Display assembly for portable object comprising two superposed display devices.

[0001] The present invention relates to a display arrangement comprising two superposed display devices. More specifically, the present invention relates to such a display assembly to be housed in a portable object such as a wristwatch.

[0002] Legibility information displayed by the display devices, such as liquid crystal display cells or the organic light emitting diode display is very dependent on the ambient light conditions. With some display devices, the displayed information may be read out in good conditions in an environment illuminated, but are difficult to read in a dark environment. Conversely, other categories of display devices provide a display of good quality within the penumbra or the dark, but are extremely difficult to read in daylight.

[0003] For example, think the liquid crystal display cells transflective type, i.e. the liquid crystal display cells capable of displaying information that will be visible day by using the phenomenon of reflection of the ambient light, and that will also be visible in the dark by transmission using a backlight device. Such liquid crystal display cells transflective type are optimized for best reflect sunlight and thus to ensure a good legibility of the information displayed in bright ambient light conditions. However, so that such liquid crystal display cells transflective type with best may reflect sunlight, their efficiency in transmission is greatly limited. Therefore, when the backlight device is activated to read the information displayed in the penumbra, a major part of the light emitted by the backlight device is lost through absorption phenomena. Energy efficiency in this situation is therefore poor. Furthermore, optical qualities of information displayed by the liquid crystal cell are strongly dependent on the viewing angle.

[0004] The emissive type display devices such as organic light emitting diode display, optical quality of which is larger than that of the liquid crystal display, these optical qualities such as luminance and the color does not dependent upon the viewing angle. However, the display devices of the transmission type of high quality do not permit reflective mode operation. The information that they display are very readable in the penumbra or the dark, but become extremely difficult to read as soon as they are observed outdoors. In order to overcome this problem, it is possible to increase the amount of current supplied to the emissive display devices to ensure a minimum of readability. Or, even in normal-use conditions, these emissive display devices consume more than a liquid crystal cell reflection type. Their power consumption is therefore such that it is hardly possible to preserve the turned on at all times, particularly when it is mounted in a small portable object such as a wristwatch in which the only power source is a battery which is usually desired that it lasts at least one year.

[0005] The present invention aims at remedying the above problems and others by providing a display assembly for a portable object such as a wristwatch and whose operation is suitable both in manner that illuminated in a dark environment.

[0006] To this end, the present invention provides a display assembly for a portable object, the display assembly comprising a first emissive display at least partially transparent located at the side of an observer, a second reflective display disposed under the first emissive display, the second reflective display is capable of capable of switching between a transparent state when it is at rest and a reflective state when activated.

[0007] complementary According to a feature of the invention, the emissive display transparency is mounted on the reflective display.

[0008] According to another feature of the invention, the emissive display transparent is bonded to the reflective display by means of an adhesive film or a layer of liquid glue.

[0009] Thanks to these characteristics, the present invention provides a display assembly for a portable object such as a wristwatch that function optimally irrespective of the conditions of ambient illumination. In daylight, the information will be preferably displayed by the reflective display. Indeed, the reflective display, which can make effective use of a phenomenon of reflection of the sunlight for displaying the information, is energy-saving. It can therefore remain on permanently and has good readability of the information. Conversely, the penumbra or in the dark, the information will be displayed by the emissive display. Such emissive display consumes more current that a reflective display device, but the information that it displays are visible in the dark night or with very good optical properties that are particularly independent of the viewing angle. Therefore, unlike a liquid crystal display cell transflective type seeking a compromise between the reflectivity of its reflective mode, and the power consumption of its backlight device in the transmissive mode, the display assembly of the invention combines two display devices, one purely reflective and the other purely emissive, without compromising the performance or one, or the other of the two display devices.

[0010] According to a first embodiment of the invention, the first display device comprises a display cell transparent emissive organic light-emitting diode, and the second display device comprises a liquid crystal display cell reflective twisted nematic type or super-twisted nematic or vertically aligned.

[0011] complementary According to a feature of the invention, the cell organic light emitting diode display is arranged between a circular polarizer and a quarter-wave, the circular polarizer being placed on the side of the observer.

[0012] Addressing electroluminescent areas cells of organic light emitting diode display is provided by transparent electrodes most often performed using metallic material or a metal oxide. The electrodes thus often cause slight optical reflection phenomena induce degradation of the contrast, which affects visibility of the information displayed by the cells of organic light emitting diode display. To cope with this, the present invention teaches arranging the display cell organic light-emitting diode between a circular polarizer and a quarter-wave, the circular polarizer being placed on the side of the observer. Therefore, one of the polarization components of the ambient light that enters the display assembly according to the invention is absorbed by the circular polarizer, while the other polarization component of the light is circularly polarized. When through the cell organic light emitting diode display, circularly polarized ambient light is partially reflected by the transparent electrodes of the cell of organic light emitting diode display, the reflected light is phase-shifted, thereby transforming its circular polarization into circular polarization opposite direction of rotation. Therefore, when the reflected light passes back through the circular polarizer, it is absorbed by the latter. In this way, it is possible to eliminate stray light that reflects on the cell electrodes of organic light emitting diode display, and only that the light passing through the display cell organic light-emitting diode without modification. As a result, the light again are linearly polarized after passing through the quarter-wave plate placed in the cell of organic light emitting diode display and will eventually be absorbed or reflected by the liquid crystal display cell according to that a search is made reflective contrast display positive or negative.

[0013] In a second embodiment of the invention, the first display device comprises a display cell transparent emissive organic light-emitting diode, and the second display device comprises a display cell free of reflective polarizers. The reflective display cell may be a cell electrophoretic display, a liquid crystal display cell or a dichroic cell cholesteric liquid crystal display.

[0014] The interest of such an embodiment is much for the use the character intrinsically reflective, for example, a cell electrophoretic display to obtain a display assembly according to the invention and whose operation is suitable both in manner that illuminated in a dark environment. The can be accomplished and reflective films absorbent, thereby saving in terms of components and assembly time. Furthermore, the set of resulting display is thinner, which is very advantageous particularly in the case where it is desired to integrate such a display assembly for example in a wristwatch in which the space available is required to be limited.

[0015] Other features and advantages of the present invention shall become apparent more clearly of the following detailed description of an embodiment of the display assembly according to the invention, this example being exemplary purely illustrative and not limiting, only in combination with the attached drawing on which:

The fig. 1 is a schematic view illustrating a display assembly according to the invention comprising a first emissive display at least partially transparent located at the side of an observer, a second reflective display disposed under the first display device

emissive;

fig. 2 is a cross-sectional view of an example embodiment of a display assembly according to the invention in which the first display is a display cell transparent emissive organic light-emitting diode, and the second display device is a liquid crystal display cell reflective twisted nematic type;

the Fig. 3A to 3D illustrate schematically the mode of operation of the display assembly shown in fig. 2 as the cell of organic light emitting diode display and the display cell twisted nematic liquid crystal are active or passive;

fig. 4 is a view similar to that of fig. 2 wherein the second display device is a liquid crystal display cell reflex vertical alignment;

the Fig. 5A to 5D illustrate schematically the mode of operation of the display assembly shown in fig. 4 as the cell of organic light emitting diode display and the liquid crystal display cell are active or passive vertically aligned;

fig. 6 is a cross-sectional view of a detailed embodiment of the display assembly according to the invention shown in Figure 4 wherein is disposed on top of the display cell transparent organic light-emitting diode a circular polarizer comprises a polarizer and a quarter-wave plate;

fig. 7 is a schematic view illustrating a display assembly having a display cell transparent emissive organic light-emitting diode, and the second display device is a liquid crystal display cell reflective, the display cell organic light-emitting diode being disposed between a circular polarizer and a quarter-wave, the circular polarizer being placed on the side of the observer;

fig. 8 is a schematic view illustrating a display assembly having a display cell transparent OLED disposed above a reflective electrophoretic display cell, and

fig. 9 is a schematic view illustrating a display assembly according to the invention comprising wherein the display cell is a cell reflective electrophoretic display glued in the emissive display cell transparent organic light-emitting diode by means of an adhesive layer.

[0016] The present invention is based on the general idea to provide an inventive display assembly capable of displaying information, so that it can be read both in daylight and the dark or the dark and whose electric power consumption is optimal. To achieve this object, the present invention teaches of combining a emissive type display device with a display device which is arranged to be switchable between a rest state in which it is transparent and an active state in which it is capable of reflecting the ambient light. The display device of the transmission type is typically a cell of organic light emitting diode display, while the display device is typically a reflection type liquid crystal display cell. For display information daytime, are preferred the use of display reflection type which, by reflecting the sunlight, to display information clearly readable and while consuming a small amount of electric power. For display information in the penumbra or the dark, are preferred the use of the emissive display. Owing to its excellent optical properties, in particular in terms of contrast and color reproduction, such a emissive display may display a large number of very readable information. In particular, visibility of the information displayed is not dependent upon the viewing angle. Furthermore, despite the penumbra or the dark, it is possible to significantly reduce the power consumption of such an emissive display while ensuring a good legibility of the information displayed. This provides a display assembly that includes a reflective display positioned at the base of the stack and which is capable of displaying information continuously consumes very low power, and an emissive display device placed on top of the stack and which is capable of displaying information to the request very readable in the penumbra or the dark.

[0017] The fig. 1 is a sectional schematic view of a display assembly according to the invention. Slated as a whole by the general digital reference 1, the display assembly comprises a first emissive display 2 at least partially transparent arranged at the side of an observer 4, and a second reflective display 6 also at least partially transparent disposed beneath the first emissive display 2. At direction of the present invention, the first emissive display 2 is capable of switching between a passive state, in which it is at least partially transparent, and an active state in which it emits light for displaying information. The second reflective display 6, it is capable of switching between a passive state, in which it is absorbent and an active state in which it is capable of reflecting the ambient light.

[0018] Preferably but not necessarily, the first emissive display 2 is mounted on the second reflective display 6 by means of a transparent adhesive layer 8. The transparent adhesive layer 8 may be formed of a film adhesive type transparent optical adhesive (also known as its-Saxon Optical Clear [...] or OCA) or a layer of liquid adhesive acrylic or silicone. The transparent adhesive layer 8 relates to avoid the problems of parasitic reflections that would arise if the two display devices 2,6 were separated by a layer of air and that would degrade the optical quality of the display assembly 1 according to the invention.

[0019] The fig. 2 is a cross-sectional view of a detailed embodiment of the display assembly 1 according to the invention in the case where the first emissive display 2 comprises a display cell 20 transparent emissive organic light-emitting diode which will be designated in all of the following per cell transparent display TOLED (diode Tranparency Organic Light [...] ). The second reflective display 6, it comprises a liquid crystal display cell 60 of reflective twisted nematic type, also known as its-Saxon or Twist [...] TN.

[0020] More specifically, the cell transparent display TOLED 20 comprises a transparent substrate 21 made of glass or plastic material and a cover cap 22 which extends parallel to and at a distance from the transparent substrate 21. The transparent substrate 21 encapsulation and the cover 22 are joined together by a sealing frame 23 which defines an enclosed volume with the exclusion of air and moisture for the containment of a electroluminescent layer stack generally designated by the digital reference 24. An upper electrode 25 made of, for example, a transparent indium tin oxide or ITO and a lower transparent electrode 26 implemented for instance by means of a metallic material such as aluminum or gold or of a metal oxide such as TITO or zinc-indium oxide are structured on both sides of the stack of light-emitting layer 24. The electrodes 25,26, made of a metallic material, are light reflective. The display cells transparent organic light-emitting diode are available with either a direct addressing in the event that it is simply displaying icons or segments, with either a passive matrix addressing of the type in the case of a dot matrix display. In the case of a dot matrix display, may also be to make a addressing of the active matrix type transistors combined with transparent type Thin Film Transistor or TFT for controlling the current and which are formed in the display pixels located on the side of the transparent substrate 21 of the cell transparent display TOLED 20.

[0021] On the other hand, the liquid crystal display cell reflective 60 includes a front substrate 61 arranged on the observer side of 4 and a rear substrate 62 which extends parallel to and away from the substrate before 61. The front and rear substrates 61 62 are joined together by a sealing frame 63 which defines a sealed chamber 64 for the containment of a liquid crystal whose optical properties are modified by applying an appropriate voltage to a cross point considered between transparent electrodes 65a provided on a lower face of the front substrate 61 and counter electrodes transparent 65b provided on an upper face of the back substrate 62. The electrodes 65a and 65b the counter electrodes are made of a transparent electrically conductive material such as indium tin oxide or indium tin oxide, the latter material is better known as its-Saxon Indium Tin oxide or ITO.

[0022] In the case of the present invention, all phases of the liquid crystal such as twisted nematic (Twist [...] or TN in Anglo-Saxon terminology), super-twisted nematic (Super Twist [...] or STN in Anglo-Saxon terminology) or vertically aligned ( [...][...] or VA in Anglo-Saxon terminology) can be contemplated. Also, all types of addressing such as direct addressing, active matrix addressed or multiplex addressing of a passive matrix can be contemplated.

[0023] A absorptive polarizer 30 is bonded on a top surface of the substrate 61 front of the liquid crystal display cell reflective 60 by means of an adhesive layer 32. The adhesive layer 32 may be formed of a film adhesive or a layer of liquid glue. The adhesive used for securing the absorbing polarizer 30 on the liquid crystal display cell reflective 60 may be transparent or slightly diffusing according to that one seeks to obtain specular reflection or diffuse. The absorptive polarizer 30, it can be, for example, iodine type or dye.

[0024] A reflective polarizer absorbent 34 is adhered to a lower face of the back substrate 62 of the liquid crystal display cell reflective 60 by means of an adhesive layer 36 which may be transparent or slightly diffusing according to that one seeks to obtain specular reflection or diffuse.

[0025] At direction of the present invention, is understood as meaning a reflective polarizer absorbent 34 polarizer which reflects the component of the light of which direction of polarization is parallel to the axis of reflection of the reflective polarizer absorbent, and the other component which absorbs light having a polarization direction transverse to the direction of polarization of the light component reflected by the reflective polarizer absorbent 34.

[0026] For example preferred, but not limiting, the reflective polarizer absorbent 34 may be formed of an absorptive polarizer arranged above a reflector b, or a transmissive reflective polarizer ç disposed above an absorption layer d. At direction of the present invention, is understood as meaning a reflective polarizer-transmissive polarizer which reflects one of the components of the light and passes without changing the other component light having a polarization direction transverse to the direction of polarization of the light component reflected by the reflective polarizer transmissive d.

[0027] is examined in connection with the maintaining Fig. 3A to 3D the principles of operation of the display assembly 1 according to the invention according to that the cell transparent display TOLED 20 and the liquid crystal display cell reflective 60 are in service. The assumed, purely illustrative example only and in no way limiting, that the liquid crystal display cell is a cell 60 reflective twisted nematic liquid crystal TN and that the transmission axis of the absorptive polarizer 30 and the reflection axis of the reflective polarizer absorbent 34 are parallel.

[0028] A Fig. 3A, the cell transparent display TOLED 20 and the liquid crystal display cell reflective TN 60 are both turned off. Ambient light, designated by the digital credential 46, passes through without changing the cell transparent display TOLED 20, and is linearly polarized by the absorbing polarizer 30. Ambient light 46 then undergoes rotation of 90° as it passes through the liquid crystal display cell reflective TN 60, so that when it falls on the reflective polarizer absorbent 34, its polarization direction is perpendicular to the axis of reflection of the reflective polarizer and absorbent 34 and is thus absorbed by the latter. The liquid crystal display cell reflective TN 60 thus appears dark when it is turned off, which means that the information that it displays will appear in clear on a dark background. The display of information is at negative contrast. Of course, a display information according to a positive contrast can be obtained simply by ensuring that the transmission axis of the absorptive polarizer 30 and the reflection axis of the reflective polarizer absorbent 34 are perpendicular.

[0029] A Fig. 3B, the cell transparent display TOLED 20 is activated, while the liquid crystal display cell reflective TN 60 is deactivated. The light emitted by the display cell transparent TOLED 20 reaches the observer 4 without modification, while the liquid crystal display cell reflective TN 60 appears dark. The information displayed by the display cell thus detach from transparent TOLED 20 on a dark background.

[0030] A Fig. 3C, the cell transparent display TOLED 20 is turned off, while the liquid crystal display cell reflective TN 60 is activated. As already explained above, the areas not switched from the liquid crystal display cell reflective TN 60 appear dark. However, in the areas switched from the liquid crystal display cell reflective TN 60, ambient light 46 passes through these areas without modification, so that ambient light 46 falls on the absorbent 34 reflective polarizer with a polarization direction parallel to the axis of reflection of the reflective polarizer absorbent 34. Ambient light 46 is therefore reflected back and passes through the liquid crystal display cell reflective TN 60, the absorbing polarizer 30 and the display cell transparent TOLED 20 without modification, so that it is perceptible by the observer 4. The information should thus in clear on a dark background.

[0031] A Fig. 3D, the cell transparent display TOLED 20 and the liquid crystal display cell reflective TN 60 will be activated. The light emitted by the display cell transparent TOLED 20 is directly perceptible by the observer 4. Ambient light 46 which passes through the areas not switched from the liquid crystal display cell TN 60 reflective reflective polarizer is absorbed by the absorbent 34, so that these areas appear dark. Finally, ambient light 46 which passes through the areas switched from the liquid crystal display cell reflective TN 60 is reflected by the reflective polarizer absorbent 34, so that the clear areas appear.

[0032] The fig. 4 is a cross-sectional view of an exemplary embodiment of the display assembly 1 according to the invention in the case where the first display device 2 comprises the emissive display cell transparent TOLED 20. The second display device 6, it comprises a liquid crystal display cell 600 reflective vertically aligned, also known as its-Saxon [...][...] or VA. The liquid crystal display cell reflective VA 600 includes a front substrate 601 arranged on the observer side of 4, and a rear substrate 602 which extends parallel to and away from the substrate before 601. The front and rear substrates 601 602 are joined together by a sealing frame 603 604 which delimits a sealed enclosure for the containment of a liquid crystal whose optical properties are modified by applying an appropriate voltage to a cross point considered between transparent electrodes 605a provided on a lower face of the front substrate 601 and counter electrodes transparent 605b provided on an upper face of rear substrate 602. The electrodes 605a and 605b the counter electrodes are for example made of indium tin oxide or ITO. The absorptive polarizer 30 is fixed on an upper face of the front substrate 601 of the liquid crystal display cell reflective VA 600. Absorbent 34 The reflective polarizer is attached to an underside of the rear substrate 602 of the liquid crystal display cell reflective VA 600.

[0033] is examined in connection with the maintaining Fig. 5A to 5D the principles of operation of the display assembly 1 according to the invention according to that the cell transparent display TOLED 20 and the liquid crystal display cell reflective VA 600 are in service. The assumed, purely illustrative example only and in no way limiting, that the transmission axis of the absorptive polarizer 30 and the reflection axis of the reflective polarizer absorbent 34 are perpendicular.

[0034] A Fig. 5A, the cell transparent display TOLED 20 and the liquid crystal display cell reflective VA 600 are both turned off. Ambient light, designated by the digital credential 46, passes through without changing the cell transparent display TOLED 20 and the liquid crystal display cell reflective VA 600, so that when it falls on the reflective polarizer absorbent 34, its polarization direction is perpendicular to the axis of reflection of the reflective polarizer and absorbent 34 and is thus absorbed by the latter. The liquid crystal display cell reflective VA 600 thus appears dark when it is turned off, which means that the information that it displays will appear in clear on a dark background. The display of information is at negative contrast. Of course, a display information according to a positive contrast can be obtained simply by ensuring that the transmission axis of the absorptive polarizer 30 and the reflection axis of the reflective polarizer absorbent 34 are parallel.

[0035] A Fig. 5B, the cell transparent display TOLED 20 is activated, while the liquid crystal display cell reflective VA 600 is deactivated. The light emitted by the display cell transparent TOLED 20 reaches the observer 4 without modification, while the liquid crystal display cell reflective VA 600 appears dark. The information displayed by the display cell thus detach from transparent TOLED 20 on a dark background.

[0036] A Fig. 5C, the cell transparent display TOLED 20 is turned off, while the liquid crystal display cell reflective VA 600 is activated.

[0037] In a liquid crystal cell vertically aligned, the alignment layers are oriented at 45° to the polarization axes of the polarizers. On the other hand, the result of the product of the birefringence of the liquid crystal molecules and the distance between the front and rear substrates is selected such that, when the liquid crystal is switched, it behaves relative to the direction of polarization of the light as a half-wave plate. Therefore, as the half-wave plate is placed at 45° relative to the polarization axis of the absorbing polarizer, it causes rotation of 90° of the direction of polarization of the light. Therefore, ambient light 46 is rotated 90° as it passes the switched regions of the liquid crystal display cell reflective VA 600, so that when it falls on the reflective polarizer absorbent 34, its direction of polarization is parallel to the axis of reflection of the reflective polarizer and absorbent 34 and is thus reflected therefrom. The ambient light 46 which passes through the areas not switched from the liquid crystal display cell reflective VA 600, is absorbed by the absorbent 34 reflective polarizer. The information should thus in clear on a dark background, i.e. with a negative contrast.

[0038] A Fig. 5D, the cell transparent display TOLED 20 and the liquid crystal display cell reflective VA 600 will be activated. The light emitted by the display cell transparent TOLED 20 is directly perceptible by the observer 4. Ambient light 46 which passes through the areas not switched from the liquid crystal display cell VA 600 reflective reflective polarizer is absorbed by the absorbent 34, so that these areas appear dark. Finally, ambient light 46 which passes through the areas switched from the liquid crystal display cell reflective VA 600 is reflected by the reflective polarizer absorbent 34, so that the clear areas appear.

[0039] The fig. 6 is a cross-sectional view of a detailed embodiment of the display assembly as claimed in the invention shown in Figure 4. In order to suppress the parasitic reflections and thereby improving the display contrast, is placed above the display cell transparent TOLED 20, on the side of the observer 4, a circular polarizer 38 which consists of a second absorptive polarizer 40 and a first quarter-wave plate 42. On the other hand, the reflective polarizer absorbent 34 is replaced by a metal mirror 44. This variation also reduces the number of components and reduce the effect of parallax because the metal mirror 44 can be positioned as near the plane of switching of the liquid crystal.

[0040] Addressing electroluminescent areas cells of organic light emitting diode display is provided by transparent electrodes most often performed using metallic material or a metal oxide. The electrodes thus often cause enough optical reflection phenomena that induce degradation of the contrast, which affects visibility of the information displayed by the cells of organic light emitting diode display.

[0041] To cope with this, the present invention teaches disposing a circular polarizer 38 above the cell transparent display TOLED 20 and a metal mirror 44 in the display cell transparent TOLED 20. Therefore, ambient light 46 which enters the display assembly 1 according to the invention is linearly polarized by the second polarizer absorbent 40, and then circularly polarized by the first quarter-wave plate 42. When through the display cell TOLED 20 transparent, ambient light 46, circularly polarized, is partially reflected by the upper and lower electrodes transparent 25,26 of the cell transparent display TOLED 20, the reflected light is phase-shifted, thereby transforming its circular polarization into circular polarization opposite direction of rotation. Therefore, when the reflected light passes back through the circular polarizer 38, is absorbed by the latter. In this way, it is possible to eliminate stray light that reflects on the electrodes 25,26 of the cell transparent display TOLED 20. Remaining The ambient light 46 passes through without changing the cell transparent display TOLED 20, and the liquid crystal display cell reflective VA 600 and is finally reflected by the metal mirror 44 which reverses the direction of the circular polarization. Therefore, after passing again through without modification the liquid crystal display cell reflective VA 600 and the display cell TOLED 20 transparent, ambient light 46 is finally absorbed by the circular polarizer 38.

[0042] The display is therefore in clear on a dark background. That is, the display assembly 1 is polishing. Indeed, as it passes the switched regions of the liquid crystal display cell reflective VA 600, ambient light 46 that has been circularly polarized by the circular polarizer 38 and which then passes through the display cell transparent TOLED 20 without modification, becomes linearly polarized. Therefore, when the ambient light 46 reflects off of the metal mirror 44, its direction of linear polarization remains. However, when back through the liquid crystal display cell reflective VA 600, ambient light 46 is circularly polarized in the same direction as the circular polarization, which has been printed by the circular polarizer 38 when it has penetrated the display assembly 1. Therefore, it can pass through the circular polarizer 38 without being absorbed and is finally perceptible by the observer 4.

[0043] The fig. 7 is a view similar to that of fig. 6 with the exception that a second quarter-wave plate 48 is disposed between the transparent display TOLED 20 and the liquid crystal display cell reflective VA 600. The second quarter-wave plate 48 is parallel to the first quarter-wave 42 or disposed at 90° with respect to the first quarter-wave plate 42.

[0044] [...] circularly by the circular polarizer 38,46 ambient light entering the display assembly 1 through the cell transparent display TOLED 20 without modification, and then is converted into linearly polarized light after passing through the second quarter-wave plate 48. [...] linearly, ambient light 46 then passes through without changing the areas not switched from the liquid crystal display cell reflective VA 600 and is finally reflected without modification by the metal mirror 44. At return, ambient light 46 follows the same path and is finally perceptible by the observer 4. In areas switched from the liquid crystal display cell reflective VA 600, ambient light 46, initially linearly polarized after passing through the second quarter-wave plate 48, is circularly polarized by the liquid crystal display cell reflective VA 600. Ambient light 46 is then reflected by the metal mirror 44, so as to be a phase shift which transforms its circular polarization into circular polarization opposite direction of rotation. In returning through the zones switched from the liquid crystal display cell reflective VA 600, ambient light 46 retrieves a linear polarization oriented at 90° with respect to the linear polarization in the original in the forward direction. It further passes through the second quarter-wave plate 48 and is circularly polarized in a direction opposite to the rotating direction as it existed in the forward direction. Ambient light 46 passes through the display cell transparent TOLED 20 without modification and is finally linearly polarized by the first quarter-wave plate 42 in an orientation to 90° with respect to the linear polarization in the original in the forward direction. It is hence absorbed by the absorbing linear polarizer 40. The display is therefore dark against a light background. That is, the display assembly 1 is of the type with positive contrast.

[0045] The fig. 8 is a view similar to that of fig. 2 except that, in order to suppress the parasitic reflections and thereby improving the display contrast, is placed above the display cell transparent TOLED 20, on the side of the observer 4, the circular polarizer 38 which is composed of the second absorptive polarizer 40 and of the first quarter wavelength plate 42. On the other hand, the second quarter-wave plate 48 is placed under the cell transparent display TOLED 20. The second quarter-wave plate 48 is parallel to the first quarter-wave 42 or disposed at 90° with respect to the first quarter-wave plate 42. The assumed that the axis of transmission of the second absorptive polarizer 40 and the axis of reflection of the reflective polarizer absorbent 34 are perpendicular.

[0046] Therefore, ambient light 46 which enters the display assembly 1 according to the invention is linearly polarized by the second polarizer absorbent 40, and then circularly polarized by the first quarter-wave plate 42. When through the cell transparent display TOLED 20,46 circularly polarized ambient light is partially reflected by the upper and lower electrodes transparent 25,26 of the cell transparent display TOLED 20, the reflected light is phase-shifted, thereby transforming its circular polarization into circular polarization opposite direction of rotation. Therefore, when the reflected light passes back through the circular polarizer 38, is absorbed by the latter. In this way, it is possible to eliminate stray light that reflects on the electrodes 25,26 of the cell transparent display TOLED 20. Remaining The ambient light 46 passes through without changing the cell transparent display TOLED 20, and is linearly polarized as it passes through the second quarter-wave plate 48 in a direction perpendicular to the transmission axis of the second absorptive polarizer 40. It is assumed that the first and second quarter-wave plates 42 and 48 are parallel to each other. Upon passage of ambient light 46 through the liquid crystal display cell 60 reflective, the polarization direction of the ambient light 46 is rotated 90 °, so that it is ultimately absorbed by the absorbent 34 reflective polarizer.

[0047] The display is therefore in clear on a dark background. That is, the display assembly 1 is polishing. Indeed, in the areas switched from the liquid crystal display cell 60 reflective, ambient light 46 passes through the liquid crystal display cell 60 reflective without modification, such that it impinges on the reflective polarizer absorbent 34 according to a direction of polarization that is parallel to the axis of reflection thereof. Ambient light 46 is therefore reflected by the reflective polarizer absorbent 34, into and through the liquid crystal display cell 60 reflective without modification. Ambient light 46 is then circularly polarized by the second quarter-wave plate 48, and then through the circular polarizer 38 without modification and is perceptible by the observer 4.

[0048] Alternatively, it is possible to dispose the second quarter-wave plate 48 between the liquid crystal display cell 60 reflective and absorbent the reflective polarizer 34.

[0049] In a second embodiment of the invention, the first display device comprises the emissive display cell transparent organic light-emitting diode 20, and the second display device comprises a display cell free of reflective polarizers. The reflective display cell may be a cell electrophoretic display, a liquid crystal display cell dichroic or a display cell cholesteric liquid crystal (for example electronic ink or e-ink). In the example illustrated in fig. 9, the display cell is a cell reflective electrophoretic display 70 glued in the emissive display cell transparent organic light-emitting diode 20 by means of an adhesive layer 50. The cell electrophoretic display 70 includes a front substrate and a rear substrate 72 71 between which is arranged the optically active layer 73 which results from mixing two different colored powders, typically white and black. The front substrate 71 is a transparent substrate on the underside of which is formed an electrode. The rear substrate 72, it may be a printed circuit board on the upper face of which are structured the counter electrodes.

[0050] Alternatively, the display cell is reflective-type cholesteric liquid crystal and a circular polarizer is arranged above the display cell transparent organic light-emitting diode to absorb parasitic reflections produced by the electrodes thereof. Indeed, a liquid crystal display cell cholesteric has the feature reflect a circular polarization of the light. The circular polarization will therefore pass through the circular polarizer without being absorbed.

[0051] Clearly, the present invention is not limited to the embodiments just described and that various modifications and simple variants may be contemplated by those skilled in the art without departing from the scope of the invention as defined by the appended claims. In particular, in the case of a liquid crystal display cell dichroic, the presence of the dichroic dyes (black or color) dispersed in the liquid crystal is provided to absorb ambient light without the need for polarizers.

[0052]

Display assembly 1

Prime emissive display 2

Observer 4

Second reflective display 6

Transparent adhesive layer 8

Transparent display cell TOLED 20

Transparent substrate 21

22 encapsulating cover

Sealing 23 frame

Stack emitting layers 24

Transparent top electrode 25

Transparent lower electrode 26

Polariser absorbent 30

Adhesive layer 32

Absorbent 34 reflective polarizer

Adhesive layer 36

Polariser absorbent

Reflector b

Transmissive reflection Polariser c

Liquid acquisition layer d

38 circular polarizer

A second absorptive polarizer 40

A first quarter-wave plate 42

Metal mirror 44

Ambient light 46

The secondary quarter-wave plate 48

Adhesive layer 50

Cell reflective liquid crystal display 60 TN

Front substrate 61

62 rear substrate

Sealing frame 63

Leaktight chamber 64

Transparent electrodes 65a

Counter transparent electrodes 65b

Cell reflective liquid crystal display 600 VA

Electrophoretic display cell 70

Front substrate 71

Rear substrate 72

Optically active layer 73