Patent Publication Number: US-2012044131-A1

Title: Flat screen and household appliance equipped therewith

Description:
The present invention relates to a flat screen, which can be used in particular but not only in a household appliance. Such a screen can be used to display operating information relating to the household appliance for the user but can also display images that are not or at least not necessarily related to the operation of the household appliance, for example television programs or internet pages. 
     Many embodiments of flat screens with an LCD matrix display element are known. Conventionally they comprise an image field with two transparent panes, between which a liquid crystal layer is enclosed. Polarizing and in some instances color-filtering layers are configured on the panes. One of the panes also has a matrix of transparent electrodes to which a voltage can be applied individually, opposite which there is a similarly transparent ground coating on the other pane. The panes are enclosed at their edges by a generally metallic frame, which holds the panes together and protects their edges against damage due to impact from a lateral direction. 
     In practice the display element must also be protected from external influence from a direction perpendicular to the panes, since any deformation of the panes which results in a change in the thickness of the liquid crystal layer impairs image representation. The image field and frame are therefore often additionally protected by a cover pane extending in front of the frame. Such a cover pane can for its part be provided with a transparent, electrically conductive coating, to shield an observer from high-frequency electromagnetic emissions from the flat screen. 
     The air gap that results between the front pane of the image field and the cover pane is conventionally as least as large as the projection of the frame beyond the front pane. A large distance between image field and cover pane means however that reflections on the surfaces of the cover pane appear with a clear offset compared with the image displayed on the image field for an observer looking at the screen from a direction at an oblique angle to the surface normal, thereby making it difficult to identify the displayed image. 
     So that reflections on the inside and outside of the cover pane cannot be seen separately, it is desirable per se to use a cover pane of small material thickness. Such a thin cover pane may curve due to inner tensions of the flat screen or its environment, with the result that the displayed image appears distorted to an observer. 
     One object of the invention is to improve a flat screen of the type mentioned in the introduction so that image errors caused by a cover pane are minimized. 
     The object is achieved in that in a flat screen with an LCD matrix display element, which comprises a frame and an image field which is surrounded by the frame and set back behind a front face of the frame, and with a cover pane disposed in front of the LCD matrix display element, the cover pane features a core zone opposite the image field and a peripheral zone opposite the frame and a flat inner surface of the core zone facing the image field runs in an intermediate space between the image field and a plane defined by the front face of the frame. In other words the core zone of the cover pane projects into the frame towards the image field, with the result that the air gap that is conventionally present between cover pane and image field becomes narrower. The visible offset between the actual image on the image field and a reflection of said image on the inner surface of the cover pane is reduced as the distance is reduced, so that the reflection can scarcely be seen even when observed at a wide angle. 
     According to a first elementary embodiment the entire core zone of the cover pane is displaced towards the image field compared with the peripheral zone, for example by deep drawing. Image distortion is then concentrated on a transition region between the core and peripheral zones, in which the cover pane is not flat. 
     In order to avoid such distortion as well, it is advantageous if the peripheral zone and the core zone of the cover pane have a common outer surface. 
     To shield electromagnetic emissions from the matrix display element, the cover pane can advantageously be provided with an electrically conductive, transparent coating, in particular of indium tin oxide. 
     To shield electromagnetic emission effectively, low-resistance grounding of the transparent coating is important. To this end a metallic contact strip can be positioned on the conductive coating—preferably in the peripheral zone. 
     The coating can be applied to the inner surface of the cover pane so that it is protected from external influences. 
     In this instance the grounding can be established simply by pressing the contact strip against the similarly metallic frame of the display element. 
     For production purposes it can be expedient to apply the coatings to the outer surface of the cover pane. This allows a continuous coating to be applied in particular to an extensive substrate, the substrate then to be separated into individual cover panes and the peripheral zones then to be formed on these cover panes. 
     In this embodiment at least the core zone of the cover pane preferably remains free of the contact strip, so that it does not restrict the visible image field. 
     A metal tape running around the cover pane can also serve to improve shielding, being in conducting contact with the contact strip. 
     In particular the metal tape can feature a limb lying flat on the contact strip. However the contact strip itself can also extend over a peripheral surface of the cover pane to touch large areas of the metal tape there. 
     If an air gap is provided between the inner surface of the core zone and the image field, said air gap preferably has a width of 50 to 250 μm, particularly preferably 100 to 200 μm to prevent the formation of Newton&#39;s rings between the opposing surfaces of image field and cover pane. 
     The subject matter of the invention is also a household appliance, in particular a refrigeration appliance, having a flat screen of the type described above. Such a flat screen can be positioned in particular in the door of the household appliance, with an outer face of the door being formed in its entirety by a glass plate, behind which the flat screen is located. 
    
    
     
       Further features and advantages of the invention will emerge from the description which follows of exemplary embodiments with reference to the accompanying figures, in which: 
         FIGS. 1 to 8  each show a partial section through a flat screen according to different embodiments of the invention; and 
         FIG. 9  shows a schematic perspective view of a refrigeration appliance having an incorporated flat screen. 
     
    
    
       FIG. 1  shows a cross section through the peripheral region of a flat screen according to a first embodiment of the invention. The side from which the screen can be observed is at the top in  FIG. 1 ; a scatter surface irradiated by a light source, by which the screen is evenly illuminated from the rear and bottom, is not shown. Two panes  1 ,  2  made of mineral glass or a crystal-clear plastic material are provided with the standard coatings for LCD display elements, such as polarization coatings (not shown), individually activatable transparent pixel electrodes  3  and an unstructured ground electrode  4 . The surface covered by the pixel electrodes  3  corresponds to the region of the screen that can be used to display images, also referred to here as the image field. In an intermediate space, which is kept free by spacers  5  and sealed at the sides, between the glass plates  1  is a liquid crystal solution  6 . The rectangular glass plates  1 ,  2  are enclosed along their edges by a continuous frame  7 , which is made up of metallic U-profiles. 
     A cover pane  8  made of a rigid, moderately elastic thermoplastic plastic material is held pressed against the outside of the frame  7  by a peripheral plastic enclosure  9 . The cover pane  8  is deformed by deep drawing, so that a core zone  10  of it engages in the frame  7  and the distance between the pane  1  and an inner surface  14  of the core zone  10  is much smaller than between the pane  1  and an outside  15  of the frame  7  facing away from the pane  1 . A shielding conductive transparent coating  13  made of indium tin oxide on the inside of the cover pane  8  is provided in a non-deformed peripheral zone  11  of the cover pane  8  extending around the core zone  10  with a metallic contact strip  12  that has been printed, chemically deposited or applied in some other suitable manner with little electrical transfer resistance to the coating  13 . This contact strip  12  rests on the frame  7  due to the pressure of the enclosure  9 , thereby ensuring effective grounding of the conductive coating  13  on the frame  7 . 
     Since in this embodiment there is practically no intermediate space between the cover pane  8  and the glass plate  1  and the width of such an intermediate space may be in the region of a few μm over large areas, it is expedient to roughen the inner surface of the cover pane  8  facing the pane  1  before applying the conductive coating  13 , for example by impressing an irregular pattern, to prevent Newton&#39;s rings becoming visible. 
     In order to prevent image distortions resulting from the unevenness of the cover pane  8  in the transition region between core zone  10  and peripheral zone  11  becoming perceptible, it is expedient with this embodiment if the entire exposed surface of the plates  1 ,  2  within the frame  7  is not used for image display. Also the enclosure  9 , as outlined with a broken line, can be made so wide that it covers the peripheral zone  11  and the uneven transition to the core zone  10  completely. 
       FIG. 2  shows a second embodiment of the flat screen in a section like the one in  FIG. 1 . The matrix display element with the glass plates  1 ,  2  and the frame  7  surrounding them is identical to the one in  FIG. 1  and is not described again. As in  FIG. 1 , a cover pane  18  has a peripheral zone  11  pressed onto the outside of the frame  7  and a core zone  10  engaging in the frame  7  but both have a common flat outer surface  19  and the material thickness of the core zone  10  is greater than that of the peripheral zone  11 . The cover pane  18  is therefore much more rigid in its core zone  10  than the cover pane of a conventional flat screen of the same incorporated depth, so that despite the reduced distance it can protect the matrix element effectively against damage due to impact by an object. The wall thicknesses of the core and peripheral zones  10 ,  11  are tailored to the dimensions of the frame  7 , to maintain an air gap  20  between the cover pane  18  and the outer glass plate  1  of typically around 100 μm width. Such a distance is sufficient to prevent the occurrence of Newton&#39;s rings but at the same time small enough for a reflection on the inside of the cover pane  18  not to be perceived as problematic by an observer. 
     In this embodiment, as in the one in  FIG. 1 , the cover pane  18  is covered completely on its inner surface with a shielding conductive coating  13 . In the peripheral zone  11  the coating  13  is completely covered by the metallic contact strip  12 , since the three-dimensional contour of the inside of the cover pane  18  makes it difficult to restrict the contact strip  12  to part of the width of the peripheral zone  11 . The contact strip  12  therefore also conceals the frame  7  and, in so far as the dimensions of this core zone  10  do not correspond exactly to the exposed surface of the glass plate  1  within the frame  7 , also a peripheral region of this. This is no longer problematic if the entire exposed surface of the glass plate  1  is not used to display images. 
     If the latter is the case, an embodiment as shown in  FIG. 3  is possible, in which the conductive coating  13  is restricted to the core zone of the cover pane  18 . To ground the coating  13  reliably, the contact strip  12  not only covers the entire peripheral zone  11  but also overlaps in a narrow peripheral region of the core zone  10  with the coating  13  applied thereto. 
     The latter embodiment has the advantage that it allows simple and economical production of the cover pane  18 , in that the conductive coating  13  is first applied continuously to a large-format plate of even thickness and the plate is then broken into pieces and a peripheral zone  11  is produced on each of these pieces by pressing or by abrasive processing, said peripheral zone  11  having a smaller material thickness than the core zone  10  that remains unprocessed. 
     If the exposed surface of the plate  1 ,  2  is used in its entirety to display images, it is not desirable for part of this surface to be covered by the contact strip  12 . An embodiment as shown in  FIG. 4  is then expedient, wherein the conductive coating  13  and with it the contact strip  12  is applied to the flat outer surface  19  of the cover pane  18 . To protect the coating  13  from wear, a scratch-resistant coating, known from eye glasses for example, can be applied to it. Such a coating may be omitted, if the entire flat screen, as illustrated in  FIG. 4 , is incorporated for its part behind a transparent pane  21  of an appliance housing. This pane  21  is expediently provided with a non-reflective coating, since otherwise, because of its distance of several mm from the matrix display element, reflections occurring at its surfaces could be perceived separately from the actual image by an observer looking at the screen from a direction away from the surface normal. 
     To establish a low-resistance electrical contact between the contact strip  12  and the frame  7 , as desired for effective electromagnetic shielding of the matrix display element, in a development shown in  FIG. 5  a thin metal tape  22  is wound around the outer edge of the frame  7  and the cover pane  18  and folded onto the outside of the cover pane  18 , where a limb  23  of the metal tape  22  is kept pressed against the contact strip  12  by the enclosure  9 . To ensure close electrical contact between the metal tape  22  and the frame  7  or contact strip  12 , an electrically conducting bonding agent can also be used, or the metal tape  22  can be held pressed against the outer edge of the frame  7  by a leaf spring  24 . 
     In the embodiment in  FIG. 6  the metal tape  22  not only has a limb  23  pressed against the contact strip  12  on the outside of the cover pane  18  but also a limb  25  held clamped between the enclosure  9  and a rear face of the frame opposite this. 
     According to a modification shown in  FIG. 7  a second limb  25  of the metal tape  22  can also be clamped between the peripheral zone  11  of the cover pane  18  and the front face of the frame  7 . This allows the thickness of the core zone  10  to be increased by the material thickness of the metal tape  22  whilst still maintaining the width of the air gap  20 , thereby making the cover pane  18  more rigid. 
     It is also possible to position the contact strip  12 , as illustrated in  FIG. 8 , not only on one of the main surfaces of the cover pane  18  but also along its narrow sides  26 . This allows low-resistance electrical contact to be established between the contact strip  12  and the frame  7  by means of a metal tape  22  which does not have an angled limb but simply extends along the outside of the frame  7  and the narrow sides  26  of the cover pane  18 . 
     The coating  13 , the frame  7  and where present the metal tape  22  connecting them together form a conductive flat shell, which is pulled over the matrix display element to shield a user from its emissions beyond the cover pane  18  and to reflect them in the counter direction or absorb them. 
     This shielding effect is also not impaired by openings in the shell, as long as these are smaller than the wavelength of the radiation to be shielded. Therefore for example the frame  7  does not have to touch the cover plate  18  over its entire periphery but for example grooves can be formed in the front face of the frame or local breaks can be formed in the contact strip  12 , through which an air exchange is possible between the air gap  20  and the environment. Therefore if moisture enters the air gap  20 , it can also quickly escape again and there is no sustained interference with the image display due to condensate deposited on the facing surfaces of the cover pane  18  and the plate  1 . 
       FIG. 9  shows a refrigeration appliance as one example of the application of the flat screen, the flat screen being incorporated in the front face of the door  27  here. The entire front face of the door  27  is taken up by a glass plate  21 , which like the pane  21  in  FIG. 4  covers the entire flat screen. The glass plate  21  is printed in a non-transparent manner on its rear face, with the exception of a central blank space  28 , behind which the flat screen is positioned. The edges of this blank space  28  correspond precisely to the region of the plates  1 ,  2  used for the image display, so that the frame  7  and the peripheral zone  11  of the cover pane  18  and together with these also the contact strip  12  and where applicable the metal tape  22  are concealed.