Abstract:
The display of a notebook computer includes a privacy filter extending over the display screen to limit the viewing angle within which the image on the screen can be discerned. To allow for normal viewing when the privacy feature is not required, the privacy filter is movable from the screen by sliding into a cavity extending from one side of the screen to be rolled onto a roller or to be stored partly behind the screen. Alternately, a privacy filter is provided that is electrically switchable between a mode in which the viewing angle is limited and a mode in which normal viewing is permitted. The movable privacy filter includes closely spaced louvers surrounded by clear plastic or, alternately, a polarizer. The switchable privacy filter includes a liquid crystal device and a polarizer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to apparatus providing for private viewing of information displayed on the LCD screen of a notebook or laptop computer, and, more particularly, to a filtering device extending in front of an LCD screen, having an active mode in which a limitation is placed on the viewing angle through which data on the screen can be read and an inactive mode, in which the screen can be viewed normally. 
     2. Background Art 
     Laptop or notebook computers are often used in crowded, public places such as airplanes for writing personal or otherwise confidential information. Under such circumstances, there is generally a concern the a nearby person, such as the person in the next airplane seat, may be reading sensitive material. This concern keeps many people from using a laptop computer in many instances when its use would be particularly convenient. If the computer is used in this way, sensitive data may be stolen. 
     Privacy can be provided during the use of a laptop computer by restricting the viewing angle through which the screen may be viewed, so that only the person sitting directly in front of the screen can read the data written on it. This angle can be limited by holding a privacy screen across the front of the computer display screen, so that the display screen can only be viewed through the privacy screen. 
     FIG. 1 is a fragmentary cross-sectional view of a first type of privacy screen  10  placed in front of a computer display screen  12 . The privacy screen  10  includes a number of partitions  14  separated by transparent light channels  16 . The partitions  14  limit the angle at which light is transmitted through the privacy screen  10  to the angle indicated by the reference numeral  18 . When a portion of the screen is viewed from outside this angle, it appears to be dark. If the partitions  14  extend only vertically, the angle of view is limited from side to side, so that a person sitting next the person using the computer is prevented from reading the data on the screen. If the partitions  14  extend horizontally, as well as vertically, the angle of view is also limited vertically, so that a person standing behind the person sitting and using the computer is also prevented from reading the data on the screen. An example of a privacy screen having closely-spaced partitions or louvers is sold by the 3M Company as a PF14.1 Notebook Privacy Filter. 
     The partitions may be small louvers, ridges, or other shapes. A number of examples from the patent literature describe ways of making and using films including such shapes. For example, U.S. Pat. No. 4,766,023 describes a louvered plastic film that can be more easily handled and installed by coating at least one surface with a solvent-free monomer composition and exposing the coating to ultraviolet radiation to polymerize the coating. U.S. Pat. No. 4,764,410 describes such a film in which the coating is polymerized to an adhesive state, preferably forming a pressure-sensitive adhesive. U.S. Pat. No. 4,812,702 describes a privacy screen using a pair of films with small louvers, placed against one another so that the image from a color CRT must pass through both the films. The louvers of each of the films are tilted in opposite directions at angles from the vertical, substantially reducing any visual Moire patterns resulting from an interaction between the pixels on the CRT screen and the louvers. U.S. Pat. No. 5,528,319 describes a privacy filter having two or more aligned and spaced-apart grids, each of which includes a array of strips blocking light transmission. U.S. Pat. No. 5,993,940 describes a composite including a polymeric film having a first generally planar major surface and an opposite second major surface defined by a multiplicity of elongate parallel contiguous ridge portions, each of which has a generally triangular cross section. The film can provide visual transmission between its first major surface and its second major surface, while coatings along obscured portions of the second major surface prevent viewing of visual images through the film. 
     The patents referenced above do not describe a means for selectively providing the effect of the louvers or partitions. Such means for selectively providing is particularly desirable in the application of a method for achieving privacy in the use of a notebook computer, since such a device is typically used both in public places, such as airplanes and waiting rooms, requiring such privacy, and in more private places, where there is no need for such privacy. In such more private places, a failure to cancel the effect of the louvers or partitions means that the notebook computer is unnecessarily difficult to use, first because the privacy film covering the screen of the computer absorbs a significant portion of the light used to form the image visible on the screen, and second because it is often more difficult to use a computer when the screen has a narrow angle of view. In particular, what is needed is a convenient way to move a louvered privacy filter into place in front of the LCD screen of a notebook computer for use in a situation requiring privacy, and to move the louvered privacy filter out of place to be stored within the notebook computer for use in a situation not requiring privacy. 
     One method for varying the angle through which an LCD screen may be viewed is described in U.S. Pat. No. 6,211,930, which describes variable viewing angle LCD systems for use in automatic teller machines where, during parts of a transaction, it is desirable that information on the screen can be read only by the person using the machine and standing directly in front of the screen. The system includes conventional elements for backlighting a transmission type LCD screen, with such conventional backlighting elements including a reflector, a light source, and an optional brightness enhancer. A collimator and a variable diffuser are interposed between these conventional backlighting elements and the LCD screen. The collimator consists of a pair of louver films through which light passes only in essentially straight lines. The variable diffuser, which can be switched between a transparent state and a scattering state by the application of an electric field, may be a polymer dispersed liquid crystal cell or reverse mode polymer stabilized collisteric texture cell. When the variable diffuser is in its transparent state, the light passing through the LCD screen remains collimated, so that the screen is only visible over a narrow viewing angle. When the variable diffuser is set to scatter the collimated light, light passes through the LCD screen at a range of angles, so a large angle of viewing is made available. 
     What is needed is a method for varying the angle of view without requiring the illumination for wide angles to be first collimated and then scattered. The process of collimating absorbs much of the illumination, in the form of light not passing through the louvers in essentially straight lines. Then, for wide angle viewing, the light is scattered by the diffuser, lowering the peak level of illumination at a viewing angle perpendicular to the screen. 
     FIG. 2 is a fragmentary cross-sectional view of a second type of privacy screen  20  placed in front of a liquid crystal display (LCD) screen  22 . This privacy screen  20  is a polarizing film, which takes advantage of the fact that the light output of the LCD  22 , is strongly polarized. The LCD screen  22  is of a conventional type, including a light source  26 , such as a flourescent light; a diffuser  27 , which directs light from the light source in the outward direction of arrow  28 ; an inner polarizing filter  30 ; a liquid crystal panel  32  including liquid crystal material  34 , extending between a pair of glass plates  36 ; and an outer polarizing filter  38 . Light emitted by the light source  26  is polarized by the inner polarizing filter in a first direction, forming a plane polarized area of illumination passing through the liquid crystal panel  32  and through the outer polarizing filter  38 , which is aligned to pass light polarized in this direction. 
     The liquid crystal panel  32  is capable of forming cross-polarized images against this plane polarized background light. For example, the liquid crystal material  34  comprises mesomorphic crystals which align themselves with an electric field. When these crystals are aligned in this way, polarized light passing through them is rotated into a condition of cross-polarization with the outer polarizing filter  38 , so that the light is absorbed by the outer polarizing filter  38 . When the electric field is removed, the alignment of the crystals returns to a random state, so that polarized light passes through the liquid crystal panel  32  without this rotation. Thus, with the electric field removed the plane polarized light passes through the liquid crystal panel  32 , retaining this plane polarization, and thus also passes through the outer polarizing filter  38 . The glass plates  36  include embedded electrodes, which are selectively driven to produce an electric field forming alphanumeric characters and graphics. A color display is formed by including patterns of red, green, and blue dots within the light path to form colored pixels. The angle through which the image formed by the liquid crystal panel  32  may be effectively viewed is limited by imposing a polarizing privacy filter  20  between the outer polarizing filter  38  and the viewer (not shown). The polarizing privacy filter  20  is aligned to be partially cross-polarizing with the plane polarized light transmitted through the liquid crystal panel  32  without rotation. While the polarizing privacy filter  38  darkens the illuminated areas of the displayed information, even when the image is viewed from a direction indicated by line  42 , perpendicular to the plane of the liquid crystal panel  32 , this darkening effect resulting from interposing the polarizing privacy filter  38  is much greater when the image is viewed from an oblique angle, as indicated by line  44 . Thus, the effective viewing angle of the displayed image is substantially reduced. U.S. Pat. No. 5,488,496 describes this kind of privacy filter in configurations in which at least one of the polarizers associated with creating a cross-polarized image on a liquid crystal display is displaceable. One of these configurations includes cylindrical rollers extending above and below an LCD, with a clear, transparent continuous belt made from a material such as acetate film being tensioned between the rollers. The polarizing privacy filter is constructed on one region of the belt, with this region being the height and width of the LCD. This privacy filter is moved into position between the LCD and the view, and subsequently out of this position, by rotation of one of the rollers using a crank or an electric motor. 
     What is needed is a method for moving a privacy filter between a position in which it is hidden and a position in which it extends in front of the LCD of a notebook computer without a need to attach the privacy filter to a transparent belt. Such a method would eliminate a need to establish a belt tracking system and would eliminate covering the LCD with a plastic sheet even when the privacy filter is moved away from the LCD screen. 
     Another method for achieving privacy when viewing a computer screen is to provide opaque members extending outward from the sides and top of the screen, as described in U.S. Pat. Nos. 4,444,465, 5,717,566, and 5,877,896. While this method has the additional advantage of minimizing glare on the computer screen, and can be used entirely to control glare, it is of limited use in crowded conditions such as airplanes because of the bulkiness of the associated devices. 
     Screens have also been described as being used to reduce glare when viewing cathode ray tube (CRT) displays, including television sets. U.S. Pat. No. 4,819,085 describes a protective screen formed of a frame and a series of woven, non-metallic fibers, which is fitted over a display surface of a CRT to reduce glare and/or to diffuse emanated radiation and static electricity. U.S. Pat. No. 4,633,322 describes a filter, composed of a woven micromesh screen, held in front of a the CRT screen of a television set, and removed from the CRT by being rolled onto a roller when the television set is not being used to prevent an accumulation of dust in the filter mesh. 
     What is needed is a mechanism providing for a conveniently removed privacy filter composed of, for example, a louvered film. 
     SUMMARY OF THE INVENTION 
     Accordingly, an objective of the invention is to provide apparatus for a notebook computer in which a privacy filter, limiting the angle at which the display screen can be viewed, is readily made effective and ineffective; 
     Another objective of the invention is to provide a privacy filter which is easily brought into place in front of the display screen of a notebook computer for use in situations requiring privacy and moved into a storage cavity within the notebook computer for use in situations not requiring privacy; 
     According to a first aspect of the invention, display apparatus for a computing system is provided, including a display panel, a display cover, and a privacy filter. The display panel includes a display surface and causes a visible image to be produced on said display surface in response to electrical signals generated within said computing system. The display cover, which extends around said display panel, includes a window outwardly exposing said display surface and a cavity extending from a first end of said window to a closed end of said cavity. The privacy filter, which includes a filter sheet, is movable between a closed position with said filter sheet extending to cover said display surface and an open position exposing said display surface through said window. A first portion of said privacy filter in said open and closed positions extends into said cavity. The filter sheet limits a viewing angle through which said visible image is discernible. 
     The display apparatus may additionally include a roll rotatably mounted within said cavity, with a first end of said privacy filter being attached to a peripheral surface of said roll. Rotating said roll in a first direction wraps an increasing portion of said privacy filter around said roll, moving said privacy filter from said closed position to said open position, and rotating said roll opposite said first direction unwraps an increasing portion of said privacy filter from said roll, moving said privacy filter from said open position to said closed position. 
     Alternately, the cavity may include a rear cavity portion extending along a side of said display panel opposite said display surface, with the display apparatus additionally including a curved deflector deflecting a portion of said privacy filter into said rear cavity portion as said privacy filter is moved from said open position into said closed position. 
     According to a second aspect of the invention, display apparatus for a computing system is provided, including a display panel, a liquid crystal device, and an external polarizer. The display panel, which includes a display surface, causes a visible image to be produced on said display surface. The visual image includes darkened areas and illuminated areas having light polarized in a first plane. The liquid crystal device extends externally across said display surface, with the display surface being visible through said liquid crystal device. The external polarizer extends externally across said liquid crystal device, which is electrically switchable between a first mode causing light from said illuminated areas to be cross-polarized with said polarizer and a second mode causing light from said illuminated areas to be plane polarized with said external polarizer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary cross-sectional view of a first type of prior art privacy filter placed in front of a computer screen; 
     FIG. 2 is a fragmentary cross-sectional view of a second type of privacy filter placed in front of a liquid crystal display screen; 
     FIG. 3 is a perspective view of a notebook computer built in accordance with a first embodiment of the present invention to include a movable privacy filter; 
     FIG. 4 is a fragmentary vertical cross-sectional view of the notebook computer of FIG. 3, taken as indicated by section lines IV—IV therein to show a storage roller holding a portion of the movable privacy filter; 
     FIG. 5 is a fragmentary vertical cross-sectional view of the notebook computer of FIG. 3, taken as indicated by section lines V—V therein to show a privacy filter held in place in front of a liquid crystal display; 
     FIG. 6 is a fragmentary cross-sectional plan view of the notebook computer of FIG. 3, taken as indicated by section lines VI—VI therein to show a latching structure holding the privacy filter in place; 
     FIG. 7 is a fragmentary vertical cross-sectional view of the notebook computer of FIG. 3, taken as indicated by section lines IV—IV therein to show an alternate storage roller holding a portion of the movable privacy filter; 
     FIG. 8 is a fragmentary cross-sectional plan view of a notebook computer built in accordance with a second embodiment of the present invention to include a movable privacy filter; and 
     FIG. 9 is a fragmentary cross-sectional plan view of a notebook computer built in accordance with a third embodiment of the present invention to include an electrically switchable privacy filter. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 3-6 show a notebook computer  50  built in accordance with a first embodiment of the present invention to include a movable privacy filter  52 . FIG. 3 is a perspective view of the notebook computer  50 , while FIG. 4 is a first fragmentary vertical cross-sectional view of the notebook computer  50 , taken as indicated by section lines IV—IV in FIG. 3 to show a storage roller  54  holding a portion of the privacy filter  52 . FIG. 5 is a second fragmentary vertical cross-sectional view of the notebook computer  50 , taken as indicated by section lines V—V in FIG. 3 to show the movable privacy filter  52  extending in front of a liquid crystal display (LCD)  56 . FIG. 6 is a fragmentary cross-sectional plan view of the notebook computer  50 , taken as indicated by section lines VI—VI in FIG. 3 to show a latch  58  holding the movably privacy filter  52  in it fully extended condition, in front of the LCD  56 . 
     Referring first to FIG. 3, a notebook computer  50  includes a privacy filter  52 , having a flexible filter sheet  59 , extending within a window  60  in a display cover  62 , in front of the LCD  56  (shown in FIG.  5 ). The privacy filter  52  extends to the left, in the direction of arrow  64 , from the window  60 , being attached by means of an adhesive or by clamping means to a peripheral surface of the roller  54 . The privacy filter  52  includes a bar  66 , adhesively attached or clamped to a right end of the filter sheet  59 , to be held in place, with the privacy filter  52  fully extended to the right, opposite the direction of arrow  64 . The flexible filter sheet  59  is understood to be composed of a flexible material, which is at least partially transparent, but which limits the angle  67 , in a horizontal plane, through which the LCD  56  may be effectively viewed. Depending on the composition of the filter sheet  59 , a viewing angle  68  in a vertical plane may also be limited. 
     The filter sheet  59  may include a number of louvers or partitions. For example, the filter sheet  59  may be formed as described in U.S. Pat. No. 4,766,023, the disclosure of which is incorporated herein by reference, to have at least one surface coated with a solvent-free monomer composition. The filter sheet  59  may comprise a pair of films, attached to one another to so that light forming an image from the LCD  56  must pass through both of the films, with the two films being aligned so that the louvers within them are rotated in opposite directions from the vertical, as described in U.S. Pat. No. 4,812,702, the disclosure of which is also incorporated herein by reference. The filter sheet  59  may include two or more aligned and spaced-apart grids, each of which includes an array of strips blocking light transmission, as described in U.S. Pat. No. 5,528,319 the disclosure of which is also incorporated herein by reference. The filter sheet  59  may include a polymeric film having a first generally planar major surface and an opposite second major surface defined by a multiplicity of elongate parallel contiguous ridge portions, each of which has a generally triangular cross section. The film can provide visual transmission between its first major surface and its second major surface, while coatings along obscured portions of the second major surface prevent viewing of visual images through the film, as described in U.S. Pat. No. 5,993,940 the disclosure of which is also incorporated herein by reference. 
     A filter sheet  59  including such louvers or partitions blocks the transmission of light at angles outside a particular viewing angle, with light within the viewing angle being transmitted directly through the individual clear spaces between the louvers or partitions. 
     Alternately, the filter sheet  59  may be formed as described within U.S. Pat. No. 6,488,496, the disclosure of which is also incorporated herein by reference, being composed of a polarizing film oriented so that it is cross-polarized with the light emitted within the illuminated areas of the LCD  56 . 
     A filter sheet  59  including a polarizing film oriented in this way darkens the illuminated areas in the image formed by the LCD  56 , with this darkening effect being markedly increased as the LCD  56  is viewed at larger angle from the normal to the surface of the filter sheet  59 . Thus, the angle through which an image formed by the LCD  56  is discernible by a viewer is again substantially limited by the filter sheet  59 . 
     The notebook computer  50  may otherwise by of conventional construction including a number of keys  70  which are depressed to provide an input from the user and conventional computer devices, such as a processor and data storage (not shown). The notebook computer  50  includes a forward-extending lower portion and an upper portion  74 , which is pivotally mounted along the rear edge of the lower portion  72  to be opened, as shown, or to be closed by pivoting movement in the direction of arrow  76 . 
     Continuing to refer to FIG. 3, and referring additionally to FIG. 4, the roller  54  includes a cylinder  78 , an upper end cap  80  and a lower end cap  82 . The roller  54  rotates on a stationary shaft  84 , which is held in place between an upper socket  86  within an upper cover member  88  and a lower socket  90  in a lower cover member  92 . A non-circular upper portion  94  of the shaft  84  extends within a non-circular upper socket  86  to prevent rotation of the shaft  84 . While the roller  54  rotates on the shaft  84 , it is held between a pair of retaining clips  96  within slots in the shaft  84 . A torsion spring  98 , having a lower end  100  resting against a pin  102 , extending through the shaft  84 , and an upper end  104  extending through the upper end cap  80  of the roller  54 , applies a torque in the direction of arrow  106  to the privacy filter  52 , which is attached to the cylinder  78  of the roller  54 , holding the privacy filter  52  under tension when it is latched in its fully extended condition, as shown in FIG. 3, and wrapping the privacy filter  52  around the roller  53  when the latch  58  is released to move in the direction of arrow  64 . This motion is limited by contact between the bar  66  and the left edge  108  of the window  60 , so that the bar  66  may be used to move the privacy filter  52  in either direction between a fully extended condition and a fully open condition. 
     Referring to FIG. 5, the privacy filter  52  extends within slots  107  along the upper and lower edges of the window  60  in the display cover  62 , being held in place in front of the LCD  56 . The LCD  56  is illuminated from the rear through a diffuser  108  extending between a pair of fluorescent lamps  110 . The diffuser  108  is, for example, composed of a white translucent plastic resin. 
     Referring to FIG. 6, the latch  58  includes a flexible section  112  allowing deflection of the tip  114  of the latch  58  in the outward direction of arrow  115  as the bar  66  of the privacy filter  52  is moved opposite the direction of arrow  64  by the inclined surface of the tip  114 . When the bar  66  is moved past the inclined surface of the tip  114 , the latch  58  snaps inward, opposite the direction of arrow  115  to hold the privacy filter  52  in its fully extended position. Then, the tip  114  can be moved outward, in the direction of arrow  115 , to release the latch  58 , allowing the privacy filter  52  to move in the direction of arrow  64 . 
     FIG. 7 is a fragmentary vertical cross-sectional view of the notebook computer  50 , taken as indicated by section lines IV—IV in FIG. 3 to shown an alternative arrangement for rolling up the privacy filter  52 . In this arrangement, the lower end cap  116  of the roller  117  includes a gear portion  118  driven by a motor  119  through a gear  119   a . Means are also provided to determine when the privacy filter  136  has reached either end of its allowed motion. For example, the bar  66  moves into a slot  119   b  at each (left and right) side of the window  108 , with a light emitting diode  119   c  directing light through the slot  119   b  toward a photodetector  119   d  and with the light path between the light emitting diode  119   c  and the photodetector  119   d  being blocked by the movement of the bar  66  into the slot  119   b.    
     Thus, to open the privacy filter  52 , so that the LCD  56  can be viewed without its influence, the motor  119  is turned on to move the roller  117  in the direction of arrow  106  (shown in FIG. 3) until the photodetector  119   d  at the left side of the window  108  indicates that the bar  66  has moved into the slot  119   b  on the left side of the window  108 . To close the privacy filter  52 , so that the viewing angle of the LCD  56  is significantly reduced, the motor  119  is turned on to move the roller  117  opposite the direction of arrow  106  until the photodetector  119   d  at the right side of the window  108  indicates that the bar  66  has moved into the slot  119   b  on the right side of the window  108 . The motor  119  is preferably operated under control of a routine executing within a processor in the computing system  50 , with an input being provided by the user through the keyboard to begin the process of moving the privacy filter  52 . The motor  119  and associated gearing preferably have enough friction to hold the privacy filter  52  in either its open or closed positions, allowing the elimination of the latch  58  (shown in FIG. 3) and the spring  98  (shown in FIG.  4 ). 
     FIG. 8 is a fragmentary cross-sectional plan view of a notebook computer  120  built in accordance with a second embodiment of the present invention to include a movable privacy filter  122 . The notebook computer  120  includes an upper section  124  having a window  126  through which an LCD  128  is visible, and a lower section  130 , having a number of keys  132  used to provide conventional user inputs. The upper section  124 , which is preferably pivotally mounted on the lower section  130 , is shown extending straight upward. The LCD  128  may again be of a conventional type, producing either monochromatic or color images, being illuminated from behind by a diffuser  133  extending between fluorescent tubes  134 . 
     The movable privacy filter  122  includes a flexible filter sheet  136  which is composed as the filter sheet  59  of the first embodiment, composed as the flexible filter sheet  59 , described above in reference to FIG.  3 . When the filter sheet  136  is held across the front of the LCD  128 , as shown, the notebook computer  120  may be used with relative privacy, since the effective angle through which the LCD  128  can be viewed is substantially reduced. The movable privacy filter  122  also includes a sliding bar  138 , attached by an adhesive or by clamping to the right end of the filter sheet  136 , which is used to move the privacy filter  122  between the closed position in which it is shown and an open position, with the filter sheet  136  moved leftward, in the direction of arrow  140 , to expose the LCD  128  for direct viewing without the intervention of the filter sheet  136 . 
     The notebook computer  120 , built in accordance with the second embodiment, is also similar to the notebook computer  50 , built in accordance with the first embodiment, in that upper and lower edges of the privacy filter  122  extend into slots  142  in the upper and lower edges of the window  126 , in the general manner described above in reference to FIG.  5 . However, in the notebook computer  120 , the left end of the privacy filter  122  extends between a curved inner guide  144  and a curved outer guide  146 . When the portion of the privacy filter  122  extending in front of the LCD  128  is moved to the left, in the direction of arrow  140 , the left end of the privacy filter  112  is moved into a cavity  148  extending behind the diffuser  133 . 
     Preferably, the privacy filter  122  is long enough to extend between the curved guides  144 ,  146  regardless of the position in which the privacy filter  122  is held, with contact between the privacy filter  122  and the guides  144 ,  146  providing sufficient frictional force to hold the privacy filter  122  in place. Thus, a latching mechanism, as explained above in reference to FIG. 6, is not required, and the sliding bar  138  is used to move the privacy filter  122  from any position. 
     The first and second embodiments of the present invention have an advantage over various prior-art devices using a privacy filter installed over the display screen in that the privacy filter is easily moved away from the display screen for ease of system use in situations where this kind of privacy is not required. This advantage is particularly important when the display system is applied within a notebook computer, which is typically used in locations requiring different levels of privacy. 
     U.S. Pat. No. 5,488,496 also describes a display device having a privacy filter which is moved away from the display screen. However, the first and second embodiments of the present invention have an additional advantage of not requiring the complexity of attaching the privacy filter to one side of a clear belt, which is stretched over a pair of rollers, and which must be moved and tracked with the rollers as they are turned. Also, these embodiments of the present invention allow the display screen to be viewed directly when the privacy filter is moved away, without an intervening clear belt surface. 
     FIG. 9 is a fragmentary cross-sectional plan view of a notebook computer  150  built in accordance with a third embodiment of the present invention. The display of the notebook computer  150  includes a conventional LCD display  152 , operating generally as described in reference to FIG. 2 to produce a color or monochrome image from light emitted by one or more light sources  154 , which illuminates the LCD display  152  from behind through a diffuser  156 . Light from the diffuser is polarized as it is transmitted through an inner polarizing filter  158 . In areas to be illuminated, this polarized light from the inner polarizing filter  158  is transmitted through the liquid crystal panel  160 , including liquid crystal material  162  between glass plates  164 , and through a second polarizing filter  166 , since this polarized light is plane polarized with the second polarizing filter  166 . However, in areas corresponding to a darkened image to be produced, an electric field is placed across electrodes within the glass plates  164 , causing liquid crystals in the liquid crystal material  162  to become aligned in a manner causing the rotation of polarized light transmitted from the diffuser  156 , so that such light becomes cross-polarized with the front polarizing filter  166 . Thus, within such image areas a substantial portion of the light is absorbed within the front polarizing filter  166 . 
     In accordance with the third embodiment of the present invention, the notebook computer  150  also includes a liquid crystal device  168  which is used to form an electrically switchable privacy filter. The liquid crystal device  168  includes liquid crystal material  170  extending between a pair of glass plates  172 . Electrodes  174 , extending within the glass plates  172 , are used to produce an electric field which is uniform within the liquid crystal material  170 , but which can be varied in intensity. This electric field is used to rotate the polarized light passing through the liquid crystal device  168 , coming from the illuminated areas of the image produced within the LCD, so that this light is at least partially cross-polarized with a third polarizing filter  176 . Preferably, a voltage source  178 , connected to the. Electrodes  174 , produces a variable voltage under control of the user of the notebook computer  150 , with the highest voltage giving the greatest degree of cross-polarization between the polarized light coming from illuminated areas of the image produced by the LCD display  152 , so that the illuminated image is darkened to the maximum extent and so that the effective viewing angle is reduced to a minimum value to afford maximum privacy. When no voltage is placed across the electrodes  174 , polarized light is not rotated as it is transmitted through the liquid crystal device  168 , providing a maximum viewing angle for ease of use of the computer  150 , while not providing for privacy. Intermediate levels of voltage produce intermediate effects, allowing the user to vary these effects, balancing the degree of privacy required with his own difficulty (if any) of reading data displayed on the LCD display  152 . 
     Alternately, the orientation of the third polarizing filter  176  may be changed so that the application of an electric field within the liquid crystal device  168  is used to bring the polarized light coming from illuminated areas of the image produced by the LCD display  152  into plane polarization with the third polarizing filter  176  through rotation of the direction of polarization of the light. Then, a maximum level of privacy would be achieved without the application of the electric field across the electrodes  174 . This type of operation may be preferable in an application where a maximum level of privacy is needed, as in an airplane seat, when the notebook computer  150  is operated on its own battery power, placing an emphasis on a need to conserve power. 
     The voltage source  178  may be made variable under the control of the user in a number of ways well known in the art of such electronic circuits. For example, a potentiometer may be used as part of a voltage dividing circuit determining the output voltage, or a subroutine executing within a processor in the computer  150  may cause a number to be stored in a register within system memory, a digital-to-analog convertor producing a voltage in response to the number, and with the voltage source  178  being driven to produce a voltage equal to that of the digital-to-analog convertor. 
     The third embodiment of the present invention has an advantage over various prior-art devices using a privacy filter installed over the display screen in that the privacy filter can effectively be switched on an off. Again, this advantage is particularly important in a notebook computer application, as such systems are typically used in situations both requiring and not requiring this kind of privacy. 
     U.S. Pat. No. 6,211,930 describes apparatus for switching the effective angle through which an LCD display can be read. However, the third embodiment of the present invention has an advantage, when compared to the apparatus of this patent, of not requiring the light used to backlight the LCD screen for viewing at a wide angle to be first collimated and then diffused. This advantage is particularly important in its application to a notebook computer, which must often be operated on battery power, instead of in the application to an ATM (automated teller machine) terminal, as described in the patent. 
     While the invention has been shown in its preferred forms or embodiments with some degree of particularity, it is understood that this description has been given only by way of example and that numerous changes in the details of construction and use, including changes in the combination and arrangement of parts, may be made without departing from the spirit and scope of the invention.