MXPA97010151A - Optico panel, able to switch between statesreflector and transmi - Google Patents

Abstract

The present invention relates to a switchable optical device, characterized in that it comprises: a transparent optically active layer, having a first and second main surfaces, a first reflective polarizer disposed on the first main surface of the optically active layer; second primary surface of the optically active layer, and means for switching the panel between a reflecting state and a transmitting state, wherein the first and second reflective polarizers each comprise a multilayer group of pairs of layers of adjacent material, each of the pairs of layers, exhibiting a refractive index difference between the adjacent layers in a first direction in the polarizer plane, and exhibiting essentially no refractive index difference between the adjacent layers in a second direction in the plane of the polarizer and orthogonal to the first direction

Description

OPTICAL PANEL, ABLE TO SWITCH BETWEEN REFLECTOR AND TRANSMITTER STATES Field of the Invention The present invention relates to an optical device, which can be switched between a reflector state and a transmitter state. The invention also relates to a switchable window and an optical transillier screen, each comprising the said switchable optical device.

Background of the Invention Windows that can be switched between open (transmissive) and closed (non-transmissive) states, are commonly used in privacy windows and privacy curtains. The current technologies used in such windows are generally based on either optical absorption or optical scattering mechanisms. When an optically absorbent window is in the closed state, most of the light incident on the window is absorbed and the window is obscured opaque. This type of window may be undesirable, because of the excessive accumulation of heat, when exposed to sunlight. Some examples of such windows are electrochromic devices and REF: 26340 liquid crystal screen (LCD) blinds, having absorbent polarizers. A window employing an optical scattering mechanism causes the light to be scattered diffusely in a frontal direction, when it is in the closed state, such that the window looks white. As a result, the window does not substantially block incident light and has no utility in controlling energy in structures such as home or office buildings. Said window is described in U.S. Pat. No. 4,435,047. Optical displays, such as LCD screens, are widely used in laptop computers, portable calculators, digital clocks and similar devices. In conventional LCD screen assemblies, a liquid crystal panel, with a matrix electrode, is located between a front absorbing polarizer and a rear absorbing polarizer. On the LCD screen, there are portions of the liquid crystal that have the optical state altered by the application of an electric field. This process generates the necessary contrast to display the elements of image, pixels or information in polarized light. Typically, absorbent polarizers use dichroic dyes, which absorb light from a polarization orientation, rather than the orthogonal polarization orientation. In general, the transmission axis of the front polarizer is crossed with the transmission axis of the rear polarizer. The angle that crosses can vary between OS and 90s. The optical screens can be classified, based on the light source. The reflecting screens are illuminated by ambient light, which enters the screen from the front. Typically, a brushed aluminum reflector is placed behind the LCD screen assembly. This reflecting surface returns the light to the LCD screen assembly, while keeping the polarization orientation of the incident light on the reflecting surface. It is common to replace a rear light assembly with a reflector surface in applications where ambient light is insufficient to see. A typical backlight assembly includes an optical cavity and a lamp or other device that generates light. The backlight is powered by a battery, in the case of a portable screen, such as those of laptops. The screens that are made to be seen with both, ambient light and backlight are called "transflectoras". One problem with the transflective screens is that the typical rear light is not such an efficient reflector with the traditional brushed aluminum surface. Also, the backlight raises the polarization of the light and also reduces the amount of light available to illuminate the LCD screen. Consequently, the addition of back light to an LCD screen assembly, makes the screen less bright when viewed, under ambient light. A passive transponder can be placed between the LCD screen and the backlight on the transfer screen, to improve the brightness of the screen under both ambient light and backlight. A passive transponder is an optical device, which in a single state, operates as a transmitter and as a reflector. Unfortunately, passive translators tend to be insufficient in both cases, typically transferring only 30% of the light from the backlight, and reflecting 60% of the ambient light, while absorbing the remaining 10%. A third type of optical screen, incorporates a special backlight, which is on whenever the screen is operating, without taking into account the level of ambient light. This rear light can produce significant wear on the battery in a device with a portable screen.

Brief Compendium of the Invention The present invention provides a device comprising an optically switchable panel, comprising an optically active transparent layer and a secondary surface, a primary reflective polarizer disposed on the first surface and a secondary reflective polarizer disposed on the second surface. The device also comprises means for switching the panel between a reflecting state and a transmitting state. In one embodiment, the optically active layer comprises a liquid crystal device, which has a pair of transparent substrates in parallel thereto and defining a cavity therebetween. Each of the substrates has an internal surface, giving the cavity and an external surface. The liquid crystal device also contains a conductive material on the inner surface of each of the substrates and liquid crystal material confined in the cavity. In this mode, the switching means is an electronic control system, connected to the conductive material, for applying the voltage through the liquid crystal device. The conductive material may be comprised of a matrix of electrons localizable in a thin film on the inner surface of each of the substrates to form a pixelated liquid crystal device, or a continuous transparent conductive layer on the inner surface of each of the substrates The liquid crystal device preferably must be rotated nematic liquid crystal device. The primary and secondary reflective polarizers preferably each comprise a large number of pairs of layers of adjacent material, each of the pairs of layers, exhibiting a refractive index difference, between the adjacent layers in a first direction in the plane of the polarizer and not exhibiting any refractive index difference, between the adjacent layers in a second direction in the plane of the polarizer and orthogonal to the first direction. More preferably, the device should comprise a switchable optical panel including a rotated nematic liquid crystal device, comprising a first and second transparent plane substrates parallel defining a cavity therebetween. Each of these substrates has an outer surface and a lower surface, and a liquid crystal material confined in the cavity. The liquid crystal device further includes continuous transparent conductive layers on the interior surfaces of the substrates, a first reflective polarizer disposed on the outer surface of the first substrate and a second reflective polarizer disposed on the outer surface of the second substrate. The first and second reflector polarizers, each comprising a set of at least 100 pairs of layers, wherein each pair of layers comprises a birefringent layer, adjacent to another layer of polymer, which may be isotropic or birefringent. The device also includes an electronic control system connected to the conductive layers, such that the panel is electronically switchable between the between a reflecting state and the transmitting state. Alternatively this device may comprise a switchable optical panel, which comprises a liquid crystal device that includes a pair of parallel reflecting polarizers, defining a cavity therebetween, the reflective polarizers, each having an inner surface, giving the cavity and an external surface. The liquid crystal device also contains a liquid crystal material confined in the cavity and transparent conductive layers on the interior surfaces of the reflective polarizers. The device also includes an electronic control system, the conductive layers, in such a way that the panel is electronically switchable between the between a reflecting state and the transmitting state. The invention also provides a switchable window comprising a switchable optical panel, previously described and means for applying an electric field to the switchable optical panel to be able to switch the panel between an open state and a closed state. Each of the reflective polarizers in the switchable optical panel should preferably be a multilayer sheet, as described above. The window may also include, at least one transparent glass, located adjacent and parallel to the switchable optical panel.

The window can be placed in a "normally open" or "normally closed" configuration. In a normally open configuration, the window is transmitter in the absence of an electric field, while in the normally closed configuration, it is non-transmitting, due to the absence of an electric field. The invention further provides a window, which is mechanically switchable between an open state and a closed state. The window comprises a first transparent glass, having first and second main surfaces, a first reflective polarizer, arranged in the first transparent glass and at least one blind including a second transparent glass, a second reflective polarizer arranged in the second glass transparent, opposite to the second reflector polarizer. The window also includes means for rotating the louver to locate, either the optically active layer or the second reflector polarizer adjacent and parallel to the first reflector polarizer. The switchable window of this invention allows electronic or mechanical control of the (light) transmission of the window, for privacy purposes, light control and power control, in buildings, home and automobiles. The window does not absorb significant amounts of external light and thus prevents excessive window heating, characteristic of optically absorbent windows.

The invention further provides a transferable optical screen, which includes a liquid crystal display device comprising a front reflective polarizer, a rear reflector polarizer, and a liquid crystal pixelated device positioned therebetween, a backlight located proximate to the device of liquid crystal display, to illuminate the liquid crystal display device, an optical diffuser located between the liquid crystal display device and the backlight, a switchable transflector located between the optical diffuser and the backlight. The switchable transflector includes a non-pixelated liquid crystal display device having a front surface, located adjacent to the rear absorbent polarizer and to the rear surface, the liquid crystal display device has an alignment in a front direction associated with the surface front and an alignment in a posterior direction, associated with the rear surface and a reflective polarizer disposed on the rear surface of the liquid crystal display device not pixilated and close to the backlight. The optical screen further includes the means for electronically switching the transducer between a reflecting state and a transmitting state. The orientation of the polarization of the back absorbing polarizer is parallel to the alignment direction of the liquid crystal device. The reflective polarizers should preferably be and each one, a multi-layer sheet, as previously described. The switchable transflector is efficient in either the transmitting or reflecting state, allowing the transferable optical screen of the present invention, to use at least 80% of the available light for illumination of the LCD screen, regardless of the source of light. Because of the efficiency of the transflector, the backlight can be turned off, under normal ambient light conditions, with the effect of increasing battery time.

Brief Description of the Drawings FIGURE 1 is a schematic perspective view of a switchable optical device according to an embodiment of the present invention. FIGURE 2 is a schematic perspective view of a portion of a reflective polarizer, for use in the present invention. FIGURE 3 is a schematic perspective view of a switchable optical panel according to an embodiment of the present invention. FIGURE 4 is a schematic perspective view of the panel of FIGURE 3, after a magnetic field has been applied.

FIGURES 6a, 6b, 7a and 7b are schematic side views, illustrating the operation of the switchable window of FIGURE 5. FIGURE 8 is a schematic perspective view of a switchable window according to an embodiment of the present invention. FIGURE 9 is a schematic side view of a transferable optical screen according to an embodiment of the present invention. FIGURES 10 and 11 are schematic side views, illustrating the operation of the transferable optical screen of FIGURE 9. FIGURES 12 through 14 show the optical performance of the reflective polarizers in Examples 1 through 3, respectively.

Detailed description A device of this invention comprises a switchable optical panel, which includes a transparent optically active layer, having two main surfaces, the first reflective polarizer arranged on one of the main surfaces of the optically active layer and a second reflective polarizer, disposed on the other main surface. The device also includes means for switching the panel between a reflecting state and a transmitting state. FIGURE 1 shows a preferred embodiment of the device. The device 8 includes a switchable optical panel 10, in which the optically active layer comprises a liquid crystal device 12. The liquid crystal device 12 comprises a pair of transparent flat substrates 14 and 16 in parallel, superimposed and spaced apart. one from another. The periphery of the substrates is adhered and sealed with an adhesive sealant (not shown) to define a closed cavity. The cavity is filled with the liquid crystal material 18. A conductive material is provided on the inner surface of the substrates to allow voltage to be applied to the liquid crystal material 18. The conductive material can be in the form of transparent conducting conductive layers. and 22, as shown in FIGURE 1, or a thin film array of locatable electrodes, to form a pixelated liquid crystal device. The pixelated liquid crystal device is composed of thousands of small image elements, or "pixels", that can be made to be black, white or possibly gray. When used as part of a standard liquid crystal display (LCD), an image can be displayed through proper manipulation of the individual pixels.

The alignment layers 24 and 26 arranged on the inner surfaces of the transparent conductive layers cause the desired orientation of the liquid crystal material 18 at its interface with each substrate. The arrows 18 and 30 show how the molecules of the liquid crystal material 18 are aligned to approximately 90s rotated by the alignment layers 24 and 26 in the absence of a magnetic field. The liquid crystal display device should preferably be a rotated nematic liquid crystal (TN) display device, having an angle of rotation of between OS and 90s, preferably still between 803 and 90d. Alternatively, the liquid crystal display device may be a rotated super nematic liquid crystal display (STN) device, having a rotation angle of between 180d and 170S. Other types of LCD screens, such as ferroelectric LCD screens, can also be used. The substrates 14 and 16 may be made of plastic or glass materials, which are optically transparent, have a low birefringence and have a reasonable dimensional stability, under the conditions found in the manufacture and use of the switchable optical devices. In order to maintain a uniform space between the substrates, one of the well known methods of spacing must be used. For example, beads or fibers, between the substrates, may be incorporated in the cavity, or at least one substrate must be molded to form an integral spacing channel, as described in U.S. Pat. No. 5,268,782. Referring again to FIGURE 1, the reflective polarizers 32 and 34 are disposed on the outer surfaces of the substrates 14 and 16, respectively. In general, a reflective polarizer of this invention has the effect of separating the randomly polarized light into its polarized flat components. The randomly polarized light can be like the sum of two orthogonal polarized plane components of equal magnitude, having polarization states (a) and (b). Under optimum conditions, the reflecting polarizer transmits all the light having the polarization state (a) which is orthogonal to the polarizer's elongated direction, and reflects the light having a polarization state (b). The polarization orientation of the reflective polarizer 32 can be oriented parallel (mode e) or orthogonally (mode o) to the alignment direction of the liquid crystal 12 as shown by the arrow 30. The polarization orientations of the reflective polarizers 32 and 34 can be orthogonal to each other (crossed) or parallel. The device 8 preferably includes a birefringent compensation film (not shown), such as an optical retarder, ie a negative birefringent optical retarder. The birefringent compensation film is provided between the substrate 14 and the reflective polarizer 32 and / or between the substrate 16 and the reflective polarizer 34. Said films allow the device 8 to maintain the desirable optical characteristics over the visible wavelength range and at unusual angles. FIGURE 2 is a schematic perspective diagram of a segment of a preferred reflective polarizer 36. The figure includes a coordinate system 38 that defines the directions x, y and z. The reflective polarizer 36 is a group of multi-layer layers alternated in two different materials. The two materials are referred to as material "A" and material "B" in the drawings and in the description. Adjacent layers 41 and 43 of material "A" and material "B", comprise a pair of example layers 44. The pair of layers 44 exhibits a refractive index difference between layers 41 and 43, associated with the y direction. In a preferred embodiment of the device of the present invention, the first and second reflective polarizers, each comprising a multi-layer sheet of alternating layers in materials A and B, wherein each of the layers has an average thickness of no more than of 0.5 μm .. the layer of material A, adjacent to the layer of material B, comprises a pair of layers. The number of pairs of layers should preferably be in the range of about 10 to 2000, and more preferably even of 200 to 1000.

The multi-layer sheet is formed by a co-bond of materials A and B in the sheet, followed by a uniaxial elongation in the x-direction. The elongation radius is defined by the dimension after elongation divided by the dimension before elongation. The elongation radius should preferably be in the range of from 1: 1 to 10: 1, and more preferably from 3: 1 to 8: 1, and more preferably even from 4: 1 to 7: 1, ie 6: 1. The leaf does not lengthen appreciably in the direction y. Material A is a polymeric material, thus chosen, to exhibit a force-induced bierfrigence or a change in refractive index with elongation. For example, a uniaxially elongated sheet of material A will have a refractive index of n ^, associated with the direction of elongation (eg, n ^ = 1.88) and a different refractive index, nAy, associated with the transverse direction (eg example, nAy = 1.64). the material A exhibits a difference in the refractive index between the elongation and the transverse directions of (^ -nay) by at least 0.05, preferably by at least 0.10, and more preferably by at least 0.20. The material B is a polymeric material, so chosen that the refractive index nByf is substantially equal to nAy after the multi-layer film is elongated. Until the elongation is made, the value of ngx decreases preferably.

After elongation, the multi-layer sheet of this embodiment shows a large difference in the refractive index between the adjacent layers associated with the elongation direction (defined as? Nx-n-v-t-nt-j). Despite this, in the transverse direction the refractive index difference between the adjacent layers must be substantially zero (defined as? Ny = nAy-nBy). These optical characteristics cause the multi-layer group to act as a reflective polarizer which will transmit the polarization component of the randomly polarized light that is parallel to the transmission axis 40, shown in FIGURE 2. The portion of light that is transmitted by the reflector polarizer 36, is referred to as having a polarization state (A). The portion of light that does not pass through the reflective polarizer 36 has a polarization state (b), which corresponds to the extinction axis 42 shown in FIGURE 2. The extinction axis 42 is parallel to the elongation direction x. Therefore, the light polarized by (b) finds a refractive index difference? NX / which results in its reflection. The reflective polarizer should preferably be at least 50% reflective of the light polarized by (b) and more preferably still, at least 90%. The third differential of the refractive index, ? nz, it is important to control the off-axis reflectivity of the reflecting polarizer. For high radii of extinction of light polarized by (b), and for a high transmission of light polarized by (a), at large angles of incidence, it is preferred that? Nz = nAz-nBz < 0.5? Nz, more preferably, less than 0.2? Nx and more preferably even less than 0.1? Nx. The optical behavior and the design of said reflective polarizers is described in more detail in the pending application to be assigned U.S. Number 08/402041, registered on March 10, 1995, entitled "Optical film". Anyone normally prepared on the subject will be able to select the materials needed to achieve the desired refractive index ratios. In general, the material A can be selected from a semi-crystalline polymeric material, such as the semi-crystalline naphthalene dicarboxylic acid (PEN) polyester and its isomers (ie, 2,6-, 1,4-, 1 , 5-, 2,7-, and 2,3- of PEN). Material A can also be selected from other semi-crystalline polymer materials, such as polyethylene terephthalate (PET), polyethylene isophthalate (PEI) and PEN, PET and PEI co-polymers. As used here, the PEN co includes the PEN polymers and the PET includes the PET co-polymers. The material B may be an amorphous or semi-crystalline polymeric material, such as syndiotactic polystyrene (sPS) and its co-polymers, ie, co PEN, co-PET and the copolymers of styrene, which is polycyclohexanedimethylene terephthalate, commercially available by Eeastman Chemical Co. The co PEN described can be a mixture of beads or dragees where at least one component is a polymer based on naphthalene dicarboxylic acid and the other components are polyesters or polycarbonates, such as PET, PEN or co PEN. Materials A and B should preferably be chosen, to have similar rheological properties (ie, incorporation viscosity), so that they can be extruded. The reflective polarizer is prepared to extrude the materials A and B to form a multi-layer film and thus to orient the film, elongating it substantially in one direction (uniaxially) at a selected temperature and optionally followed by setting them to a certain temperature. The film can be left to relax dimensionally in a cross direction of elongation (orthogonal to the direction of elongation) in the range of natural reduction of dimensions of cross elongation (equal to the square root of the elongation radius) to a reduction null of the crossed elongation dimension (corresponding to a complete fixation). The film can be lengthened in the direction of the machine, as a length guide, or in the width direction, as a tensioner.

It may be apparent to anyone ordinarily trained in the subject, selecting a combination of process variables, such as the elongation temperature, the elongation radius, the heating temperature, and the relaxation of the cross elongation, to obtain a reflective polarizer that have the desired refractor index relationship. In a particularly preferred embodiment, a multi-layer sheet comprises a group of pairs of layers of materials A and B, previously described, wherein the group is divided into one or more segments of pairs of layers. Each segment is designed to have a maximum reflection of light, having a bandwidth, by having the pairs of layers, each with a combined gauge of approximately one-half the wavelength, at the center of the bandwidth in that segment. The combination of the segments that have different calibers in the pairs of layers, allows the reflector polarizer to reflect the light, having a relatively large bandwidth. For example, the multi-layer sheet may include ten segments having pairs of layers with a combined gauge ranging from 100 nm. and 200 nm. Each segment can include between 10 and 50 pairs of layers. This polarizer is capable of reflecting light, having wavelengths that oscillate between 400 and 800 -nm. Alternatively, the caliber of the pairs of layers can be continuously graduated between 100 and 200 nm. For an optical coverage of the wavelengths between 400 and 2000 nm. , the calibers of the pairs should be between 100 and 500nm. Although the multi-layer optical film described above is preferred for reflective polarizers, other reflective polarizers may be used, such as the MacNeille micro-structured polarizers and cholesteric polarizers, having a quarter-wave plate, fixed thereto. The reflective polarizers can be laminated to the LCD screen or attached to the LCD screen, or they can be mechanically secured to it. Referring again to FIGURE 1, an electric field can be applied to the liquid crystal material 18, through the conductive layers 20 and 22, using an electronic control system such as the electric source 19, through the charges 21 and 23. When the field is applied, the liquid crystal molecules over the entire area are reoriented and rotated, due to the dielectric anisotropy of the molecules. This behavior allows molecules to turn to polarized light at 902, when in the rotated mode and transmit the light without turning when they are in the unrotated mode. When used in combination with the reflective polarizers 32 and 34, this ability to rotate in polarized light provides the means for switching to the switchable optical panel 10, e between the reflecting state and the transmitting state. The reflectivity for the pairs of the identical reflector polarizers, of the optical panel, will be approximately double when switching from the reflector state to the transmitting state (ignoring the reflections of the front and back surfaces of the polymers and the conductive material). This value of the radius of reflection, changes very little with the quality of the reflector polarizer. In spite of this, the transmission radius of the reflector and transmitter states depends largely on the extinction value of the two polarizers. For polarizers with high leakage, say 50% of the high polarization extinction (perfect extinction is 100%), the transmission of the panel in the transmitting state will be 75% and in the 50% reflection state. The transmission radius for this "leaky" optical panel is only 1.5. Optical panels that have a transmission radius of 1.5, although they are not very useful as privacy blinds, can still provide significant energy control in exterior windows of buildings and automobiles. In good polarizers with an extinction of 99.9%, the transmission in the closed state will be only 0.1%, while in the transmitting state it will be roughly 50% transmitter, producing a transmission radius of 500. The extinction value, of a given polarization, depends on the optical bandwidth of interest for the user. For laser applications, thin bandwidths are sufficient. Bandwidths for privacy windows should cover at least the entire visible spectrum, while solar control windows should desirably cover Visibility and portions that are near the infrared of the spectrum (400 to 1200 nm.), The multi-layer film reflecting polarizer, described above, is capable of covering any of the above-mentioned bandwidths. To illustrate the concept of switching, FIGURE 3 shows a schematic perspective diagram of a switchable optical panel 46, in which a ray 48 of randomly polarized light containing both of the polarization states (a) and (b), collides with the reflective polarizer 50. Of the light contained in the ray 48, the light having a polarization state (b) (represented by the ray 52), is reflected, while the light that is in a polarization state (a ) (represented by ray 54), is transmitted by the reflecting polarizer 50. In the absence of an electric field, the liquid crystal 56, causes the polarization state of beam 54, to be rotated at an inclination of 902 and then it is transmitted by the reflective polarizer 58 (which is crossed with respect to the reflective polarizer 50). Therefore, a switchable optical panel containing the reflective polarizers 50 and 58 is substantially transmissive. This is usually referred to as a "normally open" state. Under optimal conditions, the optically switchable film is transmitter at 50%. Due to the residual absorption, the incomplete rotation of the polarization, the front and rear reflections and the reflections of the conductive layers (not shown), the transmission is generally in the range of 25 to 40%. When an electric field is applied to the switchable optical panel 46, as shown in FIGURE 4, the beam 48 is once again divided by the reflective polarizer 50, in a transmitted beam (shown as the beam 55) and in the beam. reflected beam 53. Despite this situation, the beam 55 stops through the liquid crystal liquid crystal 56, without rotating and is reflected by the reflecting polarizer 58. The reflected light, shown as ray 60, for again, through the liquid crystal 56 without rotating and is finally transmitted to the reflective polarizer 50. Therefore, the liquid crystal 56 is almost fully reflective in this state. The absorption losses in the conductive layers and in the reflective polarizers are very small, that is to say * 1-5%. It should be understood, that the behavior of the 46, is transposed (ie, the panel becomes a transmitter, when an electric hood is applied and in reflective in the absence of an electric field) locating the reflector polarizers 50 and 58 in parallel the one to the other, back in cross way. This is referred to as a "normally closed" state. Alternatively, it may be desirable to have a reflection of the switchable optical panel, being adjustable along the gray scale. Said adjustability can be achieved, by using a nematic liquid crystal display device and varying the applied voltage, to adjust the intensity of the transmitted light. Achieving this can be difficult, since a uniform gray scale requires a very precise uniform spacing of the substrates, and a uniform alignment of the liquid crystal molecules across a large area, as well as a uniform temperature of the electric field. Minimal variations in conditions will cause variations in reflectivity across the screen, creating a mottled appearance. Alternatively, an effective grayscale can be implemented using pixelated liquid crystal and switching, only in certain fractions of the pixels to give the appearance of gray (at a certain distance) to a human observer. In an alternative embodiment, the switchable optical panel comprises a pair of reflective polarizers, such as those described above, located in parallel and spaced apart from each other to form a closed cavity where the liquid crystal is confined. The reflective polarizers, act as a consequence, instead of the substrates of the liquid crystal, described above. It is to be understood that this embodiment includes the conductive layers, the alignment layers, the diffusion barriers and any other suitable element and that is associated with the substrates of the above embodiment. Other embodiments of this invention may include various birefringent materials in the optically active layer, other than the previously described liquid crystal device, including uniraxially oriented birefringent thermoplastics and liquid crystal devices of switchable, dispersed polymers, such as the one discovered in the US patent Number. 4,435,047. The means for switching the film between a reflecting state and a transmitting state are chosen, based on the characteristics of the birefringent material and the application in which the film is to be used. For example, the means for switching may include elongating the optically active layer to alter its birefringence or removing the optically active layer from among the reflective polarizers, to prevent the rotation of plane-polarized light. FIGURE 5 is a schematic diagram of a switchable window 62 of the present invention. The window 62 comprises a pair of transparent glasses 64 and 66 and a switchable optical panel 68, located between the glasses. The switchable optical panel 68, as previously described, preferably comprises a liquid crystal display device 70, which in turn comprises a para crystal clear substrate 72 and 74, in parallel, the liquid crystal material 76. confined in the cavity between the substrates and conductive layers 78 and 80, disposed on the interior surfaces of the substrates 72 and 74. The reflective polarizers 82 and 84 are disposed on the outer surfaces of the substrates 72 and 74, respectively and may be crossed or parallel with respect to each other. For the purposes of the subsequent discussion, the reflective polarizers 82 and 84 will be placed in parallel. The absorbent polarizers 86 and 88 will preferably be located on the surfaces of the reflective polarizers 82 and 84, as shown in FIGURE 5, with a polarization orientation of each absorbing polarizer, parallel to the polarization orientation of the reflecting polarizer transmission. in which it is located. The conductive layers 78 and 80 are connected to an electrical source 94 by means of the loads 90 and 92, or other types of means. The transparent glasses 64 and 66 can be made of glass or other transparent material, rigid and stable at temperature, suitable for use in windows. The reflective polarizers 82 and 84 preferably each comprise a multi-layer group of layers of alternating polymeric material, as discussed previously and shown in FIGURE 2. Absorbent polarizers 86 and 88 can be of any of the different types that are good. known in the art, such as dichroic polarizers, based on iodine or dyes oriented to polyvinyl alcohol. Alternatively, the absorbent polarizers may be included in the surface layer of the reflective polarizer. FIGURES 6a, 6b, 7a and 7b illustrate the operation of a switchable window 62. In FIGURE 6a, an electric hood is applied to the window, through the charges 90 and 92, causing the liquid crystal material 76, not to rotate, as described above. An exemplary randomly polarized outer light beam 96, such as sunlight, containing equal amounts of polarization states (a) and (b), passes through the glass 64 in its entirety. A portion (about 50% for a good reflective polarizer) of the beam 96, which is reflected by the reflective polarizer 82, is shown as the beam 98, having a polarization of (a), the light residue (having a polarization (b)), shown as the beam 100, passes through the absorbent polarizer 84 and the absorbent polarizer 88 to be able to observe inwards. Because the window is about 50% transmitter in this state, it is referred to as an "open" state. In the same state, an exemplary randomly polarized interior light beam 102 passes through the glass 66, as shown in FIGURE 6b. The beam component 102, having a polarization of (a) is absorbed by the absorbent polarizer 88 before it reaches the reflective polarizer 84. The light residue, shown as the beam 104 is polarized in (b) and is transmitted through the rest of the window. The absorbent polarizer 88 thereby absorbs the light from the interior which can otherwise be reflected back into the room by the reflective polarizer 84 and thereby avoid an unwanted mirror appearance. To switch the window 62 to the reflective ("closed") state, the electric field is removed in such a way that the liquid crystal material 76 reverts to a rotated configuration. In this state, shown in FIGURE 7a, an exemplary beam 106 of outside light is reflected at about 50% by the reflective polarizer 82, as described for the "open" state. The reflected light is shown as the beam 108 having a polarization of (a), the light residue, shown as the beam 110 has a polarization of (b), is transmitted by the absorbing polarizer 86, but is rotated to a polarization (a) by the liquid crystal 70. The resulting light is reflected by the reflective polarizer 84 and rotated again by the liquid crystal 70 and transmitted by the absorbent polarizer 86, the reflective polarizer 82 and the glass 64, back towards the outside. Referring to FIGURE 7b, the polarization component (a) of the beam 112 of the inner light is absorbed by the absorbent polarizer 88, while the polarization component (b) (shown as the beam 114) is absorbed by the absorbing polarizer 86. A window 62 in the "closed" state, with this it has a mirror-like appearance, an observer from the outside with daylight and dark for an observer from the inside. In another embodiment, a switchable window 114 is illustrated in FIGURE 8. The window includes a blind 116, a transparent glass 118 and a reflective polarizer 120. The blind 116 includes a transparent glass 122 with a birefringent layer 124, a one side of the reflector polarizer 126 on the other side. The birefringent layer 124 should preferably be a polymeric sheet, such as PET. For a higher transmission, the blade will be an achromatic wavelength retarder on an LCD screen. In any event, the birefringent layer 124 must be oriented for maximum transmission. The reflector polarizer 120 and the reflector polarizer 126 are crossed.

The blind 116 is rotatably mounted at a pivot point 123, for example, to the window frame, such that the window can be set to "open" or "closed" form. Appropriate means for rotation include manual or motorized movements, ie, like Venetian blinds. Three identical blinds are shown in FIGURE 8, which are shown at a sufficient distance, so that they can rotate freely, but are capable of forming a continuous panel when they are mechanically closed. The switchable window of this invention may include either only one shutter or a plurality of shutters. The transparent glass 118, having a reflective polarizer 120 on its surface, is held in a fixed position. In an example of an "open" position, the blind is rotated in such a way that the birefringent layer 124 is adjacent and parallel to the reflective polarizer 120. In this position, the birefringent layer 124 is located between the reflective polarizers 120 and 126. The randomly polarized light rays striking the window 114 are thus partially transmitted and partly reflected due to the rotation of plane polarized light by the birefringent layer 124, in the same manner as described in the previous window mode switchable. In the corresponding "closed" position, the shutter 1165 is rotated in such a way that the reflecting polarizer 120 is adjacent and parallel to the reflective polarizer 126 and the birefringent layer 124 moves away from the front of the reflective polarizer 120. In this position, the birefringent layer 124 is not in a position that affects the rotation of the plane polarized light, transmitted by the reflective polarizers 120 and 126. Because the reflective polarizers 120 and 126 are crossed, the plane-polarized light, transmitted by the reflector polarizer, is reflected by the other reflective polarizer, achieving a substantially reflective window when viewed either from the inside or outside. Optionally, at least one absorbent polarizer may be placed in the interior (observer side) of the reflective polarizer 120 or between the reflective polarizer 126 and the glass 122, or both. The polarization orientation of the absorbing polarizer is parallel to the polarization orientation of the reflective polarizer adjacent to it. The absorbent polarizer provides antireflective properties as described in the previous modality. A particular feature of this mode is that, whether the window is in an "open" or "closed" state, the blinds are always physically closed to form a continuous panel. This feature gives the window a good transmission from any angle of observation and provides better thermal insulation if the windows were physically open. FIGURE 9 is a schematic diagram of a transflectoral optical screen28, including a liquid crystal display (LCD) 130, a backlight 132, an optical diffuser 134 and a switchable transponder 136. Typically, the transflectoral optical screen28 in its The whole will be flat and rectangular in a plan view, as seen by the observer 129 and will be relatively thin, in view of cut, with the components very close together. The optical screen 128 also includes electronic means (not shown) for switching the switchable transponder 136 between a reflecting state and a transmitting state, such as the electrical source and the loads, previously described. The LCD screen device 130, which is of a well known construction, includes an absorbent polarizer 138, a back absorbent polarizer 140, and a pixellated liquid crystal panel 142. The liquid crystal display device is designed to deploy information and images through pixel areas, which can be switched on or paid by a matrix of locatable electrodes, in a way that is already well known by art. The backlight 132 may be an electroluminescent panel, a cold cathode fluorescent lamp in a reflective behavior or fixed to a light guide. The backlight must be of low absorbency and be diffuse. The optical diffuser 134 is typically a polarization conservation sheet, such as transparent spherical particles in a non-birefringent base film. If the diffuser does not retain the polarization, more light will be absorbed by the back absorbing polarizer 140. The switchable transponder 136 includes an optional reflective polarizer 144, a non-pixelized liquid crystal display device 146 and a reflective polarizer 148 the orientation of the polarization of the reflective polarizer 144 (if used) should be parallel to the orientation of the polarization of the rear absorbent polarizer 140. The liquid crystal display device comprises a front substrate 150 and a rear substrate 152 storing the liquid crystal material 154. The non-pixelated liquid crystal display device also includes continuous transparent conductive layers 156 and 58, which allow the entire area of the switchable transponder 136 to be electronically switched between a reflecting state and a transmitting state in the manner previously described. The liquid crystal display device 146 also includes alignment layers (not shown), which provide an alignment in the front direction associated with the front substrate and a subsequent alignment direction associated with the subsequent substrate. The reflective polarizers 144 and 148 will each preferably be a multi-layer group of alternating layers of two different materials, as previously described in relation to FIGURE 2. Most preferably, the reflective polarizers 144 and 148 each comprise a group of alternating layers of PEN and co PEN in the configuration described above. In general, it is intended that the switchable transponder 136 be transmitter when the liquid crystal display device 130 is illuminated by the backlight 132. When the backlight 132 is off, the liquid crystal display device 130 will be seen with the ambient light, the switchable transflector 136 becomes a reflector, in such a way as to increase the brightness and contrast of the screen. The operation of the transflectoral optical screen 28 is illustrated in FIGURES 10 and 11. In a preferred mode of backlighting of the transflectoral optical screen 28, shown in FIGURE 10, an electric field is applied to the transflector 135 and the reflective polarizers 144 and 148 They are parallel. An exemplary beam 164, of the randomly polarized light, containing the polarization states (a) and (b) is produced by the backlight 132. The portion of the beam 164 having the bias (b) is transmitted without the rotation of the switchable transflector 136, because the electric field has been applied to the switchable trans- fector "tears" the liquid crystal material therein and the polarization orientations of the reflector polarizers44 and 148 are parallel. The transmitted light, shown as the beam 168, passes through the diffuser 134 and has the correct polarization to be transmitted by the back absorbent polarizer 140. Meanwhile, the portion of the beam 164, having a polarization (a), shown as the beam 166, is reflected by the reflector polarizer 148 and returned to the backlight 132, where it is scattered and depolarized. This light will re-emerge from the backlight 132, as well as the beam 170, which will be partially transmitted and partially reflected by the switchable transponder 136. With the repeated reflections and depolarizations in this manner, a large percentage of the light coming from the backlight 132 is eventually recycled and so on, it goes through the 136 with the correct polarization. It should be noted that the reflective polarizer 144 is not required in the switchable transponder 136, if the liquid crystal panel 146 is completely and optically inactive in the on state (ie, all the light transmitted by the reflective polarizer 148 is not rotated). In spite of this, the liquid crystal panel 146 remains somewhat birefringent, when an electric field has been applied to it, then some components in general of the visible light transmitted by the switchable transponder 136 will have the wrong polarization, with respect to the back absorbing polarizer 140. In this case, the reflective polarizer 144 needs to reorient those components through the recycling process previously described in such a way that they are not absorbed by the back absorbent polarizer 140. In an ambient light mode of the same transflective optical screen, shown in FIGURE 11, the backlight 132 is off, and no electric field is applied to the switchable transponder 136. The switchable transponder 136 is thus, in a reflector state as previously described with reference to the FIGURE 4. An exemplary ray 172 of randomly polarized ambient light, is partially trans mitigated and partially reflected by the absorbent polarizer 138 (shown as beam 174, having a polarization state (b)) and will also be transmitted by the 140. Ray 174 is transmitted back, through the LCD screen 130, creating a bright pixel, to the eye of the observer. If beam 172 collides with the black pixel (not shown), beam 174 could be absorbed by the back absorbing polarizer 140. Thus, diffuser 134 needs to make the bright pixels appear from various viewing angles. As in the backlight mode, if the liquid crystal panel 146 rotates most of the light correctly, the reflective polarizer 144 can be eliminated. The apparent alternation between the reflective polarizer 148 and the rear absorbing polarizer 140 can cause a significant loss of brightness, due to the absorption of light in proximity to the black pixels, so it is important to make the diffuser 134 and the panel liquid crystal 146, as thin as possible. Accordingly, it is an advantage to eliminate the reflective polarizer 144, with the intention of placing the reflective polarizer 148 closer to the back absorbent polarizer 140. In a preferred embodiment, the switchable transflector comprises a pair of reflective polarizers that act as substrates to confine the liquid crystal material in these. This construction provides the smallest possible distance between the reflective polarizer 148 and the rear absorbent polarizer 140. The transferable optical screen of this invention may also be designed in a configuration in which the reflective polarizer 144 and the reflective polarizer 148 are crossed, or wherein the rear absorbent polarizer 140 and the reflective polarizer 148 are crossed if the reflective polarizer 144 is not used. In this case, the switchable transmitter is not electrically powered in the backlight mode and powered in the ambient light mode. In the optical screen of FIGURE 9, the switchable transflector may be laminated or otherwise similarly adhered or fixed to the backlight and / or the back of the LCD device. Laminating the switchable transflector to the backlight eliminates the air portion between these and thus reduces surface reflections, which might otherwise occur at the limit of the switchable transflector and the gap. These reflections reduce the total transmission of the desired polarization. The invention will now be illustrated by the following examples. All measurements are approximate.

Example 1 A reflective polarizer for use in the present invention was constructed. The reflective polarizer comprised two 601-layer polarizers, laminated together with an optical adhesive. Each of the 601-layer polarizers was produced by extruding the membrane and orienting the membrane in a grocer two days later. A polyethylene naphthalate (PEN) with an intrinsic viscosity of 0.5 dl / g (60 wt.% Phenol / 40 wt.% Dichlorobenzan) was sent to an extruder at a speed of 34 kg. per hour and one coPEN (70 mol%, 2.6 NDC (naphthalene dicarboxylic acid), and 30 mol% DMT (dimethyl terephthalate) with an intrinsic viscosity of 0.55 dl / g (60% phenol weight / 40% dichlobenzane weight) were sent to another extruder at a rate of 34 kg per hour.The PEN was in the surface layers, which were extruded as thin outer layers through the same feed block and are bent in both layers, internal and external, by the multipliers.The internal and external surfaces, comprised 8% of the polarizer caliber.The feeding block method was used to generate 151 layers, which were passed through two multipliers, producing an extrusion of 601 US Pat. No. 3,565,985 describes similar extrusion multipliers.All elongations were made in the shopkeeper.The film was preheated to about 140 C about 20 seconds and thrown in a cross section at a radius of 4.4 to a degree of 6% per second. The film was then relaxed to 2% of its maximum width in a heating oven set to 240dC the caliber of the finished film was 46 μm. The transmission of a 601-layer film is shown in FIGURE 12. The curve a shows the transmission of the polarized light in (a) at an incidence of 602. The non-uniform transmission of the polarized light in (a) is denoted. a both normal and 602 incidences. It also denotes the non-uniform extinction of the polarized light in (b) in the visible range (400 to 700 nm.) shown by the curve c.

Example 2 Another reflective polarizer for use in the present invention was constructed. The reflective polarizer comprised a polarizer of 603 and was made in a line to make sequential flat films, through an extrusion process. A polyethylene naphthalate (PEN) with an intrinsic viscosity of 0.47 dl / g (60% weight of phenol plus 40% by weight of dichlorobenzan) was delivered by an extruder at a speed of 34 kg. per hour. The coPEN was a 70% molar co-polymer, 2.6 NDC of dicarbonoxylate naphthalene methyl ester, 15% molar of DMT (dimethyl terephthalate) and 15% molar of isophthalate with ethylene glycol. The feed block method was used to generate 151 layers. The power block was designed to produce a gradual distribution of the layers with a gauge radius in the optical layers of 1.22 for the PEN and 1.22 for the coPEN. This optical group was multiplied by two sequential multipliers. The nominal multiplication radius of the multipliers was 1.2 and 1.4, respectively. Between the final multiplier and the die, surface layers were added, composed of the same coPEN, described above, delivered by a third extruder at a speed of 48 kg. Per hour. The film was preheated to about 150 c for about 30 seconds and thrown in a direction transverse to an initial draft radius of 6 to a degree of 20% per second. The caliper of the finished film was 89 μm. FIGURE 13 shows the optical performance of this reflective polarizer. Curve a shows the transmission of polarized light in a non-elongated direction at a normal incidence, curve b shows the transmission of light, having the incidence and polarization both flat, parallel to the non-elongated direction at an angle of 502 of incidence and curve c shows the transmission of polarized light in an elongated direction at a normal incidence. It is denoted that there is a high transmission of polarized light in a non-elongated direction. The average transmission of the curve to about 400 to 700 nm. It is 87%. It is also denoted that there is a high extinction of polarized light in an elongated direction, in the visible range (400 to 700 nm.) shown by curve c. The film had an average transmission of 2.5% for curve c, between 400 and 700 nm. The RMS color percentage for curve b is 5%. The RMS color percentage is the square root of the transmissibility over the wavelength range of interest.

Example 3 Another reflective polarizer for use in the present invention was constructed. The reflective polarizer comprised an extruded film, containing 481 layers, made by extruding the mold membrane in one operation and then orienting the film in a laboratory film elongation apparatus. The feed block method was used with a 61-layer feed block and three multipliers (2x). Thin surface layers were added between the final multiplier and the die. A polyethylene naphthalate (PEN) with an intrinsic viscosity of 0.47 dl / g (60 wt.% Phenol / 40 wt.% Dichlorobenzan) was delivered by an extruder at a speed of 11.4 kg. per hour. A modified polyethylene cyclohexane dimetane terephthalate glycol (PCTG 5445 DE Eastman) was delivered by another extruder at a rate of 11.4 kg. per hour. Another PEN jet from the above-mentioned extrusion was added as surface layers at a speed of 11 kg. per hour. The mold membrane was 0.2 mm. wide and 30 cm long. The membrane was oriented uniaxially using a laboratory elongation device that uses a pantograph to hold a section of the film and thus lengthen in one direction to a uniform range while allowing itself to relax freely in the other direction. The sample of the loaded membrane was 5.40 cm. wide (in the direction without shrinkage) and 7.45 cm. long between the pantograph tensioners. The membrane was charged to an extender at 1002C and heated to 1352C for 45 seconds. The elongation was then started at 20% per second (based on the original dimensions) until the sample was elongated at a ratio of 6: 1 (based on measurements from tensor to tensor). Immediately after elongation, the sample was cooled by the ordinary air at room temperature. In the center of the sample, a relaxation factor of 2.0 was found.

FIGURE 14 shows the transmission of this multi-layer film wherein curve a shows the transmission of polarized light in a non-elongated direction at a normal incidence, curve b shows the transmission of light, having flat incidence and polarization both, parallel to the direction not elongated at an angle of incidence 602 (polarized light in p) and curve c shows the transmission of polarized light to the direction of elongation with a normal incidence. The average transmission of the curve to about 400 to 700 nm. It is 89.7%, the average transmission for curve b from 400 to 700 nm. it is 96.9% and the average transmission for the curve c from 400 to 700 nm. it is 4.0%. The RMS color percentage for curve a is 1.05% and the RMS color percentage for curve b is 1.44%.

Example 4 A switchable optical panel of the present invention was prepared to fix a reflective polarizer, comprising a multi-layer optical group, as previously described herein to frame a pixelized STN liquid crystal display, lacking the absorbent polarizers. The reflective polarizers were secured to the LCD screen with an adhesive tape along the corners of the polarizers. The orientation of the polarization of each one was located parallel to the direction of alignment of the liquid crystal in each substrate, in such a way that the maximum visible extinction was obtained in the reflection mode when the reflective polarizers are crossed. The optical panel was placed in ambient light and visually monitored. With no applied voltage, the panel appeared to be partially transparent, when voltage was applied, the panel switched to a mirror appearance.

Example 5 A mechanically switchable window was constructed in the following manner: a birefringent film of H of wavelength at 560 nm. of Polaroid Corp. was laminated to the side of a transparent glass plate of 10 x 10 x 0.16 cm. A first reflector polarizer prepared as in Example 1 was laminated on the opposite side of the plate. A second reflector polarizer of the same construction as the first was laminated to a second transparent glass plate. The plates were fixed in parallel slots and were switched manually. The switchable window, was evaluated by measuring the transmission of light through the window in both positions, "closed" and "open". The light source was a 12 volt tungsten halogen lamp. The intensity of the transmitted light was measured with an amorphous silica photodiode, which is sensitive only to visible light. In the "closed" position, the first plate was located parallel to the second plate with the birefringent film towards the front or furthest from the second plate. To switch to the open position, the first plate was flipped to 1802 in such a way that the birefringent film was backward, or closer to the second plate and between the two polarizers. Two control transmissions were also measured through 1) two glass plates without polarizers or birefringent film and 2) two polarizing reflectors with polarization orientations in parallel, each laminated to the glass plate. The second control was with the intention of simulating the presence of a birefringent film perfectly, between the two polarizers. The results are shown in the following table: The transmission of the two glass plates was reported as 100%. The relative percentage of transmission for 1), 2) and 4) were compared with that value. The window proved to be mechanically switchable between 5% and 35% transmission. For the theoretically perfect birefringent film, as demonstrated in position 4, the transmission was 42%. It is noted that, with regard to this date, the best method known by the requested, to carry out the present invention, is that which is clear from the present, discovering the invention. Having described the invention as above, the content of the following is claimed as property.

Claims (9)

2. A switchable optical device, characterized in that it comprises: a transparent optically active layer, having a first and second main surfaces; a first reflector polarizer disposed on the first main surface of the optically active layer; a second reflective polarizer disposed on the second main surface of the optically active layer; and means for switching the panel between a reflecting state and a transmitting state, wherein the first and second reflective polarizers each comprise a multi-layer group of pairs of layers of adjacent material, each of the pairs of layers, exhibiting a refractive index difference between the adjacent layers in a first direction in the polarizer plane, and exhibiting essentially no refractive index difference between the adjacent layers in a second direction in the plane of the polarizer and orthogonal to the first direction.
3. A switchable optical device, according to claim 1, characterized in that the optical panel is a liquid crystal device, comprising transparent first and second plane substrates, in parallel, defining a cavity therebetween, each substrate having an outer surface and an inner surface facing the cavity, and a liquid crystal material, confined in the cavity. 3. A switchable optical device according to any of claims 1 and 2, characterized in that the difference of the refractive index between the adjacent layers in the first 15 direction, exceeds the refractive index difference between the adjacent layers in the second direction by at least 0.05.
4. 4. An optical device switchable in accordance with any of claims 1 and 2, characterized in that the first and second reflective polarizers each comprise a multi-layer sheet of alternating layers of the first and second material, wherein the first material exhibits a birefringence induced and the leaf is uniaxially elongated.
5. A switchable optical device according to claim 4, characterized in that the first material is a polyester of dicarboxylic acid naphthalene and the second material is selected from the group of polystyrene, polyethylene naphthalate, polyethylene terephthalate and cyclohexanedimethylene terephthalate.
6. A switchable optical device according to claim 4, characterized in that the first material is selected from the group of polyethylene naphthalate, polyethylene terephthalate, polyethylene isophthalate and co-polymers thereof.
7. A switchable optical device according to any of claims 1 and 2, characterized in that a refractive index difference between the adjacent layers in a third direction, orthogonal to the plane of the polarizer is less than about 0.2 times of the refractive index difference between the adjacent layers in the first direction.
8. 8. A switchable optical device according to claim 1, characterized in that the optical panel comprises: a rotated nematic liquid crystal device, comprising transparent first and second substrates in parallel, defining a cavity therebetween, each substrate having an outer surface and an inner surface facing the cavity, and a liquid crystal material, confined in the cavity; and continuous transparent conductive layers disposed on the interior surfaces of the substrates.
9. 9. A switchable window, characterized in that it comprises: a transparent glass, having first and second main surfaces; a first reflector polarizer disposed in the first transparent glass; at least one shutter comprising a second transparent glass, a second reflective polarizer disposed in the second transparent glass, a birefringent layer disposed on one side of the second transparent glass, opposite the second reflective polarizer; and means for rotating the shutter and locating either the birefringent layer or the second reflective polarizer, parallel and adjacent to the first reflector polarizer, such that the window is mechanically switchable between a reflector state and a transmitter state; wherein the first and second reflector polarizers each comprise, to a multi-layer group of pairs of layers of adjacent material, each of the pairs of layers, exhibiting a difference of refractive index between the adjacent layers in a first direction in the polarizer plane, and exhibiting essentially no difference in refractive index between the adjacent layers in a second direction in the plane of the polarizer and orthogonal to the first direction. A transferable optical screen, characterized in that it comprises: a liquid crystal device comprising an absorbent polarizer, a rear absorbing polarizer and a pixelated liquid crystal panel, located therebetween; a backlight to illuminate the liquid crystal display device; an optical diffuser located between the liquid crystal display device and the backlight; and a switchable transflector located between the optical diffuser and the backlight, the switchable transflector comprising: a non-pixelated liquid crystal display device comprising: a front substrate, facing the optical diffuser and a subsequent substrate, in parallel and defining a cavity between these, each of the substrates having an inner surface facing the cavity and an outer surface; a conductive material on the inner surface of each substrate; and a liquid crystal material confined in the cavity; the non-pixelated liquid crystal display device having a front alignment direction associated with the front substrate and a subsequent alignment direction associated with the subsequent substrate; a reflective polarizer disposed in the rear substrate of the liquid crystal display device not pixilated and close to the backlight; and means for electronically switching the transducer between a reflecting state and a transmitting state wherein the orientation of the polarization of the rear absorbing polarizer is parallel to the front alignment direction of the non-pixelated liquid crystal display device; wherein the first and second reflector polarizers each comprise, to a multi-layer group of pairs of layers of adjacent material, each of the pairs of layers, exhibiting a difference of refractive index between the adjacent layers in a first direction in the polarizer plane, and exhibiting essentially no difference in refractive index between the adjacent layers in a second direction in the plane of the polarizer and orthogonal to the first direction. OPTICAL PANEL, ABLE TO SWITCH BETWEEN STATES REFLECTOR T TRANSMITTER Summary of the Invention A device, comprising a switchable optical panel and means for switching the panel between a reflecting state and a transmitting state. The switchable optical panel includes an optically active layer, having first and second main surfaces, a first reflective polarizer, disposed on the first main surface and a second reflective polarizer, disposed on the second main surface. The optically active layer preferably comprises a liquid crystal display device and the switching means preferably comprise an electronic control system for applying the voltage across the liquid crystal display device. The invention also includes a switchable window and a transflective optical screen.