . Also, in FIGS. 3-6, the liquid crystal reflector  302  and λ/4 retarder  304  are illustrated as being separated, however, as components of an LCD, these elements would normally be bonded together. 
     FIG. 3 illustrates the operation of the tunable mirror  300  in the reflective mode in which the liquid crystal reflector  302  is in the reflecting state. In this state, the liquid crystal reflector  302  is capable of reflecting circularly polarized light of one particular rotational direction, e.g. right handed circular polarization but not left handed, indicated by the circular reflection vector  306 . 
     First, linearly polarized light  308  (e.g. in the horizontal direction), as indicated by the polarized light vector  312 , is converted by the λ/4 retarder  304  to a circular right handed polarization, indicated by the polarized light vector  314 . The light is then reflected by the liquid crystal reflector  302 , maintaining its circular right handed polarization  316 , and travels back through the λ/4 retarder  304  which converts it back to a horizontal linear polarization  318 . 
     FIG. 4 illustrates the operation of the tunable mirror  300  in the transmissive mode, in which the liquid crystal reflector  302  is in the transmitting state. In this state the liquid crystal reflector is transmits light, as indicated by the absence of a reflection vector  400 . Randomly polarized light  402 , indicated by the unpolarized light vectors  404 , is transmitted through both the liquid crystal reflector  302  and λ/4 retarder  304 , maintaining its random polarization indicated by vectors  406  and  408 . 
     FIGS. 5 and 6 illustrate the operation of the LCD  500  which incorporates the tunable mirror  300 . The elements of the LCD  500  are essentially the same as those in the previous figures, with the exception of the tunable mirror, and tunable mirror control. These elements are a liquid crystal cell  502  located in front of the λ/4 retarder  304  of the tunable mirror  300 ; a backlight  506  which is located behind the liquid crystal reflector  302  of tunable mirror  300  and can be switched between emissive and non-emissive states; a first polarizer  508  in front of the cell  502 ; a second polarizer  510  between the cell  502  and the λ/4 retarder  304 ; and a control  520  for the backlight  506 . Again, for the purpose of illustration, the liquid crystal cell  502  is assumed to be a 90° TN cell operating in the normally white mode, and in its ON state wherein it rotates the plane of polarization of linearly polarized light by 90°. Also for the purpose of illustration, the first and second polarizers  508  and  510  have mutually orthogonal planes of polarization in the vertical and horizontal directions, respectively. 
     FIG. 5 illustrates the LCD  500  operating in the reflective mode, in which the liquid crystal reflector  302  is in the reflecting state, as indicated by the circular reflection vector  306 , and the backlight  506  is in the non-emissive state  526 . First, randomly polarized ambient light  526 , indicated by unpolarized light vectors  530 , travels through the first polarizer  508 , where it is linearly polarized, as indicated by the polarized light vector  532 . The light then travels through the liquid crystal cell  502 , where its is rotated to have a plane of polarization approximately parallel to the transmission axis of the second polarizer, as indicated by the polarized light vector  534  and through the second polarizer  510 , maintaining its linear polarization  536 . The light then passes through the λ/4 retarder  304  wherein it is circularly polarized in the right handed direction  538 . The circularly polarized light  538  is then reflected by liquid crystal reflector  302 , following a reversed path with successive polarizations  540 ,  542 ,  544 ,  546 , and  548  to emerge as an LCD output  550 . (When the liquid crystal cell  502  is in the OFF state, the light entering through the first polarizer  508  travels through the cell  502 , where it is rotated to have a plane of polarization approximately orthogonal to the transmission axis of the second polarizer  510  and is blocked.) 
     FIG. 6 illustrates the LCD  500  operating in the transmissive mode, in which the liquid crystal reflector is in the transmitting state, as indicated by the absence of a reflection vector  400 , and the backlight is in its emissive state  600 . First, randomly polarized backlight  602 , indicated by unpolarized light vectors  606 , is transmitted through the liquid crystal reflector  302  and λ/4 retarder  304 , having unpolarized light vectors  607  and  608 . The resultant light then travels through the second polarizer  510  and is linearly polarized  610 . Its polarization is then rotated by the liquid crystal cell  502  to an approximately linear polarization  612 , parallel to the transmitting axis of the first polarizer  508 . Finally the light passes through the first polarizer  508  with linear polarization  614 , to emerge as the LCD output  616 . (When the liquid crystal cell  502  is in the OFF state, the light exiting the cell  502  has a polarization direction orthogonal to the transmitting axis of the first polarizer  508  and is blocked.) 
     FIGS. 7-12 illustrate a second tunable mirror assembly  700 , and LCDs  900 ,  1100  and  1200  incorporating the tunable mirror  700 . The tunable mirror  700  includes a reflective polarizer  702 , a tunable liquid crystal zero to half-wave (0-λ/2) retarder  704 , and a control  705  for the 0-λ/2 retarder  704 . The reflective polarizer  702  reflects linearly polarized light of one direction, (e.g. the horizontal direction) and transmits linearly polarized light of another direction (e.g. the vertical direction). Commercially available reflective polarizers (supplied through Merck and 3M) have a specular rather then a diffuse reflectance. 
     The 0-λ/2 retarder  704  is controllably switchable between 0λ and λ/2 states of operation, for rotating the plane of polarization of linearly polarized light by 90° in the λ/2 state, and for transmitting light with no change in the 0λ state. (see Meadowlark Optics) Such retarder is typically of a nematic type, and is switchable to the 0λ state with the application of a voltage (approximately 10V) at a speed of 20 ms, and works in analogue mode. 
     Furthermore, the reflective polarizer  702  and retarder  704  are oriented with respect to each other such that in the reflective mode, the reflective polarizer  702  reflects light received from the retarder  704 , as illustrated in FIGS. 7-12. Also, in FIGS. 7-12, the reflective polarizer  702  and retarder  704  are illustrated as being separated, however, as components of an LCD, these elements would normally be bonded together. Additionally, in an LCD where a diffuse reflectance is desired, an additional light diffusing element may be added with this type of tunable mirror. In this case it is preferable to use a holographic diffuser that does not scramble the polarization. 
     FIG. 7 illustrates the operation of the tunable mirror  700  in the reflective mode, in which the 0-λ/2 retarder  704  is in the 0λ state  708 . In the simplest case, light having horizontal linear polarization  710  as indicated by the polarized light vector  712 , is transmitted through the retarder  704  and reflected by the reflective polarizer  702 , back through the retarder  704 , having successive polarizations  714 ,  716 , and  718 . It is also possible for the light entering to have both vertical and linear polarization components. In this case, still only the component having horizontal linear polarization will be reflected back. 
     FIG. 8 illustrates the operation of the tunable mirror  700  in the transmissive mode in which the retarder  704  is in the λ/2 state  800 . First, randomly polarized light  802 , as indicated by the unpolarized light vectors  804  is transmitted through the reflective polarizer  702 , wherein only the vertical component is transmitted, as indicated by the polarized light vector  806 . Thereafter, the light passes through the retarder, wherein its polarization is rotated 90°, as indicated by polarized light vector  808 . 
     FIGS. 9 and 10 illustrate the operation of the LCD  900  which incorporates the tunable mirror  700 . The elements of the LCD  900  are essentially the same as those in the previous figures, with the exception of the tunable mirror, and tunable mirror control. These elements are a liquid crystal cell  902  located in front of the 0-λ/2 retarder  704  of the tunable mirror  700 ; a backlight  906  which is located behind the reflective polarizer  702  of tunable mirror  700  and can be switched between emissive and non-emissive states of operation; a first polarizer  908  in front of the cell  902 ; a second polarizer  910  between the cell  902  and the retarder  704 ; and a control  920  for the backlight  906 . Again, the liquid crystal cell  902  is assumed to be a 90° TN cell operating in a normally white mode, and in its ON state, wherein it rotates the plane of polarization of linearly polarized light by 90°. Also for the purpose of illustration, the first and second polarizers  908  and  910  have mutually orthogonal planes of polarization in the vertical and horizontal directions, respectively. 
     FIG. 9 illustrates the LCD  900  operating in the reflective mode, in which the retarder  704  is in the 0λ state  708 , and the backlight  906  is in its non-emissive state  926 . First, randomly polarized ambient light  928 , indicated by unpolarized light vectors  930 , travels through the first polarizer  908 , where it is linearly polarized, as indicated by the polarized light vector  932 . The light then travels through the liquid crystal cell  902 , where it is rotated to have a plane of polarization approximately parallel to the transmission axis of the second polarizer  910 , as indicated by the polarized light vector  934 , and passes through the second polarizer  910 , maintaining its linear polarization  936 . The light is then transmitted through the 0-λ/2 retarder  704  maintaining its linear polarization  938 , and is reflected back by the reflective polarizer  702 . The light then follows a reversed path with successive polarizations  940 ,  942 ,  944 ,  946 , and  948  to emerge as an LCD output  950 . (When the liquid crystal cell  902  is in its OFF state, the light entering through the first polarizer  908  travels through the cell  902 , where it is rotated to have a plane of polarization approximately orthogonal to the transmission axis of the second polarizer  910 , and is blocked.) 
     FIG. 10 illustrates the LCD  900  operating in the transmissive mode, in which the 0-λ/2 retarder  704  is in the λ/2 state  800 , and the backlight  906  is in the emissive state  1000 . First, randomly polarized backlight emission  1002 , indicated by unpolarized light vectors  1004 , is transmitted through the reflective polarizer  702 , wherein only the vertical component is transmitted, as indicated by the polarized light vector  1006 . Thereafter, the light passes through the retarder  704  wherein its polarization is rotated 90°, as indicated by polarized light vector  1008 . The light then passes through the second polarizer  910  maintaining its linear polarization  1010 . Its polarization is then rotated by the liquid crystal cell  902  to an approximately linear polarization  1012 , parallel to the transmitting axis of the first polarizer  908 . Finally the light passes through the first polarizer  908  with linear polarization  1014 , to emerge as the LCD output  1016 . (When the liquid crystal cell  902  is in its OFF state, the light exiting the cell  902  has a polarization direction orthogonal to the transmitting axis of the first polarizer  908  and is blocked). 
     FIG. 11 illustrates an LCD  1100 , similar to the LCD  900  in which a λ/4 retarder  1102  is located between the backlight  906  and reflective polarizer  702 . As is known in the art the placement of a λ/4 retarder  1120  between a backlight  906  and a reflective polarizer  702  can improve the brightness of the display. This is because horizontally polarized light not transmitted by the reflective polarizer  702  in the transmissive mode is reflected back through the λ/4 retarder  1120 . The light is then circularly polarized in the right handed rotational direction, and is reflected back through the retarder as left handed circularly polarized light. The light then emerges through the retarder  1120  with vertical linear polarization, and passes through the reflective polarizer  702 . (See Taber, U.S. Pat. No. 5,731,886). 
     It is also possible to assemble the LCD  900  without the second polarizer  910 . This is because the reflective polarizer  702  performs the function of the second polarizer  910  by filtering out the light not rotated by the liquid crystal cell. FIG. 12 illustrates this LCD assembly  1200 . 
     FIGS. 13-17 illustrate a third tunable mirror assembly  1300 , and LCDs  1500  and  1700  incorporating the tunable mirror  1300 . The tunable mirror  1300  includes a cholesteric reflector  1302 , a negative quarter-wave to positive quarter-wave (+/−λ/4) liquid crystal retarder  1304 , and a control  1306  for the +/−λ/4 retarder. 
     The cholesteric reflector  1302  reflects circularly polarized light having a polarization of one rotational direction, and transmits circularly polarized light of the opposite rotational direction. The cholesteric reflector  1302  may be a diffuse reflecting cholesteric liquid crystal polymer film, which diffuses light. Such reflector may be made according to the process described in Wacker-Chemie, R. Maurer, F. H. Kreuzer, and J. Stohrer, “Cholesteric Reflectors with a Color Pattern”, SID 94 Digest, p. 399 (1994). 
     The +/−λ/4 retarder functions similarly to the λ/4 retarder in that it converts light between circular and linear polarization forms. However, the +/−λ/4 retarder  1304  is controllably switchable between −λ/4 and +λ/4 states of operation, wherein the optical phase delay between the two states differs by half a wavelength (λ/2). In the +λ/4 state, the retarder  1304  converts horizontal or vertical linearly polarized light to right-handed or left-handed circularly polarized light, respectively. Conversely, it will convert right-handed or left handed circularly polarized light to horizontal or vertical linearly polarized light, respectively. In the −λ/4 state, the retarder  1304  converts horizontal or vertical linearly polarized light to left-handed or right-handed circularly polarized light, respectively, and conversely, it will convert right-handed or left handed circularly polarized light to vertical or horizontal linearly polarized light, respectively. Such retarder can be either of a nematic or ferroelectric type. The nematic type can be made by combining a λ/4 retarder with a 0−λ/2 wave retarder. There may be other ways of constructing a suitable tunable retarder. This invention contemplates the use of any suitable retarder which can be controllably switched between two states, with the optical phase delay between the two states differing by λ/2. 
     Furthermore the cholesteric reflector  1303  and retarder  1304  are oriented with respect to each other such that in the reflective mode, the cholesteric reflector  1302  reflects light received from the retarder  1304 , as illustrated in FIGS. 13-17. Also, in FIGS. 13-17, the cholesteric reflector  1302  and +/−λ/4 retarder  1304  are illustrated as being separated, however, as components of an LCD, these elements would normally be bonded together. 
     FIG. 13 illustrates the operation of the tunable mirror  1300  in the reflective mode, in which the retarder  1304  is in the +λ/4 state  1308 , for converting horizontal linearly polarized light to circularly polarized light of a right rotational direction. Also, the cholesteric reflector reflects right handed circularly polarized light and transmits left handed circularly polarized light. In the simplest case, light having horizontal linear polarization  1312  as indicated by the polarized light vector  1314 , is transmitted through the retarder  1304  and converted to right handed circularly polarized light, as indicated by polarization vector  1316 . The light is then reflected by the cholesteric reflector  1302 , maintaining its polarization  1318 , and is then linearly polarized back through the retarder  1304 , to its original horizontal polarization  1320 . It is also possible for the light entering to have both horizontal and vertical polarizations. In this case, still only the component having horizontal linear polarization will be converted into circularly right handed polarization and reflected back. 
     FIG. 14 illustrates the operation of the tunable mirror  1300  in the transmissive mode, in which the +/−λ/4 retarder  1304  is in the −λ/4 state  1400 . First, randomly polarized light  1402 , as indicated by the unpolarized light vectors  1404  is transmitted through the cholesteric reflector  1302 , wherein only the left handed circularly polarized component is transmitted, as indicated by the polarized light vector  1406 . Thereafter, the light passes through the retarder  1304  wherein it is linearly polarized to a horizontal linear polarization  1408 . 
     FIGS. 15 and 16 illustrate the operation of the LCD  1500  which incorporates the tunable mirror  1300 . The elements of the LCD  1500  are essentially the same as those in the previous figures, with the exception of the tunable mirror and tunable mirror control. These elements are a liquid crystal cell  1502  located in front of the +/−λ/4 retarder  1304  of the tunable mirror  1300 ; a backlight  1506  which is located behind the cholesteric reflector  1302  of tunable mirror  1300  and can be switched between emissive and non-emissive states; a first polarizer  1508  in front of the cell  1502 ; a second polarizer  1510  between the cell  1502  and the +/−λ/4 retarder  1304 ; and a control  1520  for the backlight  1506 . Again, the liquid crystal cell  1502  is assumed to be a 90° TN cell operating in a normally white mode, and in its ON state wherein it rotates the plane of polarization of linearly polarized light by 90°. Also, for the purpose of illustration, the first and second polarizers  1508  and  1510  have mutually orthogonal planes of polarization  1522  and  1524  in the vertical and horizontal directions, respectively. 
     FIG. 15 illustrates the LCD  1500  operating in the reflective mode, in which the +/−λ/4 retarder  1304  is in the +λ/4 state  1308 , and the backlight  1506  is in the non-emissive state  1526 . First, randomly polarized ambient light  1528 , indicated by unpolarized light vectors  1530 , travels through the first polarizer  1508 , and is linearly polarized, as indicated by the polarized light vector  1532 . The light then travels through the liquid crystal cell  1502 , where it is rotated (approximately 90°) to have a plane of polarization approximately parallel to the transmission axis of the second polarizer, as indicated by the polarized light vector  1534 , and then passes through the second polarizer  1510 , maintaining its linear polarization  1536 . The light is then transmitted through the retarder  1304  and is circularly polarized in the right handed direction, as indicated by polarization vector  1538 . Thereafter, the light is reflected back by the cholesteric reflector  1302 . The light then follows a reversed path with successive polarizations  1540 ,  1542 ,  1544 ,  1546 , and  1548  to emerge as an LCD output  1550 . 
     (When the liquid crystal cell is in its OFF state, the light entering through the first polarizer  1508  travels through the cell  1502 , where it is rotated to have a plane of polarization approximately orthogonal to the transmission axis of the second polarizer  1510  and is blocked.) 
     FIG. 16 illustrates the LCD  1500  operating in the reflective mode, in which the retarder  1304  is in the −λ/4 state  1400 , and the backlight  1506  is in the emissive state  1600 . First, randomly polarized backlight emission  1602 , indicated by unpolarized light vectors  1604 , is transmitted through the reflective polarizer  1302 , wherein only the left handed circularly polarized component is transmitted, as indicated by the polarized light vector  1606 . Thereafter, the light passes through the retarder  1304  wherein it is linearly polarized to a horizontal linear polarization  1608 . The light then passes through the second polarizer  1510  maintaining its linear polarization  1610 . Its polarization is then rotated by the liquid crystal cell  1502  to an linear polarization  1612 , approximately parallel to the transmitting axis of the first polarizer  1508 . Finally the light passes through the first polarizer  1508  with linear polarization  1614 , to emerge as the LCD output  1616 . (When the cell is in its OFF state, the light exiting the liquid crystal cell  1502 , has a polarization direction orthogonal to the transmitting axis of the first polarizer  1508  and is blocked). 
     As is known in the art, (see Taber, U.S. Pat. No. 5,731,886) the right handed light that is reflected from the cholesteric reflector  1302  will undergo a 180° phase change upon reflection from the backlight surface causing the right handed circular polarization to change to left handed circular polarization which improves the brightness of the display. 
     It is also possible to assemble the LCD  1500  without the second polarizer  1510 . This is because the cholesteric reflector  1302  performs the function of the second polarizer  1510  by filtering out the light not rotated by the liquid crystal cell. FIG. 17 illustrates this LCD assembly  1700 . 
     In addition to the reflective and transmissive modes described, the LCD and tunable mirror may optionally include an intermediate mode of operation, in which the tunable mirror and backlight are operated at intermediate states. 
     An LCD with a tunable mirror according to any of the embodiments described above may be incorporated into many types of operating systems, including but not limited to: Global Positioning Satellite (GPS) receiver units; computers including the laptop and notepad units; personal digital assistants; calculators; personal calendars; cellular telephones; watches and clocks; automobile, aircraft, and boat displays. 
     Three examples of operating systems embodying the present invention are shown in FIGS. 18 a - 18   c . FIG. 18 a  is a cellular telephone  1800  with an LCD  1802 , according to the present invention. FIG. 18 b  is a watch  1804  with an LCD  1806 , according to the present invention, and FIG. 18 c  is a laptop  1808  with an LCD screen  1810  according to the present invention, attached to the keypad section  1812 . Since cellular telephones, watches and laptops can be battery operated, using an LCD in accordance with the present invention is desirable as it saves battery life, as well as increase the contrast and brightness of the display.