Abstract:
The present invention concerns a display assembly including two superposed, respectively top ( 52 ) and bottom ( 54 ), display devices, the top display device ( 52 ) including a double structure formed of a display cell ( 56 ) arranged above an optical valve ( 58 ), said double structure being arranged so that said cell ( 56 ) and said valve ( 58 ) are transparent in a first state to make the bottom display device ( 54 ) visible, and so that the cell ( 56 ) displays an item of data and so that the valve ( 58 ) is opaque and at least partially masks the bottom display device ( 54 ) in a second state, control means supplying a control voltage to cause the display cell ( 56 ) and the optical valve ( 58 ) to pass from the first state to the second state and vice versa, characterised in that the display cell ( 56 ) is of the reflective type in the switched state, and in that the optical valve ( 58 ) is formed of a cell having an absorbent state in the switched state.

Description:
BACKGROUND OF THE INVENTION 
     The present invention concerns a display assembly including two superposed display devices and, more particularly, an assembly of this type including means allowing one of the display devices to appear to an observer selectively, to the exclusion of the other. 
     The present invention also concerns a timepiece including a display assembly of the aforementioned type and, more particularly, such a display assembly wherein an analogue display device is combined with a digital display device. 
     A display assembly including two superposed display devices is already known from European Patent No. EP 0 926 574 in the name of the Applicant. This display assembly is shown in FIG. 1 annexed to the present Patent Application. 
     FIG. 1 shows a timepiece of the wristwatch type designated as a whole by the general numerical reference  1 . This watch  1  includes, in a conventional manner, a case  2  in the back cover  4  of which are arranged an electronic clockwork movement  6  and a battery  8  which rest on back cover  4  via a contact spring  10 . Movement  6  includes electronic timekeeping circuits associated, via a control circuit, to a drive device (not shown) for a second hand  12 , a minute hand  14  and an hour hand  16  which move above a dial  18  which bears hour symbols (not shown). Case  2  is also closed in a conventional manner by a crystal  20  covering all of dial  18 . 
     Watch  1  further includes a display assembly including two superposed display devices respectively top device  22  and bottom device  24 . 
     In the example shown in FIG. 1, bottom display device  24  includes analogue time display means formed by hands  12 ,  14  and  16  and dial  18 . 
     According to the invention disclosed in European Patent No. EP 0 926 574, top display device  22  includes a double structure formed of a display cell  26  arranged above an optical valve  28 , this display device  22  extending between bottom display device  24  and crystal  20 . 
     Top display device  22  is arranged, on the one hand, so that display cell  26 , arranged between crystal  20  and optical valve  28 , like the latter, is transparent in a first switching state of cell  26  and valve  28 , so as to make the data displayed by bottom display device  24 , i.e. hands  12 ,  14  and  16  and dial  18 , visible. 
     On the other hand, top display device  22  is arranged so that display cell  22  displays an item of data, for example of the alphanumerical type, and so that valve  28  masks bottom display device  24  in a second switching state. 
     As a result of these features, it is possible to use selectively either bottom display device  24 , or top display device  22 . If one chooses to use bottom display device  24 , top display device  22  can be switched into its first state in which it is transparent, so that the data displayed by bottom display device  24  is visible. If, conversely, one chooses to use top display device  22 , optical valve  28  is then switched into the second state in which it is opaque and reflective to completely mask bottom display device  24 , while cell  26  of top display device  22  can display the desired data. 
     According to the particular embodiment shown in FIG. 1, cell  26  is a liquid crystal type display cell. Cell  26  includes a transparent front substrate  30 , a back substrate  32  which is also transparent and a sealing frame  34  forming spacing and closing means which, with substrates  30  and  32 , delimit a closed cavity in which is located a layer of liquid crystals. The opposite faces of substrates  30  and  32  include transparent electrodes respectively  36  and  38 , made, for example, of indium/tin oxide. In the example illustrated, front substrate  30  carries electrodes configured in digits each formed of segments allowing alphanumerical characters to be displayed, while back substrate  32  carries an electrode extending over its entire surface. 
     When a voltage is applied or removed across electrode  38  and certain of electrodes  36 , the liquid crystals located between these electrodes  36  and  38  are switched alternately from an absorbent state to a transparent state or vice versa, according to the type of liquid crystals in question and/or the presence and arrangement of polarisers associated with the cell. It is thus possible to display data in a light colour on a dark background or in a dark colour on a light background. 
     In the following description, elements which are identical to those previously described will be designated by the same numerical references. 
     In the example shown in FIGS. 2A and 2B annexed to the present patent Application, display cell  26  is a liquid crystal cell of the twisted nematic type of which the respective outer surfaces of substrates  30  and  32  are provided with absorbent polarisers  40  and  42  of crossed orientation. Thus, cell  26  is completely transparent in the first switching state (FIG.  2 A), when no voltage is applied across the terminals of its electrodes  36 ,  38  (non-switched state), and is absorbent or diffusing in the second switching state (FIG.  2 B), when a voltage is applied across the terminals of its electrodes  36 ,  38  (switched state). In the example shown, the optical valve includes, starting from the side of dial  18 , a reflective polariser  44  and a liquid crystal cell of the twisted nematic type. Valve  28  is thus transparent in the non-switched state (FIG. 2A) and opaque and reflective in the switched state (FIG.  2 B). 
     In the first switching state shown in FIG. 2A the natural non-polarised light, designated by the numerical reference  46 , is vertically polarised by first absorbent polariser  40 . The direction of polarisation of the light, designated by the reference  48 , is then rotated by 90° during its passage through display cell  26 , then it is transmitted unmodified by second absorbent polariser  42 . Polarised light  48  is then again rotated through 90° during its passage through optical valve  28 , then it is transmitted unmodified by reflective polariser  44  to dial  18 . The light follows the same path during its return travel, so that the dial is visible to an observer  50 . 
     Conversely, when cell  26  and valve  28  are in the switched state, the dial is totally masked. Indeed, as is seen in FIG. 2B, the vertically polarised light which passes through cell  26  in the switched zones of the latter is not modified, so that it is absorbed by absorbent polariser  40 . In the same time, the light which passes through cell  26  outside the switched zones of the cell is rotated through 90° as already explained above, then transmitted without being modified by absorbent polariser  42  up to valve  28 . Valve  28  being in the switched state, the polariser light passes through it without being modified is totally reflected by reflective polariser  44 . Dial  18  is thus masked, and the data is displayed in a dark colour on a light background with a mirror-like appearance. 
     The embodiment described above has several drawbacks. 
     The mirror-like appearance of the background on which the data displayed by display cell  26  appears is dazzling and thus makes it difficult to read this data, and this is all the more so when the ambient lighting is strong. 
     Moreover, since the light is reflected on reflective polariser  44 , and thus at a certain distance from display cell  26 , typically of the order of two to three millimetres, the images of the switched segments of cell  26  appear in projection on said reflective polariser  44 , which leads to double vision of the displayed data. This is detrimental not only to the aesthetic appearance of the watch, but also, of course, to the legibility of the displayed data. 
     Finally, when one looks at the watch from certain angles, bottom display device  24  remains visible although top display device  22  is in the switched state in which it should normally completely mask said bottom device  24 . 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to overcome the aforementioned drawbacks of the prior art by providing a display assembly including two superposed, respectively top and bottom, display devices, wherein it is possible to make data displayed by one of the display devices visible to an observer selectively, to the exclusion of the data displayed by the other display device, and with improved legibility of the data displayed by the display device in question. 
     Another object of the present invention is to provide a display assembly wherein the top display device is a liquid crystal device with excellent display contrast. 
     Another object of the present invention is to provide a timepiece fitted with such a display assembly having an improved aesthetic appearance. 
     The present invention therefore concerns a display assembly including two superposed, respectively top and bottom, display devices, the top display device including a double structure formed of a display cell arranged above an optical valve, said double structure being arranged so that said cell and said valve are transparent in a first state to make the bottom display device visible, and so that the cell displays an item of data and so that the valve is opaque and at least partially masks the bottom display device in a second state, control means supplying a control voltage to cause the display cell and the optical valve to switch from the first state to the second state and vice versa, characterised in that the display cell is of the reflective type in the switched state, and in that the optical valve is formed of a cell having an absorbent state in the switched state. 
     As a result of these features, the reflection of the incident light penetrating the display assembly according to the invention no longer occurs on a reflective polariser placed behind the optical valve as was the case in the prior art, but directly on the display cell when the latter is switched into the second state in which it displays a data item. This thus allows any parasitic projection of the switched zones onto the reflective polariser to be eliminated, and thus prevents double display of these zones to the observer. 
     Moreover, unlike the prior art wherein the data was displayed in a dark colour on a totally reflective background, only the switched segments of the display cell are reflective and are displayed in a light colour on a dark background formed by the optical valve which is absorbent in the second switching state. Such a structure is thus less dazzling for the observer. It also allows the luminosity of the displayed data to be increased, and thus its legibility. 
     Finally, since the optical valve is absorbent in the switched state, it is no longer possible to see the bottom display device through it, from whatever angle one looks at the display assembly. 
     According to an advantageous embodiment, the display device and optical valve are in a transparent state in the absence of any voltage applied by the control means. Thus, the data displayed by the bottom display device is permanently visible, without the top display device consuming any power. This is particularly advantageous within the scope of an application to a portable object such as a wristwatch. 
     According to another feature of the invention, the display cell is formed of a twisted nematic type cell, an absorbent polariser placed in front of the cell, and a reflective polariser placed behind the cell. 
     According to yet another feature of the invention, the optical valve is formed of a liquid crystal cell of the twisted nematic type and an absorbent polariser placed behind the cell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the present invention will appear more clearly upon reading the following detailed description of an embodiment example of the display assembly according to the invention, this example being given purely by way of illustrative and non-limiting example, with reference to the annexed drawings, in which: 
     FIG. 1, which has already been cited, is a cross-section of a wristwatch fitted with a display assembly according to the prior art, the assembly being switched into a first state allowing the bottom display device to appear; 
     FIGS. 2A and 2B, which have already been cited, are schematic views of the display assembly of FIG. 1 switched respectively into a first state in which it is transparent, so that the data displayed by the bottom display device is visible, and into a second state in which the optical valve is opaque and reflective to totally mask the bottom display device, while the display cell of the top display device can display the desired data; and 
     FIGS. 3A and 3B are schematic views of the display assembly according to the invention switched respectively into a first state in which it is transparent, so that the data displayed by the bottom display device is visible, and into a second state in which the optical valve is absorbent to totally mask the bottom display device, while the switched segments of the display cell of the top display device are reflective to display the desired data. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention proceeds from the general inventive idea which consists in superposing two, respectively top and bottom, display devices, one of these display devices being able to appear selectively to an observer to the exclusion of the other. The top display device therefore includes a double structure formed of a display cell arranged above an optical valve. This top display device is arranged so that, in a first state, the display cell and the optical valve are both transparent to allow the data displayed by the bottom display device to appear. Moreover, the top display device is arranged so that, in a second state, the display cell operates in negative contrast, i.e. the switched segments are reflective and stand out on a dark background formed by the optical valve which is reflective and which masks the bottom display device. 
     The description of the invention is made within the scope of an application to a timepiece such as a wristwatch. It goes without saying however that the invention is not limited to this application and it could advantageously be used within the scope of any other application requiring the display of data such as, for example, measuring instruments. 
     In the example shown in FIGS. 3A and 3B, the display assembly according to the invention includes, in a conventional manner, two superposed display devices, respectively top device  52  and bottom device  54 . Bottom display device  54  includes analogue time display means formed by hands moving above a dial which bears hour symbols. It goes without saying however that bottom display device  54  may be formed by any display device, for example a liquid crystal type digital display device. This bottom display device  54  may also include a combination of analogue and digital display means or even a decorative element, for example a picture, a figure or something else. As regards top display device  52 , this includes a double structure formed of a display cell  56  arranged above an optical valve  58 . 
     With reference to FIGS. 3A and 3B, display cell  56  is a twisted nematic type liquid crystal cell (TN) of which the outer surfaces of the front and back substrates are provided with an absorbent polariser  60  and a reflective polariser  62  respectively. The optical valve includes, starting from the side of dial  54 , an absorbent polariser  64  and a twisted nematic type liquid crystal cell. 
     In the first switching state shown in FIG. 3A, natural non-polarised light  46  is polarised vertically by absorbent polariser  60 . Polarised light  48  is then rotated through 90° during its passage through display cell  56 , then it is transmitted without modification by reflective polariser  62 . The polarised light it then again rotated through 90° during its passage through optical valve  58 , then it is transmitted without modification by absorbent polariser  64  to dial  54 . The light follows the same path during its return travel, so that dial  54  is visible to observer  50 . 
     Conversely, when display cell  56  and optical valve  58  are in the switched state, dial  54  is totally masked. Indeed, as is seen in FIG. 3B, the vertically polarised light which passes through cell  56  in the switched zones thereof, is not modified, so that it is reflected by reflective polariser  62 . At the same time, the light which passes through cell  56  outside the switched zones thereof is rotated through 90°, then it is transmitted without modification by reflective polariser  62 . Since valve  58  is in the switched state, the polarised light passes through it without being modified and is then totally absorbed by absorbent polariser  64 . Dial  54  is thus masked, and display cell  56  operates in negative contrast, i.e. the switched segments are reflective and stand out on a dark background formed by optical valve  58  which absorbs the light and which masks dial  54 . 
     Absorbent polarisers  60  and  64  used within the scope of the present invention may be those manufactured by the Japanese companies Nitto Denko, Sanritz or Sumitomo. In an advantageous manner, reflective polariser  62  can be, for example, of the multi-layered type marketed under the name of DBEF (Dual Brightness Enhancement Film) by the 3M company or of the holographic type marketed by the Polaroid company. 
     It goes without saying that cell  56  and valve  58  may be of another type provided that in a first switching state, cell  56  and valve  58  are transparent, and that in a second switching state, cell  56  is reflective and valve  58  is absorbent. 
     One could in particular select cells of the Super Twist-Nematic (STN), Guest-Host (GH), electrically controlled birefringence (ECB), cholesteric texture, ferroelectric, electrochromic, electrolytic, electrocapillary type. Optical valve  28  may be of the same type as display cell  56 . Two cells of different types selected from the above group of cells may also be combined. 
     The structure and operation of these types of cells are well known to those skilled in the art and will not be described in detail in this description. 
     For a complete description of the structure and operation of an STN type cell, reference will be made for example to European Patent No. EP 0 131 216 which is incorporated here by reference. 
     For a complete description of the structure and operation of a GH type cell, reference will be made for example, for a positive contrast GH cell, i.e. which displays in a dark colour on a light background, to U.S. Pat. No. 4,257,682 which is incorporated by reference here, and, for a negative contrast GH cell, i.e. which displays in a light colour on a dark background, to the publication by G. H. Heilmeier and L. A. Zanomi entitled “Guest-Host Interaction in Nematic Liquid Crystals A New Electro-optical Effect” published in Appl. Phys. Lett. vol. 13, No 3, pages 91-92, year 1968. 
     For a complete description of the structure and operation of a ECB type cell, reference will be made for example to volume 3 of the work entitled “Liquid Crystals Applications and Uses” edited by B. Bahadur at pages 254-300, which is incorporated here by reference. 
     For a complete description of the structure and operation of a cholesteric texture type cell, reference will be made for example to U.S. Pat. No. 5,188,760 and European Patent No. EP 0 451 905 which are incorporated here by reference. 
     For a complete description of the structure and operation of a ferroelectric type cell, reference will be made for example to volume 1 of the work entitled “Liquid Crystals Applications and Uses” edited by B. Bahadur at pages 307-356, which is incorporated here by reference. 
     All the types of cells described above may of course be addressed passively by a matrix electrode network or actively addressed by non linear elements such as thin film transistors (TFT, MIM) or diodes. 
     Optical valve  58  may, for example, be formed of an electrolytic cell which is transparent in the non switched state, and which is absorbent in the switched state. Such a valve  58  includes an electrolytic solution of a metal salt dissolved in a solvent placed between two electrodes. When valve  58  is in a non switched state, the metal in the solution is deposited over the entire surface of one of the electrodes, and thus makes valve  58  absorbent. When valve  58  is in the non switched state, the metal dissolves again in the solution, and the cell becomes transparent again. Such a valve  58  is for example described in the publication by J. Duchêne et al. entitled “Electrolytic Display” published in SID 1978 at pages 34 to 37 which is incorporated here by reference. 
     Of course, according to an alternative embodiment, optical valve  58  may be formed of a mercury electrocapillary cell including a capillary film arranged between two substrates bearing electrodes which, in the cell&#39;s switched state, do not wet the surface of the substrates, so that the cell is transparent and which, in the cell&#39;s non switched state, wets the entire surface of the substrates to make it absorbent. Such a cell is for example described in U.S. Pat. No. 4,583,824 which is incorporated here by reference. 
     According to another alternative embodiment, the optical valve may, for example, be formed of an electrochromic cell known to those skilled in the art and described in the publication by C. M. Lampert and C G Granquist entitled “Large-area Chromogenics: Materials and Devices for Transmittance Control” published in SPIE press, volume 15. 4. 1988 which is incorporated here by reference. 
     According to another alternative embodiment, optical valve  58  may be formed of a switchable reflector including a stack formed of a polariser, a twisted nematic (TN) type liquid crystal cell, a quarterwave plate, and a cholesteric film such as the reflector described in the publication by T. J. Scheffer entitled “Twisted Nematic Display with Cholesteric Reflector” published in J. Phs. Appl. Phy., vol. 8, 1975 which is incorporated here by reference. Preferably, the polariser is of the high polarisation and transmission efficiency type, for example such as the polariser marketed by the Sanritsu company, Japan, under the reference LLC 2 5618SF. Advantageously, the quarterwave plate and the cholesteric film can be integrated in a single element, for example, such as the TRANSMAX® marketed by Merck. 
     It will also be noted in this regard that, according to another variant, the twisted nematic (TN) liquid crystal cell described in the publication by Scheffer may be replaced by a colour display device such as those described in European Patent No. EP 0 600 349 which is also incorporated here by reference. 
     Within the scope of the invention, the switchable reflector is advantageously transparent in the non switched state, and absorbent in the switched state. 
     Optical valves  58  which have just been described are all transparent in the non switched state, and may advantageously be combined with cells  56  which are also transparent in a non switched state. Thus, within the scope of the application to a timepiece, bottom display device  54  may be permanently visible with minimum power consumption, insofar as said bottom display device  54  needs to be powered, top display device  52  only being powered when the user wishes to read data supplied by display device  52 . 
     According to another variant, optical valve  58  may be made by means of a switchable cholesteric film which is transparent in a switched state, and absorbent in a non switched state. Such a cholesteric film is for example described in European Patent No. EP 0 643 121 which is incorporated here by reference. 
     The present invention also concerns a timepiece including a case closed by a crystal and a back cover in which is housed a clockwork movement associated with a display device of time related information, this timepiece including a display assembly as described above, bottom display device  54  being formed by said display device of time related information, and top display device  52  extending between the crystal and said display device of time related information. This display device of time related information may include a dial as well as an hour hand and a minute hand which move above the dial. 
     Finally, according to a particularly advantageous embodiment, the crystal is formed by top display device  52 . 
     It goes without saying that various simple variants and modifications fall within the scope of the present invention.