Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
1998-11-18
2001-04-03
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C349S064000
Reexamination Certificate
active
06211930
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to visual displays and, in particular, to displays enabling variation of the relationship between brightness and viewing angle.
It is well-known that in connection with any flat screen display, there is a relationship between the perceived brightness of the display and the viewing angle relative to the plane of the flat screen. This variation can be of value or a disadvantage, depending upon circumstances. Thus, in connection with a retro-reflective beaded cinema screen, the relatively narrow angle over which a satisfactorily bright image may be seen forces the design of narrower cinemas deeper from front to back, which may not always be the best use of space. On the other hand, in applications where the information to be displayed on screen may be needing to be kept secure, for example on the screen of a video display unit, it may be desirable to ensure that the image displayed on the screen can only be seen over a highly restricted range of viewing angles, usually only if the viewer's eyes are in a relatively narrow angled cone having its axis normal to the plane of the screen.
It is well-known to provide security screens for fitting over cathode ray tube displays. These consist of a screen incorporating some form of louvre, the components of the louvre running essentially in planes normal to the plane of the screen. The louvre may consist of a plurality of parallel planes in which case the restriction in viewing angle will apply, e.g. horizontally but not vertically or vice versa, or it may consist of a set of cells, e.g. formed by the notional intersection of two sets of orthogonal louvre panels, or, for example, the louvre may take the form of a honeycomb type construction.
In all such cases, the restriction on viewing angle imparted by the louvre type screen is permanent and invariable once the screen has been placed over the face of the display. While, in many cases, this may be entirely satisfactory, equally it is often not satisfactory at all.
In a screen display system is disclosed in which the range of viewing angles can be altered by the use of a screen which has an actuatable louvre. When unactuated, the screen is transparent and the display screen which it overlies can accordingly be viewed from a relatively wide range of viewing angles. If the actuatable louvre screen is actuated, on the other hand, planes within the screen running substantially perpendicular to the display screen are rendered opaque, and accordingly the display screen can only be viewed when those planes are substantially edge on to the observer. Thus, by simply actuating this type of louvre screen, the display viewing angle may be varied from wide to narrow and vice versa. Such a actuatable louvre screen is described in more detail in the above-noted application, and may be straightforwardly constructed using standard liquid crystal display technology.
One of the disadvantages of the system proposed in EP-A-0802684 is that the louvre screen must be placed over the display, which inevitably reduces the brightness of that display. This is naturally disadvantageous.
The problem underlying the invention is to produce a variable viewing angle display which does not suffer from the disadvantages of the prior suggestions.
SUMMARY OF THE INVENTION
We have now found that variable viewing angle displays may be constructed by interposing between a standard diffuse backlight illumination source and a transmission type liquid crystal display means for collimating the illumination from the backlight display to a greater or lesser extent.
Preferably, the means consist essentially of a collimator device adjacent a standard diffuse backlight unit and, between the collimator device and the transmission type liquid crystal display, a transparent/translucent member of controllable diffusivity. This last member should preferably be controllable between a state in which it is entirely transparent, i.e. allows the collimated light simply to pass through it without lateral dispersion or diffusion and one in which it is highly diffusive so that it acts to scatter and re-emit the collimated light, the re-emission occurring over a wide range of angles relative to the plane of the device itself.
The collimator device may be any convenient device which will act to channel the light from the diffuse backlight source so as to emerge effectively as a collimated beam. Preferred are thin films containing an internal louvre structure with the planes of the louvres being essentially perpendicular to the plane of the film. The louvre spacing is generally substantially less than the thickness of the film, preferably ten percent of the thickness of the film. The lower the percentage the greater the collimation, but the greater, usually, the expense of manufacturing the louvre film and/or the greater the transmission losses through the film.
It is often found useful to use a pair of adjacent louvre films, each with a relatively low ratio of louvre spacing to film thickness, for example from 1 to 4 to 1 to 7, since the combined louvre spacing to thickness ratio is then half that of either film but, more importantly, the tendency to form moiré patterns between the collimation device and the transmission type liquid crystal display is perceptibly reduced.
As noted above, between the transmission type liquid crystal display and the collimating device, there is a variable diffusibility device. Preferably, this takes the form of a so-called scattering cell which can be rendered optically transparent or optically scattering by the application of an appropriate electrical field to material, usually a liquid crystal material, comprised within the cell. Two known types of scattering cell may be used, viz. polymer dispersed liquid crystal cells or reverse mode polymer stabilized cholesteric texture cells. Both types are commercially available.
As will be apparent from consideration of the principle on which the present invention operates is that of varying the collimation/diffusion of the backlighting, the transmission type liquid crystal display may be selected from any such display, several varieties being already commercially available. Thus, the present invention may be used in combinations where the transmission type liquid crystal display overlying the backlighting unit, collimating unit and variable diffusing unit is, for example, one of the following types:
Super twisted nematic
Twisted nematic
In-plane switching
Ferro-electric liquid crystal
Passive matrix displays
Active matrix displays using a thin film transistor matrix or a metal insulator metal matrix
The invention is of particular value in connection with display systems including transmission type display screens (usually LCD type screens) wherever security considerations are important. Thus, one particular area of application is in connection with displays used in automatic teller machines, i.e. fixed installations usually located in public places and having a screen display in a public place, the display giving information to a person who wishes to carry out some form of banking or other financial transaction, interacting with the machine via their observation of the display screen and, usually, an input device such as a touch screen, touch pad, keypad, keyboard or the like. In particular, the present invention provides automatic teller machines including variable viewing angle displays as set out above, wherein the viewing angle for the display is changed in accordance with the operating status of the automatic teller machine. By way of example, when no-one is attempting to transact any business with the machine, the viewing angle of the screen may be wide and the screen may carry, e.g. advertising information, animated cartoons to attract attention, or the like. However, when a transaction involving, e.g. a customer's balance is attempted, the balance may be displayed on the screen, but only after the display has been switched to narrow viewing angle mode, thus enabling only the user of the machine to see the bal
Bader Gerhard
Kallfass Traugott
Knight Kenneth R.
Koenig Walter
Lueder Ernst
Dudek James A.
NCR Corporation
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